Constance Dubant

Aurore Meugnot
email: aurore.meugnot@universite-paris-saclay.fr

Keywords: Motor Imagery, Mental Practice, Spinal Manipulation, Chiropractic, Motor Skill

Motor imagery practice paired with physical practice led to a similar improvement of thoracic spinal manipulation than physical practice alone

Constance Dubant, Aurore Meugnot

Introduction

Spinal manipulation (SM) is a complex bimanual skill widely used by chiropractors to treat their patients [START_REF] Beliveau | The chiropractic profession: a scoping review of utilization rates, reasons for seeking care, patient profiles, and care provided[END_REF] . Expertise in spinal manipulation relies on strength dosage and reproducibility to be administered safely and effectively by controlling several biomechanical variables (e.g. speed and magnitude/duration of applied force). The acquisition of this skill requires intensive practice before it can be adaptive and context dependent, with graduated targeted learning outcomes [START_REF] Descarreaux | Learning spinal manipulation skills: assessment of biomechanical parameters in a 5-year longitudinal study[END_REF][START_REF] Ndetan | Prevalence of musculoskeletal injuries sustained by students while attending a chiropractic college[END_REF] . For instance, in their 4th and 5th years of study, students practice SM in order to be able to control and adapt their strength to the patient's morphology. However, this intensive learning can sometimes lead to injuries. The prevalence of musculoskeletal injuries amongst students when learning and performing SM is well documented in the literature [START_REF] Ndetan | Prevalence of musculoskeletal injuries sustained by students while attending a chiropractic college[END_REF][START_REF] Triano | Biomechanics -Review of approaches for performance training in spinal manipulation[END_REF][START_REF] Bisiacchi | Physical injury assessment of male versus female chiropractic students when learning and performing various adjustive techniques: a preliminary investigative study[END_REF][START_REF] Holm | Work-related injuries of doctors of chiropractic in the United States[END_REF][START_REF] Macanuel | Characterization of side effects sustained by chiropractic students during their undergraduate training in technique class at a chiropractic college: a preliminary retrospective study[END_REF][START_REF] Stock | Workplace ergonomic factors and the development of musculoskeletal disorders of the neck and upper limbs: a meta-analysis[END_REF][START_REF] Thomsen | Risk factors for hand-wrist disorders in repetitive work[END_REF] . The authors especially point out the impact of repetitive forceful work on lower back, wrist and shoulder disorders in manual therapists. A recent study proposed a strength and conditioning program based on preventative exercises to reduce the risk of injuries in chiropractic students learning spinal manipulation [START_REF] Descarreaux | Learning spinal manipulation: the importance of augmented feedback relating to various kinetic parameters[END_REF] . Other chiropractic scholars also seek to develop specific didactic strategies to optimise chiropractic teaching techniques [START_REF] Ndetan | Prevalence of musculoskeletal injuries sustained by students while attending a chiropractic college[END_REF][START_REF] Triano | Biomechanics -Review of approaches for performance training in spinal manipulation[END_REF][START_REF] Bisiacchi | Physical injury assessment of male versus female chiropractic students when learning and performing various adjustive techniques: a preliminary investigative study[END_REF][START_REF] Holm | Work-related injuries of doctors of chiropractic in the United States[END_REF][START_REF] Macanuel | Characterization of side effects sustained by chiropractic students during their undergraduate training in technique class at a chiropractic college: a preliminary retrospective study[END_REF] . For instance, several studies based on motor learning research investigated the potential of augmented feedback using an instrumented mannequin to facilitate the learning of SM [START_REF] Descarreaux | Learning spinal manipulation: the importance of augmented feedback relating to various kinetic parameters[END_REF][START_REF] Scaringe | The effects of augmented sensory feedback precision on the acquisition and retention of a simulated chiropractic task[END_REF][START_REF] Triano | Quantitative feedback versus standard training for cervical and thoracic manipulation[END_REF] . Practicing with such a device provides immediate quantitative feedback on specific biomechanical aspects of SM, which may be useful for both the student and the teacher, who would be able to individualise teaching by focusing on aspects of spinal manipulation that have not been acquired fully.

In the same vein, motor imagery practice (MIP), which is "the act of mentally repeating movements (without physically doing it) several times with the intention of learning a new ability or perfecting a known skill" [START_REF] Jackson | Potential role of mental practice using motor imagery in neurologic rehabilitation[END_REF] , could be a promising tool to optimise SM training. The seminal theory proposed by Marc Jeannerod [START_REF] Jeannerod | Neural simulation of action: a unifying mechanism for motor cognition[END_REF] , and supported by robust neurophysiological and behavioural studies, suggests the existence of shared neural representations; i.e. motor imagery (MI) activates neural substrates that partially overlap those activated by movement execution. On the functional level, the same processes involved in the planning and programming of motor skills appear to underly MI [START_REF] Guillot | The Neurofunctional Architecture of Motor Imagery in "Advanced Brain Neuroimaging Topics in Health and Disease -Methods and Applications[END_REF][START_REF] Dickstein | Motor Imagery in Physical Therapist Practice[END_REF][START_REF] Cuenca-Martínez | The Role of Movement Representation Techniques in the Motor Learning Process: A Neurophysiological Hypothesis and a Narrative Review[END_REF][START_REF] Jeannerod | Neural simulation of action: a unifying mechanism for motor cognition[END_REF] . Indeed, a bulk of evidence from fundamental to applied research in various domains such as cognitive neuroscience, sport science and neurohabilitation show the benefits of MIP paired with physical practice (PP) to enhance the efficacy of motor training or recovery [START_REF] Guillot | The Neurofunctional Architecture of Motor Imagery in "Advanced Brain Neuroimaging Topics in Health and Disease -Methods and Applications[END_REF][START_REF] Malouin | Towards the integration of mental practice in rehabilitation programs. A critical review[END_REF][START_REF] López | Motor imagery as a complementary technique for functional recovery after stroke: a systematic review[END_REF][START_REF] Dickstein | Motor Imagery in Physical Therapist Practice[END_REF][START_REF] Cuenca-Martínez | The Role of Movement Representation Techniques in the Motor Learning Process: A Neurophysiological Hypothesis and a Narrative Review[END_REF] . For instance, in an experimental study, Allami and collaborators (2008) [START_REF] Allami | Visuo-motor learning with combination of different rates of motor imagery and physical practice[END_REF] investigated the optimal proportions of MIP paired with PP to improve the movement time of an action that involved grasping an object and to inserting it in an appropriate slot. The results showed that participants who performed 120 imagined trials of the action followed by 120 physical repetitions reached the same level of performance as the participants who performed 240 physical repetitions. These findings illustrate the potential of MIP paired with PP, suggesting that MIP could be used to partially replace physical practice when learning a motor skill. Therefore, MIP paired with PP appears to be a suitable technique for learning SM to reduce the risk of injuries associated with excessive physical repetitions. The present study aimed to examine whether MIP paired with a reduced number of PP might lead to a similar improvement of thoracic spinal manipulation as PP alone.

Method

Participants

Forty-two 4th-and 5th-year chiropractic students at the Franco-European Institute of Chiropractic (IFEC) in Paris (France) were recruited through advertisements sent via IFEC's student mailing lists. Exclusion criteria included suffering musculoskeletal pain in the shoulder, neck or low back region currently or in the past 3 months, or any reported history of chronic physical disability.

The study was approved by the Ethics Committee of IFEC (#CE 2018-03-15-3) and participants gave written informed consent to take part in this study. They were randomly assigned to the PP group (mean age = 23 years, SD =1.5, range = 21-26 years, 9 women) or the PP+MI group (mean age = 23 years, SD =1.5, range = 21-26 years, 16 women).

Material and Tasks

Thoracic Spinal Manipulation Task. In pre-test and post-test, the participants performed a thoracic spinal manipulation (TSM) task using a mannequin (H.A.M. series; CMCC, Toronto, Ontario) resting on a force-sensing table (Leander 900 Z series table equipped with an AMTI force plate; see Triano et al., 2012 for similar material). The task comprised 2 phases: a warm-up phase composed of 2 TSM at 3 intensities (low: force applied = 300 Newton (N); medium = 450 N; high = 600 N) = 6 pseudo-randomised TSM. And, an experimental phase composed of 12 pseudo-randomised TSM at the same 3 intensities: 1 TSM = 1 trial; 4 trials per intensity (low, medium, high) = 12 TSM in total in the experimental phase. For each trial, the experimenter verbally told the participants which TSM intensity to apply. In the experimental phase of each trial, the force applied by the participant on the mannequin's contact point was recorded in real time on a computer connected to the force plate using Labview software (National Instruments, Austin, TX). Visual feedback on performance (i.e. force-time profile displayed in real time) was hidden from the participants. Each session began with 2 video clips displaying a teacher performing TSM from either a 1stperson perspective (the camera was attached to his forehead) or a 3rd-person perspective (the camera faced the teacher). Then, training involved 2 phases: a warm-up phase and a practice phase. For each session, the participants were given a technical form with instructions and the training sequence. For the PP + MI group, there was also an MI form with instructions on mental practice (see supplementary files).

Movement of Imagery Questionnaire short-version (MIQ-SV).

For the PP group, the warm-up phase was the same as the TSM task. The practice phase was composed of 12 physical repetitions for each intensity (low: force applied = 300 N; medium = 450 N; high = 600 N) for a total of 36 physical repetitions. The session was organised in blocks of 3 repetitions with the same intensity. The order of the blocks was pseudo-randomised by the experimenter and was changed at each session.

For the PP + IM group, the warm-up phase included 6 TSM with 4 physical repetitions and 2 mental repetitions. The practice phase was composed of 8 physical repetitions and 8 mental repetitions for each intensity (low: force applied = 300 N; medium = 450 N; high = 600 N) for a total of 48 TSM with 24 physical repetitions and 24 mental repetitions. The session was organised in blocks of 2 physical repetitions followed by 2 mental repetitions at the same intensity. The order of the blocks was pseudo-randomised by the experimenter and was changed at each session (see a sample session for each group in supplementary files).

Fig. 2 approximately here

Data analysis

Thoracic Spine Manipulation Task. The peak force (in Newton, N) corresponding to the maximal value of force application was computed for each TSM. Then, absolute error (AE) was calculated for each trial using the formula:

AE (N) = !"#$ !"#$% -!"#$% !"#$%!"&
The force required corresponded to the intensity of a trial: low, medium or high (see the description of the TSM task above). High AE represented low performance, while low AE represented high performance.

MIQ.

A score was calculated for each perspective (IV, IK) from the sum of corresponding items.

High scores represented difficulty performing motor imagery, while low scores represented motor imagery with ease. All statistical analyses were conducted using SPSS (version 19). A mixed analysis of Variance (ANOVA) was performed on AE with the intensity of each trial (low, medium, high) as a within-subject factor and the group (PP, MI+PP) as a between-subject factor. A Greenhouse-Geisser correction was used when the sphericity assumption was violated (p < .05 for Mauchly's sphericity test). Alpha was set at .05. In the event of significance, pairwise comparisons using the Bonferroni correction were applied. Shapiro-Wilk tests showed that IK and IV scores were normally distributed (p >.05) and there was homogeneity of variances, as determined by Levene's test for equal variances (p > .05). Paired t-tests were performed on the IK and IV scores to compare MI abilities in pre-test vs. post-test.

Results

Thoracic Spine Manipulation Task. Mixed ANOVA on AE revealed a main effect of intensity [F(1.7, 67.6) = 4.05, p = .027, η² = .94] and a main effect of test [F(1,39) = 28.9, p < .0001, η² < .42]. As illustrated by Fig. 1, post-hoc tests with the Bonferroni correction revealed that AE was significantly higher for high intensity (mean = 72 N, SD =49 ) than low (mean = 57 N, SD =34.5, p=.03) or medium intensities (mean = 59 N, SD =32 , p=.02). There was no significant difference between low and medium intensities (p= .61). Fig. 2 shows that AE was significantly lower in post-test (mean = 51 N, SD =27) than in pre-test (mean = 74 N, SD =46). There was no significant interaction between the test and the group [F(1.39) = 0.46, p = .46, η² < .012].

Discussion

This study aimed to examine the potential of MIP as a learning technique in chiropractic training, particularly in improving strength dosage in thoracic spinal manipulation in 4th and 5thyear chiropractic students. The results show that the number of physical repetitions is reduced when PP is paired with MIP while reaching a similar level of performance compared with PP alone. Therefore, MIP appears to be a suitable tool to reduce physical practice to limit the risks related to repetitive spinal manipulation practice in students. The results observed in the present study showed that the participants in both the MI+PP and PP groups improved the accuracy of strength application significantly during SM, as shown by a lower difference between the intensity of SM required and the force applied during the TSM task. Moreover, the significant improvement of MI abilities in the MI+PP group appear to suggest that the chiropractic students were able to apply mental practice efficiently. Overall, these findings confirm that MIP can be used to partially reduce PP in the learning of thoracic spinal manipulation. This study agrees with previous experiments exploring the potential of MIP paired with PP in surgical training, although the results observed in these works might be debatable [START_REF] Sanders | Comparing the effects of physical practice and mental imagery rehearsal on learning basic surgical skills by medical students[END_REF][START_REF] Sanders | Learning basic surgical skills with mental imagery: using the simulation centre in the mind: Learning basic surgical skills with mental imagery[END_REF][START_REF] Immenroth | Mental Training in Surgical Education[END_REF] .

Another question may be raised when combining MIP with PP. Indeed, the authors investigated whether motor learning can be optimised by adding MIP, i.e. MIP could be considered a supplement to physical practice [START_REF] Hidalgo-Perez | Effectiveness of a motor control therapeutic exercise program combined with motor imagery on the sensorimotor function of the cervical spine : a randomized controlled trial[END_REF][START_REF] Dana | Internal and External Imagery Effects on Tennis Skills Among Novices[END_REF][START_REF] Slimani | Effects of mental imagery on muscular strength in healthy and patient participants: A systematic review[END_REF] . These studies imply a similar amount of physical practice in both training groups, with one of the 2 groups performing additional MIP.

For instance, Hidalgo-Perez and collaborators (2015) investigated the impact of MI combined with PP when healthy participants practiced motor control exercises (generally used in a therapeutic program to manage neck pain) to improve sensorimotor function [START_REF] Immenroth | Mental Training in Surgical Education[END_REF] . Their results showed that craniocervical neuromotor control, and the subjective perception of fatigue after effort were significantly higher in the group for which MI was added to PP, compared with the group that trained only physically. Thus, it would be of interest to test the impact of MIP+PP compared with PP alone on the learning of spinal manipulation, while maintaining a similar amount of PP in each group.

Moreover, the students who participated in this experiment were in 4th and 5th year at the chiropractic college. Therefore, they would be in the 2nd stage of SM learning, which consists of reducing trial-to-trial variability of force production, indicating automated control of movement.

At the end of this stage, students will be able to vary precisely the degree of force production between successive trials. This ability is critical, and ensures that the chiropractor is able to deliver effective SM and adapt SM to patient morphology. MI focused mainly on the sensations associated with the force applied to the mannequin (i.e. kinaesthetic imagery). It is likely that MI would have facilitated the integration of proprioceptive inputs used by the central system to update internal models or memories of motor programs [START_REF] Wolpert | Computational principles of movement neuroscience[END_REF] . In the future, it would be interesting to investigate the potential of MI to enhance the acquisition of SM in the 1st stage of learning, i.e. when students learn the correct posture in the different phases of SM (e.g. pre-load force, time to peak force) so as to stabilise the rate of force production over time [START_REF] Descarreaux | Learning spinal manipulation skills: assessment of biomechanical parameters in a 5-year longitudinal study[END_REF][START_REF] Triano | Biomechanics -Review of approaches for performance training in spinal manipulation[END_REF] . Other MI modalities might be also considered, such as visual imagery, which might be interesting very early in the learning of SM skills.

There are some limitations to this study. Firstly, the contents of the training sessions were similar to the TSM task used when evaluating SM performance (i.e. in pre-test and post-test sessions) since each training session consisted of repetitions of SM at the same intensities as those used in the TSM task. Moreover, only 3 levels of force intensity were used within a training session, which may have reinforced these specific motor programs of SM, and probably favoured the development of automaticity in performance. However, the lack of variability in training sessions and skill transfer evaluation did not allow us to conclude that the students improved their ability to regulate the rate of force production when performing SM trials. Thus, although the results showed that the students did reduce the mean variability of SM peak at the given intensities, the task was probably quite simple with respect to their level of expertise. It would have been more suitable to use other intensities from one training session to another, and to propose a TSM task with other intensities than those used during practice. Secondly, performance was evaluated quantitatively, based on knowledge of results (i.e. difference between the force required compared to the force applied), which was appropriated to evaluate the effect of the training program. However, it did not make it possible to specify which aspects of SM skill were improved consecutive to practice, and whether certain aspects would benefit more from MI than others. Future studies including qualitative indicators of SM performance and learning (namely, biomechanical variables usually used to describe SM 4 ) will contribute to the understanding of the effect of MI paired with PP in the learning of SM skills. Moreover, a recent study [START_REF] Touche | Instruction Modes for Motor Control Skills Acquisition: A Randomized Controlled Trial[END_REF] demonstrated that MI may be an effective strategy (used as a teaching method) to explain a motor task. It would be pedagogically interesting to use MI as a teaching method to encourage the understanding of biomechanical aspects of SM skill, which are not always easy to verbalise.

Conclusion

The results of this study highlighted the potential of MIP paired with MI practice to improve the learning of the spinal manipulation technique by chiropractic students. It encourages the use of MI as a teaching tool in the context of the learning of manual therapies, notably to limit the risk of injury caused by intensive practice. Future investigations are needed to clarify the effectiveness of MIP in the context of SM skills learning, e.g. by examining which biomechanical variables would be sensitive to mental practice. Moreover, a recent study [START_REF] Touche | Instruction Modes for Motor Control Skills Acquisition: A Randomized Controlled Trial[END_REF] demonstrated that MI may be an effective strategy (used as a teaching method) to explain a motor task. It would be pedagogically interesting to use MI as a teaching method to encourage the understanding of biomechanical aspects of SM skill, which are not always easy to verbalise.

Practical Applications

• Potential of MIP paired with PP was examined to reduce physical repetitions while reaching the same level of performance.

• Despite fewer physical repetitions, MI+PP participants improved as much as PP participants.

• Motor Imagery should be considered in the development of didactic strategies related to SM skills.

  The MIQ-SV(Hall & Pongrac, 1983 [START_REF] Hall | The measurement of imagery ability[END_REF] ) consists of 16 items relating to either the visual imagery (VI) of movement (i.e. movement is visualised from an external, 3rd-person perspective) and the kinaesthetic imagery (KI) of movement (i.e. movement simulation based on proprioceptive and internal sensations associated to the movement). For each imagery perspective, the difficulty of movement simulation was assessed on a 7-point Likert-type scale (from 1 = movement very easy to visualise or feel, to 7= movement very difficult to visualise or feel).Training program. The training program was composed of 2 20-minute sessions per week(Tuesday and Thursday afternoon) for 5 weeks for a total of 10 sessions. During the Tuesday session, the participants worked in pairs and practiced on the sensor table used in pre-test or posttest with visual feedback being visible. During the Thursday session, the participants were in groups of 5 and practiced on classical tables with no feedback on performance.

Fig. 1

 1 Fig. 1 approximately here

Fig. 3

 3 Fig. 3 and 4 approximately here

Acknowledgments. The authors thank chiropractic students for their participation in the experiment, and individuals (teachers/administration) of IFEC for their assistance to complete this study.

Funding sources and conflict of interest. No funding sources and conflicts of interest to declare for this study.

 : H,H,H,L,L,L,H,H,H,M,M,M,L,L,L,etc.. M,M,L,H : H,H,H,H,L,L,L,L H,H,H,H M,M,M,M,L,L,L,L,etc

Video clips displaying a teacher

WARM-UP = 6 TSM (4 physical, 2 mental) (2 x 3 intensiHes) e.g. : L,


Phuong Vuong

Suresh Sundaram

Adama Mballo

Gilles Patriarche

Stefano Leone

Fouad Benkhelifa

Soufiane Karrakchou

Tarik Moudakir

Simon Gautier

Paul L Voss

Jean-Paul Salvestrini

Abdallah Ougazzaden

Control of the Mechanical Adhesion of III-V Materials Grown on Layered h-BN

Keywords: 2D boron nitride, III-nitrides, semiconductors, mechanical transfer, flexible (opto) electronics, transferrable nanodevices

établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

■ INTRODUCTION

Optoelectronic and electronic applications of hexagonal boron nitride (h-BN), a two-dimensional (2D) ultrawide-bandgap semiconductor, have attracted extensive attention in the last decade. Van der Waals (vdW) epitaxial growth of 2D h-BN layers and subsequent three-dimensional (3D) III-nitride device structures-based h-BN has been demonstrated. [START_REF] Kobayashi | Layered Boron Nitride as a Release Layer for Mechanical Transfer of GaN-Based Devices[END_REF][START_REF] Hiroki | Suppression of Self-Heating Effect in AlGaN/GaN High Electron Mobility Transistors by Substrate-Transfer Technology Using h-BN[END_REF][START_REF] Ayari | Wafer-Scale Controlled Exfoliation of Metal Organic Vapor Phase Epitaxy Grown InGaN/GaN Multi Quantum Well Structures Using Low-Tack Two-Dimensional Layered h-BN[END_REF][START_REF] Ayari | Gas Sensors Boosted by Two-Dimensional h-BN Enabled Transfer on Thin Substrate Foils: Towards Wearable and Portable Applications[END_REF][START_REF] Motala | Transferrable AlGaN/GaN High-Electron Mobility Transistors to Arbitrary Substrates via a Two-Dimensional Boron Nitride Release Layer[END_REF][START_REF] Glavin | Flexible Gallium Nitride for High-Performance, Strainable Radio-Frequency Devices[END_REF][START_REF] Lee | Wafer-Scale Single-Crystal Hexagonal Boron Nitride Film via Self-Collimated Grain Formation[END_REF] Kobayashi et al. demonstrated the successful transfer of GaN-based blue light-emitting diodes (LEDs) from h-BN onto foreign substrates using h-BN as a mechanical release layer. [START_REF] Kobayashi | Layered Boron Nitride as a Release Layer for Mechanical Transfer of GaN-Based Devices[END_REF] A goal of subsequent research was the wafer-scale transfer of III-N devices to flexible substrates. Transfers of different device structures such as AlGaN/GaN high-electronmobility transistors (HEMTs), [START_REF] Hiroki | Suppression of Self-Heating Effect in AlGaN/GaN High Electron Mobility Transistors by Substrate-Transfer Technology Using h-BN[END_REF][START_REF] Glavin | Flexible Gallium Nitride for High-Performance, Strainable Radio-Frequency Devices[END_REF] AlGaN/GaN-based gas sensors, [START_REF] Ayari | Gas Sensors Boosted by Two-Dimensional h-BN Enabled Transfer on Thin Substrate Foils: Towards Wearable and Portable Applications[END_REF] InGaN/GaN multi-quantum-well LEDs, and solar cells [START_REF] Liang | Quasi van Der Waals Epitaxy Nitride Materials and Devices on Two Dimension Materials[END_REF][START_REF] Gautier | Structural and Compositional Characterization of MOVPE GaN Thin Films Transferred from Sapphire to Glass Substrates Using Chemical Liftoff and Room Temperature Direct Wafer Bonding and GaN Wafer Scale MOVPE Growth on ZnO-Buffered Sapphire[END_REF][START_REF] Kelly | Large Free-Standing GaN Substrates by Hydride Vapor Phase Epitaxy and Laser-Induced Liftoff[END_REF] have been then demonstrated at the wafer scale.

Ultrawide-bandgap (UWBG) h-BN also reveals a new generation of integration between 2D materials and bulk materials for physical coupling, such as deep ultraviolet (DUV) sources, [START_REF] Caldwell | Photonics with Hexagonal Boron Nitride[END_REF][START_REF] Li | Millimeter-Scale Hexagonal Boron Nitride Single Crystals for Nanosheet Generation[END_REF] neutron detectors, [START_REF] Ahmed | Solid-State Neutron Detectors Based on Thickness Scalable Hexagonal Boron Nitride[END_REF][START_REF] Mcgregor | Thermal Neutron Detection with Pyrolytic Boron Nitride[END_REF] and a hybrid III-N/2D ultraviolet (UV) photodetection. [START_REF] Bae | Integration of Bulk Materials with Two-Dimensional Materials for Physical Coupling and Applications[END_REF][START_REF] Journot | Self-Assembled UV Photodetector Made by Direct Epitaxial GaN Growth on Graphene[END_REF] In these cases, the h-BN layer should adhere to the neighboring layers. However, it has been challenging to control spontaneous delamination and the degree of force needed for lift-off of III-N device hetero-structures on h-BN. [START_REF] Ahmed | Solid-State Neutron Detectors Based on Thickness Scalable Hexagonal Boron Nitride[END_REF][START_REF] Snure | GaN and AlGaN/GaN Heterostructures Grown on Two Dimensional BN Templates[END_REF][START_REF] Ayari | Novel Scalable Transfer Approach for Discrete III-Nitride Devices Using Wafer-Scale Patterned h-BN/Sapphire Substrate for Pick-and-Place Applications[END_REF][START_REF] Chugh | Flow Modulation Epitaxy of Hexagonal Boron Nitride[END_REF] Researchers have reported spontaneous delamination of h-BN during growth owing to the mismatch in the thermal expansion coefficient between h-BN and substrates during cooldown. [START_REF] Chugh | Flow Modulation Epitaxy of Hexagonal Boron Nitride[END_REF][START_REF] Bera | Strain Distribution and Thermal Strain Relaxation in MOVPE Grown HBN Films on Sapphire Substrates[END_REF] The delamination of h-BN is also observed during front-end device processing with some liquids during cleaning steps. [START_REF] Ahmed | Solid-State Neutron Detectors Based on Thickness Scalable Hexagonal Boron Nitride[END_REF][START_REF] Chugh | Flow Modulation Epitaxy of Hexagonal Boron Nitride[END_REF] These issues influence the performance of the devices and may ultimately block the route to large-scale commercial device fabrication on h-BN.

An essential component of III-N on h-BN is the use of aluminum-based buffer layers on h-BN for growth of highquality GaN device structures. [START_REF] Liu | Substrates for Gallium Nitride Epitaxy[END_REF] It has been demonstrated that an AlN buffer greatly improves the surface morphology and crystalline quality of subsequent nitride layer growth, compared to an AlGaN buffer. [START_REF] Kim | Principle of Direct van Der Waals Epitaxy of Single-Crystalline Films on Epitaxial Graphene[END_REF][START_REF] Sundaram | MOVPE van Der Waals Epitaxial Growth of AlGaN/AlGaN Multiple Quantum Well Structures with Deep UV Emission on Large Scale 2D h-BN Buffered Sapphire Substrates[END_REF][START_REF] Wu | Growth Mechanism of AlN on Hexagonal BN/Sapphire Substrate by Metal-Organic Chemical Vapor Deposition[END_REF][START_REF] Chugh | Improving the Morphology and Crystal Quality of AlN Grown on Two-Dimensional HBN[END_REF] However, the conflict in the mechanical lift-off of AlN/h-BN structures emerges, where the lift-off process is not always successful. [START_REF] Wu | Growth Mechanism of AlN on Hexagonal BN/Sapphire Substrate by Metal-Organic Chemical Vapor Deposition[END_REF] Further research on this topic is required to clearly understand the mechanism and conditions needed for lift-off.

In this article, we report a growth technique that eliminates spontaneous delamination and prevents lift-off of 3D III-N layers grown on h-BN. This is accomplished by controlling the diffusion of Al atoms into h-BN from AlN buffers grown on thin (<5 nm thick) h-BN on sapphire substrates. First, we study mechanical lift-off of GaN/AlN/h-BN structures with AlN layer grown conventionally (1100 °C) and at a high temperature (1280 °C). Detailed structural analysis of these samples was performed using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and energy-dispersive X-ray spectroscopy (EDX) measurements, allowing for characterization of crystal structure and Al content. We also test lift-off with two BAlN alloys, Al 0.03 B 0.97 N/sapphire and Al 0.17 B 0.83 N/sapphire. Then, we test lift-off and measure Al content in 3 nm thick h-BN and 25 nm thick h-BN topped with high-temperature AlN to show the role of diffusion. Finally, we tested that management of Al diffusion controls the mechanical adhesion of HEMT devices grown on AlN/h-BN/sapphire.

■ RESULTS AND DISCUSSION

We first address the mechanical lift-off of the GaN/AlN/h-BN structures grown on sapphire with different growth temperatures of the AlN layer. One set is grown at 1100 °C (conventional AlN). The second set is grown at 1280 °C (high-temperature AlN). The mechanical lift-off comparison between the two sets of samples is presented in Figure 1, which involves camera photograph, optical microscope, and scanning electron microscope (SEM) images. The GaN/conventional AlN/h-BN structure enables to lift from their substrates using a transparent scotch tape, as seen in Figure 1a-c. A high contrast between lifted and no-lifted areas is clear from the camera photograph and optical microscope (Figure 1a,b). The SEM image clearly shows the three-dimensional (3D) morphology of the top GaN layer compared to the flat surface of the released area (Figure 1c). On the other hand, using the same procedure with scotch tape, the lift-off of the GaN/hightemperature AlN/h-BN structures was not possible despite several attempts. Figure 1d-f presents photograph, optical microscope, and SEM images showing the uniform surface of the sample after lift-off attempts. The improved morphology smoothness of the GaN layer compared to the previous case is due to the better quality of the buffer layer with improved AlN grown at high temperatures, as demonstrated in refs 1, 22, 26. The high-resolution X-ray diffraction (HR-XRD) scans of the two sets of samples are shown in Section 1 in the Supporting Information. These adhesion behaviors of h-BN as a function of buffer layer has never been reported and raises questions about the role of AlN layer in the lift-off mechanism of III-V structures.

Detailed structural analyses of the conventional (Figure 2a) and high-temperature (Figure 2d) AlN on h-BN were performed using transmission electron microscopy (TEM), showing different areas of interest in the structures. In two sets of samples, a two-dimensional (2D) h-BN layer with the lattice oriented along the c-axis was observed. The insets of Figure 2a,d present the fast Fourier transform (FFT) pattern, corresponding to the selected areas (green boxes), identifying clearly a single-crystalline hexagonal phase. The interlayer distance of the h-BN layers is calculated to be 0.349 ± 0.003 nm, which is larger than the reported value (d = 0.33 nm [START_REF] Kobayashi | Hexagonal BN Epitaxial Growth on (0001) Sapphire Substrate by MOVPE[END_REF] ). This 5.7% increase can be attributed to residual strain.

However, the h-BN layer of conventional AlN/h-BN structures is found to be continuous in contrast to the discontinuous h-BN layer of high-temperature AlN/h-BN structures. In the second set of samples, we detect the presence of w-AlN within the h-BN layer, as proved by the FFT (inset of Figure 2dred box), and the calculation of interlayer distance (∼0.25 nmcomparable with the reported value [START_REF] Nilsson | Lattice Parameters of AlN Bulk, Homoepitaxial and Heteroepitaxial Material[END_REF] ). A possible interpretation is that Al diffusion into the h-BN layer causes the discontinuous h-BN layer as well as the w-AlN inclusions. Previous studies reported that high growth temperature of AlN (>1100 °C) causes Al diffusion into other materials such as GaN or GaAs. [START_REF] Matsumoto | Origin of Acceptor Diffusion into Silicon Substrate during GaN Growth by Metal Organic Chemical Vapor Deposition[END_REF][START_REF] Kumagai | Self-Separation of a Thick AlN Layer from a Sapphire Substrate via Interfacial Voids Formed by the Decomposition of Sapphire[END_REF][START_REF] Chaaben | Study of Al Diffusion in GaN during Metal Organic Vapor Phase Epitaxy of AlGaN/GaN and AlN/GaN Structures[END_REF][START_REF] Cai | Abruptness Improvement of the Interfaces of AlGaN/GaN Superlattices by Cancelling Asymmetric Diffusion[END_REF][START_REF] Haneda | SIMS Analysis of Impurities and Nitrogen Isotopes in Gallium Nitride Thin Films[END_REF] In particular, Haneda et al. have demonstrated that the diffusion coefficient of Al (D Al ) is increased by 2 orders of magnitude when the growth temperature is increased by 100 °C. [START_REF] Haneda | SIMS Analysis of Impurities and Nitrogen Isotopes in Gallium Nitride Thin Films[END_REF] In our study, high-temperature AlN/h-BN structures, which were grown at 1280 °C (almost 200 °C higher than conventional AlN/BN structures), enable Al atoms to have more kinetic energy and diffuse into h-BN layers, increasing the Al concentration there. The decomposition rate of AlN is very low at temperatures below 1300 °C, as reported in ref 31. Therefore, in our study, the Al diffusion into h-BN layer may occur from the atomic Al on the surface during the AlN growth and not from the AlN layer. We speculate that the diffused Al atoms create some bonding between Al and h-BN, changing the structure of h-BN and h-BN/sapphire interface. In some areas, with a high Al concentration, a w-AlN layer can be formed, as seen in Figure 2d. w-AlN inclusions may act as a "glue", which anchors the h-BN layer and thus the full structure on the substrate disabling the mechanical lift-off and any delamination during processing. In the same way, thick high-temperature AlN will lead to longer growth, and more time consequently leads to more Al atoms diffusing into the h-BN layer, thus making lift-off more difficult. On the other hand, the mechanical lift-off of IIInitride devices on 2D h-BN on the AlN template with successful lifting was reported in refs 5, 27. To control the liftoff of such a structure, AlN should be subjected to temperatures higher than 1300°C for Al to diffuse into h-BN.

In addition, it is interesting to note that the sapphire/h-BN interfaces between the two sets of samples are different. For the conventional AlN/h-BN structures, a 1-2 nm amorphous AlOxN layer is observed, as identified by the FFT pattern (Figure 2ayellow box). A similar observation has been mentioned and studied by Amano's group. [START_REF] Yang | Interface Amorphization in Hexagonal Boron Nitride Films on Sapphire Substrate Grown by Metalorganic Vapor Phase Epitaxy[END_REF] Conversely, the high-temperature AlN/h-BN structures exhibit a clearly ordered crystalline arrangement of a few w-AlN layers (2-3 nm in thickness) at the interface of the layered h-BN and sapphire substrate (Figure 2d). The diffraction pattern (inset of Figure 2dyellow box) and the 0.25 nm of interlayer distance indicate the wurtzite crystal structure of this AlN layer. We believe that the AlN at the interface sapphire/h-BN is due to the nitridation and its structural quality probably deeply depends on the growth temperature of the above structure, which will need further study to confirm.

To better understand the different impact of the AlN growth temperature on the AlN/h-BN structures, the complementary cross-sectional high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image and the energy-dispersive X-ray spectroscopy (EDX) element mappings of Al have been performed for conventional AlN growth (Figure 2b,c) and for high-temperature AlN growth (Figure 2e,f). The Al concentration in the 3 nm h-BN region of the high-temperature AlN/h-BN structure is 19 atom %, whereas the Al concentration of the h-BN region in the conventional sample is less than 3 atom %. This provides strong support that temperature-dependent diffusion of Al explains the Al content in the h-BN region. It also provides direct proof for the role of Al concentration in the lift-off process. At a low Al concentration, 2D-layered h-BN still exhibits a uniform crystalline structure that enables the mechanical lift-off of the GaN/conventional AlN/h-BN structures. When the Al concentration is increased to the critical value, some bonding between Al, B, and N can be created and the layered structure of h-BN has been changed.

We have recently studied 2D boron-rich BAlN alloys and found phase separation for an Al content above 17% in the presence of wurtzite Al-rich BAlN phases in a matrix of layered hexagonal B-rich BAlN together with the pure w-AlN. [START_REF] Vuong | Single Crystalline Boron Rich B(Al)N Alloys Grown by MOVPE[END_REF] Checking lift-off in BAlN alloys would be expected to confirm that the presence of aluminum content would prevent lift-off and delamination. We examined the mechanical lift-off of single hexagonal phase boron-rich BAlN alloys with Al contents of 3.5 and 17%. The Al content was estimated by secondary-ion mass spectrometry (SIMS), as reported in ref 36. We found that only BAlN alloys with 3.5% Al could be lifted off, whereas samples with an average Al content of 17% could not be lifted off. Figure 3 displays the atomic force microscopy (AFM) images of these alloys after lift-off. The thickness difference between the lift-off part and the non-liftoff part of 3.5% Al BAlN alloys was measured to be 18 ± 1 nm. The morphology of these alloys exhibits honeycomb-like wrinkles on the surface, which is comparable to the standard morphology of un-alloyed 2D h-BN. [START_REF] Li | Large-Area Two-Dimensional Layered Hexagonal Boron Nitride Grown on Sapphire by Metalorganic Vapor Phase Epitaxy[END_REF] In additional to wrinkles, particles (white spots) were observed on the surface of the two samples (more pronounced in 17% Al BAlN alloys), confirming the Al incorporation. When the presence of Al in h-BN is high (≥17%), h-BN is no longer a layered structure and mechanical lift-off is impossible. These results are in agreement with 3 atom % (19 atom %) of Al concentrations of conventional (high-temperature) AlN/h-BN structures, as mentioned in the previous section, confirming the role of Al content in the mechanical lift-off. This study brings the unique approach of controlling the mechanical adhesion of this layered material, which can result in both lift-off controllable-layered h-BN and robust h-BN layers.

If the hypothesis that lift-off is disabled by Al diffusion at high temperatures into the h-BN layer is correct, then it should be possible to have lift-off for high-temperature AlN buffer growth if the h-BN layer is sufficiently thick to prevent Al from diffusing completely through the h-BN layer. To test this, we grew the GaN/high-temperature/AlN/h-BN structure with an h-BN thickness of 25 nm (other growth conditions are kept the same) and then studied the mechanical lift-off of this set of samples. Figure 4 displays a camera photograph of the sample after lift-off, TEM and HAADF-STEM images, and EDX mapping for detailed structural analysis. In contrast to the GaN/high-temperature AlN/3 nm h-BN structures, the GaN/ high-temperature AlN/25 nm h-BN structures had been successfully lifted from the sapphire substrate. Identification of homogeneous and continuous h-BN layers comes from the HAADF-STEM FFT pattern (Figure 4a,b). The bright-field HAADF-STEM image (Figure 4c) shows a high contrast between h-BN, AlN layers and sapphire substrate. A quantitative analysis of various positions (green rectangles in Figure 4c) of this structure was done. The Al concentrations at the sapphire/h-BN interface and inside the h-BN layer are estimated at 2.0 atom % (zone 1) and 1.2 atom % (zone 2), respectively. However, at the h-BN/w-AlN interface up to 4 nm, a high Al content was found with the estimation of 26.7 atom % (zone 3). This large value of Al content indicates the presence of Al at this interface, confirming the limited diffusion of Al.

We first conclude that high-temperature growth directly affects Al diffusion, which plays an important role in the control of mechanical lift-off of the h-BN layer in particular and of III-nitride semiconductor in general. In addition, variation of the thickness of h-BN can open a new, flexible approach on the control of the lift-off process, expanding potential applications in mechanical transfer with the quality improvement of III-nitride device-based h-BN. These investigations can provide valuable insights into the remote control of the epitaxy growth of III-device-based h-BN using AlN as a buffer layer.

This understanding of the mechanism lift-off process further points out the way to avoid the h-BN delamination problem during growth and processing and to control mechanical lift-off of devices. With the intention of illustrating this, AlGaN/GaN HEMTs/high-temperature AlN/3 nm h-BN structures were grown. A set of these devices were subjected to mechanical liftoff, and we could not be able to accomplish it as seen in the inset of Figure 5a (left). Figure 5 displays the cross-sectional high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image and the energy-dispersive X-ray spectroscopy (EDX) element mappings of Al of this sample, focusing on high-temperature AlN/h-BN/sapphire areas in the structure. The HAADF-STEM images (Figure 5a) clearly show the high contrast between sapphire substrate, h-BN, and AlN. Furthermore, at the sapphire/h-BN interface, we observed a 1-1.5 nm thick w-AlN layer. Besides, EDX element mappings of Al (Figure 5b) indicate the presence of Al in the h-BN layer and at the h-BN/sapphire interface. The Al concentrations at different areas (green rectangles in Figure 5b) were estimated at around 26, 10, and 18 atom %, corresponding to zones 1, 2, and 3. The high value of Al

■ CONCLUSIONS

In conclusion, we have shown how to control the adhesion of 2D h-BN layers, which can result in either lift-off of layered h-BN and devices grown on h-BN or mechanically inseparable robust h-BN(Al) layers. We found that high-growth-temperature (∼1280 °C) AlN on h-BN leads to Al diffusion into h-BN, which enhances the surface and interface dangling bonds and anchors the layered h-BN. Detailed structural analyses of these samples were performed using TEM measurements to explore the lift-off mechanism. The effect of Al concentration into h-BN layers on the mechanical lift-off is verified by studying carefully BAlN alloys with different Al contents. It has been observed that for an Al content above 17%, the layered BAlN sample cannot be lifted off, supporting our investigation. The control of the mechanical adhesion of different device structures such as HEMT-based h-BN layers has been examined to confirm the important role of Al diffusion for controlling the lift-off of the subsequent device structure grown on h-BN. These results reveal the perspective in understanding layered materials and controlling this layer-transfer technique. Moreover, they break through the blocked route to the largescale fabrication of III-V devices grown on h-BN, bringing them close to industrial applications.

■ METHODS Sample Preparation. The growth was performed in an Aixtron MOVPE close-coupled showerhead (CCS) reactor on the (0001) sapphire substrate. Triethylboron (TEB), trimethylaluminum (TMAl), trimethylgallium (TMG), and ammonia (NH 3 ) were used as B, Al, Ga, and N precursors, respectively. First, an h-BN layer (3 nm) was grown on the sapphire substrate at 1280 °C and 90 mbar pressure. Then, an AlN layer (200 nm) was grown directly on the h-BN layer at high temperature (1280 °C) and low temperature (1100 °C) to study the impact of AlN layer on mechanical lift-off under different growth conditions. A 300 nm GaN layer was subsequently grown on top of the AlN layer. In addition, to have a comprehensive understanding, the BAlN alloys were grown directly on the c-plane sapphire substrate with the variation of TMAl/III ratio. Detailed growth conditions of these heterostructures have been reported elsewhere. [START_REF] Vuong | Single Crystalline Boron Rich B(Al)N Alloys Grown by MOVPE[END_REF] Finally, full heterostructures of AlGaN/GaN HEMTs were grown on the GaN/AlN/h-BN buffer for the mechanical lift-off final test. After growth, the transparent scotch was used to verify the mechanical lift-off possibility of all of the samples.

Characterizations. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to study the surface morphology of the samples before and after mechanical lift-off. For the crystallography studies, high-resolution transmission electron microscopy (HR-TEM) characterizations were performed on an aberration-corrected JEOL 2200FS electron transmission microscope. Prior to this study, cross-sectional lamellae were prepared using focused ion beam (FIB) thinning and ion milling after coating a 100 nm thick carbon for layer protection.

■ ASSOCIATED CONTENT

* sı Supporting Information

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsami.0c16850.

High-resolution X-ray diffraction (HR-XRD) and Al concentration depth profile (PDF)

■ AUTHOR INFORMATION

Figure 1 .

 1 Figure 1. Photograph, optical microscope, and SEM images of GaN/conventional AlN/BN (a-c) and GaN/high-temperature AlN/BN (d-f) grown on sapphire, respectively. Insets of (a) and (d): schematic illustration of the grown structures.

Figure 2 .

 2 Figure 2. Cross-sectional high-resolution TEM and bright-field HAADF-STEM images, and energy-dispersive X-ray spectroscopy (EDX) elemental mappings of Al of GaN/conventional AlN/h-BN (a-c) and GaN/high-temperature AlN/h-BN (d-f) grown on sapphire, respectively. Insets of (a) and (d): schematic illustration of the grown structure.

Figure 3 .

 3 Figure 3. Atomic force microscopy (AFM) images for the BAlN alloys grown on sapphire after lift-off with different Al contents: (a) 3.5% and (b) 17%.

Figure 4 .

 4 Figure 4. Cross-sectional high-resolution TEM and bright-field HAADF-STEM images, and the energy-dispersive X-ray spectroscopy (EDX) elemental mappings of Al of GaN/high-temperature AlN on 25 nm h-BN grown on sapphire, respectively. Inset: schematic illustration of the grown structure and the photograph of the sample after lift-off.

Figure 5 .

 5 Figure 5. Cross-sectional bright-field HAADF-STEM image (a) and energy-dispersive X-ray spectroscopy (EDX) elemental mappings of Al (b) of HEMTs/high-temperature AlN/3 nm h-BN grown on sapphire. Inset in (a): schematic illustration of the grown structure photograph of the device after attempted lift-off.

  Abdallah Ougazzaden -Georgia Tech-CNRS, UMI 2958, Georgia Tech Lorraine, 57070 Metz, France; School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States; orcid.org/0000-0002-9959-5280; Email: abdallah.ougazzaden@georgiatech-metz.fr Georgia Tech-CNRS, UMI 2958, Georgia Tech Lorraine, 57070 Metz, France; orcid.org/0000-0003-2775-7081 Suresh Sundaram -Georgia Tech-CNRS, UMI 2958, Georgia Tech Lorraine, 57070 Metz, France; School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States Adama Mballo -Georgia Tech-CNRS, UMI 2958, Georgia Tech Lorraine, 57070 Metz, France Gilles Patriarche -Centre de Nanosciences et de Nanotechnologies, Université Paris-Saclay, F-91460 Marcoussis, France Stefano Leone -Fraunhofer IAF, Fraunhofer Institute for Applied Solid State Physics, 79108 Freiburg, Germany Fouad Benkhelifa -Fraunhofer IAF, Fraunhofer Institute for Applied Solid State Physics, 79108 Freiburg, Germany Soufiane Karrakchou -Georgia Tech-CNRS, UMI 2958, Georgia Tech Lorraine, 57070 Metz, France; School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States Tarik Moudakir -Institut Lafayette, 57070 Metz, France Simon Gautier -Institut Lafayette, 57070 Metz, France Paul L. Voss -Georgia Tech-CNRS, UMI 2958, Georgia Tech Lorraine, 57070 Metz, France; School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States

	Corresponding Author

Authors

Phuong Vuong -

Author Contributions

A.O. contributed to design the study and supervised the whole project. P.V. performed SEM, AFM, and mechanical lift-off; analyzed the results; and wrote the manuscript. S.S., P.V., and A.M. performed the epitaxial growth and prepared the BN samples. G.P. carried out HAADF-STEM experiments. S.L., F.B., S.S., T.M., and A.O. contributed with the growth and characterization of the AlGaN/GaN HEMTs heterostructure on the GaN/AlN/h-BN buffer. S.K., S.G., and J.-P.S., were responsible for the processing of HEMTs heterostructure. All authors contributed to discussion of the results. All authors reviewed and commented on the manuscript.

Notes

The authors declare no competing financial interest. All data generated or analyzed during this study are included in this published article.

■ ACKNOWLEDGMENTS

This study was partially funded by the French National Research Agency (ANR) under the GANEX Laboratory of Excellence (Labex). C. Manz and L. Kirste at Fraunhofer IAF are acknowledged for the characterization of the AlGaN/GaN HEMTs heterostructure on the GaN/AlN/h-BN buffer. The authors gratefully acknowledge Marcel Reis Soubkovsky (marcelsoubk.com) for his help in creating the Table of Contents (TOC) graphic.


Sara Bekaddour
email: bekaddour.sara@gmail.com

Nassim Ait-Mouheb

Tarik Hartani

Re-emergence of dry toilets and fecal nutrient reuse in M'zab cities

Keywords: arid regions, dry toilets, human excreta, oasis, recycling, water and nutrient cycles

In the M'zab valley, dry toilets represent an ancestral dry sanitation system, serving as a source of fertilizer thanks to human excrement valorization. However, in the 20th century, local populations began to shun these systems. The objective of this article is to illustrate the importance of dry toilets on agricultural and environmental scales in ancient M'Zab, and the renewal of these systems in response to sanitation problems in the oasis after their decline. The hypothesis put forward is that dry toilets can act as a complementary system to conventional sanitation systems. Data were collected through interviews with the local population. Our results show that the use of dry toilets, and the resulting use of human excrement as fertilizer, has gone through three phases. First, a phase of strong recycling dynamics, followed by a second phase of decline in dry toilet use which is linked to the discovery of the Albian aquifer and flush toilet adoption.

The third phase is characterized by dry toilet reuse in response to oasis degradation caused by sanitation and environmental problems. Some oasesians have taken the initiative to revert to dry toilets to ensure oasis system sustainability and to revive the practice of recycling human waste.

INTRODUCTION

In developing countries characterized by poor soil conditions and scarce water resources, human excreta used as fertilizer can address two problems: low food productivity and a lack of sanitation services [START_REF] Sugihara | Reuse of human excreta in developing countries: agricultural fertilization optimization[END_REF]. Dry sanitation enables the treatment and recycling of excreta, i.e., to dispose of human urine and feces without water use [START_REF] Peasy | Health aspects of dry sanitation with waste reuse Peasey[END_REF]. One of the most popular types of low-cost, on-site, dry sanitation systems is pit latrines [START_REF] Peasy | Health aspects of dry sanitation with waste reuse Peasey[END_REF]. Pit latrines are common in rural and peri-urban areas in developing countries. They are considered as a closed-loop sustainable ecological sanitation system, which combines sanitation and agriculture [START_REF] Langergraber | Ecological sanitation: a way to solve global sanitation problems[END_REF]. This is in sharp contrast to waterbased sewage systems that have been designed and built on the premise that human excreta should be considered as waste. Dry sanitation, on the other hand, is based on the principle that human excreta should not be disposed of, and that the environment can assimilate them [START_REF] Langergraber | Ecological sanitation: a way to solve global sanitation problems[END_REF]. These systems are more advantageous in areas of water scarcity as they preserve groundwater from contamination by pathogens and microorganisms. In addition, human excreta contain high levels of nitrogen, phosphorus and potassium needed for plant growth [START_REF] Prasad Devkota | Urine diversion dry toilet: a narrative review on gaps and problems and its transformation[END_REF]. One study showed that in 2009, the total available phosphorus in feces was about 1.68 million tons, which could meet 22% of the total global demand for phosphorus [START_REF] Mihelcic | Global potential of phosphorus recovery from human urine and feces[END_REF]. The use of human excreta to fertilize agricultural land can reduce the need for artificial fertilizers and would improve human excreta disposal efficiency through the use of composting toilet systems [START_REF] Sugihara | Reuse of human excreta in developing countries: agricultural fertilization optimization[END_REF].

In the M'zab valley, an Algerian Saharan region characterized by strong agricultural and social dynamics, dry toilets demonstrate the inhabitants' adaptation to water scarcity, while also providing a source of profitable organic fertilizer and ecological balance in a growing urban environment. Some authors have indeed mentioned these toilets in their books such as Didillon et al. (1977) and [START_REF] Benyoucef | The M'zab: a Millinary Journey (le M'zab: un parcours millénaire[END_REF]. They gave a brief description of the model of dry toilet used in the M'Zab valley. Despite the environmental, agricultural and economic role of these systems in the M'zab, no previous studies had been conducted on them.

In this paper, we will discuss the history of dry toilets and the reintroduction of this process. Our hypothesis is that the reintroduction of small-scale human waste recycling could help revitalize societies in the M'zab valley oases, and even beyond.

We put forward the idea that a model, which is respectful of the Saharan environment and economical in terms of water and financial resources, can be part of a future decentralized public policy designed to modernize the sanitation sub-sector in Algeria. This model could provide an alternative or complementary system to conventional human waste treatment.

MATERIALS AND METHODS

Study area

The M'zab valley is located in the northern Ghardaia region, which is located in the northern part of the Algerian Sahara. It is located south of the capital of Algeria (600 km) and occupies an area of 86,560 km 2 [START_REF] Hamdani | Study and effect of orientation two room of buildings located in Ghardaïa, Algeria[END_REF]) (Figure 1).

A population census from 1896 to 1966 shows that the population of the M'zab valley increased from 18,892 to 46,530 with a coefficient of variation equal to 2.46 [START_REF] Josse | Urban growth in the Sahara: Ghardaïa[END_REF]). Between 1966[START_REF] Adad | La participation de la population à son Habitat: Cas d'un nouveau Ksar au M'zab[END_REF], the population of the pentapolis almost multiplied by 3, with a variation rate of 1.13% per year [START_REF] Maachou | The M'zab: from the Pentapolis to the Metropolis (le M'zab: de la Pentapole à la Métropole)[END_REF]. During the period 2008-2018, the population of the M'zab reached 196,019 inhabitants with an average annual growth rate varying between 2.14 and 3.58 (DSA 2021).

In the 11th century, five towns called 'ksour' grew around the M'zab river (from upstream to downstream: Ghardaïa, Melika, Béni Isguen, Bounoura and El Atteuf), along with six dams and five palm groves [START_REF] Cote | A city fills its valley: Ghardaïa (une ville remplit sa vallée)[END_REF]. Outside the valley, Figure 1 | Geographic localization of the M'zab cities [START_REF] Bensalah | Urbanization of the M'zab Valley and the urban sprawl of the palm grove of Ghardaïa (Algeria): a threatened oasis heritage (Urbanisation de la vallée du M'zab et mitage de la palmeraie de Ghardaïa (Algérie): un patrimoine oasien menacé)[END_REF].

two other 'ksour' towns were founded in the 17th century: Guerrara in 1631, which is located north-east of the M'Zab valley, on the course of the Zegrir river, and Berriane in 1679 located 48 km north of the Sudan river [START_REF] Didillion | Living in the Desert: Mozabite Houses[END_REF].

The region is characterized by a hot and dry climate in summer, with significant temperature variations, intense solar radiation and strong winds [START_REF] Hamdani | Study and effect of orientation two room of buildings located in Ghardaïa, Algeria[END_REF]. It is a hyper-arid region where average rainfall does not exceed 160 mm per year, and temperatures can reach 50 °C in the summer period [START_REF] Hamdani | Study and effect of orientation two room of buildings located in Ghardaïa, Algeria[END_REF]. Groundwater is the main irrigation source for the palm tree, and its recharge is mainly provided by flood rainwater infiltration. This water is stored in dams locally called 'Ahbas' [START_REF] Ouled Belkhir | Cleanup and valuation of waters of the aquifer of M'zab Valley (Algeria)[END_REF]. The aquifers of the terminal complex and continental interlayer are also exploited (Albian aquifer) whose depths vary between 250 and 1,000 m depending on the location [START_REF] Bouamar | The effects of the flood of October 2008 on the water quality in the M'zab valley (Algeria)[END_REF].

Beni Isguen was chosen as the main site for the study because the palm grove and its extensions are not connected to the sewerage system; therefore, sanitation rely on on-site treatment. Ancestral dry toilets still exist there and are in good condition, there are many septic tanks, and finally, we observed a renewal of dry toilet use in this area. The surveys carried out in neighboring villages Berriane and Guerrara show a similarity in dry toilet design.

Research methodology

To carry out this work, we used bibliographical research, observational analysis, and surveys of different types. After defining the study area and the main actors, 40 semi-structured and open-ended interviews were conducted with selected local people: farmers, tourist guides, the owners of houses equipped with dry toilets, and finally with civil society actors ('friends of the oasis' association members and 'environmental protection' association members). From the 40 interviews we conducted, there are three former artisans who worked in the cleaning of dry toilets and in the marketing of human waste. The survey that was applied to all the actors in this study was designed to determine: the history of latrines and human waste recycling; the means and methods of cleaning and emptying latrines; the collection and use of human waste in agriculture; the benefits of using human manure in agriculture; the crops most often fertilized with this manure; the problems associated with the transition from dry toilets to flush toilets; and finally the reasons for the return to dry toilets, and whether dry toilets could offer an alternative system to conventional sanitation systems in the future.

The questions asked to the actors during the interview sessions are listed in Table 1.

RESULTS

Umbilical cord linking the domestic habitat and dry toilets in the M'zab

Ancient dynamics around dry toilets

In this section, through a chronological approach, we will attempt to shed some light on the importance of dry toilets in the ancient M'zab, by first describing them, and then by determining the processes involved in the cleaning, recycling and recovery of human waste from these toilets for use in agriculture.

Figure 2 is a timeline that was established after interviews with local actors (farmers, dignitaries, and tourist guides). It illustrates changes in the use of dry toilets by the local population. The reason for these changes can be summarized as the discovery of the deep groundwater (after 1950), its subsequent exploitation and its impact on the oasis environment. The • How do you prepare human manure?

• What is the difference between human manure and animal manure? Which is better according to your experience? Why?

Artisans (former dry toilet cleaning specialists) 3

• Why did you choose to become a dry toilet cleaner?

• What was the cleaning process for these dry toilets? At which cost?

Associations and civil society 15

• What is the history of dry toilets in the M'Zab region?

• Why did the forefathers choose this sanitation concept? And what are its advantages and disadvantages?

• Why was there a decline in the use of dry toilets? What were the consequences of this decline?

availability of water offered the local population a level of comfort that has allowed them to gradually abandon dry toilets. These have been replaced by flush toilets, without any consideration of the long-term impacts on the oasis health (1970 to today).

In the M'Zab, dry toilets have been an integral part of the design and construction of Mozabite houses, which are called Ajmi or Goma (in the Berber language) since the foundation of the Mozabite cities and up to 1950, as shown in Figure 2. Given its availability in the region, local stone is widely used in house construction, where it is the only material used for building walls, as it can withstand the compressive forces to which it is exposed [START_REF] Adad | La participation de la population à son Habitat: Cas d'un nouveau Ksar au M'zab[END_REF]. Generally, the latrines are 1.8-2 m high; the slit leading to the pit, which acts as a fecal storage tank, is 60 cm long and the pit is nearly 1 m 2 (Figure 3). The town planning rules defined by the local population, who belong to the Mozabite ethnic group, require that the width of the rooms in the house should not exceed 2 m and the height of the ceilings should not exceed 2.30 m [START_REF] Didillion | Living in the Desert: Mozabite Houses[END_REF]. There are private and public dry toilets in the region, of which there are more than 20 public and private dry toilets in the Beni Isguen palm grove.

• Private: in each house in the ksar, and in the secondary house located in the palm grove, there was (to this day) a dry toilet for each family. Figure 4(b) shows the dry toilets that exist in the heart of the Beni Isguen palm grove and other oases of the M'zab which are located in the secondary houses of the inhabitants. We counted 12 private dry toilets but there are more in the region.

• Public: they are located along the palm grove. Eight dry toilets were accounted. They are built for collective use, i.e., anyone passing through the palm grove can use them. For example, the laborers of the palm grove can use these dry toilets. They have a double role: to preserve the palm grove by keeping it clean and to ensure the collection and use of a certain quantity of human manure as an organic soil improver. There are also public dry toilets next to the mosques for people who come to pray (Figure 4(a)).

The old dry toilets are mainly public ones built within the oasis. In addition, the new dry toilets are private built either by farmers on their farms or by owners of local tourist lodges.

In the M'zab community, water plays an important role in the oasis context. It is a symbol of life, purity and blessing. Its scarcity has forced local populations to seek the most rational and ingenious ways of ensuring optimal management of this resource [START_REF] Benyoucef | The M'zab: a Millinary Journey (le M'zab: un parcours millénaire[END_REF]). As a result, dry toilets are used to minimize water consumption. Indeed, dry toilets used to be the only sanitation system in urban areas locally known as 'ksour' (fortified town) and in rural areas in 'the palm grove'. According to a former artisan collecting human waste from dry toilets, and other farmers and local actors, recycling human waste for agricultural purposes was an ancient and common local practice since the foundation of the cities (Figure 2). As indicated by several of the people interviewed, the situation in the past was simple: gray water was collected in buckets, and then they carried it to the pits near the river that ran by the agricultural plots so that the trees could draw this water. As there were no detergents, this water could not do any damage to the trees. In addition, the washing of clothes was done on weekends in the oases where the used water was poured into the palm tree basins.

In urban agglomerations, dry toilet cleaning is traditionally carried out each year during the summer period, and at night. During this period, because of the great heat, the inhabitants prefer to live in their secondary houses in the palm grove. According to three former specialists in dry toilet cleaning: 'the collection of human waste was done at night so as not to disturb the inhabitants of the town'. The cleaning was carried out on a chain work basis by three to four workers. Each worker was responsible for a specific task: two people took turns to clean the pit, the third person was positioned at the pit exit to remove the waste matter and the fourth person was in charge of transporting this material to the palm grove (Figure 1). Money was one of the reasons given by the specialist workers for choosing this trade. However, as two former specialist workers say, 'it is a very difficult job that comes with mortal risks. When you are in a pit, and once you start to empty it, there are gas emissions; so you can't breathe, and you have to get out of the pit and breathe in fresh air to cope with the situation'. For this reason, it was mandatory to have two people in the pit to be able to take it in turns. In addition to paying the cleaning costs, offering a meal and a shower, it was customary for the owner of the house to offer perfume and soap to the workers because the people exercising this profession were mostly poor, according to interviews with local actors.

The management and commercialization of human manure, according to several farmers and local actors, can be done directly or indirectly. In the first case, the owner himself reuses the human waste collected from his house for his own farming purposes. In this case, he pays for the costs of cleaning and of transporting the waste to his farm. In the second case, the owner of the house did not need the manure, so he let the specialized workers manage the waste and sell it on to farmers. The owner linked the cost of selling the product to the price of cleaning by paying the specialized workers half the cost of cleaning and the other half in the form of human manure. The specialized workers sold it to supplement their income. There was a contract between the owner, the seller and the farmer for the collection of a certain amount of this manure.

The intermediary buys the human waste from the owners and sells it to the farmers who ordered it. In this case, the specialist pit cleaners would only benefit from the cleaning costs.

Advantages

The dry toilet made it possible to collect human waste at a single, well-defined place, thus avoiding the dispersion of this waste into the environment and the resulting spread of diseases. Also, it was much more advantageous for the inhabitants who did not need to deal with the problem of sewage disposal, especially in urban agglomerations where the quantities of human waste were significant. In the past, this practice helped the water table protection, which is between 10 and 50 m deep depending on rainfall [START_REF] Bouchair | Decline of urban ecosystem of Mzab valley[END_REF]), from domestic wastewater-related pollution. According to the local actors, what is much more problematic is the installation of a wastewater treatment system requiring heavy investments, and even more so when it is ill-conceived and thus inefficient. They also added that the local context should have been studied more before implementing such a system. The hydro-chemical assessment of groundwater quality reveals excessive salinity (0.647 S•m À1 ) due to discharge of untreated industrial wastewater in the M'zab valley, and specifically in the oasis of el Atteuf, which is located downstream of the valley (Baba Amer et al. 2016). These waters are also unsuitable for domestic consumption and sometimes even for irrigation because of their excessive salinity [START_REF] Baba Amer | Impact of industrial waste on the quality of ground water in the M'Zab valley and Evaluation of the salinity of the ground water used for irrigation in the region of El Atteuf, South Algerian[END_REF].

Local farmers also stated that 'human fertilizer is the best because humans are omnivorous and eat a lot of good quality, nutrient-rich food. In addition, it also prevents weeds from growing, which saves on weeding costs.' Human waste has the advantage of being inexpensive, as it offers savings in manure, transport and labor costs. It is also organic and available all year round, which ensures good yields. On the other hand, cattle manure or poultry manure is expensive as it has to be imported from other regions outside Ghardaia, and this means high transport costs.

Decline of dry toilets after the emergence of deep wells

The results of the interviews with all actors reveal that the transition from a circular system, based on the recycling of human waste in agricultural production, to a linear 'all-sewer' system is the result of a rational evolution. This trend was mainly due to population growth and the exploitation of wells in the Albian aquifer varying from 250 to 1,000 m in depth. It should be noted that the population in the M'zab valley increased from 18,000 in 1986 to 135,000 in 2002 [START_REF] Cote | A city fills its valley: Ghardaïa (une ville remplit sa vallée)[END_REF], and it was estimated to be 196,019 inhabitants in 2018. According to Abdellah Haba Aina, an agro-ecologist and former member of the local association for the environment protection of Beni Isguen, 'Before 1985, the only water supply source was the subsurface aquifer and the flood waters that enabled the recharge of the artificial water table.' Water was scarce and used to satisfy basic needs, which meant that dry toilets were mainly perceived as a way to save water rather than as a way of soil improvement. In the M'zab valley, the palm grove was irrigated with water from the subsurface aquifer, contained in the Mio-Pliocene formations, which was fed by the rains and periodically raised by the infero-flow of the river [START_REF] Josse | Urban growth in the Sahara: Ghardaïa[END_REF]. However, the discovery of the Albian aquifer in the late 1930s (the first Albian borehole was drilled between 1937 and 1938) and the development of a modern hydraulic infrastructure resulted in significant economic and social transformations [START_REF] Josse | Urban growth in the Sahara: Ghardaïa[END_REF]. These transformations, which are confirmed by the actors concerned by this study, reveal that the local population started to use water in abundance, and from the 1970s running water made its presence felt in almost all Mozabite houses, according to the statements of the inhabitants. These 'modern wells' provide considerable quantities of water compared to the modest volumes of the old Khettaras (traditional wells): without them, any urban growth would have been impossible [START_REF] Josse | Urban growth in the Sahara: Ghardaïa[END_REF]). However, the exploitation of the deep aquifer has a destabilizing effect and has undermined social water practices and cultures by causing ecological and environmental upheavals due to the difficulties involved in disposing of the enormous quantities of wastewater [START_REF] Bensaad | Water and Its Challenges in the Sahara (L'eau et ses enjeux au Sahara)[END_REF].

According to local actors (farmers, environmental associations and tourist guides), this high water use is associated with hydraulic system modernization and the resulting use of flush toilets. This led the local population to abandon dry toilets in the M'zab valley. Subsequently, wastewater was disposed of in septic tanks or discharged into wells providing the local population's drinking water. Dry toilets disappeared from the 'ksour' of the M'zab in the 1980s, except for those in the ancient palm grove of Beni Isguen, which still exist. However, the dynamics that used to govern recycling have all but disappeared in view of the small quantities of dry matter now available, and due to the emergence of chemical fertilizers in agriculture. As a result, high quantities of wastewater were discharged into the environment without any treatment prior to the construction of a collective sewerage system.

In the 2000s, according to the local Water Resources Administration, the state launched the study of a project to build the El Atteuf wastewater treatment plant, which was designed to receive all the wastewater from the valley. However, the old palm groves and their extensions within the Mzab valley are not connected to the collective sewerage system and that is why wastewater is disposed of in septic tanks. In Beni Isguen, the multiplication of tourist lodgings has amplified the use of non-standard septic tanks. The latter are septic tanks made in an artisanal way by the owners of the houses located in the palm groves, without any proper study. The wastewater collected in these tanks infiltrates into the water table. Concerning the sludge, it is collected and rejected in the public dumps.

Dry toilets in Beni Isguen: a revival or just a survival? Despite the introduction of flush toilets, and in the absence of a collective sewage evacuation system in the palm grove, some farmers and local actors in the town of Beni Isguen have built new latrines in their farms. These initiatives aim to exploit human manure and to protect the oasis from pollution, namely pollution of the water table because the palm grove and its extensions are not connected to the collective sanitation system. This situation has led to a multiplication of septic tanks in the heart of the palm grove. These septic tanks are built in an artisanal way. We put forward the hypothesis that the resulting re-emergence of dry toilets will play an important role in minimizing the quantities of domestic wastewater discharged. According to some members of local community, i.e., 'the notables of the region', houses are built in the palm grove without planning permission, which has led to an increase in the number of septic tanks due to the absence of a sanitation network. It is worth noting that these notables are equally against the construction of a collective sanitation network in the palm grove. Their main argument for this refusal is: 'the installation of a sewerage network will encourage people to build houses in the palm grove, which will degrade the palm grove because each new house means the removal of several palm trees. Moreover, there is a fear that flood waters might burst the sewerage pipes, and consequently, cause environmental damage'.

The local notables and Water Monetaries Omanas El Ma are the wise men of the region. They are in charge of managing the sharing of floodwaters among farmers according to rules drawn up by the community. They have taken several initiatives to raise the awareness of young farmers, and farmers operating in the extensions of the palm grove of Beni Isguen, about the importance of dry toilets in rural and oasis environments (Figure 5). They have decided to take part in this awareness campaign because of their great influence on the local population who take into account the opinions of these wise people. There are farmers who have built new private dry toilets on their farms, and there are more under construction, which will result in 10 new dry toilets.

A farmer in 'Ntissa' (an extension of the Beni Isguen palm grove) offers a concrete example of dry toilet usage and human excreta recycling. He owns 2 ha of farmland which is mainly taken up by date palms, fruit trees and horticultural crops. He only has accessed a subsurface aquifer for irrigation. He revived the use of dry toilets as a system that provides natural and organic fertilizer. He illustrates the modern example of a farmer who practices 100% organic farming based on the combination of small livestock and agriculture. As shown in Figure 6, he uses sawdust instead of sand as a drying agent in its latrine, and its pit design is different from the traditional ones shown in the previous figures.

This farmer stated that the use of human manure is just one of several agro-ecological practices (manual weeding, use of mulching to avoid herbicides, crop rotation) he uses on his farm. He fertilizes using a compost made of animal manure (sheep, goat and poultry), plant debris and human manure. For the latter, he built a dry toilet in his garden and encourages visitors to use it. He then dries the waste using the sawdust, which also eliminates bad odors (Figure 6).

For the preparation of manure, he adopts anaerobic composting by digging a 1 m deep pit in the soil. Then he mixes human, animal and vegetable waste from his garden, adding lime to speed up the fermentation process, and he dampens this mix with water. Finally, he covers the mixture with plastic and lets it sit for 6-12 months (Figure 6). In addition to compost, he buys bovine manure for fertilization.

According to the farmer, who has a long experience in agriculture, the compost is a good fertilizer that is: easy to apply, reduces odors, improves soil structure and even if used in large quantities does not adversely affect the plant. Generally, human excrement is mixed inside the toilet with either soil or sand in order to dry it. However, some farmers use sawdust and ashes to eliminate bad odors and to dry the human excrement. According to the statements of those surveyed, the local inhabitants have two ways of using human waste as manure in agriculture. It is either used directly, after drying in a dedicated area of the farm for 20 days to a month, given the high temperatures in the region and the aridity of the area [START_REF] Jenkins | The Humanure Handbook[END_REF]. It is used to fertilize the land, especially for vegetable crops. Otherwise, anaerobic composting is performed for a period between 3 and 6 months or even up to a year during which the matter is mixed it with plant debris (leaves and tree trunks).

DISCUSSION

In the M'zab valley, dry toilets were a precious commodity in the Mozabites way of life. There, for centuries, they have helped to ensure an agricultural and ecological balance. They allow the optimization of human waste in agriculture, the protection of the population against diseases, as well as the protection of the environment and the aquifer from pollution. Thanks to the circular system offered by dry toilets, the inhabitants of the M'zab valley were able to avail of organic, low-cost, and locally produced manure to fertilize their agricultural plots and obtain higher yields. Dry toilets also have the advantage of allowing direct valorization of human waste after composting, without having to dry the material, thereby providing an organic fertilizer for agriculture [START_REF] Koanda | Co-composting of solid residues from mobile dry toilets in the presence of green waste (co-compostage de résidus solides de toilettes sèches mobiles en présence de déchets verts)[END_REF]. A study was carried out in Ghana to determine the importance of human waste on soil fertility and agricultural production showed that human manure improved soil fertility and increased sorghum and maize yields [START_REF] Cofie | The use of human waste for peri-urban agriculture in Northern Ghana[END_REF]. 8.2 kg of nitrogen, 1.1 kg of phosphorus, 2.2 kg of potassium and 21.3 kg of carbon were found in 1 m 3 of human manure [START_REF] Cofie | The use of human waste for peri-urban agriculture in Northern Ghana[END_REF]. Although human excreta have proven their potential as a source of nutrients for plants, they are still treated as waste to be disposed of by sewage treatment plants. The standards that sewage treatment plants must meet regarding the removal of nitrogen and phosphorous are increasingly demanding, which means that higher energy and chemical needs [START_REF] Spangberg | Environmental impact of recycling nutrients in human excreta to agriculture compared with enhanced wastewater treatment[END_REF]. Therefore, the recycling of human waste can be considered as an alternative to wastewater treatment plants [START_REF] Spangberg | Environmental impact of recycling nutrients in human excreta to agriculture compared with enhanced wastewater treatment[END_REF]. They can offer an alternative small-scale system, i.e., instead of investing in the extension of collective networks, decentralized systems such as dry toilets could be installed in rural areas. For example, in the old oasis of Beni Isguen and its extensions, the sewerage network does not reach this remote location, and the inhabitants have resorted to the use of artisanal septic tanks, especially the tourist lodgings which are numerous in the region. In some cases, effluent discharged from the septic tank contains pollutants that could have adverse effects on public health and the environment (Ninsiima 2019).

Nevertheless, if fecal material is used without composting, it can cause problems such as pathogens and heavy metals [START_REF] Moya | Challenges to the use of fertilisers derived from human excreta: the case of vegetable exports from Kenya to Europe and influence of certification systems[END_REF]. Anaerobic composting is a very popular traditional method for the composting of feces and waste, as it offers many advantages. It protects the compost from adverse climatic conditions, minimizes nutrient losses caused by wind and high heat, and reduces the discomfort caused by flies [START_REF] Nagy | The utilization of human excreta and human urine in the agriculture as a fertilizer[END_REF]). In our case, people practice anaerobic composting because it eliminates odors while offering good easy-to-apply fertilizer that improves soil structure. Anaerobic composting can help balance the relationship between carbon and nitrogen when there is excess nitrogen (Heinonen-Tanski & Wijk-Sijbesma 2005). However, the introduction of human excreta management into the agricultural and food systems means paying particular attention to several aspects. These aspects are primarily: health security, soil protection, and the quality of nutrients provided by human excreta [START_REF] Harder | Reframing human excreta management as part of food and farming systems[END_REF]. In our case study, according to the 'agricultural statistics service of the Ghardaia region', the available agricultural area of the region has increased from 3,146 ha in 1984 to 58,508 ha in 2018. This makes the sustainability of dry toilets questionable, given that the small quantities of fertilizers currently provided by these toilets when compared to the fertilizer requirements of the cultivated areas. Nevertheless, the notables of the region, and the local oasis protection associations, want to revive this practice in rural areas, on a pearl scale, to protect the oasis and ensure the reestablishment of ancestral practices that are based on recycling. This effort to revive these ancestral systems comes after their almost total abandonment by the local population due to the arrival of modernization and to the abundance of available water thanks to the exploitation of the Albian aquifer. The decline in the use of dry toilets has led the local population to become more innovative in the field of waste treatment and they are considering the implementation of environmentally friendly treatment systems. Indeed, in Beni Isguen, they have three sanitation systems: conventional treatment by wastewater treatment plants, dry toilets in the palm grove and a decentralized system to treat wastewater using natural processes based on water filtering vegetation. Each system has its advantages and drawbacks.

The association of the three sanitation systems could be considered positive for the development of the oases, but there is a lack of expertise and adequate system monitoring to successfully operate all three systems at once. In Beni Isguen, all these systems currently exist. If these three systems, and their potential association, were given greater consideration, it would help to ensure the protection of the existing oasis environment and to extend these oases, thereby improving agricultural production in the region. However, the acceptability of dry toilets by the younger generations, who became accustomed to flush toilets, is the main obstacle that could hinder their appropriation and development by future generations. Real dialog between the local population and the institutions must be fostered to ensure the development of a sanitation system that is adapted to the local context. What the local actors demand is the right to participate in discussions and to be able to offer their opinions on this kind of project. Unfortunately, especially in isolated oases, there is a lack of knowledge and support for the development of this type of system. In some countries (such as the United Kingdom, Sweden, Australia and the United States), specific certification processes or insurance systems have been developed for these dry toilet and fertilizer solutions, which have improved public perception of biosolids [START_REF] Moya | Challenges to the use of fertilisers derived from human excreta: the case of vegetable exports from Kenya to Europe and influence of certification systems[END_REF]. The development of such a certification process could provide guarantees on the conformity of fertilizers produced from human excreta [START_REF] Moya | Challenges to the use of fertilisers derived from human excreta: the case of vegetable exports from Kenya to Europe and influence of certification systems[END_REF].

CONCLUSION

For centuries, the recycling of nutrients present in human excreta from dry toilets has been an important source of fertilizer for the M'zab oases. However, these ancestral systems, which were well established and well maintained to fulfill their roles in the agricultural domain, have been almost totally abandoned by the local populations due to the arrival of modernization and the availability of water in significant quantities from the deep Albian aquifer. Today, the agricultural systems, especially in the old oases, have become practically obsolete and are seriously threatened by urbanization that is transforming the old palm groves into anarchic settlements [START_REF] Houichiti | Dynamics of Agriculture and Food Security in the Ghardaïa Region (Algerian Northern Sahara)[END_REF]. What the notables of the region are trying to do to encourage the rural population to install dry toilets on their farms to fight the ever-increasing numbers of septic tanks linked to uncontrolled urbanization. Nevertheless, dry toilets remain a tried and trusted solution that can both protect the old oasis from pollution while reducing the costs involved in connecting homes to the sewerage system. Through this case study, we were able to show the importance of dry toilets in the protection of an arid oasis, such as the Ghardaia region. However, the sustainability and acceptability of these systems by the future generations remain unknown because the construction of dry toilets is a private initiative taken by the local population without any collaboration with the local authorities. The coordination between relevant institutions (e.g., the sanitation office), local actors and scientific research should lead to the adoption of dry toilets as an official small-scale alternative sanitation system. There will also be the possibility to export the use of these systems to other arid regions by adapting them to the context of each region. In view of this, it seems necessary to take into consideration several aspects such as the sanitary and environmental aspects by ensuring the elimination of all pathogens and pollutants before using human manure in agriculture.

Figure 2 |

 2 Figure 2 | Chronology of the different phases of dry toilet use (according to interviews with the local population).

Figure 3 |

 3 Figure 3 | Dry toilet architecture in the M'zab valley.

Figure 4 |

 4 Figure 4 | Private and public dry toilets in the M'zab Valley: (a) private and (b) public.

Figure 5 |

 5 Figure 5 | New dry toilets in Beni Isguen, 2019.

Figure 6 |

 6 Figure 6 | Example of dry toilet use and recycled human excreta (May, 2019): (a, b) cultivation system, (c, d) dry toilet and sawdust and (e, f) compost.

Table 1 |

 1 Examples of questions to the actors in Ghardaia, 2019

	Actors	Number Example of questions
	Farmers	22

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ACKNOWLEDGEMENTS

This study was partly funded within the framework of the following IFAD project: Massire Strengthening the agricultural and rural innovation systems in oases and arid regions of the Maghreb. We would also like to express our gratitude to the DGRSDT, and in particular their Algerian research director, for its support.

DATA AVAILABILITY STATEMENT

All relevant data are included in the paper or its Supplementary Information.


Cécile-Marie Doyen

Fabien Montel

Thierry Gautier

Hervé Menoni

Cyril Claudet

Marlène Delacour-Larose

Dimitar Angelov

Ali Hamiche

Jan Bednar

Cendrine Faivre-Moskalenko

Dissection of the unusual structural and functional properties

  Figure1


est destinée au dépôt

  Fig.1