Reduced Subsurface Lateral Flow in Agroforestry System Is Balanced by Increased Water Retention Capacity: Rainfall Simulation and Model Validation

Soil hydrology controls the terrestrial water cycle and the transport of substances to influence the environmental quality of Earth's critical zone (CZ). Soil and water management in agroforestry systems (AF) is able to reduce soil nitrogen losses and to alleviate secondary salinity in some regions of the world by reducing subsurface lateral flow. Compared to monocropping (MC) system, the reduction of subsurface lateral flow in AF has been attributed not only to the enhanced evapotranspiration and canopy interception but also to changes in soil structure and related hydraulic properties. However, for AF, it remains unclear how changes in soil structure and hydraulic properties occur and can act to reduce the subsurface later flow. Rainfall simulation experiments were conducted in the field and soil matric potential was measured to determine the effect of AF and MC on the dynamics of rainfall infiltration, subsurface lateral flow, and soil water storage in the soil profile. The calculated isolines of soil matric potential showed that a physical domain of water saturation occurred in the subsoils during the rainfall and diminished after the rainfall. The water saturation domain was larger during the rainfall and drained more slowly after the rainfall in AF than in MC. These results illustrated that AF increased vertical preferential flow and retarded the subsurface lateral flow, resulting in increased water retention capacity in the soil profile, compared to MC. The changed water mass and flow distribution was attributed to the deep roots, which increase macropores oriented in the vertical direction and modify micro- and mesopores in the lateral direction, resulting in changes in anisotropy of soil hydraulic properties along transects of slope. These proposed mechanisms were successfully verified by mathematical modeling. Numerical experiments using the Hydrus-2D mathematical modeling code at the virtual condition of the same antecedent soil moisture condition along the slope at different rainfall events ruled out the effect of antecedent soil moisture or evapotranspiration on generation of subsurface flow. These findings suggest that land use has strong effects on water distribution not only above the ground but also in the subsurface. The changes in soil structure and hydraulic properties need to be considered in understanding landscape hydrology related to agricultural practices and their impacts on Earth's CZ.