Modeling soil-root water transport and competition for single and mixed crops

A knowledge of above and below ground plant interactions for water is essen tial to understand the performance of intercropped systems. In this work, r oot water potential dynamics and water uptake partitioning were compared be tween single crops and intercrops, using a simulation model. Four root maps having 498, 364, 431 and 431 soil-root contacts were used. In the first an d second cases, single crops with 'deep' and 'surface' roots were considere d, whereas in the third and fourth cases, roots of two mixed crops were sim ultaneously considered with different row spacing (40 cm and 60 cm). Two so ils corresponding to a clay and a silty clay loam were used in the calculat ions. A total maximum evapotranspiration of 6 mm d(-1) for both single or m ixed crops was considered, for the mixed crops however, two transpiration d istributions between the crops were analyzed (3:3 mm d(-1), or 4:2 mm d(-1) for each crop, respectively). The model was based on a previous theoretica l framework applied to single or intercropped plants having spatially distr ibuted roots in a two-dimensional domain. Although water stress occurred mo re rapidly in the loam than in the clay, due to the rapid decrease of the s oil water reserve in the loam, the role of the root arrangement appeared to be crucial for water availability. Interactions between the distribution o f transpiration among mixed crops and the architecture of the root systems which were in competition led to water movements from zones with one plant to another, or vice versa, which corresponded to specific competition or fa cilitation effects. Decreasing the distances between roots may increase com petition for water, although it may determine greater water potential gradi ents in the soil that increase lateral or vertical water fluxes in the soil profile. The effects of the root competition on water uptake were quite co mplicated, depending on both environmental conditions, soil hydrodynamic pr operties, and time scales. Although some biological adaptive mechanisms wer e disregarded in the analysis, the physically 2-D based model may be consid ered as a tool to study the exploitation of environmental heterogeneity at microsite scales.