Jérome Ngao

B Longdoz
email: longdoz@nancy.inra.fr

D Perrin

G Vincent

Daniel Epron

Valérie Le Dantec

Kamel Soudani

M Aubinet

F Willm

A Granier

Jérôme N

Bernard L

Dominique P

Gaëlle V

Daniel E

Valérie L D

Kamel S

Marc A

François W

André G

Cross-calibration functions for soil CO2 efflux measurement systems

INTRODUCTION

After photosynthesis, soil CO 2 efflux (R S ) is the second CO 2 forest flux of importance [START_REF] Raich | The global carbon dioxide flux in soil respiration and its relationship to vegetation and climate[END_REF] and soil constitutes the major carbon reserve in terrestrial ecosystems [START_REF] Dixon | Carbon pools and flux of global forest ecosystems[END_REF]. Even small R S responses to global climatic change can induce important variation in CO 2 atmospheric concentration [START_REF] Watson | Land use, land-use change, and forestry[END_REF]. Accurate measurements are required for improving the understanding of the soil respiration process and its modeling.

Different methods have been used for R S measurements, such as static chamber systems (alkali solution, soda lime [START_REF] Anderson | Carbon dioxide evolution from two temperate deciduous woodland soils[END_REF]), closed or open dynamic chambers connected to infrared gas analyzers (IRGA) [START_REF] Norman | Soil surface CO 2 fluxes and the carbon budget of grassland[END_REF][START_REF] Rayment | An improved open chamber system for measuring soil CO 2 effluxes in the field[END_REF], eddy covariance measurements below the canopy [START_REF] Law | Below-canopy and soil CO 2 efflux in ponderosa pine forest[END_REF] and soil CO 2 concentration gradient analysis [START_REF] Russell | Carbon dioxide efflux from the floor of a boreal aspen forest. II. Evaluation of methods -Verification by infrared analysis of a dynamic closed chamber[END_REF]. The choice of the measurement system mostly depends on the specific spatial and temporal resolution requested, as underlined for chamber techniques by Savage and Davidson [START_REF] Savage | A comparison of manual and automated systems for soil CO 2 flux measurements: trade-offs between spatial and temporal resolution[END_REF].

This study only includes closed dynamic chamber systems (CDC). Indeed, these systems are easy portable allowing a high number of measurement repetitions and therefore are able to integrate the intra-plot spatial variability. When frequent measurement campaigns are performed within a stand (every or http://dx.doi.org/10.1051/forest:2006028 week or every two weeks), these systems are also able to capture the seasonal variation of R S [START_REF] Epron | Interannual variation of soil respiration in a beech forest ecosystem over a six-year study[END_REF][START_REF] Reichstein | Modeling temporal and large-scale spatial variability of soil respiration from soil water availability, temperature and vegetation productivity indices[END_REF]. They are most appropriate to estimate the annual soil respiration of plots and, thus, can be used for the comparison of annual respiration between different forest sites.

Five different CDC systems are usually employed to measure the R S on the different sites taking part in our plot comparison project. The R S data recorded with these systems cannot be directly compared since the inter-plot variability can be masked by significant systematic deviations already observed among the different closed dynamic systems in number of studies [START_REF] Janssens | Forest floor CO 2 fluxes estimated by eddy covariance and chamber-based model[END_REF][START_REF] Dantec | Soil CO 2 efflux in a beech forest: comparison of two closed dynamic systems[END_REF], mainly due to differences in air circulation and pressure conditions in the chamber headspace. It had also been shown that (i) most of these deviations were linear and (ii) correction coefficients could be applied for more accurate comparisons of soil CO 2 efflux values, revealing the necessity of cross-calibrations [START_REF] Janssens | Assessing forest soil CO 2 efflux: an in situ comparison of four techniques[END_REF][START_REF] Lankreijer | Measurement of soil respiration[END_REF][START_REF] Pumpanen | Comparison of different chamber techniques for measuring soil CO 2 efflux[END_REF]. Unfortunately, each of these studies has been performed on one single site or using a calibration device in standardized conditions. In addition, the influence of site characteristics as soil moisture, soil type or texture on deviations between systems has not yet been taken into account [START_REF] Liang | In situ comparison of four approaches to estimating soil CO 2 efflux in a northern larch (Larix kaempferi Sarg.) forest[END_REF][START_REF] Pumpanen | Comparison of different chamber techniques for measuring soil CO 2 efflux[END_REF].

In addition to the soil chamber type, the soil-chamber contact mode (i.e. inserted into soil or laid on a pre-inserted collar) is another methodological point that has been well discussed [START_REF] Lankreijer | Measurement of soil respiration[END_REF][START_REF] Nay | Biases of chamber methods for measuring soil CO 2 efflux demonstrated with a laboratory apparatus[END_REF]. Both of these methods present advantages and drawbacks. The direct insertion of a chamber into the soil potentially disturbs the litter-soil layer at a short-term (within 24 h) and does not allow multiple measurements in the same location, but it allows a large number of measurements. The use of collars is suspected to cut the litter and superficial soil fine root networks and thus to suppress a significant part of the root respiration [START_REF] Lankreijer | Measurement of soil respiration[END_REF][START_REF] Wang | Effect of collar insertion on soil respiration in a larch forest measured with a LI-6400 soil CO 2 flux system[END_REF] whilst it avoids the short-term soil disturbance as the collar is inserted several days before measurements. Despite these considerations, both methods are still used.

In this study we aimed to compare five CDC systems and three measurement methodology parameters. The objective was to establish cross-calibration functions between the different systems and methodologies usually used by the team involved in this paper. We first tested the impact of measurement methodology on R S measured, and focused on the soilchamber contact mode. In a second step, we cross-calibrated these systems on three sites that mainly differ in their soil and humus type and tree-species compositions. Comparing systems and methodologies under different site conditions provided calibration functions, which can be used to correct the systematic divergences among the tested systems, also taking into account the influence of the site characteristics. In the final stage, annual soil respiration values of the forests studied were compared after the use of cross-calibration functions to erase the experimental set up impact.

MATERIALS AND METHODS

Sites

Our study was conducted in three forests sites. The Vielsalm forest (Belgium, 50 • 18' N, 6 • 00' E) and the Hesse forest (France, 48 • 40' N, 7 • 05' E) were described in Aubinet et al. [START_REF] Aubinet | Long term carbon dioxide exchange above a mixed forest in the Belgian Ardennes[END_REF] and in Granier et al. [START_REF] Granier | The carbon balance of a young Beech forest[END_REF], respectively. Soil type in Vielsalm is classified as drystic cambisol (FAO classification) covered by a moder humus type. Soil type in Hesse is a stagnic luvisol covered by an oligo-mull humus type. The third site located in the Chaux forest (France, 47 • 07' N, 05 • 42' E) in a mixed deciduous stratified stand; the dominant species are oaks (Quercus robur L., Quercus petraea (Mattuschka) Liebl.), and several other deciduous species as Carpinus betulus L., Fagus sylvatica L., Populus tremula L. and Betula verrucosa Ehrh. are mainly in coppice. Mean annual temperature and precipitations averages 10.3 • C and 950 mm respectively. The soil is a gleyic luvisol with a meso-mull humus type.

Presentation of tested systems

There were five CDC systems involved in our cross-calibration experiment. The principle of the closed dynamic system is to calculate the soil CO 2 efflux from the rate of increase of the CO 2 concentration in a chamber that is hermetically in contact with a small area of soil [START_REF] Lankreijer | Measurement of soil respiration[END_REF]. The five systems were divided into two groups. The first group was made up of three systems based on the Licor company products (Licor, Lincoln, USA): -the "Li-Gx" system (Faculté Universitaire des Sciences Agronomiques, Gembloux), described by Longdoz et al. [START_REF] Longdoz | Soil CO 2 efflux measurements in a mixed forest: impact of chamber disturbances, spatial variability and seasonal evolution[END_REF] consisted of a Li-6252 (Licor, Lincoln, USA) IRGA connected to a homemade respiration chamber (185 mm height × 80 mm diameter) built following Norman et al. [START_REF] Norman | Soil surface CO 2 fluxes and the carbon budget of grassland[END_REF], -the "Li-He" and "Li-62" systems (Unité Écologie et Écophysiologie Forestières, Nancy) both consisted of a Li-6252 IRGA connected with a Li 6000-9 chamber (Licor, Lincoln, USA).

For these three systems, the pump of the Li-6252 (flow rate 1.5 L min -1 ) provides air circulation inside the chamber and drives the air inside the chamber by a drilled ring that allowed air mixture and homogenization. The second group consisted of two systems based on the PP-systems company products (PP-Systems, Hutchin, UK): -the "PP-Ch" system (Université de Franche-Comté, Besançon) consisted of a upgraded EGM-4 IRGA connected with a modified version of the SRC-1 chamber including a metal mesh in the lower part of the chamber, -the "PP-Or" system (Université Paris-Sud XI, Orsay) consisted of a upgraded CIRAS-1 IRGA connected to the first version of the SRC-1 chamber that is not equipped with the metal mesh. In these two last chambers, a vertical fan provided air mixture and homogenization inside the chamber. The flux data provided by all systems were compared to those from the "Li-62" that has been arbitrarily chosen as the common comparative system at each site.

Methodology parameters tested during the intercomparison

The methodology measurement usually used by each operator for the R S survey of their forest plots (therefore called "particular protocol") differs by (a) the time-lag between the placement of the chamber and the beginning of the record of the CO 2 increase (defined as the "waiting time"), (b) the duration of this record and (c) the chambersoil contact mode. These three modalities are set up by the operators and are not imposed by the system itself.

Table I. Description of the modalities used during the methodology comparison. Particular protocols corresponded to the protocols usually used by the teams for their studies of the temporal variations of soil respiration. For the soil-chamber contact mode, the chamber can be directly inserted in the soil ("Inserted") or laid on collars ("Collar").

Particular protocols

Common Protocol "Li-62" "Li-Gx" "Li-He" "PP-Ch" "PP-Or"

In situ comparisons

Four campaigns of soil respiration measurements took place at the Vielsalm (May 2003), Chaux (June 2004) and Hesse (mid and end of September 2003) forest sites. These campaigns, aiming at the comparison of methodologies and systems, were divided into two steps. The first step dealt with the impact of the different measurement methodologies on R S values as related to the system characteristics and the operators. The second step comprised the system comparisons and cross-calibrations.

Step 1: Methodology comparisons

Table I shows the different parameters tested for each system. The "waiting time" varied from 10 to 15 s or the record started when CO 2 concentration in the chamber was 15 ppm below the ambient concentration by using a soda lime scrub for the Licor systems.

The rate of CO 2 concentration increase was recorded during a constant time that ranged from 60 s to 120 s or during an increase of 30 ppm or 50 ppm of the CO 2 concentration in the chamber [START_REF] Lankreijer | Measurement of soil respiration[END_REF] (Tab. I). The impact of the "waiting time" and the record duration was tested at Vielsalm forest campaign for all systems except for the "PP-Or" system. For this purpose, measurements performed with the different particular protocols were compared to measurements obtained with a common protocol (Tab. I) employed by all systems. For this comparison, R S was measured on 24 PVC collars (60 mm height, 119 mm diameter) inserted into the soil (15 mm depth) 2 weeks before measurements in order to avoid R S measurements perturbations.

Two chamber-soil contact modes were used. The chamber can be either directly inserted in the soil or laid on collars pre-inserted in the soil. The chamber-soil contact mode was only assessed for both, "PP-Ch" and "Li-He" systems, because the three other systems were never used with the insertion mode (Tab. I). We compared two successive flux measurements, the first one performed with the chamber laid on a larger PVC collar (60 mm height, 119 mm diameter, a foam gasket ring provided an airtight seal between chambers and the collar), and the second one with the chamber inserted into the soil (15 mm depth) inside the collar. This test was realized at all sites, with 7 to 8 collars per site. The collars were inserted into the soil 2 weeks before measurements in order to avoid R S measurement perturbations.

Step 2: Cross calibration

The four campaigns for system comparison and cross-calibration were performed in the Vielsalm (May 2003) forest with "Li-62", "Li-He", "Li-Gx" and "PP-Ch", on 12 PVC collars. We performed campaigns in the Hesse forest with "Li-62" and "Li-He" on 23 PVC collars (mid-September 2003), and with "Li-62", "Li-He", "Li-Gx" and "PP-Ch" on 23 collars (end of September 2003). The forth campaign was performed in the Chaux forest (June 2004) with "Li-62", "Li-He", "PP-Ch" and "PP-Or" on 29 PVC collars. The collars were inserted in the forest soil 2 weeks before measurements. A foam gasket ring provided an airtight seal between chambers and the collar. For each system, a R S value corresponded to the mean of three measurements on the same collar, the measurements on a same collar being alternated with those made on other collars.

Before the Vielsalm and Chaux campaigns, the pressure difference between the chamber headspace and the atmosphere (PDC) was verified for each system with a FCO42 (Furness Controls Ltd, Bexhill, UK). Each PDC measurement was performed with chambers laid on a collar that was hermetically sealed with a PVC disc. A PDC, induced by a leak in the air circulation circuit, is known to cause extensive measurement errors on R S by pressure pumping or blocking [START_REF] Davidson | Minimizing artifacts and biases in chamber-based measurements of soil respiration[END_REF][START_REF] Longdoz | Soil CO 2 efflux measurements in a mixed forest: impact of chamber disturbances, spatial variability and seasonal evolution[END_REF]. PDC was not checked before the Hesse campaign. Because no significant variation have been observed between the values obtained at Vielsalm and Chaux, the PDC impact can be considered as constant for all the campaign (see Results).

Impact of cross-calibration on annual soil CO 2 efflux

Soil respiration was monitored within the Hesse forest in 2003 and 2004 in three plots every 2-3 weeks on 36 collars in each plots with "Li-He". Soil temperature at -10 cm (T S ) and volumetric soil water content of the 0-6 cm layer (θ VSurf ) were measured simultaneously with R S by using homemade copper-constantan thermocouples (Faculty of Agronomy of Gembloux, Belgium) and a capacitive ML2x ThetaProbe (Delta-T Device, Cambridge, UK), respectively. In Vielsalm forest, R S was monitored between August 1997 and August 1998 with "Li-Gx" in 29 collars inserted within a beech dominated patch (see [START_REF] Longdoz | Soil CO 2 efflux measurements in a mixed forest: impact of chamber disturbances, spatial variability and seasonal evolution[END_REF] for more details). For the Hesse data, an empirical model was fitted to either measured or corrected R S values:

R S = R S10 Q (T S -10)/10 10 e -e (a-b * θSurf) (1)
where R S10 is the soil CO 2 efflux at 10 • C, Q 10 the temperature sensitivity of soil respiration and a and b are the parameters for the Gompetz function [START_REF] Janssens | Forest floor CO 2 fluxes estimated by eddy covariance and chamber-based model[END_REF]. For the Vielsalm data, an Arrhenius-type function was fitted to data:

R S = R S10 e Ea(T S -283.2) 283.2RT S (2)
with E a = E a0 10 3 T S T S -T 0 where R S10 is the soil CO 2 efflux at 10 • C, R is the universal gas constant (8.314 J mol -1 K -1 ), T S is the soil temperature (K), T 0 is a reference temperature and E a0 a reference activation energy (J mol -1 ).

Following Lloyd and Taylor [START_REF] Lloyd | On the temperature dependence of soil respiration[END_REF], the parameters T 0 and E a0 were fixed to 227.13 K and 12970 J mol -1 , respectively [START_REF] Longdoz | Soil CO 2 efflux measurements in a mixed forest: impact of chamber disturbances, spatial variability and seasonal evolution[END_REF]. An influence of soil water content was not taken into account, as it had not been observed. We simulated daily R S values by applying equations ( 1) or ( 2) with continuous measurements of T S (Hesse and Vielsalm) and θ VSurf (Hesse). Then these daily values were cumulated for 2003 and 2004 (Hesse) and 1997-1998 (Vielsalm) providing annual soil CO 2 efflux (R SA ). At Hesse site, equation ( 1) was fitted to R S , T S and θ VSurf datasets measured on 3 plots (HesseA, HesseB and HesseC), leading to one R SA value per plot. Relationships deduced from crosscalibrations among systems were used to assess the impact of systems on R SA (see Results). For the Hesse dataset, we converted each value measured by the "Li-He" system into a corrected value that represented R S as it would be if measurements were performed with either the "Li-62", "Li-Gx" or "PP-Ch" systems. For the Vielsalm dataset, we converted each value measured by the "Li-Gx" system into a corrected value that represented R S as it would be if measurements were performed with either the "Li-62", "Li-He" or "PP-Ch" systems. Depending on the site and the measuring system, the corrected values were either fitted to equation ( 1) or ( 2), and corrected annual soil respiration ( C R SA ) was obtained following the same procedures as presented above.

Statistical analysis

Linear regressions (Statview 5.0, SAS Institut Inc., N.C., USA) were used to cross-compare data from each system to the "Li-62" system used as the comparative system. Comparisons tests of the mean between the systems, measurement methodologies and campaign were performed by two-way ANOVA tests and post-hoc Fisher's Protected Least Significant Difference tests. Least square non-linear regression analyses were performed to fit empirical models to R S data (Statgraphics Plus 4.1).

RESULTS

Pressure difference between the chamber headspace and the atmosphere (PDC)

Measurements of PDC in the center of the chambers gave values lower than 0.05 ± 0.01 Pa (mean ± standard deviation) in the "Li-62", "Li-Gx" and "Li-He" systems. PDC values reached 0.01 ± 0.001 Pa in "PP-Ch" and 0.92 ± 0.35 Pa in "PP-Or".

Impact of the different measurement methodologies (Step 1)

Measurements obtained with the methodologies usually used by the different teams (particular protocol) did not differ significantly (PLSD, p > 0.05) from those obtained with the common measurement methodology (common protocol, Tab. I). Thus, during system comparison (step 2), each system measured R S with its particular protocol without introducing methodological divergences.

A significant impact of the chamber-soil contact mode (use of "collar" or "insertion" in the soil) was found for the two tested systems ("Li-He" and "PP-Ch"). Figure 1a shows a significant linear relationship between "collar" and "insertion" values for "Li-He", when the data from the different campaigns were regrouped. The "insertion" values gave fluxes 28% higher than "collar" values. A similar result was observed for "PP-Ch" with an increase due to insertion amounting to between 2% and 13% (Fig. 1b) when "collar" values ranged from 1 µmol CO2 m -2 s -1 to 6 µmol CO2 m -2 s -1 (range usually measured, data not shown). However, when considering the campaign separately, the "PP-Ch" system showed a decrease of about -11% when passing from "collar" to "insertion" during the Chaux campaign. This decrease has been verified for this site during other campaigns (data not shown).

= A × R SLi-62 + B,
where R SX and R SLi-62 are the R S value of the "X" system and the R S value given by "Li-62", respectively. The regression analysis was performed on the pooled data from the three campaigns. Each point represents the mean of the three replicates made on each collar. For "Li-He", additional data from 23 other collars were added. Each parameter was significant for p = 0.05 level (NS indicates non-significant parameter in the regression analysis).

Parameters "Li-Gx" "Li-He" "PP-Ch" "PP-Or"

A

System cross calibration (Step 2)

Table II summarizes the mean R S values measured with the different systems for each campaign. At the Vielsalm site, mean R S values did not differ among systems. At the Hesse site, the "PP-Ch" system measured significantly higher mean R S values (p < 0.05) than the other systems. At the Chaux site, a significant difference of mean R S values was found (p < 0.05) among systems, but the "Li-62" and "Li-He" systems did not record significantly different mean R S values. When considering the variability among sites, the "Li-Gx" and "PP-Ch" systems recorded mean R S values that significantly differed among the three sites (Fisher's PLSD, p < 0.05) whereas both, "Li-62" and "Li-He" systems, recorded a significantly higher mean R S value (p < 0.0001) at Chaux compared to the other sites. Significant differences among mean values of R S calculated with all the systems were also observed among sites (p < 0.0001).

Figure 2 shows the linear relationship between the R S values of different systems and those of "Li-62". The values for slopes (Tab. III) revealed very low deviations for "Li-He" and "Li-Gx". However the relationship between "Li-62" and "Li-Gx" measurements presented a larger variability than with "Li-He", in addition with a significant intercept from the regression analysis found for "Li-Gx". Considering the same system characteristics, values given by "Li-He" were very close to those of "Li-62". Figure 2 shows a higher deviation for "PP-Ch" and "PP-Or". For "PP-Ch", a constant discrepancy of about 20% was found. According to the higher mean R S value recorded by "PP-Or" at the Chaux site, the slope revealed a high deviation from "Li-62" (Tab. III).

Annual soil CO 2 efflux

Actual annual estimates of R S calculated directly from measurements (R SA ) exhibited pronounced spatial variability among plots and sites (Tab. IV), even if R SA values were not determined for the same time periods in both sites. Annual estimates of R S from corrected values ( C R SA ) for the "Li-62" were very close to annual estimates from R S values measured in Hesse A, Hesse B and Hesse C plots. Deviations for a given "X" system were calculated as: Deviation "X" = (A X -A Li-62 )/A X × 100, where A is the actual or corrected annual soil respiration of the "X" system and A Li-62 refers to the corrected annual soil respiration for the "Li-62" system. Deviations for the "Li-He" were very (between -0.6% and -0.15%, Tab. IV) despite the different site characteristics. Deviations for the "Li-Gx" were higher (between +8.2% and +21%). Deviations for the "PP-Ch" were relatively constant (around +19%) but higher than for the "Li-He".

DISCUSSION

PDC

Preliminary PDC measurements showed that there were no major pressure differences in the tested systems except for the "PP-Or" system. The higher PDC value obtained for "PP-Or" (overpressure of 0.92 ± 0.351 Pa) was probably due to the air mixing by a fan placed inside the chamber. Following the PDC influence on R S found by Longdoz et al. [START_REF] Longdoz | Soil CO 2 efflux measurements in a mixed forest: impact of chamber disturbances, spatial variability and seasonal evolution[END_REF] for the Vielsalm forest soil, the impact of "PP-Or" overpressure leads to a blockage of the R S flux and corresponds to a R S underestimation of 69%. However a higher R S values is measured with "PP-Or" compared to those of "Li-62". This discrepancy could be attributed to the presence of a possible negative PDC (leading to a R S overestimation) measured between the atmosphere and the points located near the collar walls. Another explanation would be an excessive turbulence within the SRC-1 chamber due to the fan [START_REF] Hanson | Seasonal and topographic patterns of forest floor CO 2 efflux from an upland oak forest[END_REF]. This action is prevented in "PP-Ch" by the addition of the metal mesh at the bottom of the chamber. Our results confirmed that the addition of a metal mesh in the SRC-1 chamber was a benefit. "PP-Ch" and Licor-based systems did not induce any pressure pumping or blocking effects, and gave a good confidence in the air tightness of these systems. As a consequence, the comparisons of measurement methodologies and systems were realized for these four systems without any biases coming from a pressure problem. Indeed, if closed dynamic chamber techniques are unable to reproduce wind conditions prevailing in the forest floor and boundary layer conditions inside the chamber [START_REF] Nay | Biases of chamber methods for measuring soil CO 2 efflux demonstrated with a laboratory apparatus[END_REF], it seems that the unmodified SRC-1 configuration implies artificial and unrealistic conditions. In previous tests, measurements of mean wind speed inside the chamber gave 0.9 m s -1 [START_REF] Dantec | Soil CO 2 efflux in a beech forest: comparison of two closed dynamic systems[END_REF] and 0.13 m s -1 (unpublished data) for the unmodified and modified version of the SRC-1, respectively. This large difference in wind speed and the PDC could explain the R S values divergence between "PP-Or" and the other systems. Such deviations between systems using the first version of SRC-1 and Li 6000-9 chambers have already been reported [START_REF] Janssens | Assessing forest soil CO 2 efflux: an in situ comparison of four techniques[END_REF][START_REF] Dantec | Soil CO 2 efflux in a beech forest: comparison of two closed dynamic systems[END_REF][START_REF] Norman | Soil surface CO 2 fluxes and the carbon budget of grassland[END_REF].

Chamber-soil contact mode

The chamber-soil contact mode has a significant impact on the R S measurements. Three hypotheses could explain the general higher R S values obtained for the "insertion" mode. First, a transient change of diffusion conditions of the CO 2 in the litter and the upper mineral soil layers might occur when inserting the chamber. For the soil types dealt with in this study, the insertion would have perturbed the soil aggregates and leaf litter structure increasing the vertical diffusivity coefficient, thus, inducing a rapid release of upper soil CO 2 . Second, the collar placement could affect R S measurements over the long-term due to the cutting of fine roots [START_REF] Hanson | Separating root and soil microbial contribution to soil respiration: A review of methods and observations[END_REF][START_REF] Lankreijer | Measurement of soil respiration[END_REF]. Wang et al. [START_REF] Savage | A comparison of manual and automated systems for soil CO 2 flux measurements: trade-offs between spatial and temporal resolution[END_REF] showed that a reduction in R S values for a larch forest occurs when measurements were performed at least 12 h after collar installation. Third, the distance between the soil and the drilled ring ("Li-He") or the fan ("PP-Ch") insuring air mixing inside the chambers, differs between the "insertion" and "collar" situation. Since this distance was lower without collars, the thinner boundary layer could have lead to higher R S values. This argumentation shows also that both chamber-soil contact modes have advantages and disadvantages and none of them could be considered a reference method.

In opposition to all other observations, the "PP-Ch" system during the Chaux campaign (Fig. 1b) was the only one giving lower values for "collar" than for "insertion". This has been confirmed by other campaigns at the same site (data not shown). A possible cause is that the "insertion" of the SRC-1 chamber might not have trenched the broadleaf litter layer at Chaux, but only have compressed it. Then air tightness between the chamber's edge and the soil-litter interface may not have been sufficient, thus, leading to CO 2 leaks and an underestimation of the fluxes.

The highly significant linear relationships between "collar" and "insertion" R S values (Figs. 1a and1b) suggest that scaling coefficients could be used when "collar" and "insertion" data have to be compared. The difference in the relationship parameters between the two systems tested shows that the scaling coefficient is dependent on the system. Further experiments are needed to recommend coefficients specific for each site (soil type).

Cross-calibration

The R S values measured during the cross-calibration experiments were in good agreement with the range observed during a seasonal evolution survey performed on each site (data not shown). Differences in mean R S values among sites could be partially explained by the influence of soil temperature variation among sites and campaigns, especially for the high R S during the Chaux campaign where the soil was exceptionally warm (Tab. II).

The linear regression presented in the Figure 2 shows that systematic deviations existed between in situ measurements performed with different systems. However these deviations could be corrected with a linear equation, even though the accuracy of corrections depends on the similarity of measurements performed by the different systems, and varies according to the R 2 values.

Logically, when the same system constituted with same material are considered ("Li-62" and "Li-He") R S values are very close in all campaigns. The lower R 2 of the relationship between measurements of "Li-62" and "Li-Gx" may be due to higher measurement variability as a consequence of differences in the foam gaskets assuring the airtight seal of the chambers of these two systems. This may induce a lack of air tightness in one of the two systems when the chambers are placed on a not perfectly horizontal collar's edge. Consequently an over-or underestimation could be the result, depending on the impact importance of the small PDC or/and potential CO 2 leak.

The higher R S values given by "PP-Ch" compared to those of "Li-62" could be explained by a thicker boundary layer resistance in the Li 6000-9 chamber compared to the SRC-1 modified chamber. In spite of a higher wind speed in the Li 6000-9 (0.4 m s -1 [START_REF] Law | Below-canopy and soil CO 2 efflux in ponderosa pine forest[END_REF]) and the presence of a grid mesh in the SRC-1, the position of the drilled ring in the Li 6000-9 chamber might allow airflow to move more parallel to the soil surface compared to the more vertically directed airflow induced by the fan placed in the SRC-1 chamber. Therefore a thicker boundary layer might be more easily induced in the Li 6000-9 chamber. This hypothesis, while explaining the cause of the divergence, does not allow a conclusion on which system offers measuring conditions closer to the natural situation. The slope of the linear regression for the comparison between "PP-Ch" and "Li-62" (1.21, Fig. 2c and Tab. III) is in the range of the results presented by Pumpanen et al. [START_REF] Pumpanen | Comparison of different chamber techniques for measuring soil CO 2 efflux[END_REF] for equivalent systems and performed on a calibration tank (slopes ranged from 1.16 for coarse sand to 1.33 for wet fine sand). Finally, as explained in the first section of this discussion, the PDC problem could explain the large deviation between R S measurements of "Li-62" and those of "PP-Or" (Fig. 2d and Tab. III).

Impact of corrections on annual soil respiration

R SA values calculated directly from measurements exhibited pronounced spatial variability among plots and sites, even when data were cross-calibrated to obtain corrected flux (Tab. IV). However the impact of this cross-calibration on the spatial variability was important. For example, the actual R SA of Vielsalm that is the higher one became one of the lowest after the cross-calibration for "Li-62", (Tab. IV). Beyond this spatial variability, the results showed that, logically, the two Figure 2. Comparison of R S efflux "Li-Gx" (a), "Li-He" (b), "PP-Ch" (c) and "PP-Or" (d) with "Li-62". The dashed line represents the linear regression from the overall data set. Equations and main parameters of the regressions are also presented. For analysis conveniences, for "Li-He", we gathered together data from the two campaigns performed at Hesse (see Materials and Methods).

Table IV.

In the upper panel: mean annual soil CO 2 efflux (in g C m -2 ) for 3 plots at Hesse and 1 plot at Vielsalm. Bold values correspond to the annual means that were directly calculated from measurements (R SA in the text). For other values, the measurements have been transformed (using cross-calibration equations of Tab. III) to simulate the corrected annual mean ( C R SA in the text) that would be obtained with the system listed in the first column. In the lower panel: deviations were calculated as: Deviation "X" = (A X -Li-62)/A X × 100, with A X the actual or corrected annual soil respiration of the "X" system, Li-62 referring to the corrected annual soil respiration for the "Li-62" system. identical systems ("Li-62" and "Li-He") gave very close annual soil respiration an all plots. The values obtained from the "Li-Gx" system were slightly higher, due to the particularities of this system (air circulation, foam gasket). The difference with the "Li-62" is able to partly mask the natural inter-plot variability, especially when considering Vielsalm forest. Finally, the comparison "PP-Ch" with "Li-62" showed clearly that the discrepancies between two different materials do not allow inter-plot comparison without cross-calibration functions.

Hesse

CONCLUSION

We confirmed that the unmodified SRC-1 chamber induced system specific deviations, but confidence in the measured values was improved by including a grid mesh in the chamber ("PP-Ch" system) as proposed by the manufacturer. The discussion on the possible causes of differences among systems and the choice of soil contact mode revealed that properties such as soil texture, soil-litter interface porosity and chamber design influenced R S values, with a strong dependence on the study site.

Our study showed that systematic deviations exist among in situ measurements performed with different systems; however these deviations are in the range of the already published results. Deviations were explainable and could be corrected with simple linear equations. Thus, R S values obtained with different systems for different study sites can be used to compare soil respiration effluxes after corrections using cross-calibration results. Otherwise, difference of annual soil respiration between sites could be hold against (partly when the compared systems were built one the same model or completely when the compared systems came from different manufactories) the deviations among the systems used. These deviations could also affect our estimation of the forest annual carbon sequestration because soil respiration data could be used for the NEE correction procedure and deviations are of the same order that the NEE uncertainties.

Figure 1 .

 1 Figure 1. Comparison between R S measured with the chamber laid on collars ("Collar" R S ) and R S measured with the chamber inserted in the soil ("Insertion" R S ) for "Li-He" (a) and "PP-Ch" (b). The solid line is the 1:1 and the dashed line shows the general linear regression on all values. Regression equations are (a) ["Insertion" R S ] = 1.28 × ["Collar" R S ] (R 2 = 0.95; n = 39); (b) ["Insertion" R S ] = 1.15 × ["Collar" R S ] -0.13 (R 2 = 0.91; n = 39).

Table II .

 II Mean soil respiration efflux (R S , in µmol CO2 m -2 s -1 ) of the five tested systems for the three campaigns (n = 12, n = 46 and n = 30 for Vielsalm, Hesse and Chaux sites respectively) and mean soil temperature (T s , in • C) during the measurement duration. Values in brackets are the corresponding standard errors. During the Hesse Campaign each collar was measured twice.

	Site	R S "Li-62" "Li62"	R S "Li-Gx"	R S "Li-He"	R S "PP-Ch"	R S "PP-Or"	R S Mean	T s
	Vielsalm	1.54 (0.18)	1.50 (0.17)	1.59 (0.17)	1.55 (0.15)	-	1.53 (0.08)	7.7 (0.07)
	Hesse	1.82 (0.08)	2.17 (0.15)	1.80 (0.09)	2.37 (0.13)	-	1.98 (0.06)	11.8 (0.20)
	Chaux	3.93 (0.18)	-	3.87 (0.17)	4.68 (0.27)	5.21 (0.40)	4.42 (0.14)	16.0 (0.07)

Table III .

 III Linear regression parameters between R S values of the relationship: R SX

  Deviation "PP-Ch" (%) 18.6 19.2 18.8 19.3 18.4 18.8 17.3

		A	Hesse B	Hesse C	Vielsalm
	System	2003 2004 2003 2004 2003 2004 1997-1998
	Li-62 (g C m -2 )	608 552 700 631 813 742	685
	Li-He (g C m -2 )	599 544 696 627 808 737	683
	Li-Gx (g C m -2 )	687 638 774 714 885 823	867
	PP-Ch (g C m -2 )	747 683 862 782 996 913	828
	Deviation "Li-He"(%) -1.5 -1.4 -0.6 -0.6 -0.6 -0.6	-0.2
	Deviation "Li-Gx" (%) 11.4 13.5 9.5 11.7 8.2 9.8	21.1

Acknowledgements:

We gratefully thank M. Michel Yernaux for his technical knowledge on the soil respiration systems, and M. Laurent Vanbostal for helping us to make measurements at the Chaux forest. This work was supported by the European programme Car-boEurope IP ("Assessment of European Ecosystem Carbon balance") and the Belgian-French TOURNESOL Programme Grant (No. 06718WG: "Étude des flux nocturnes de CO 2 dans les écosystèmes forestiers"). This study is a part of the GERS (Group Studying the Soil Respiration) activities. We greatly thank the two anonymous reviewers for their constructive suggestions for improving this paper.


Paule Moustier

MoISA Cirad

Michelle Holdsworth

Cirad Patrick Caron

This paper helps fill gaps in the literature on urban food security and food policy which currently focuses on high-income countries and lacks a systemic approach. We characterise urban food systems in low and middle-income economies highlighting the connections between urban food consumption, food environments, food supply chains and their outcomes in terms of employment, food security and environmental preservation. We bring to the fore the diversity of consumer profiles, consumer environments and provisioning chains. Urban food systems involve the combination of at least six urban food chains: a subsistence chain, one short relational, one long relational, one value-oriented small and medium enterprise (SME)-driven, one value-oriented supermarket-driven, and one value-oriented e-commercedriven. We identify overlaps, combinations and interactions between each type. Such diversity and interactions are keys to the resilience of urban food systems, yet they are not supported by public governance of urban food systems. We recommend interventions by national, regional and city authorities pertaining to regulations, resources and incentives, education and awareness, institutional capacity, mostly in support of SMEs and low-income consumers.

Introduction

As the world is becoming increasingly urbanised, urban food security and provisioning are the focus of growing attention. The inability of current food systems to meet the food demands of urban residents in sub-saharan Africa has been referred to as "the emerging development issue of this century" (Crush and Frayne, 2010, p. 6). While the literature on food security has for long been dominated by discourses on the need to increase food production, the need to consider consumers' access to food is increasingly relevant in an urban environment where most consumers do not produce their food. Food purchased in markets represents more than 80% of food consumption in sub-Saharan African cities, compared with 50% in rural areas [START_REF] Tschirley | Feeding Africa's cities: Opportunities, challenges, and policies for linking African farmers with growing urban food markets[END_REF].

As demonstrated by [START_REF] Frayne | A bibliometric analysis of urban food security[END_REF], the literature on urban food security has bifurcated in two foci. The first one relates to supply-side dynamics, including research on urban agriculture, supermarketisation and the informal food economy. The second foci is household food access, related to the food crisis and particularly food deserts. Both areas are seldom considered together. Likewise, food policy is either considered through the lens of productivist approaches focused on innovations to raise agricultural productivity while limiting environmental change, or demand-led approaches bringing to the fore multi-scaled inequitable relations [START_REF] Sonnino | The challenge of systemic food change: Insights from cities[END_REF].

As argued by Doherty et al, 2019: 4, "it has become something of a truism in the burgeoning field of food studies to describe food as constituting a 'system' (…). Yet this concept is invoked far more often than defined satisfactorily ». Among the proliferating definitions of food systems, the most challenging ones bring to the fore the interaction between diets, food environments and food chains 2 . It is now acknowledged that the urban food environment, in particular regarding location of retailing and catering capacities, is crucial in determining what food consumers buy and how it translates into outcomes in terms of food and nutritional security. Together with [START_REF] Sonnino | The challenge of systemic food change: Insights from cities[END_REF], we believe that a systemic approach regarding food issues is useful when it considers interactions between the various private and public actors within the food sector, and between food and other domains, including the employment situation and the environment in terms of natural resources. It should also consider in a dynamic way the resilience of food systems, i.e., the adaptive capacities of the stakeholders in face of various shocks [START_REF] Doherty | Food systems resilience: towards an interdisciplinary research agenda[END_REF]. In Australia and France, this resilience is related to four major factors: the geographical scale of supply, the diversity of actors and skills, the cohesion of the chains in terms of intra-network and extranetwork collaborations, including the relationships with public institutions and the interactions between food chains [START_REF] Smith | The resilience of long and short food chains: a case study of flooding in Queensland, Australia[END_REF][START_REF] Chiffoleau | Coexistence of supply chains in a city's food supply: a factor for resilience? Review of Agricultural[END_REF].

Most of the literature on urban food systems relates to high-income countries (HICs) [START_REF] Zhong | Urban food systems: a bibliometric review from 1991 to 2020[END_REF]. In low-and middle-income countries (LMICs), the poor's access to sufficient healthy food is even more challenging. While there are now more studies on urban households' food consumption and environments, there is still a lack of insight on how these connect to food provisioning. Urban agriculture in LMICs has been the object of many studies and reviews, yet it it only a small component of urban food provisioning. The governance of urban food systems is also under-researched, in particular the way public and private 2 "A food system gathers all the elements (environment, people, inputs, processes, infrastructures, institutions, etc.) and activities related to the production, processing, distribution, preparation and consumption of food, and the outputs of these activities, including socio-economic and environmental outcomes"; with "three constituent elements (…): food supply chains; food environments; and consumer behaviour (HLPE, 2014:11). The food environment is "the physical, economic, political and socio-cultural context in which consumers engage with the food system to make their decisions about acquiring, preparing and consuming food" (HLPE, 2017:28). Food supply chains (or food chains) are sub-systems of food systems as they relate to the stakeholders and activities involved in sourcing, making, and delivering the products or services to final consumers [START_REF] Du Toit | Supply chain management: A framework of understanding[END_REF]. stakeholders could -or should-combine their actions to achieve sustainability and inclusion. Urban food security is indeed rarely considered with a public policy perspective. This is because food policies are mostly considered with a rural lens, while urban authorities usually put housing and the attraction of business on top of their priorities so that food "is considered as a stranger in the field of urban planning" (Wiskerke and Viljoen, 2012:20).

It is the purpose of this paper to contribute to characterizing the connections between urban food consumption, food environments, food supply chains and their outcomes in terms of employment, food security and environmental preservation. We also aim at guiding the public governance of urban food systems in low-income countries. Drawing from [START_REF] Smith | The resilience of long and short food chains: a case study of flooding in Queensland, Australia[END_REF], we consider the following connections: the spatial organisation of food supply, the nature of stakeholders involved in food consumption and chains, and the interactions between these stakeholders. The spatial organisation relates to the origin of food and places where food is exchanged, as these are crucial determinants of costs, availabilty and quality of food. This is the basis of a foodshed approach, i.e. the mapping of food flows from the source of production to consumption at the city level (Karg et al,206), accounting for local, regional as well as global flows. This analysis goes well beyond the city-region concept, the limitations of which were brought to the fore by [START_REF] Watson | The return of the city-region in the New Urban Agenda: Is this relevant in the Global South?[END_REF]. Interactions between stakeholders involve the exchange of information, resources and commitments that are key to matching supply and demand, together with the creation and distribution of value along the chain [START_REF] Haggblade | A conceptual framework for promoting inclusive agricultural value chains[END_REF]. We also pay attention to the public governance of food systems, i.e. how public decisions interfere with their functioning. We highlight the diversity of consumers' profiles, consumers' environments and provisioning chains. Such a diversity is key to the resilience of urban food systems [START_REF] Zhong | Urban food systems: a bibliometric review from 1991 to 2020[END_REF]), yet it lacks public support.

Urban food consumers' behavior

Urban food consumption relates to the demographic and economic conditions of cities. These differ considerably in size, and a high proportion of urban growth is happening in secondary and tertiary cities, especially in sub-Saharan Africa where, in 2015, half the population lived in cities of less than 500,000 inhabitants (OECD/WAC, 2019). Compared to the rural population, urban populations have more diverse cultural, economic, and social profiles, and this diversity is more marked in primary cities relative to secondary ones. Latin America has the highest income inequality, including in urban areas (BBVA Research, 2017;OECD, 2019). In sub-Saharan Africa, income growth, benefiting urban areas, started in 2000 but has faltered since 2013 (Tschirley et al., 2020 based on World Bank data). Cities in LMICs lack stable employment, which explains why poverty is increasingly an urban phenomenon [START_REF] Ravaillon | The economics of poverty[END_REF]. The informal sector still provides most employment (especially for women), accounting for up to 90% in low-income countries and 67% in middle-income countries [START_REF] Bonnet | Women and men in the informal economy: A statistical brief[END_REF]. Besides the majority of low-income dwellers, a middle class is emerging, defined as an income of 12-50 US$ per capita/day in Africa, accounting for 13% of the population (Neveu-Tafforeau, 2017).

Patterns of urban food consumption have common and differentiated patterns, mostly according to income or culture. The basic pattern is a combination of starchy staples (cereals, roots, tubers) which represent more than half of expenditure, animal protein (meat, milk, eggs and fish), vegetables, and in varying amonts (lower than the recommended amounts), fruits, legumes and nuts. The consumption of animal products, fruit and vegetables is highly sensitive to income [START_REF] Tschirley | Feeding Africa's cities: Opportunities, challenges, and policies for linking African farmers with growing urban food markets[END_REF][START_REF] Pingali | Food systems transformation in Asia. A brief economic history[END_REF][START_REF] Kovalskys | Energy intake and food sources of eight Latin American countries: results from the Latin American Study of Nutrition and Health (ELANS)[END_REF]. With rising incomes, urban residents eat more animal-source foods and processed foods, which may be low in micronutrients, high in calories and fat [START_REF] Yaya | Differentials in prevalence and correlates of metabolic risk factors of non-communicable diseases among women in sub-Saharan Africa: evidence from 33 countries[END_REF][START_REF] Holdsworth | Unhealthy eating practices of city-dwelling Africans in deprived neighbourhoods: Evidence for policy action from Ghana and Kenya[END_REF][START_REF] Rousham | Dietary behaviours in the context of nutrition transition: a systematic review and meta-analysis in two African countries[END_REF]. Likewise, the consumption of imported food by urban dwellers is increasing -although the proportion is still limited: only 5% in Africa, mostly imported cereals, according to Bricas et al. (2016) and [START_REF] Tschirley | Africa's emerging food transformation, Eastern and Southern Africa[END_REF]. Consumers commonly combine local and imported products in meals, resulting in a hybridization of cooking (Soula et al., 2020). In Latin American cities, food security improved for many years, partly as result of "zero hunger" strategies first developed in Brazil in the late 1990s and later in neighboring countries. However, food insecurity has started to rise again due to increased social inequality and the Covid-19 pandemic. At the same time Latin America is facing escalating obesity rates, affecting 24% of the regional -mostly urban -population, almost double the global level of 13.2%, which is explained by unhealthy diets and poverty (FAO, RUAF, 2019).

In parallel, food safety has become a major public health issue. Food safety crises are regularly reported in the media, especially in South-East Asia, where consumers' fears are linked to chemical products in fruit and vegetables and antibiotic residues in meat (Figuie et al., 2004;[START_REF] Ortega | Demand for food safety in emerging and developing countries: a research agenda for Asia and Sub-Saharan Africa[END_REF]Ferrand et al, 2019). This is due to new industrial and domestic sources of pollution close to agricultural production areas, and the increasing use of chemical inputs by farmers [START_REF] De Bon | Sustainable urban agriculture in developing countries. A review[END_REF][START_REF] Reynolds | Environmental impacts and constraints associated with the production of major food crops in Sub-Saharan Africa and South Asia[END_REF]. The lengthening of food supply chains and the lack of knowledge about hygiene also creates risks of contamination in the processing, marketing, handling and consumption stages [START_REF] Jaffee | The safe food imperative: Accelerating progress in low-and middle-income countries[END_REF]. Consumer concerns about food safety have potential nutritional consequences as they may reduce consumption of fruit and vegetables because of concerns about pesticides, or push consumers towards packaged (often highly processed) foods because they are perceived as safer [START_REF] Liguori | How do food safety concerns affect consumer behaviors and diets in low-and middle-income countries? A systematic review[END_REF].

Urban food environments

In cities, food is sold through a range of food retailers including street mobile vendors, roadside stalls, open/covered markets, shops, kiosks, supermarkets (including mini-marts and hypermarkets) and e-commerce. Urban consumers are mainly supplied by small-scale market vendors, street vendors and neighbourhood shops, even though supermarkets and convenience stores are increasing their market share. This is evidenced by many studies in Africa [START_REF] Melesse | Understanding urban consumers' food choice behavior in Ethiopia: Promoting demand for healthy foods[END_REF]Wanyana et al, 2019), Asia [START_REF] Wertheim-Heck | Food safety and urban food markets in Vietnam: The need for flexible and customized retail modernization policies[END_REF][START_REF] Downs | The interface between consumers and their food environment in Myanmar: an exploratory mixed-methods study[END_REF] and Latin America (Guarin, 2013). E-commerce has been spurred by sanitary crises (including SARS/Covid-19) and is developing particularly rapidly in Asian countries, including China, India and Vietnam [START_REF] Reardon | E-commerce's fast-tracking diffusion and adaptation in developing countries[END_REF]Vietnam news, 2021;[START_REF] Dao | Developing a sustainable safe food value chain in Vietnam. Vietnamese version[END_REF].

Together with food retailing, food catering is an essential component of the urban food environment. Food consumption outside the home is increasing. The proportion varies across African cities, ranging from 6% in Freetown and Conakry to 25% in cities of Nigeria/Tanzania, and 30% in Cotonou, Lomé and Abidjan [START_REF] Tschirley | Feeding Africa's cities: Opportunities, challenges, and policies for linking African farmers with growing urban food markets[END_REF]. Street food is especially convenient for urban workers and low-income households who may not have the resources and facilities to purchase raw ingredients and prepare dishes at home, especially in slums (Soula et al., 2020;[START_REF] Pradeilles | Urban physical food environment drives dietary behaviours in Ghana and Kenya: a Photovoice study[END_REF]. In Latin America, between 2000 and 2013, the consumption of ultra-processed foods increased by more than 25%, and fast food consumption by almost 40% (PAHO, 2015).

Informality, i.e. absence of registration and licensing is a major characteristic of market, shop and street vendors in low-income countries. These are major providers of food and livelihoods to poor urban residents, especially women, in Africa and Asia. Yet they lack public support, are regularly harrased by police and may have their goods confiscated [START_REF] Young | The urban informal food sector in the Global South[END_REF] based on alleged traffic and visual nuisances, non compliance with licensing and food safety laws [START_REF] Turner | Street Vendor Livelihoods and Everyday Politics in Hanoi, Vietnam: The Seeds of a Diverse Economy?[END_REF][START_REF] Ogunkola | While flattening the curve and raising the line, Africa should not forget street vending practices[END_REF][START_REF] Giroux | Informal vendors and food systems planning in an emerging African city[END_REF]). Yet, as argued by Giroux et al (2021 :3), « vendors are not merely surviving but are actively engaged with operating a business, drawing on multiple types of resources to do so and creating additional economic links in the city in the process ». Informal vendors serve consumers who are not able to travel to central market-places, farmers' markets or supermarkets.

Urban food chains in relation to food environments and consumption

One important determinant of food provisioning chains' organization is the distance between production and consumption. At one extreme lies subsistence agriculture, with global food chains at the other. In between are local market-oriented chains, which come from urban/peri-urban areas and are short in terms of distance and number of intermediaries; and from rural areas, which come together with longer chains. Another distinctive feature is who governs the chain in terms of the distribution of value, which is closely linked to the control of processing and quality standards. Cities are largely supplied by what may be called conventional agriculture and chains, where quality -food safety in particular -is not labeled or rewarded, versus chains with some elements of quality differentiation. « Quality chains » may be driven by supermarkets, but also by farmer organisations, small and medium enterprises (SMEs); and recently by e-commerce companies. Hence, urban food systems involve a combination of at least six urban food chains (see Figures 1 and2): subsistence, short conventional relational ('short relational'), long conventional relational 'long relational'), value-oriented SME-driven, value-oriented supermarket-driven, and value-oriented ecommerce-driven. The three first types mostly belong to the informal sector when defined as including businesses which are not formally registered nor licensed. The term « informal » may be misleading because these businesses/chains contribute to the public resources through various taxes collected during transport and in market-places [START_REF] Fafchamps | Market institutions in sub-saharan Africa. Theory and evidence[END_REF][START_REF] Young | The urban informal food sector in the Global South[END_REF].

Subsistence/non-market chains

This refers to the involvement ofurban residents in food production, mostly for their own consumption. Examples include home gardens, community gardens, cultivation of maize on idle land, or raising poultry in backyards. Even though subsistence agriculture is of minor importance in terms of total urban food consumption, it can play an important role in livelihoods and social inclusion of vulnerable inhabitants, as shown in Tamale and Ouagadougou [START_REF] Bellwood-Howard | The role of backyard farms in two West African urban landscapes[END_REF], Cape Town [START_REF] Olivier | Beyond food security: women's experiences of urban agriculture in Cape Town[END_REF], Hanoi [START_REF] Pulliat | Food securitization and urban agriculture in Hanoi (Vietnam)[END_REF], Quito and Rosario (Renting and Dubelling, 2013). Urban gardens also have important pedagogical functions, e.g. through schooling programmes or community gardens [START_REF] Hou | Urban Community Gardens as Multimodal Social Spaces[END_REF]. Direct provisioning enables resilience at times of crisis when market provisioning is unreliable, as observed in Cuba [START_REF] Buchmann | Cuban home gardens and their role in social-ecological resilience[END_REF] and Sri Lanka [START_REF] Dissanayake | COVID-19 Outbreak and Urban Green Space, Food Security, and Quality of Life: Case of Urban Home Gardens in Kandy, Sri Lanka[END_REF]. More generally, non-market food provisioning makes a crucial contribution to food security of the urban poor, be it from relatives in rural or other urban areas (Crush and caesar, 2020). Likewise cash remittances from migrant relatives may represent important contributions to the food security of the urban poor [START_REF] Buchmann | Cuban home gardens and their role in social-ecological resilience[END_REF][START_REF] Dissanayake | COVID-19 Outbreak and Urban Green Space, Food Security, and Quality of Life: Case of Urban Home Gardens in Kandy, Sri Lanka[END_REF]. The contribution of wild food and meat to the livelihoods of the poor has been mostly documented in rural areas [START_REF] Wunder | Forests, livelihoods, and conservation: broadening the empirical base[END_REF]), but it is also observed in urban areas, e.g. collecting wild plants contributing to dietary diversification of low-income dwellers in Kampala [START_REF] Mollee | Into the urban wild: Collection of wild urban plants for food and medicine in Kampala, Uganda[END_REF].

Short relational chains

We refer to short urban food chains relative to elements of distance and chain organisation. These are food systems originating from, urban and peri-urban agriculture, with one or two intermediaries between farmers and consumers, e.g. market retailers and wholesalers. The farmers themselves or one of their relatives are frequently involved in wholesale and/or retail distribution. In line with the Von Thünen model which has some relevance when rural-urban transportation is poor, short food chains predominate in the supply of perishable produce, e.g. leafy vegetables, milk, eggs and chicken (Moustier, 2017). These commodities are nutrient-dense and commonly under-consumed relative to nutritional recommendations. The relationships between farmers, wholesalers and retailers are characterised by long-term acquaintance and regular interaction. The shortness of chains in terms of distance and intermediaries may lead to lower final prices (and environmental costs) than longer chains but this is not systematically the case because long chains may enable economies of scale [START_REF] Cara | Local Food, Urbanization, and Transport-Related Greenhouse Gas Emissions[END_REF].

Urban and peri-urban agriculture may generate problematic outcomes in terms of food safety and environmental pollution. The urban environment is responsible for major air, water and soil pollution (Amegali et al., 2017;[START_REF] Adimalla | Heavy metals pollution assessment and its associated human health risk evaluation of urban soils from Indian cities: a review[END_REF][START_REF] Douglas | Flooding in African cities, scales of causes, teleconnections, risks, vulnerability and impacts[END_REF][START_REF] Pervin | Adapting to urban flooding: a case of two cities in South Asia[END_REF], Farmers may be pushed to use excessive amounts of chemicals on their small plots. At the same time many public and international programmes support agro-ecological or organic agriculture in cities, and, if handled safely, waste can be recycled within agriculture [START_REF] De Bon | Sustainable urban agriculture in developing countries. A review[END_REF]. The multi-functionality of urban agriculture means it is a 'cheap' producer of public goods, including greening of cities, limiting floods, providing employment and food [START_REF] Moustier | Local economic development and marketing of urban produced food[END_REF]. However, systems based on urban and peri-urban agriculture, are threatened by land insecurity [START_REF] Hatab | Urban sprawl, food security and agricultural systems in developing countries: a systematic review of the literature[END_REF].

Long relational chains

These chains originate from local or regional rural areas, and also from global markets. Local long chains involve rural collectors, rural wholesalers, urban wholesalers and urban retailers who supply all types of urban consumers (Moustier, 2017a;Karg et al., 2018[START_REF] Karg | Food-and feed-based nutrient flows in two West African cities[END_REF]Lemeilleur et al., 2020). Urban wholesalers may also import food from neighbouring or global markets. For local products, transactions take place in wholesale and retail markets located to minimise traders' and consumers' transport costs [START_REF] Blekking | Adaptive Governance and Market Heterogeneity: An Institutional Analysis of an Urban Food System in Sub-Saharan Africa[END_REF][START_REF] Lemeilleur | Les systèmes de distribution alimentaire dans les pays d'Afrique méditerranéenne et Sub-Letcher[END_REF][START_REF] Balineau | Food Systems in Africa: Rethinking the Role of Markets[END_REF]. Some processing may take place in SMEs at an artisanal scale (Tshirley et al., 2020) in various locations within and outside cities. With the development of transport, credit and mobile phones, these chains may be shortened, and the roles of rural collectors and wholesalers reduced. This transformation is termed the 'quiet revolution' in agrifood value chains in LMICs by [START_REF] Reardon | The hidden middle: the quiet revolution in the midstream of agrifood value chains in developing countries[END_REF].

Long relational chains of staple food crops are characterised by long-term acquaintanceship and reciprocity, together with competition between hundreds of vendors resulting in a certain degree of price homogeneity, even though oligopolies of wholesalers are observed because of limited access to credit and storage facilities [START_REF] Fafchamps | Market institutions in sub-saharan Africa. Theory and evidence[END_REF]). This provisioning system is dominant, as it relates to the majority of staple crops including cereals, tubers, pulses, and vegetables that can be stored (either fresh or processed) and some animal products. This type of chain is also observed for the marketing of wild meat originating from forests.

Rural-based long chains are primarily influenced by the public investments in transport and market infrastructures, which are commonly inadequately maintained and generate high marketing costs and risks. Badly maintained lorries are also responsible for air pollution as well as food losses. Long chains were particularly affected by the covid crisis because of transport restrictions, and also when urban markets were closed [START_REF] Dury | Les systèmes alimentaires aux défis de la crise de la Covid-19 en Afrique : enseignements et incertitudes[END_REF].

Value-oriented SME-driven chains

These chains originate from rural, urban and peri-urban farms, driven by small-scale farmer organisations or entrepreneurial medium-scale farmers who upgrade their technologies and improve product quality in response to consumer expectations in terms of food safety and convenience. At the same time, they create new chain organisation patterns with increased interactions and different forms of vertical integration regarding direct sales or contracts with their customers [START_REF] Moustier | Urban agriculture and short chain food marketing in developing countries[END_REF][START_REF] De Brauw | Food system innovations for healthier diets in low and middle-income countries[END_REF]Tefft et al., 2019). This is the case of farmer organisations that sell food grown without or with limited chemicals in shops or farmer markets in Laos, India, Ecuador, Colombia, Brazil, or Kenya, or by subscription in Dakar and, in some South African cities [START_REF] Freidberg | Alternative food in the global south: reflections on a direct marketing initiative in Kenya[END_REF][START_REF] Joshi | Farmers' participation in weekly organic bazaars in Aurangabad, India[END_REF]Renting and Dubelling, 2013). Entrepreneurial producers, e.g. le Terroir in Abidjan, can sell dairy products and cold cuts to wealthy urban consumers thanks to processing and cold storage (Neveu-Tafforeau, 2017). Caterers, both private companies, restaurants, and school canteens are developing strategies to ensure food safety and promote local products by signing contracts with local producer groups (Moustier and Renting, 2025). Food caterers and processing SMEs also innovate to supply processed local food to urban dwellers [START_REF] Ferré | Innover dans le secteur de la transformation agroalimentaire en Afrique de l'Ouest[END_REF]Reardon et al., 2021a). These initiatives come together with concerns for consumers' health and environment commonly translated into good practices and some form of certification. Municipalities and regional governments may be involved in supporting these initiatives to promote provisioning of safe food to vulnerable households and to promote agro-ecology. This is the case for public programmes targeting the urban poor, e.g. the food purchase programme in Brazil [START_REF] Berchin | The contributions of public policies for strengthening family farming and increasing food security: The case of Brazil[END_REF] or farmers' organic markets in Laos. Without such public support, these initiatives are precarious because of the cost of access to shops, lack of product diversity and of guaranteed food safety.

Supermarket-driven chains

Supermarkets source food from global, regional and local sources. They carry both local and international brands and develop strategies for quality control and guaranteed origin, including using dedicated wholesalers and contracts, but they still face difficulties concerning quality control and traceability. Overall, supermarkets vary in their supply strategies, including whether they favour linkages with local food chains, in their pricing and in the payment conditions offered to local farmers, as well as in the training and logistics they may provide to farmers [START_REF] Minten | Agricultural value chains: How cities reshape food systems[END_REF].

Modern distribution systems, driven by supermarkets, are characterized by labour-saving and capital-intensive technologies in terms of logistics, refrigeration, self-service, packaging, cash registers [START_REF] Hagen | Causes and consequences of food retailing innovation in developing countries: supermarkets in Vietnam[END_REF]. They are judged to be efficient in terms of logistics and quality [START_REF] Nuthalapati | Supermarket procurement and farmgate prices in India[END_REF], but with potential negative effects on nutrition because they supply a wide range of highly processed food rich in fats and sugar [START_REF] Demmler | Supermarket shopping and nutritional outcomes: A panel data analysis for urban Kenya[END_REF]Gomez and Ricketts, 2013;[START_REF] Wertheim-Heck | A cross-disciplinary mixed-method approach to understand how food retail environment transformations influence food choice and intake among the urban poor: Experiences from Vietnam[END_REF]. Regarding affordability for the poor, modern systems are usually presented as less poor-friendly because of higher prices and transport constraints. Modern systems also create less employment per unit of product [START_REF] Moustier | Are Supermarkets Poor-friendly? Debates and Evidence from Vietnam[END_REF][START_REF] Wertheim-Heck | Food safety and urban food markets in Vietnam: The need for flexible and customized retail modernization policies[END_REF]. Regarding differences in prices between supermarkets and traditional vendors, when controlling for quality differences, results are country-specific. When supermarkets gain a substantial market share, they can reduce their logistic costs and provide food at lower prices, especially food that can be stored [START_REF] Reardon | Links between supermarkets and food prices, diet diversity and food safety in developing countries[END_REF][START_REF] Nuthalapati | Supermarket procurement and farmgate prices in India[END_REF]. Prior to that stage, food is usually cheaper and more accessible in open markets and small shops than in supermarkets [START_REF] Moustier | Are Supermarkets Poor-friendly? Debates and Evidence from Vietnam[END_REF][START_REF] Wanyama | How important are supermarkets for the diets of the urban poor in Africa?[END_REF]. Moreover, supermarkets favour the use of plastics for wrapping fresh food, which is a major environmental concern (Letcher, 2020). However, supermarkets are usually supported by city and national governments on the grounds of modernity and hygiene.

E-commerce-driven chains

New large-scale capital-intensive e-commerce companies have developed their activities related to food provisioning in Asia, Latin America and some African countries since the 2000s, which has accelerated due to covid. Their sourcing strategies are similar to those of supermarkets. Middle and high-income consumers order food from their platforms that is delivered at home, mostly for processed or prepared food. This is the case of Alibaba, which started its business in China and then expanded to South-East Asia. The development of these firms is spurred by policies that favor foreign direct investment and communication investments [START_REF] Reardon | Quiet Revolution by SMEs in the midstream of value chains in developing regions: wholesale markets, wholesalers, logistics, and processing[END_REF]. As they are based on capital-intensive technologies, they are less favorable to employment than other forms of food distribution. The balance between environmental costs and benefits of digitalization is subject to debate (Coronoa and Mattern, 2019).

Interactions between food systems

There are patterns of competition but also interactions and complementarities between the various food provisioning systems, bringing dynamism and resilience to the overall food system. Consumers commonly combine purchases from open markets especially for fresh food, with less frequent purchases in supermarkets for groceries [START_REF] Tran | Shopping and cross-shopping practices in Hanoi Vietnam: An emerging urban market context[END_REF] for Hanoi; Skinner, 2019 for East Africa; Si et al, 2019 for Nanjing). Informal vendors source their food from a variety of places, including wholesale markets, grocery shops and even supermarkets in Windhoek, Namibia [START_REF] Nickanor | The informal food sector and cohabitation with supermarkets in Windhoek, Namibia[END_REF]. There may be some symbiotic relationships between supermarkets and street vendors, as street vendors may sell around supermarkets (Battersby and Watson, 2019) ; or between e-commerce and shops : in India, Amazon uses shops as delivery points [START_REF] Reardon | Quiet Revolution by SMEs in the midstream of value chains in developing regions: wholesale markets, wholesalers, logistics, and processing[END_REF]. Farmers' commonly combine selling to collectors without a price premium and selling to supermarkets with a price premium, because supermarkets have limited ordering capacity or have stringent quality requirements. It has been documented in the case of tomato in Colombia that the complementarity of farmers' outlets helped reduce waste [START_REF] Chaboud | The role of diverse distribution channels in reducing food loss and waste: The case of the Cali tomato supply chain in Colombia[END_REF]. Innovations spread from one type of food chain to the other. This is not necessarily from the so-called 'modern' to the other types. For instance, supermarkets benefited from innovations brought by farmer organisations selling 'safe vegetables' to small shops in Vietnam [START_REF] Moustier | The role of farmer organizations in supplying supermarkets with quality food in Vietnam[END_REF].

Conclusions and recommendations

The paper has highlighted the diversity of consumers' socio-economic profiles, which results in varying food consumption patterns in terms of the type and sources of food. Such diversity is reflected in consumer environments, with the coexistence of a diversity of retailing and catering points, with varying levels of physical and financial accessibility. According to the nature of food, the type of access to food (market/ non-market) and the nature of retailing points (market/street vendors, shops, supermarkets or e-commerce), food chains vary in their organisation in terms of the origin of food, nature of intermediaries, interactions between stakeholders, as well as outcomes (see Table 1). Such a diversity is key to the resilience of urban food systems, yet it is not supported by an adequate public governance of urban food systems. "In the absence of clear urban food policy, urban food system governance is the unacknowledged outcome of the contestations and collaborations of local government with other actors in the urban system, including traders, market associations, and private capital and development agencies" (Battersby and Muwowa, 2019:130). National and urban public authorities commonly favor investments directed towards costly market structures and support to supermarket development, e.g. in terms of land allocation, and have negative perceptions and interventions towards the informal sector. We recommend that national, regional and city governements also support the informal operators and SMEs involved in the chains who supply food to low-income consumers, so that their business becomes more efficient in terms of food safety and income generation. This implies different layers of public interventions, in close collaboration with the private stakeholders targeted by the actions. According to Smit (2016 :81), key areas of interventions with impact on urban food systems are: (i) infrastructures; (ii) regulations ; (iii) resources and incentives ; (iv) education and awareness to which we add (v) institutional capacity.

Infrastructures

The lack of basic market infrastructures and services is an important constraint hampering food quality and traders' business environment. Urban marketplaces are frequently characterized by congestion, difficulty moving around and lack of hygiene. Some past projects to replace urban marketplaces with wholesale markets located outside ofcity boundaries were underused due to limited transport facilities as well as the high cost of market stalls (Moustier, 2017b ;Battersby and Muwowa, 2019). We thus recommend municipalities upgrade existing markets, by covering them, concreting the ground and making clean water available. Planning new markets and designing market regulations, e.g. on hygiene, should include in-depth consultation with market users' representatives, especially wholesalers and retailers in traders' associations [START_REF] Hubbard | Improving urban food supply and distribution in developing countries: the role of city authorities[END_REF][START_REF] Smit | Urban governance and urban food systems in Africa: Examining the linkages[END_REF]. Food markets can also be combined with a 'food hub' function, thereby creating new market linkages with food producers in the region, as developed in Colombia [START_REF] Dubbeling | Assessing and Planning Sustainable City Region Food Systems: Insights from Two Latin American Cities[END_REF].

Rural-urban transportation should be a priority of national and regional governments to improve both food availability and quality and to reduce food losses, especially for long relational chains. Roads between cities and the rural areas, which play a major role in supplying food to cities, need to be expanded and maintained, along with alternative transport routes by rail or water (Poppoola et al., 2021).

Resources and incentives

National programmes should improve access to training on food processing and storage, as well as food practices for food safety, targeting food SMEs. Existing small-scale food storage and processing technologies are available to improve the safety and nutritional quality of food, and to reduce food losses (Kitinola and Thomson, 2010;Tefft et al. ,2019;[START_REF] Ferré | Innover dans le secteur de la transformation agroalimentaire en Afrique de l'Ouest[END_REF][START_REF] Pallet | Systèmes de transformation durables : quelles nouvelles stratégies pour les filières ?[END_REF]. Innovation in the artisanal sector needs to be supported by public programmes providing credit to increase the working capital and to enable investment in semi-industrial processing. Financial literacy and bookkeeping skills should be improved for MSMEs [START_REF] Young | The urban informal food sector in the Global South[END_REF].

Poverty is the major constraint to the food security of the urban poor rather than food availability [START_REF] Frayne | Growing out of poverty: Does urban agriculture contribute to household food security in Southern African cities?[END_REF]. Social programmes allowing households to reduce expenditure on healthcare, education, transport and food are hence recommended, including consumer purchases from local food vendors [START_REF] Young | The urban informal food sector in the Global South[END_REF].

Regulations

The major constraint for urban and peri-urban agriculture to provide adequate food and employment is farmers' long-term access to land. If market forces are left unrestricted, urban and peri-urban agriculture is doomed to disappear given the forces of pressure on land and water. This is detrimental to urban food security and livelihoods and may create environmental problems. We consequently recommend municipalities and regional governments protect land for agriculture in areas where it can play a major role in both food supplies and livelihoods, and where pollution is not an issue. Access to land can be secured through regulations (protecting agricultural parks or zoning measures) and formal contracts between municipalities and farmer' organisations. How urban planning is enforced needs to be closely monitored as it has frequently been observed that legal protection of land is regularly trespassed because of the attraction of private investors' urban development schemes [START_REF] De Bon | Sustainable urban agriculture in developing countries. A review[END_REF]Valette and Philibert, 2014;[START_REF] Ayambire | A review of practices for sustaining urban and peri-urban agriculture: Implications for land use planning in rapidly urbanising Ghanaian cities[END_REF][START_REF] Dao | Development of sustainable peri-urban agriculture in Vietnam[END_REF].

The specific case of informal market mobile vendors should be tackled given their importance in the livelihoods of vulnerable urban populations (especially women), as vendors and consumers. Their business should be acknowledged and supported aiming at "semiformality", i.e., a self-regulating system with some light third-party regulatory enforcement of agreed-upon specifications (Cross, 2010). Examples of successful integration of street vending in the city can be found in Vietnam [START_REF] Loc | Toward a restricted tolerance of street vending of food in Hanoi districts: the role of stakeholder dialogue[END_REF], India [START_REF] Srivastava | Formalising the Informal Streets: A Legislative Review of the Street Vendors (Protection of Livelihood and Regulation of Street Vending) Bill[END_REF], China [START_REF] Dai | From overt opposition to covert cooperation: Governance of street food vending in Nanjing, China[END_REF]Thailand (Tangworamonycon, 2014).

The design and enforcement of food safety regulations should be adapted to the characteristics of the informal sector and the SMEs in general, in the sense that they should be at the same time preventing major health risks, not generating unaffordable investments and coupled with educational initiatives and the provision of clean water and disposal facilities [START_REF] Smit | Urban governance and urban food systems in Africa: Examining the linkages[END_REF]. The public sector also needs to invest human resources in food quality control, with random checks of food safety and labelling frauds, and graduated sanctions for noncompliance, at various points of the chain, including wholesale and retail markets [START_REF] Hawkes | 42 policies and actions to orient food systems towards healthier diets for all[END_REF][START_REF] Dao | Developing a sustainable safe food value chain in Vietnam. Vietnamese version[END_REF]. Trajectories towards formalisation of businesses imply that authorities work on how to make formality more attractive in terms of benefits, including legal protection and insurance [START_REF] Young | The urban informal food sector in the Global South[END_REF].

Education and awareness

We recommend collaboration between ministries of health and agriculture to promote local nutrient-dense foods, e.g. fruit, vegetables, nuts and legumes (Eat Lancet Commission, 2019). They may be available to consumers locally, but are not always purchased because consumers may have little knowledge of their health benefits or of how to include them in their meals and dietary practices. Different ways to increase public awareness about healthy food and promote traditional food cultures are discussed in [START_REF] Hawkes | 42 policies and actions to orient food systems towards healthier diets for all[END_REF].

Institutional capacity

Municipalities can be actors in the development of sustainable food systems, particularly through their governance of urban agriculture, school canteens, and waste management [START_REF] Bricas | Urbanization Issues Affecting Food System Sustainability[END_REF][START_REF] Fages | Food for Cities. What roles for local governments in the Global South? Paris, AFD[END_REF], together with the convening role that they can play [START_REF] Haysom | Food and the City: Urban Scale Food System Governance[END_REF]. Through the Milan food policy pact (https://www.milanurbanfoodpolicypact.org/), city officials are invited to commit to 31 actions aimed at sustainable food provisioning and consumption. Governing urban food systems in an inclusive way is facilitated by establishing urban food policy councils/platforms, for example the Belo Horizonte Council for Food Security's inclusion of government and civil society was crucial for its success [START_REF] Haysom | Food and the City: Urban Scale Food System Governance[END_REF]. Other cities of Latin America have set similar initiatives (see https://ruaf.org/), sometimes on the basis of urban agriculture programmes, like in Quito. The coordination of decisions of the various actors' governing food systems, including local, regional and national governments involved in agriculture, food, health, and employment; farmers, traders and consumers' representatives, and NGOs, is seen as necessary but challenging because of possible antagonistic values, visions and interests [START_REF] Smit | Urban governance and urban food systems in Africa: Examining the linkages[END_REF][START_REF] Haysom | Food and the City: Urban Scale Food System Governance[END_REF][START_REF] Haysom | Food systems sustainability: An examination of different viewpoints on food system change[END_REF].

In terms of research, we highlight the need for more accurate and updated data on food consumption, food environments, foodsheds and food chains. This requires inter-disciplinary research, including geographers, economists, nutritionistsand statisticians. Data on food consumption should account for food consumed away from home, and seasonal food, including fruit and vegetables [START_REF] Rousham | Dietary behaviours in the context of nutrition transition: a systematic review and meta-analysis in two African countries[END_REF]2020). Accurately appraising the role of different production areas and intermediaries in urban food supply requires periodic surveys of wholesale and retail markets, and of the origin and quantities of products traded. With the growing of food imports, a better knowledge of how local food chains interact with global food chains is required. Comparative studies of the varying importance of the different sub-systems across different cities is also recommended to better appraise the comparative advantages and dynamics of each [START_REF] Zhong | Urban food systems: a bibliometric review from 1991 to 2020[END_REF].

of Joachim von Braun, Kaosar Afsana, Louise Fresco and Mohamed Hag Ali Hassan (eds), Science and Innovations for Food Systems Transformation. Springer. We thank the reviewers of the brief including Ninon Sirdey and Jemimah Njuki. We are also grateful to the reviewers of GFS for their insightful comments on a first version of the paper.

saharienne. Repenser le rôle des marchés dans l'infrastructure commerciale. Notes techniques de l'AFD. Notes, (51). http://admin.riafco.org/Images/Ressources/Pulication/64/51-notes-techniques.pdf.

Acknowledgements

The paper is a substantially revised version of a research brief produced for the World Food Summit as Moustier, P., Holdsworth, M., Dao The Anh, Pape Abdoulaye Seck, Renting, H., Caron, P., Bricas, N. 2021. Priorities for urban food systems transformations in the global South. United Nations Food Systems Summit 2021 Scientific Group (link). This brief will be published as a forthcoming book chapter

The diverse and complementary components of urban food systems in the global South: characterization and policy implications 1