Slope position and biochar influence soil properties and seed displacement in a tropical agroecosystem

Soil erosion due to land use change is a major concern in agricultural areas. There is an urgent need to develop appropriate agricultural practices that promote higher biodiversity and improve soil properties. Little is known about the processes of natural vegetation recovery after disturbance by human activities in tropical humid climates. This is particularly the case for dispersion by water (hydrochory) that affects seeds. The displacement of native plant seeds on the soil surface by runoff may be influenced by soil properties and by the use of biochar in agriculture. As these effects have not been documented in tropical agroecosystems and on steep slopes, we present here an in-field experiment on a steeply sloping maize field affected by rainfall and tillage erosion in Northern Thailand. Our objectives were to test the effect of catena position and biochar use on seed displacement, soil loss and nutrient losses. We used 24 plots of 1 m² located at four positions in the catena. Two treatments were tested: (1) conventional system with tillage and (2) biochar incorporated into the soil. We measured the displacement of plastic beads (to imitate round seeds), runoff volume, soil and nutrient losses and soil surface features during in-situ rainfall simulations. After two simulated rainfall events and four natural rainfall events, total bead exportation was 7.2%. We found that catena position and biochar had an impact on runoff, soil and nutrient losses. In this study, previously incorporated biochar reappeared at the soil surface after 8 months under natural conditions and acted like a crust at the soil surface. This biochar crust increased runoff, increased potassium concentration and decreased phosphorus concentration in runoff, decreased bead displacement and had no effect on soil loss. This study, under tropical climate and steep slope conditions, highlights differences in soil surface features and runoff along the catena. We should consider catena position for improving soil management and using appropriate agroecological practices. Highlights: The effect of catena position and biochar use on seed displacement, soil loss and nutrient losses were tested. These effects have not been documented in tropical agroecosystems and steep slopes. Catena position and biochar use had an impact on runoff, soil and nutrient losses. Catena position for improving soil management and using agroecological practices should be considered.