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International audience; The sensitivity of C and N mineralization in soil to water potential is mostly described in simulation models as a linear function independent of the pedoclimatic conditions. We hypothesized that water sensitivity could be sitespecific and dependent of climate or soil properties. In this study, we characterized the responses of C and N mineralization to water stress in ten soils representing a range of French arable cropping systems and evaluated whether the responses differ between soils and pedoclimatic contexts. C and N mineralization kinetics were quantified in laboratory incubations at four soil water potentials (pF) ranging from pF = 2.0 (~field capacity) to 4.2 (~permanent wilting point). The C and N mineralization rates, calculated by curve fitting, were linearly correlated with pF or relative water content (RWC). The slope of the linear regression, representing the sensitivity to water potential, differed significantly between sites, ranging from 0.12 to 0.35 pF-1 for C mineralization and 0.20 to 0.44 pF-1 for N mineralization. The sensitivity of C or N mineralization rate to pF or RWC could be well predicted by a couple of two soil properties: either microbial quotient (ratio of microbial biomass-C to total organic C) and soil pH or soil organic C:N ratio and soil pH. The sensitivity of soil to water stress was more accurately predicted by these site-specific variables than a model common to all pedoclimatic conditions. These results open the possibility of improving soil and soil-crop models for a more accurate prediction of water stress on C and N mineralization particularly in the context of climate change.

International audience; Cover crops are a potential component of agroecological cropping systems, since they may render crop rotations more sustainable. They simultaneously provide multiple ecosystem services, such as decreasing nitrate leaching, decreasing erosion, and increasing soil organic matter. However, cover crops increase evapotranspiration and reduce drainage, which results in a potential disservice for groundwater recharge. Little attention has focused on management of cover crop residues after destruction or their influence on water flux dynamics, particularly in dry and temperate climates. The objective of our study was to analyze and quantify the impact of cover crop management on soil water content and water flux dynamics to understand the main mechanisms of system functioning. We combined a two-year field experiment with crop-model simulations. We performed the field experiment in southwestern France that compared three cover crop treatments, with bare soil as the control. The treatments included (1) living cover crops lasting ca. 9 months from August-April, (2) crushing cover crops in November and leaving them as mulch on the soil, and (3) plowing up cover crops in November to promote residue decomposition and the green manure effect. The STICS soil-crop model was used to predict water fluxes that were not measured and to perform a 20-year independent simulation study based on recent climate series for the experimental site. Our main results indicated that cover crops (1) always reduce water drainage by 20-60 mm compared to that under bare soil; and (2) could significantly reduce soil water content (0-120 cm deep) for the next cash crop by a mean of 20-50 mm, and up to 80 mm in dry spring conditions, but early destruction could decrease this negative impact. The simulations clearly showed that the interaction between climate variability, i.e., rainfall distribution during the fallow period, and cover crop management should be considered to explain the impact of inter-annual variability on the water balance. Thus, destroying cover crops mechanically in late autumn and retaining the residues as mulch could be a good compromise between the multiple services the cover crop provides during the fallow period and avoiding the negative impact on soil water availability for the next cash crop.

There is increasing concern that due to land pressure and the need to maximize income, smallholder coffee farmers are increasingly being forced to cultivate in areas which are considered to be sub-optimal for coffee. Little is known about optimal coffee and tree combinations in these conditions and the degree to which crops and trees compete or are synergistic. In environmental conditions which were sub optimal for coffee cultivation in Nicaragua (1470 mm annual rainfall, 27 °C mean annual temperature and 455 m altitude compared to optima of 2000 mm, 23–24 °C and altitude between 1000 and 1400 m at that latitude, respectively), coffee and shade tree transpiration and soil evaporation were directly and separately measured in agroforestry (AFS) and full sun systems (FS). AFS was found to be a more efficient water user than FS because a greater proportion of rainfall was used by plant transpiration rather than being lost by soil evaporation. Plant transpiration accounted for 83% and 69% of evapotranspiration while soil evaporation represented 17% and 31%, in AFS and FS respectively. In AFS most of the water transpiration was due to coffee (72.5%) and much less by deciduous Tabebuia rosea (19%) and evergreen Simarouba glauca shade trees (8.5%). Furthermore, the study demonstrated the vastly different behaviour in water use by the shade trees. When in leaf, Tabebuia rosea transpired at four to six times the rate of evergreen Simarouba glauca, although crown sizes were similar. Contrasting precipitation between two consecutive years of study demonstrated that competition for water between coffee and shade tree occurred only in a severe dry season when coffee leaf water potential (LWP) reached its lowest values of −2.33 MPa in AFS. It was concluded that in most circumstances there was sufficient water for both coffee and trees, that coffee in AFS was a more efficient user of water than FS coffee, and that evergreen Simarouba glauca was more suitable as coffee shade tree compared to deciduous Tabebuia rosea in the sub optimal environmental condition studied. (Resume d'auteur)

Background and aims: In temperate conifer forests, soil temperature is an important driver of fine root growth and winter root growth can occur during aerial dormancy. We hypothesize that in conifers, stocks of non-structural carbohydrates (NSC) in fine roots are high enough to provide energy for root growth and production throughout the year, even when photosynthesis is reduced. Methods: We measured monthly root production (i.e. the number of roots undergoing elongation) and their elongation rate (RER) in mature Picea abies for one year, along a soil temperature gradient (three altitudes of 1400, 1700 and 2000 m). Every two months we harvested needles, branches, stem, large, medium and fine roots, and quantified starch and soluble sugars in each organ using analytical methods and near infra-red spectroscopy (NIRS). Soil water potential was monitored continuously. We analysed RER data with regard to climate variables and NSC levels of the current and preceding month. Results: NIRS was a reliable method for measuring starch and soluble sugars. NSC was high in the crown and roots but very low in the trunk all year round. Soil temperature was positively correlated to RER (of the current month) between 0 and 8 °C, above which RER stabilised and was not explained by NSC levels or soil water potential. However, mean RER of fine roots in the month following the measurement of NSC was significantly and negatively correlated to soluble sugar and positively correlated with starch content. Very fine root starch content was also positively correlated with root production in the month following the starch measurement. Conclusion: Soil temperature was a major driver of fine root elongation, but at low temperatures only. At soil temperatures >8 °C, no particular driver was dominant. NSC levels were negligible in the stem and root-bases, suggesting that wood production is a major sink that depletes carbohydrates. A large pool of NSC, principally in the form of starch, existed in fine roots of P. abies, and acted as an energy supply for root production throughout the year, even when photosynthesis was limited. Soluble sugars were depleted in fine roots during the growing season, but no relationship was found between fine root production and soluble sugars. The physiological mechanism by which NSC accumulation actively or passively occurs in fine roots is not known but could be due to the symplastic pathway of phloem unloading in conifer root tips, which suggests a passive mechanism.

Plants tend to decrease transpiration under water deficit and/ or high evaporative demand by closing stomata and reducing leaf growth. Identification of sources of genetic variability for underlying mechanisms is necessary to design genotypes adapted to stressing climatic scenarios. A series of four experiments was performed in the PhenoArch image-based phenotyping platform (M3P, France) with contrasting soil water status and evaporative demand. We used a diversity panel of 255 maize hybrids genotyped with 832K polymorphic SNPs. Equivalent stomatal conductance at plant level was estimated in the studied 1680 x 4 plants by inversion of the Penman Monteith equation. It changed with light intensity and vapor pressure deficit, with different thresholds and slopes between genotypes. Maximum values ranged from 52 to 76 mmol m-2 sec-1 depending on hybrids. The sensitivity of leaf expansion to soil water potential was calculated over the four experiments as the slope of the relationship of leaf expansion rate to soil water potential. For each hybrid, a common linear relationship applied to the four experiments. The x-intercepts of these relationships, which indicate the driest soil in which a plant still has an appreciable leaf growth, ranged from -0.6 to -1.6 MPa depending on hybrids. A GWAS analysis was performed on all variables presented above, suggesting interesting candidate genes related to hydraulics and other mechanisms. Surprisingly, no co-location was observed between QTLs of stomatal conductance and of sensitivity to soil water deficit, supporting the idea that the controls of stomatal opening/photosynthesis and of leaf expansion are largely independent.

National audience; Tolerance to soil water deficit is a priority of many maize breeding programs. Its genetic analysis requires estimators that characterize each genotype in a reproducible way. For that, we have tested and compared the abilities of hybrids to maintain leaf growth in a large range of soil water potentials (i) in a phenotyping platform that measured leaf elongation rate and (ii) in a network of field experiments via an estimator of the sensitivity of grain number to water deficit. Tolerance to soil water deficit was estimated in a network of 14 field experiments via the regression between yield components and soil water potential measured with tensiometers. A drought index was obtained by averaging soil water potential, measured with tensiometers, for the phenological phase during which grain number was determined (evaluated for each individual hybrid in each site). It closely correlated with grain number in each of the 19 hybrids that were analyzed over 14 environmental situations in France, Hungary and Chile. The slope of the regression line between drought index and grain number, established for each hybrid, was taken as an estimate of the sensitivity to soil water deficit of that hybrid. Sensitivity estimated in this way varied 2-fold in the set of studied hybrids, and correlated with that of leaf growth to soil water deficit in short-term experiments in the phenotyping platform. This method is promising for the evaluation of hybrids in the context of a breeding program with a minimum amount of environmental measurements and can be extended to a large number of genotypes compatible with a genetic analysis.

Conservation tillage is one strategy whereby both sustainability and productivity can be achieved by improving the soil quality. Although reduced tillage (RT) is widely practiced, more conservative strategies, such as direct-seeding mulch-based cropping systems (DMC), are less frequent. Here we assessed the effect of conversion from RT to DMC in three commercial fields in southern France (inland Mediterranean climate). Two fields were cropped with maize (Zea mays L.) and one with sorghum (Sorghum bicolor L.), and monitored during 15 months. We found higher soil water potential retention in DMC than in RT at 10, 30, and 60 cm soil depth. Conversely, nitrogen availability was slightly higher in the RT treatment. Crop development was not affected by the soil practices, but crop yields were higher in the DMC treatment for one maize field and the sorghum field (plus 3.04 t/ha harvested in the DMC treatment when compared to the RT treatment in the maize field and plus 2.105 t/ha in the sorghum one). This study demonstrates that DMC can provide short-term benefits in farm fields in southern France, but these benefits are not automatic and the conditions under which they are obtained remain to be clarified.

Crosses of Asian rice, #Oryza sativa#, with African rice, #0. glaberrima#, aim at reducing the tradeoffs between yield potential, weed competitiveness and drought resistance. Physiological studies were initiated to develop a crop model that simulates the performance of plant-types combining the traits of the two species. The present study characterizes the control of stomatal conductance and leaf rolling under drought in one #0. glaberrima# cultivar (CG14), one #0. sativa# tropical-japonica cultivar (WAB56-104), one inter-specific descendant of the two cultivars, and the #0. sativa# indica semi-dwarf-type Bouake 189. An upland field experiment (Exp. 1) with two sowing dates was conducted during the 1997 wet season under naturally occurring drought at Mbe, Cote d'Ivoire. Patterns of growth and yield were observed, and five cycles of diurnal (0800-1600 h) measurements of leaf water potential (LWP), stomatal conductance (gs) and leaf rolling score (LRS) were conducted in drought situations. In a second experiment at the same site during the 1997 dry season, irrigation was suspended for 35 days, beginning 15 days after sowing. During and after the drought period, LWP, gs, LRS and soil water tension of the topsoil were observed. Soil-matric potential (SMP) was determined from soil water content using a pF curve. Values of gs had no stable relationship with LWP across different times of day but were roughly constant during the day and depended on the LWP observed at 0800 h. LWP measured in the morning was similar to SNIP as determined from pF curves, but soil tensiometers gave lower absolute values. Leaf rolling was closely related to LWP regardless of the time of day. The cultivars did not differ in the gs versus soil water tension or gs versus LWPMorning relationship, but differed significantly in the LRS versus LWP relationship. It is concluded that gs was controlled by a soil-moisture-dependent root signal, whereas LRS was controlled directly by leaf water status. Simple models to simulate these relationships in drought-sensitive crop models are discussed. (Resume d'auteur)

Seasonal variations in environmental conditions influence the functioning of the whole ecosystem of tropical rain forests, but as yet little is known about how such variations directly influence the leaf gas exchange and transpiration of individual canopy tree species. We examined the influence of seasonal variations in relative extractable water in the upper soil layers on predawn leaf water potential, saturated net photosynthesis, leaf dark respiration, stomatal conductance, and tree transpiration of 13 tropical rain forest canopy trees (eight species) over 2 yr in French Guiana. The canopies were accessed by climbing ropes attached to the trees and to a tower. Our results indicate that a small proportion of the studied trees were unaffected by soil water depletion during seasonal dry periods, probably thanks to efficient deep root systems. The trees showing decreased tree water status (i.e., predawn leaf water potential) displayed a wide range of leaf gas exchange responses. Some trees strongly regulated photosynthesis and transpiration when relative extractable water decreased drastically. In contrast, other trees showed little variation, thus indicating good adaptation to soil drought conditions. These results have important applications to modeling approaches: indeed, precise evaluation and grouping of these response patterns are required before any tree-based functional models can efficiently describe the response of tropical rain forest ecosystems to future changes in environmental conditions. (Resume d'auteur)

National audience

Soil water potential and nitrate content in soil solution were measured simultaneously, using Tensionic tensiometers, in farmers’ fields at three different sites for assessing nitrate leaching. The above measurements were made every 10 days at two depths below the rooting zone, for three different cropping systems; namely, grassland, field crops, vineyard in 1994 and 1995, and under field-grown and greenhouse-grown salads during the winter 1996-1997. A nitrate leaching risk indicator was calculated by using the soil water potential gradient and the nitrate concentration of the soil solution below the rooting zone. This approach can be used to analyse nitrate leaching under different management practices for a range of cropping systems. It was also possible to diagnose the contribution of groundwater containing a relatively high level of nitrate to nitrogen and water supply to crops. (© Inra/Elsevier, Paris.), Le potentiel hydrique et la teneur en nitrate de la solution du sol ont été mesurés en parcelles agricoles, à l’aide de tensiomètres Tensionic, pour calculer un indicateur de risque de lessivage du nitrate. Trois sites comprenant deux tensiomètres Tensionic situés sous la zone racinaire ont été suivis tous les 10 jours sous trois systèmes de culture (prairie, grandes cultures, vigne enherbée) en 1994 et 1995, et au cours de l’hiver 1996-97 sous des cultures de salade de plein champ et sous abri. L’indicateur de risque est défini par la teneur en nitrate de la solution du sol pendant les périodes de drainage, identifiées à l’aide du gradient de potentiel hydrique. Cette approche permet de porter un diagnostic sur des parcelles agricoles différant par l’espèce, le type de sol et l’itinéraire technique. Elle met également en évidence des situations où la nappe contribue à l’alimentation hydrique et azotée de la culture. (© Inra/Elsevier, Paris.)

La mesure de la teneur en eau des sols et la quantification des flux hydriques sont des informations nécessaires à la compréhension du fonctionnement hydrodynamique du milieu et à l'estimation du bilan hydrique des cultures. La réflectométrie temporelle est une des méthodes qui pourrait remplacer efficacement l'humidimètre à neutrons. Son principe repose sur la mesure du temps de réflexion d'une onde électromagnétique de haute fréquence dont la vitesse de propagation est proportionnelle à l'humidité volumique. Cet article montre que la mesure du profil hydrique à partir de la décomposition du signal semble possible

La réflectométrie temporelle permet de mesurer l'humidité volumique du sol grâce à l'analyse de la propagation d'ondes électro-magnétiques en hyper-fréquences. Un étalonnage unique fournit une bonne précision pour la plupart des sols, et un étalonnage particulier a été établi pour les andosols, remarquables par leur comportement hydrique. Sur ces sols, étudiés à la Réunion, les méthodes classiques de mesure ne conviennent pas. Cette technique permet de réaliser facilement des mesures de surface et de profondeur, éventuellement en multiplexage. Les possibilités d'utilisation sont nombreuses et cette méthode est particulièrement pertinente pour les suivis de transferts hydriques. On obtient des représentations graphiques en deux dimensions de l'état hydrique du sol, ou un suivi dans le temps. Ainsi des bilans d'infiltration, réalisés sur un verger de litchis et une culture de tomates irriguée par goutte à goutte, ont permis d'ajuster les techniques d'irrigation

L'infiltrométrie multidisques à succion contrôlée est utilisée pour caractériser l'hydrodynamique des souches superficielles d'un sol gravillonnaire soumis à trois modes contrastés de gestion du profil cultural : semis direct de maïs et de cotonnier, labour avant chaque culture, labour récent sur jachère. On montre que cette technique permet d'avoir accès de façon simple aux deux paramètres phénoménologiques de l'infiltrabilité (la conductivité hydraulique et la sorptivité capillaire), à partir desquels il est possible d'inférer d'autres grandeurs physiques explicatives du comportement hydraulique différencié des trois traitements. Les résultats semblent mettre en évidence un effet positif mais relativement fugace du labour sur l'infiltrabilité (augmentation de la conductivité, mais surtout de la sorptivité) et sur le temps d'apparition du ruissellement. Ils permettent également de proposer un schéma d'évolution du comportement hydrophysique des couches de surface en liaison avec le travail du sol et le développement d'un réseau de macropores conduisant à des écoulements préférentiels, invalidant ainsi les essais de type Muntz à charge positive pour caractériser l'infiltrabilité. Il apparaît également que, pour ce sol, la sorptivité capillaire est une propriété importante à déterminer pour décrire son infiltrabilité

The survival of a rifampicin-resistant mutant of Dermatophilus congolensis in vertisol and oxisol soils from Guadeloupe and in their constitutive clays was studied using a pneumatic device for controlling water potentials (pF). Experiments were carried out at two pF values simulating the wet season and the dry season. Survival time depended on the type of soil and its water content. Organic matter had a protective effect on the microorganism in oxisol but not in vertisol. The pathogenicity of D. congolensis was preserved in the soils which could therefore act as temporary reservoirs of this pathogen. Long-term survival of this organism in soils mixed with water suggests that ponds and dipping tanks may constitute sources of infection for cattle.

Une technique de placement du 32P dans le sol a été mise au point pour la détermination in situ de la profondeur d'enracinement du riz pluvial, la caractérisation du profil racinaire, vertical et latéral, et l'étude des relations entre l'absorption du 32P d'une part, la densité racinaire et le potentiel hydrique du sol d'autre part. Cette technique a permis de décrire assez finement la distribution racinaire. La corrélation entre l'absorption du 32P et la densité racinaire est faible et variable, elle est plus forte avec les variations du potentiel hydrique. Ces résultats sont discutés dans l'optique de la sélection pour la tolérance à la sécheresse.

National audience

Document publié dans le cadre du projet "Caractérisation par les techniques de la télédétection de la dynamique de la désertification à la périphérie Sud du Sahara", par la Commission des Communautés Européennes

La transpiration et les échanges de CO2 de plants de riz cultivés sur sol en chambre de culture sont suivis en continu pendant une séquence dessèchement-réhydratation. La fermeture des stomates se produit pour une valeur seuil de potentiel hydrique du sol. Cette valeur est plus basse pour les variétés de type pluvial que pour les variétés de type aquatique. La relation entre la photosynthèse et la transpiration présente une réponse hystérétique dans le cas d'une variété de type pluvial. (Résumé d'auteur)