Ms Ratha Muon

Ratha Muon
email: ratha.muon@itc.edu.kh

Pinnara Ket

David Sebag

Hanane Aroui Boukbida

Pascal Podwojewski

Vincent Hervé

Vannak Ann

Pascal Jouquet

Addresses

Termite constructions as patches of soil fertility in Cambodian paddy fields

Keywords: Bioturbation, ecosystem services, water holding capacity, C sequestration, gley soil

In Cambodia, termite mounds are commonly used by farmers as amendments to increase the fertility of their paddy fields. However, despite their utilization, their chemical and physical properties have not been described yet. Therefore, the aim of this study was to analyze the chemical and physical properties of two termite constructions commonly found in paddy fields:

(a) termitaria built and occupied by the fungus-growing termite Macrotermes gilvus and (b) lenticular mounds that are initially built by termites but host a large diversity of other invertebrates and plants. This study shows that these biogenic structures have very specific properties. Termitaria were characterized by higher clay, phosphorus and electrical conductivity than the surrounding soil. However, their effect on carbon dynamics was limited to a modification of the interactions between soil organic matter and minerals and to the presence of carbonates. At the same time, lenticular mounds appeared as patches of nutrients in paddy fields because they were always enriched in carbon, nitrogen, and phosphorus in comparison with the surrounding cultivated soil. Lenticular mounds were also enriched in clay, although this effect was only measured when the sand content in the surrounding environment was > 60%. Together with these changes, lenticular mounds were characterized by a lower bulk density, higher saturated hydraulic conductivity (Ksat), and higher water holding capacity. In conclusion, this study shows that termite constructions can be considered fertility and biogeochemical hotspots in paddy fields, thus explaining their use by farmers for improving the fertility of their lands.

Introduction

Termites are among the most important soil invertebrates across the tropics [START_REF] Wood | The role of termites in ecosystems[END_REF] due to their high abundance and impact on the decomposition of litter and other plantderived materials [START_REF] Collins | The role of termites in the decomposition of wood and leaf litter in the Southern Guinea savanna of Nigeria[END_REF][START_REF] Schuurman | Decomposition rates and termite assemblage composition in semiarid Africa[END_REF][START_REF] Griffiths | Termites can decompose more than half of deadwood in tropical rainforest[END_REF]. However, what makes termites almost unique among soil invertebrates is their capacity to regulate soil biological, chemical, and physical properties through the construction of soil biogenic structures, such as galleries, sheeting, and mounds (Jouquet et al. 2016b). For this reason, termites are commonly considered key soil bioturbators and soil engineers [START_REF] Dangerfield | The mound-building termite Macrotermes michaelseni as an ecosystem engineer[END_REF][START_REF] Black | Agricultural intensification, soil biodiversity and agroecosystem function in the tropics: the role of termites[END_REF][START_REF] Jouquet | Soil invertebrates as ecosystem engineers: Intended and accidental effects on soil and feedback loops[END_REF].

Through their bioturbation activity, termites impact the dynamics of soil and water at different spatial and temporal scales [START_REF] Bottinelli | Why is the influence of soil macrofauna on soil structure only considered by soil ecologists?[END_REF]Jouquet et al. 2016a). At a small scale, termites have been compared to weathering agents because they influence the mineralogy of clay and the properties of soil aggregates, which in turn influence soil organic matter and nutrient dynamics [START_REF] Abbadie | The role of subterranean fungus comb chambers (isoptera, macrotermitinae) in soil nitrogen cycling in a preforest savanna (côte divoire)[END_REF][START_REF] Jouquet | Effect of termites on clay minerals in tropical soils: fungus-growing termites as weathering agents[END_REF] as well as soil water-holding capacity [START_REF] Konaté | Influence of large termitaria on soil characteristics, soil water regime, and tree leaf shedding pattern in a West African savanna[END_REF][START_REF] Suzuki | Improvement in waterholding capacity and structural stability of a sandy soil in Northeast Thailand[END_REF][START_REF] Traoré | Termite mounds impact soil hydrostructural properties in southern Indian tropical forests[END_REF]. At the soil profile scale, termites use soils from various soil depths or specifically select soil particles of small sizes (i.e., clays) for the construction of their nest structures [START_REF] Abe | Soil-particle selection by the mound-building termite Macrotermes bellicosus on a sandy loam soil catena in a Nigerian tropical Savanna[END_REF][START_REF] Jouquet | Abundance and impact on soil properties of cathedral and lenticular termite mounds in Southern Indian woodlands[END_REF][START_REF] Jouquet | Effect of termites on clay minerals in tropical soils: fungus-growing termites as weathering agents[END_REF].

If the capacity of termites to enrich the soil in clay particles seems to be the rule [START_REF] Holt | Termites: Evolution, Sociality, Symbioses[END_REF], their impact on soil organic matter is highly variable. For instance, several studies showed higher C content in termite mounds than in the surrounding soil [START_REF] De Souza | Termite participation in the soil-forming processes of "murundus" structures in the semi-arid region of Brazil[END_REF][START_REF] Chen | Accumulation and spatial homogeneity of nutrients within termite (Odontotermes yunnanensis) mounds in the Xishuangbanna region, SW China[END_REF][START_REF] Wakbulcho | Impact of termite activities on soil physicochemical characteristics in different land use types in Lalo Asabi District[END_REF], while [START_REF] Cheik | Fungus-growing termite foraging activity increases water infiltration but only slightly and temporally impacts soil physical properties in southern Indian woodlands[END_REF] found a negative or neutral effect of termite bioturbation activity. Finally, termites act as heterogeneity drivers and shape the distribution of natural resources such as water and nutrients at the landscape scale through the edification of mounds that are comparable to nutrient patches or fertility islands [START_REF] Dangerfield | The mound-building termite Macrotermes michaelseni as an ecosystem engineer[END_REF][START_REF] Seng | Termite-induced heterogeneity in African savanna vegetation: mechanisms and patterns[END_REF][START_REF] Van Der Plas | Functional traits of trees on and off termite mounds: understanding the origin of biotically-driven heterogeneity in savannas[END_REF][START_REF] Cramer | The distribution and spatial patterning of mima-like mounds in South Africa suggests genesis through vegetation induced aeolian sediment deposition[END_REF][START_REF] Davies | Termite mounds alter the spatial distribution of African savanna tree species[END_REF]Jouquet et al. 2016a[START_REF] Jouquet | Influence of termites on ecosystem functioning. Ecosystem services provided by termites[END_REF][START_REF] Muvengwi | Termite diversity along a land use intensification gradient in a semi-arid savanna[END_REF][START_REF] Muvengwi | Cascading effects of termite mounds in African savannas[END_REF][START_REF] Chen | Accumulation and spatial homogeneity of nutrients within termite (Odontotermes yunnanensis) mounds in the Xishuangbanna region, SW China[END_REF].

The specific soil biological, chemical, and physical properties of termite mounds have largely been described, especially in Africa, where they play a key role in the dynamics of vegetation (e.g., [START_REF] Traoré | Impact of Macrotermes termitaria as a source of heterogeneity on tree diversity and structure in a Sudanian savannah under controlled grazing and annual prescribed fire (Burkina Faso)[END_REF]) and increase the robustness and resilience of African dryland ecosystems against water shortages and desertification [START_REF] Bonachela | Termite mounds can increase the robustness of dryland ecosystems to climatic change[END_REF]. In Brazil, termite activity is considered to be at the origin of the "murundus" structures, which represent a quantity of soil equivalent to approximately 4,000 great pyramids of Giza (De Souza and Delabie 2018; [START_REF] Martin | A vast 4,000-year-old spatial pattern of termite mounds[END_REF][START_REF] De Souza | Termite participation in the soil-forming processes of "murundus" structures in the semi-arid region of Brazil[END_REF]. In Asia, studies considering the properties of termite mounds have mainly been restricted to India [START_REF] Jouquet | The distribution of Silicon in soil is influenced by termite bioturbation in South Indian forest soils[END_REF][START_REF] Jouquet | Abundance and impact on soil properties of cathedral and lenticular termite mounds in Southern Indian woodlands[END_REF][START_REF] Jouquet | Influence of soil type on the properties of termite mound nests in Southern India[END_REF] and China [START_REF] Chen | Accumulation and spatial homogeneity of nutrients within termite (Odontotermes yunnanensis) mounds in the Xishuangbanna region, SW China[END_REF]). However, termite mounds are also conspicuous features of the landscapes in the Lower Mekong Basin [START_REF] Choosai | Termite mounds and dykes are biodiversity refuges in paddy fields in north-eastern Thailand[END_REF][START_REF] Miyagawa | Indigenous utilization of termite mounds and their sustainability in a rice growing village of the central plain of Laos[END_REF]. In Laos, lenticular mounds provide numerous ecosystem services, and their soil is also used for growing vegetables or as an amendment for increasing the soil fertility of paddy fields [START_REF] Miyagawa | Indigenous utilization of termite mounds and their sustainability in a rice growing village of the central plain of Laos[END_REF]. In northeastern Thailand, their higher concentrations of C and nutrients and higher clay content in comparison with the surrounding topsoil explain why farmers spread termite mound soil in their lands to increase their soil fertility [START_REF] Noble | Enhancing the agronomic productivity of degraded soils in Northeast Thailand through caly-based interventions[END_REF][START_REF] Suzuki | Improvement in waterholding capacity and structural stability of a sandy soil in Northeast Thailand[END_REF][START_REF] Choosai | Biological activity in paddy fields: the role of soil engineers in ecosystem functioning[END_REF]. Despite the fact that termite mounds are also commonly observed in paddy fields in Cambodia and that their soil is often used as an amendment for increasing the soil fertility of paddy fields or for growing vegetables (Muon pers. com.), there is a dearth of information on their chemical and physical properties. The question of the impact of termites on soil properties is, however, very important in paddy fields in Cambodia, where soils are mainly sandy and are characterized by low soil fertility and poor soil physical properties (i.e., soils are highly compact with a low water-holding capacity) [START_REF] White | The rice soils of Cambodia. I. Soil classification for agronomists using the Cambodian Agronomic Soil Classification system[END_REF][START_REF] Seng | Termite-induced heterogeneity in African savanna vegetation: mechanisms and patterns[END_REF]. In this environment, any enrichment in clay particles is likely to have a positive impact on soil chemical and physical properties, especially the retention of organic matter and water [START_REF] Noble | Enhancing the agronomic productivity of degraded soils in Northeast Thailand through caly-based interventions[END_REF].

Therefore, the aim of this study was to analyse the impact of termite bioturbation on soil properties in Cambodian paddy fields. Our main hypothesis was that termite mounds can be considered biogeochemical hotspots in paddy fields with higher clay, organic matter, and nutrient contents, as well as higher water hydraulic conductivity and a higher water-holding capacity.

Materials and methods

Study site and sampling procedure

The study was carried out in the Chrey Bak catchment, Cambodia, a long-term observatory and research infrastructure of the Institute of Technology of Cambodia. This catchment is located in the watershed of Tonle Sap Lake (12°15'3.62"N, 104°40'6.21"E) and in the southwest of the provincial capital of Kampong Chhnang Province. It is covered by two districts, namely Tuek Phos and Rolea Biér, approximately 700 km 2 . 34% of the catchment is covered by forests, and the remaining is used for agriculture (32%) or consists of shrublands (27.4%) and grasslands (5.4%) (JICA, 2003). The catchment is influenced by the tropical monsoon and has two distinct seasons: the dry season from November to April and the rainy season from May to October, with annual rainfall varying between 1400 and 2000 mm (MOWRAM 2014). Soils are mainly Gleysols (Bridges et al. 1998) with varying sand contents [START_REF] Muon | Termite bioturbation in Cambodia -From characterization to application[END_REF]. Soils were sampled in three positions in the catchment (see supplementary file 1). Three soil categories were identified based on their sand contents: Group I (GI, very sandy soils with > 80% sand), Group II (GII, moderately sandy, ~60% sand), and Group III (GIII, low sandy soils, ~40% sand) (see supplementary file 2. GI was found in the lower part of the catchment, while GIII was found in the upper reaches of the soil gradient. GII was sampled in both the lowland and upland. The slope of the catchment is variable and high (~37% on average) in the mountainous area in the southwest region, while it is very gentle (slope < 12%) or almost flat in the cultivated area (Mekong River Commission 2003), which is dominated by rain-fed paddy fields. Rice plots were separated by small dikes for irrigation purposes. In this environment, soil bioturbation is mainly carried out by termites, especially Macrotermes gilvus [START_REF] Bathellier | Contribution à l'étude systématique et biologique des termites de l'Indochine[END_REF] which builds termite mound nests, also called termitaria (TN) that are only occupied by M. gilvus (i.e., they do not host inquiline species) and which are usually found on large lenticular mounds (LM, ~6 m 3 and 16.88 m 2 on average) (see [START_REF] Josens | Lenticular mounds in the African savannahs can originate from ancient Macrotermes mounds[END_REF][START_REF] Harit | Origin and dynamics of termite mound soils in Southern India[END_REF] for examples of 'termite lenticular mounds'; see supplementary file 3 for a description of the method used for the identification of M. gilvus). Although the origin and dynamics of LM are unknown, it is commonly considered by local villagers that they come from the degradation and colonization of abandoned TN by other termite species (e.g., Odontotermes spp. and Globitermes sulphureus), invertebrates (e.g., ants and earthworms) and plants (Figure 1; and see [START_REF] Inoue | Mound structure of the fungus-growing termite Macrotermes gilvus in Thailand[END_REF] for a description of LM), then suggesting very different properties and dynamics than those of TN.

Soil sampling occurred during the dry season in February 2020 because sampling sites are unavailable during the rainy season when paddy fields are flooded. Five LM were selected per soil group based on the presence of TN (n = 5 per soil group). The influence of termite bioturbation on soil properties was measured via the sampling of soil in three locations: (i) in the topsoil (0-10 cm depth) of LM, (ii) from the outer wall of TN located on LM, and (iii) in the surrounding soil environment in the paddy field (CTRL) at 0-10 cm depth and at a distance of ~5 m ahead of each LM (Figure 1). The same procedure was repeated in the three soil groups (GI, II and III), giving a total of 3 soil groups × 3 locations (LM, TN and CTRL) × 5 replicates = 45 soil samples. Soil samples were air-dried in shade at air temperature for one week and then sieved to 2 mm and stored in air tight plastic bags.

Soil chemical and physical analyses

The soil pH and electrical conductivity (EC) were determined in a 1:5 soil:water suspension [START_REF] Pansu | Handbook of soil analysis: Mineralogical, organic and inorganic methods, Handbook of Soil Analysis: Mineralogical, Organic and Inorganic Methods[END_REF] with portable pH and EC devices. The soil particle size distribution was measured after the destruction of organic matter by 30% hydrogen peroxide (H2O2) and dispersion with sodium hexametaphosphate (Na6O18P6). The proportion of sand (50-2000 µm) was obtained by sieving, while the proportions of silt (2-50 µm) and clay (< 2 µm) were obtained by using ASTM 152H hydrometers [START_REF] Gee | 2.4 Particle-Size Analysis[END_REF]. The bulk densities of LM and CTRL were measured using 100 cm 3 sample rings and dried at 105°C for 48 h.

Because of the hardness of TN soil, this method could not be used for measuring the soil bulk density of TN, that is why the paraffin-clod method [START_REF] Blake | Bulk Density 1. Methods soil Anal[END_REF]) was used for TN using large size soil samples (~20-25 g in average). Concentrations of C and N were analysed using a Thermo Scientific Flash 2000 organic elemental analyser. Total P was analysed by a spectrophotometer using a blank and standards prepared in the Olsen P extracting solution [START_REF] Estefan | Methods of soil, plant, and water analysis[END_REF]) with a Thermo Scientific GENESYS 30 Visible Spectrophotometer.

Exchangeable K was measured by the flame photometer method [START_REF] Black | Methods of soil analysis, Part 2[END_REF]) using S20 Superspec Series flame photometers. The saturated hydraulic conductivity (Ksat) of CTRL and LM soils was analysed in the laboratory using the falling head method [START_REF] Reynolds | Falling head soil core (tank) method[END_REF] with a Ksat device. Finally, the soil water holding capacity was determined at pF 0 by the sand box (Eijkelkamp) and pF 2.5, 3, 3.2, and 4.2 by the pressure plate method [START_REF] Dane | Methods of Soil Analysis: Part 4 Physical Methods[END_REF] using "Richards" pressure plate extractors. Because of the hardness of TN soil and the difficulty to collect non-disturbed TN soil samples, Ksat and the soil water holding capacity were measured only for CTRL and LM.

Soil samples were analysed at IFPEN laboratory (Institut Français du Pétrole Énergies

Nouvelles) with a Rock-Eval 6 device. This technique has been recommended for characterizing soil organic matter [START_REF] Derenne | Analytical pyrolysis as a tool to probe soil organic matter[END_REF][START_REF] Disnar | Soil organic matter (SOM) characterization by Rock-Eval pyrolysis: Scope and limitations[END_REF]) and has proven relevance in various contexts [START_REF] Malou | The Rock-Eval® signature of soil organic carbon in arenosols of the Senegalese groundnut basin. How do agricultural practices matter?[END_REF][START_REF] Romanens | Soil diversity and major soil processes in the Kalahari basin, Botswana[END_REF][START_REF] Sebag | Dynamics of soil organic matter based on new Rock-Eval indices[END_REF][START_REF] Thoumazeau | A new in-field indicator to assess the impact of land management on soil carbon dynamics[END_REF], and for studying the impact of soil engineering organisms [START_REF] Schomburg | Composition and superposition of alluvial deposits drive macro-biological soil engineering and organic matter dynamics in floodplains[END_REF][START_REF] Schomburg | Rock-Eval pyrolysis discriminates soil macro-aggregates formed by plants and earthworms[END_REF][START_REF] Mer | Inferring the impact of earthworms on the stability of organo-mineral associations, by Rock-Eval thermal analysis and 13C NMR spectroscopy[END_REF]. The method uses ramped pyrolysis of organic matter under an artificial air supply (N2) between 200 and 650°C and then the combustion of residual carbon under oxidative conditions between 200 and 850°C. The released gases are quantified using a flame ionization detector (FID) for hydrocarbon compounds (HC) and infrared detectors (IR) for CO and CO2. Qualitative and quantitative parameters are calculated by integrating the amounts of HC, CO, and CO2 produced during thermal cracking of organic matter between defined temperature limits [START_REF] Behar | Rock-Eval 6 Technology: Performances and Developments[END_REF][START_REF] Lafargue | Rock-Eval 6 applications in hydrocarbon exploration, production, and soil contamination studies[END_REF]. Carbon contents were calculated by integrating the C moieties produced during thermal cracking and combustion-defined temperature limits, with TOC (%) calculated as the sum of all released carbon released at low temperatures from organic compounds and OrgC and MinC (%) consisting of organic and mineral carbon pools released at high temperatures. Soil organic matter quality was assessed with the hydrogen index (HI in mg HC/g TOC), calculated as the total amount of HC normalized to the TOC content. Soil organic matter thermal stability was assessed by combining two indices (denoted R-and I-index) calculated from five subdivided areas of the S2 thermograms related to HC [START_REF] Sebag | Dynamics of soil organic matter based on new Rock-Eval indices[END_REF]. By construction, the R-index relates to the thermally resistant and refractory pools of organic matter, while the I-index is related to the ratio between the thermally labile and resistant pools.

As derived from a mathematical construct, these two indices may be inversely correlated when OM stabilization results from progressive decomposition of labile organic compounds and relative enrichment in refractory compounds. Then, in the I/R diagram, the same "decomposition line model" describes the decreasing labile pools and concomitant increase in more thermally stable pools, as observed in compost [START_REF] Albrecht | Organic matter decomposition: bridging the gap between Rock-Eval pyrolysis and chemical characterization (CPMAS13C NMR)[END_REF] and in soils [START_REF] Sebag | Dynamics of soil organic matter based on new Rock-Eval indices[END_REF][START_REF] Matteodo | Decoupling of topsoil and subsoil controls on organic matter dynamics in the Swiss Alps[END_REF][START_REF] Thoumazeau | A new in-field indicator to assess the impact of land management on soil carbon dynamics[END_REF]. I/R diagrams were used to calculate the deviation of I-index values from LM and TN to those predicted from the regression line of the control soil.

Statistical analyses

Since soils were sampled in different locations in the catchment, linear mixed models with elevation as random factor and Tukey tests were performed to assess differences between means for the different soil structures (TN, LM and CTRL) and soil groups (GI to III). Prior to running linear mixed models, data were tested for homogeneity of variance and normality using Levene and Shapiro-Wilk tests. Differences among treatments were declared significant at the P <0.05 probability level. All statistical analyses and visualizations were carried out with R version 4.1.1 (https://www.r-project.org/) using mainly lme4, ggplot2 and emmeans packages.

Results

Soil physical and chemical properties

The three soil groups were mostly differentiated from their soil particle size distribution (i.e., no significant differences in C, N, or P contents, C:N ratio, pH or EC) (Figure 2 and Table 1).

The K content was higher in GIII than in GI, with intermediate value for GII. Conversely, the amount of sand was the highest in GI and the lowest in GIII, while intermediate sand contents

were measured in GII. The opposite was found for the clay and silt contents, which were the highest in GIII and the lowest in GI. 1).

The C, N and P contents and EC were only influenced by termite bioturbation (i.e., not influenced by soil groups). Higher C, N and P contents were measured in LM than in CTRL (Figure 2). LM was also characterized by intermediate EC value between those measured in CTRL and TN (P > 0.05 for both CTRL and TN). TN had similar C content to CTRL but intermediate N content to those measured in CTRL and LM (P > 0.05 for both). TN was also characterized by a higher P content and a higher EC than that of CTRL (P < 0.05 for both).

Termite bioturbation did not influence the soil pH, except in the sandiest soil group (GI)

where a higher soil pH was measured in TN than in CTRL (Figure 2). In GI, LM had intermediate soil pH values between those of CTRL and TN (P > 0.05 in both cases). Clay and sand contents were also influenced by termite bioturbation. The clay content was always higher in TN than in CTRL, while the enrichment in clay was only measured in GI and GII for LM.

Conversely, the sand content was reduced in TN in comparison with CTRL, although this impact was only measured in the first soil group (i.e., P > 0.05 for GII and GIII).

Carbon pools measured by Rock-Eval ® analysis

Soil groups had no influence on the soil properties measured by Rock-Eval (Table 2). Higher

OrgC content was measured in LM than in TN and CTRL (P > 0.05 between TN and CTRL).

In general, MinC content was very low. However, their concentrations were the highest in TN in comparison with the other treatments for all soil groups (P > 0.05 in all cases). While HI was lower in TN than in LM and CTRL, the differences were nonsignificant. Finally, no significant differences in R-index values were measured (P > 0.05).

Comparatively, I/R values from LM and CTRL were lower than those of the general regression model (Figure 3). The distribution of the data was more variable for TN, with values both higher and lower than those of the general model. This difference in variability shows a higher deviation of the I-index from control soils for TN than for LM (Figure 4).

Soil porosity and water dynamics

The soil bulk density (BD) reached between 1.4 and almost 1.7 g cm -3 in CTRL. The BD value was influenced by both the location (i.e., soil group effect, P < 0.001) and the termite bioturbation (P < 0.001) (Table 3). CTRL and TN had similar BD values, independent of the soil groups. Conversely, the soil BD was lower in LM than in CTRL and TN, except in GI, where BD in TN was similar to that in LM and CTRL. The lower BD in LM was also associated with a higher Ksat in LM soil than in CTRL (P = 0.005), independent of the soil groups (P = 0.084) (Figure 5). Water-holding capacity was also significantly higher in LM than in CTRL at pF 4.2, 3.2 and 2.5, independent of the location (soil group effect, P > 0.05) (Table 4 and Figure 6).

Discussion

Termite constructions as nutrient patches

In the Lower Mekong Basin, LM are conspicuous features of the landscape. At our study site, their density was not influenced by the location within the catchment or by the soil types, reaching 2 mounds ha -1 , which corresponds to ~6 m 3 soil ha -1 on average. One striking conclusion of this study is that LM always constitutes patches of nutrients in paddy fields, with higher C and nutrient (N and P) contents than the surrounding topsoil. TN was also enriched in P in comparison with the reference soil, but no difference in C or N content was found.

Enhancement in C content in LM was confirmed by Rock-Eval ® analysis. However, termite bioturbation did not influence the R-index and HI value, suggesting similar soil organic matter quality [START_REF] Barré | The energetic and chemical signatures of persistent soil organic matter[END_REF][START_REF] Sebag | Dynamics of soil organic matter based on new Rock-Eval indices[END_REF]. Nevertheless, the I/R diagram showed specific SOM-mineral interactions in TN in comparison with both LM and CTRL. Indeed, while LM and CTRL samples were distributed along the 'decomposition line' (i.e., the red line in Figure 3), TN samples were characterized by high variability. This result suggests the presence of variable proportions of labile but unstable organic matter fractions in TN, most likely of saliva origin and/or of organic fractions stabilized by adsorption on mineral surfaces. Such changes could be attributed to the total reorganization of the interactions between SOM and minerals during the molding of individual soil pellets by termites, as well as the enrichment of clay particles in TN. Indeed, TN was always enriched in clay in comparison with the surrounding soil, although a reduction in sand was only measured when the sand content of the paddy field was > 80% (i.e., GI). LM was also enriched in clay, but this effect was only measured when the sand content of the paddy field was > 60% (i.e., GI and GII). Therefore, these findings confirm the general assumption that termites' impact on soil texture is preponderant in sandy and poor soils, while it is more limited in more clayey soil (e.g., [START_REF] Jouquet | Influence of soil type on the properties of termite mound nests in Southern India[END_REF][START_REF] Bera | Termite mound soil properties in West Bengal, India[END_REF]. In sandy soil, the construction of termitaria usually requires the selection of clay particles, which might come from the deeper soil layers [START_REF] Abe | Soil-particle selection by the mound-building termite Macrotermes bellicosus on a sandy loam soil catena in a Nigerian tropical Savanna[END_REF][START_REF] Jouquet | Abundance and impact on soil properties of cathedral and lenticular termite mounds in Southern Indian woodlands[END_REF], and the incorporation of organic matter (C and N), i.e., mostly saliva that fungus-growing termites use for ensuring cohesion between particles [START_REF] Holt | Termites: Evolution, Sociality, Symbioses[END_REF][START_REF] Jouquet | The impact of termites on soil sheeting properties is better explained by environmental factors than by their feeding and building strategies[END_REF][START_REF] Zachariah | Building mud castles: a perspective from brick-laying termites[END_REF].

As observed by others [START_REF] Bera | Termite mound soil properties in West Bengal, India[END_REF][START_REF] Cheik | Fungus-growing termite foraging activity increases water infiltration but only slightly and temporally impacts soil physical properties in southern Indian woodlands[END_REF][START_REF] Jouquet | Influence of soil type on the properties of termite mound nests in Southern India[END_REF]; Subi and Sheela 2020), a higher soil pH was found in termite constructions in comparison with the surrounding soil in GI. Interestingly, this higher soil pH was associated with the highest MinC values in TN. Although its concentration was very low (< 1%), the presence of mineral C was highly surprising in the Chrey Bak catchment, where the soil is acidic. Since a Pisoplinthic horizon was found at approximately 80 cm depth, we consider it unlikely that mineral C came from the deeper soil layers. Examination of the thermograms measuring the fluxes of CO2 and CO during pyrolysis shows the presence of 2 synchronous peaks between 500 and 525 °C, observed in oxalate-rich samples (see supplementary file 4). These peaks are not observed in CTRL and suggest that high MinC value in TN could correspond to biogenic carbonate, which could be derived from the oxalotrophic activity of termite-associated fungi and bacteria feeding on decaying oxalogenic plant tissues [START_REF] Cailleau | Turning sunlight into stone: The oxalate-carbonate pathway in a tropical tree ecosystem[END_REF][START_REF] Mujinya | The origin of carbonates in termite mounds of the Lubumbashi area, D.R. Congo[END_REF][START_REF] Suryavanshi | Eubacterial diversity and oxalate metabolizing bacterial species (OMBS) reflect oxalate metabolism potential in Odontotermes gut[END_REF][START_REF] Francis | Calcite accumulation in a South African heuweltjie: Role of the termite Microhodotermes viator and oribatid mites[END_REF][START_REF] Jouquet | The impact of termites on soil sheeting properties is better explained by environmental factors than by their feeding and building strategies[END_REF]. Additionally, these oxalate crystals could also have a bacterial [START_REF] Hervé | Diversity and ecology of oxalotrophic bacteria[END_REF] or fungal origin [START_REF] Hervé | Functional diversity of the litterassociated fungi from an oxalate-carbonate pathway ecosystem in madagascar[END_REF]. Since termite constructions harbour microbial communities distinct from those present in the surrounding soil [START_REF] Anklin-Mühlemann | Morphological, microbiological and biochemical studies of the gut flora in the fungusgrowing termite Macrotermes subhyalinus[END_REF], oxalogenic microorganisms could be more abundant in termite constructions than in soil. To form CaCO3 from the oxidation of calcium oxalate, the soil pH needs to reach the stability of calcite, i.e., pH 8.4 under conventional surface conditions of temperature and pressure. This pH is likely to be reached during the digestion of organic residues by termites (pH = 9.5 in the paunch of Macrotermes subhyalinus, [START_REF] Anklin-Mühlemann | Morphological, microbiological and biochemical studies of the gut flora in the fungusgrowing termite Macrotermes subhyalinus[END_REF]. Even if the soil pH is lower than 8.4 in TN, it is also possible that such a value could be reached at a very small scale (e.g., [START_REF] Kim | Microscale pH variations during drying of soils and desert biocrusts affect HONO and NH3 emissions[END_REF] within the soil pellets produced by termites by building their nests. Finally, the oxidation of oxalate by oxalotrophic bacteria can also locally increase the soil pH, making CaCO3 precipitation possible in an overall acidic environment. Enhancement in soil pH increases the cationic exchange capacity of soil organic matter and 1:1 minerals in soil [START_REF] Brady | Elements of the nature and properties of soils[END_REF] and therefore its ability to hold nutrients.

Indeed, together with these enrichments in clay, C and N contents, termites also enhanced the concentrations of exchangeable basic cations and nutrients in their constructions [START_REF] Jouquet | Influence of termites on ecosystem functioning. Ecosystem services provided by termites[END_REF][START_REF] Abe | Nutrient storage in termite (Macrotermes bellicosus) mounds and the implications for nutrient dynamics in a tropical savanna Ultisol[END_REF][START_REF] Bera | Termite mound soil properties in West Bengal, India[END_REF][START_REF] Chisanga | Prospects of using termite mound soil organic amendment for enhancing soil nutrition in Southern Africa[END_REF]. Our study confirmed this increase in soil fertility with higher amount of total P in termite constructs. The impact of termite bioturbation activity on the amount of available P is usually related to the functional groups to which termite species belong [START_REF] López-Hernández | Effect of different termite feeding groups on P sorption and P availability in African and South American savannas[END_REF]. Previous studies showed that fungus-growing termites increase the sorption of P in soil (i.e., reduce the amount of available P) in comparison with the surrounding soil [START_REF] López-Hernández | Effect of different termite feeding groups on P sorption and P availability in African and South American savannas[END_REF][START_REF] Bera | Termite mound soil properties in West Bengal, India[END_REF].

Therefore, more research is needed to determine whether the higher amount of total P in LM and TN is associated with a higher availability of P in soil.

Termite bioturbation improves soil water dynamics

As shown in other ecosystems, sandy soils can be highly susceptible to compaction [START_REF] Huang | Soil and environmental issues in sandy soils[END_REF]. This is especially true in the Lower Mekong Basin when the ploughing of soil and the harvesting of rice are mechanized [START_REF] Gürsoy | Soil compaction due to increased machinery intensity in agricultural production: its main causes, effects and management[END_REF]. At our study site, the high compaction of soil is unlikely to be a problem for the cultivation of rice when it is submerged by water, and it can even be looked at to avoid the leaching of nutrients [START_REF] Patel | Changes in bulk density and water intake rate of a coarse textured soil in relation to different levels of compaction[END_REF].

However, a high bulk density can become a real issue in periods of water shortage [START_REF] Hoque | Effect of soil compaction on the grain yield of rice (Oryza sativa L.) under water-deficit stress during the reproductive stage[END_REF] because of the hardness of soil, which can reduce the ability of rice roots to grow [START_REF] Hasegawa | Root behavior: field and laboratory studies for rice and nonrice crops[END_REF][START_REF] Iijima | Effects of soil compaction on the development of rice and maize root systems[END_REF]. In Cambodia, agriculture is hampered by erratic rainfall at the beginning of the rainy season, with a temporary period of drought threatening the seedlings and the growth of rice and thus, rice productivity [START_REF] Tsubo | Frequency of occurrence of various drought types and its impact on performance of photoperiod-sensitive and insensitive rice genotypes in rainfed lowland conditions in Cambodia[END_REF]. Therefore, the lower bulk density measured in lenticular mounds is interesting because it shows that bioturbation (i.e., the production of subterranean tunnels by termites and other invertebrates and the production of macropores by roots) associated with a limited increase in clay particles and organic matter can significantly improve soil structure and both the infiltration of water and the water-holding capacity of soil. This was evidenced by the higher values of Ksat measured in lenticular mounds in comparison with the surrounding topsoil, as well as by the significantly higher water retention measured at pF 2.5, 3.2, and 4.2.

The high compaction (i.e., extremely low soil porosity) of Macrotermes spp. termitaria has been described in several ecosystems [START_REF] Holt | Termites: Evolution, Sociality, Symbioses[END_REF][START_REF] Abe | Soil-particle selection by the mound-building termite Macrotermes bellicosus on a sandy loam soil catena in a Nigerian tropical Savanna[END_REF][START_REF] Traoré | Termite mounds impact soil hydrostructural properties in southern Indian tropical forests[END_REF][START_REF] Bera | Termite mound soil properties in West Bengal, India[END_REF]) and explains the extremely high resistance of these soil structures to the attack of predators and to the weathering impact of rain [START_REF] Jouquet | The soil structural stability of termite nests: role of clays in Macrotermes bellicosus (Isoptera, Macrotermitinae) mound soils[END_REF]). Our study confirmed the very high bulk density of M. gilvus termitaria. The water retention curves and Ksat could not be measured from TN samples. However, the higher clay content in TN but similar soil bulk density than in CTRL suggested a very specific organization of the soil porosity and a very low diffusion of water within termitaria.

Conclusion

Two key conclusions and perspectives came out from this study. First, we confirmed that termite constructions could be seen as nutrient patches in paddy fields, explaining why farmers use these soil types to improve the fertility of their lands [START_REF] Noble | Enhancing the agronomic productivity of degraded soils in Northeast Thailand through caly-based interventions[END_REF][START_REF] Suzuki | Improvement in waterholding capacity and structural stability of a sandy soil in Northeast Thailand[END_REF][START_REF] Sarcinelli | Soil modification by termites in a sandy-soil vegetation in the Brazilian Atlantic rain forest[END_REF][START_REF] Subi | Review on termite mound soil characteristics and agricultural importance[END_REF][START_REF] Chisanga | Prospects of using termite mound soil organic amendment for enhancing soil nutrition in Southern Africa[END_REF]). In addition to increasing the chemical fertility of soil, the amendment of LM and TN by farmers is also likely to improve the physical properties of soil. More research is now needed to determine if the amendment of LM soil can also enhance the water holding capacity of cultivated soils. This impact could be very important in Cambodian paddy fields, where the management of water is considered to be a key issue [START_REF] Chem | Hydrological analysis in support of irrigation management[END_REF].

Second, we evidenced the presence of carbonates in TN, most likely due to the stimulation of specific bacteria and fungi associated with the production of carbonate nodules within microhabitats. This result is especially interesting because, although the concentration of carbonate is very low, it can favourably impact both the soil fertility and the sequestration of C in paddy fields, which are known to be important emitters of methane [START_REF] Li | Quantifying greenhouse gas emissions from soils: Scientific basis and modeling approach[END_REF]. Therefore, more research is also needed to identify the ecological processes and biological actors associated with the production of carbonates in TN and to determine if they could be used to increase the soil fertility and the sequestration of C in cultivated soils.

Sample P7_TN3

525°C on the S3CO and S3CO2 themograms of the N sample is characteristic of the presence of oxalates, most likely CaC2O₄.

Figure 1 .

 1 Figure 1. Termite constructions in paddy fields in the Chrey Bak catchment, Cambodia: (a) Lenticular mounds covered by vegetation (LM), (b) Termitaria built by M. gilvus on the top of a lenticular mounds (TN), (c) control soil (photos: R. Muon, 2021; Jouquet, 2022).

Figure 2 .

 2 Figure 2. Histograms showing the impact of termite bioturbation (control soil "CTRL" in orange, lenticular mounds "LM" in green, and termitaria "TN" in blue) in the three soil groups (GI, GII and GIII) on the C, N, and P contents, pH and electrical conductivity (EC), and soil particle sizes (clay and sand contents) (n = 5), and on K and silt contents (no termite bioturbation effect, n = 10). Histograms with different letters are significantly different at P < 0.05 (mixed linear model with elevation as random factor).

Figure 3 .

 3 Figure 3. I-vs. R-index diagram for termitaria and lenticular mounds, and control soils. The dashed line represents the 'humic trend' measured by Sebag et al. (2016). The I-index is shown on the vertical axis (control 'CTRL' in orange, lenticular mounds 'LM' in green, and termitaria 'TN' in blue). Soil groups are GI in circles, GII in triangles, and GIII in squares.

Figure 4 .

 4 Figure 4. Histogram showing the deviation of I-index values of lenticular mounds (LM) and termitaria (TN) from those of the control soils (CTRL) (average values for the three soil groups). Vertical bars are standard errors (n = 15). Differences are significantly different

Figure 5 .

 5 Figure 5. Saturated hydraulic conductivity (Ksat) in lenticular mounds (LM) and control soil (CTRL) (average values for the three soil groups). Vertical bars are standard errors (n = 13). Histograms with different letters for the same location are significantly different at P < 0.05.

Figure 6 .

 6 Figure 6. Soil water content at different pFs for the lenticular mound (LM) in green and control soils (CTRL) in orange in the different soil groups. Vertical bars are the standard errors, and the stars are significantly different at P < 0.05.

1S2.

  Soil texture triangle showing the different soil groups (GI to GIII) of thermograms (S2HC, S3CO and S3CO2) obtained by Rock-Eval® pyrolysis 4 of (A) CTRL samples and (B) TN samples. The existence of a sharp peak between 500 and 5

Table 1

 1 Results of the linear mixed models testing the influence of termite bioturbation (i.e., termite mound nests, lenticular mounds, and reference soil) and soil groups (i.e., GI, GII and GIII) on soil properties with elevation as random factor.

	*** < 0.001, ** < 0.01, * < 0.05
		Bioturbation (a)	Soil Groups (b)		(a) × (b)
		F-values	P-values	F-values	P-values	F-values P-values
	C (%)	5.53	0.008 **	2.30	0.302	32.87	0.829
	N (%)	5.20	0.011 *	5.42	0.156	0.46	0.760
	C:N	1.68	0.201	4.64	0.177	0.10	0.980
	P (ppm) K (meq 100g -1 ) EC (µS cm -1 )	9.028 3.67 6.32	< 0.001 *** 0.036 * 0.005 **	0.15 2.05 1.18	0.899 0.328 0.458	1.09 4.09 1.60	0.377 0.008 ** 0.197
	pH	8.92	< 0.001 ***	4.47	0.182	3.21	0.024 *
	Clay (%)	35.02	< 0.001 ***	11.75	0.078	3.15	0.026 *
	Silt (%)	0.45	0.637	32.18	0.030 *	1.61	0.193
	Sand (%)	16.48	< 0.001 *** 115.74 < 0.001 ***	2.14	0.096

Bioturbation by termites significantly influenced soil properties apart from the C:N ratio, which was influenced neither by termite bioturbation nor by the soil groups (Table

Table 2 .

 2 Results from the Rock Eval analysis showing the percentages of organic (OrgC) and

	mineral C contents (MinC) and HI (in mg HC g -1 TOC) and the R index for termite mound
	nests (TN), lenticular mounds (LM), and the surrounding paddy field (control, CTRL) found
	in the three soil groups (GI, GII, and GIII) in the Chrey Bak catchment. Numbers in parentheses
	are standard errors of the means (n = 5).
	P-values correspond to the results of the linear mixed models testing the influence of termite
		Soil types OrgC (%)	MinC (%)	HI (mg HC g -1 TOC)	R index
		CTRL	0.49 b (0.12) 0.01 b (0.00)	238.6 (9.15)	0.58 (0.02)
	GI	LM	1.06 a (0.16) 0.00 b (0.01)	215.8 (10.28)	0.60 (0.01)
		TN	0.62 b (0.06) 0.07 a (0.02)	191.0 (20.60)	0.61 (0.01)
		CTRL	0.85 b (0.09) 0.00 b (0.00)	253.4 (14.22)	0.53 (0.02)
	GII	LM	1.63 a (0.40) 0.02 b (0.02)	215.4 (23.63)	0.56 0.01)
		TN	0.74 b (0.19) 0.03 a (0.00)	158.6 (10.43)	0.55 (0.03)
		CTRL	1.08 b (0.11) 0.00 b (0.00)	191.2 (7.96)	0.61 (0.01)
	GIII	LM	1.94 a (0.45) 0.01 b (0.00)	197.4 (18.72)	0.58 (0.02)
		TN	1.60 b (0.59) 0.05 a (0.01)	136.6 (28.48)	0.61 (0.01)
	Bioturbation (a)		P = 0.011 * P < 0.001***	P = 0.7531	P = 0.490
	Soil groups (b)		P = 0.285	P = 0.117	P = 0.139	P = 0.092

bioturbation (i.e., LM, TN, and CTRL) and soil groups (i.e., groups I, II and III) on soil properties with elevation as random factor. Values with similar letters are significantly similar at P > 0.05. *** P < 0.001, * P < 0.05

Table 4 .

 4 Results of the linear mixed models testing the influence of termite bioturbation (i.e., termite mound nests, lenticular mounds, and reference soil) and soil groups (i.e., GI, GII and GIII) on water the content at pF 0, 2.5, 3, 3.5 and 4.2 with elevation as random factor.

	*** < 0.001, ** < 0.01, * < 0.05
		Bioturbation (a)	Soil Groups (b)		(a) × (b)
		F-values	P-values	F-values P-values F-values P-values
	pF0	0.38	0.545	0.69	0.590	0.81	0.455
	pF2.5 8.24	0.009 **	7.79	0.114	0.02	0.975
	pF3	3.31	0.082	13.74	0.068	0.02	0.978
	pF3.2 4.95	0.036 *	12.66	0.073	0.11	0.893
	pF4.2 14.71	< 0.001 *** 5.96	0.144	0.46	0.638

Acknowledgements

We are particularly grateful to Boreborey Ty for her access to the laboratory and to Daniel Pillot and Herman Ravelojaona (IFPEN, France) for their technical supports in Rock-Eval® analysis, a trademark registered by IFP Energies Nouvelles. This work is part of the PhD of Ratha Muon who was supported by a grant co-funded by the French Embassy (BGF) in Cambodia, the Cambodian Ministry of Education, Youth and Sports, the Institute of Technology of Cambodia (ITC) and the French Institute of Research for Development (IRD, program ARTS).

Conflict of interest

Authors declare no competing interests.

Supplementary file.

S1. Map of the Chrey Bak catchment showing the location of the sampling plots for the three soil groups. The coordinate positions of the centre of the plots are displayed.