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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)

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)

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.)