Interception of Subsurface Lateral Flow Through Enhanced Vertical Preferential Flow in an Agroforestry System Observed Using Dye-Tracing and Rainfall Simulation Experiments

Abstract

Abstract Surface soil hydrology is a major control on the terrestrial water cycle in Earth's Critical Zone (CZ). Partitioning of vertical preferential flow and subsurface lateral flow is commonly attributed to the heterogeneity of slope, soil profile horizon, and soil structure, but the influences of land-use types are largely unknown. Agroforestry systems (AF) can intercept subsurface lateral flow for reducing nitrogen losses to drainage water and for alleviating secondary salinity in central China and southeast Australia. These effects have been attributed to enhanced evapotranspiration and canopy interception in the agroforestry systems compared to monocropping systems (MC). Here, we show the differences in lateral and vertical soil hydrological pathways between AF and MC with dye-tracing experiments before and during a simulated rainfall event. Before the rainfall, the vertical and horizontal dye-staining patterns demonstrated that preferential flow occurred through isolated macropores with fine tree roots in AF and through connected cracks in MC. The dye coverage area and depth indicated greater vertical preferential flow in AF than in MC. During the 2-h rainfall event, the dye-staining area at different depths indicated that the preferential flow contributed to greater near-surface lateral flow in MC than in AF. The changes in the hydrological pathways were attributed to deep roots and no physical barrier from plough pan in AF and the presence of the plough pan in MC. These results suggest that land use has strong water partitioning effects not only above ground but also in the subsurface, and that understanding the landscape hydrology in the Earth's Critical Zone required quantification of the considering coupled pedological and biological processes.