Effects of agroforestry planting systems on top-layer soil hydraulic characteristics and soil water storage on red soil slopes

Abstract: In-depth studies on soil water storage in red soil slopes in south China are of important scientific significance and practical application value for flood control, drought relief and conservation of soil and water. Agricultural cultivation under the fruit forest is an important way of development and utilization of red soil slopes in southern China, and the properties of top-layer soil would be affected by biological process, soil erosion, artificial tillage, etc. Hence, it is essential to understand difference of top-layer soil hydraulic characteristics of several agroforestry systems, which may affect soil water storage. The study area lies in Jiangxi Provincial Eco-Science Park of Soil and Water Conservation (115°42′38″-115°43′06″ E、29°16′37″-29°17′40″ N), which is located in De'an County, Jiangxi Province, China and belongs to Boyang River watershed of Poyang Lake Basin. A total of 4 treatments of agroforestry systems, namely, agriculture-forestry-grass, protective farm-forest, common farm-forest, and pure forest were studied. The top layer was defined as 0-0.30 m subsurface depth in this paper, and soil samples of the top-layer soil were collected by foil sampler for each plot. For each sample, the bulk density was measured by drying method, water retention curve was test by pressure membrane meter, and saturated hydraulic conductivity was measured by saturated infiltration experiment. The van Genuchten-Mualem model was used to describe water retention curve, and the other 4soil hydraulic parameters were reciprocal of air entry pressure, grain size distribution parameter, saturated soil water content, and residual soil water content besides saturated hydraulic conductivity. The one-dimensional Richards' equation based on the Darcy's law was used to describe soil water flow due to its solid physical foundation, and the Ross method was used to solve the Richards' equation owing to the higher computational efficiency and accuracy. For the sake of simplicity, several assumptions were considered for simulation. Firstly, the difference of top-layer soil hydraulic characteristics was the only various factor of the 4 plots, and the soil profile for simulation was also divided into 2 layers, and the top layer was from soil surface to 0.30 m depth. Secondly, the simulation period was set to from August 1st to December 31st, 2010 since runoff wasn't observed in this period. Thirdly, the upper boundary condition was considered as atmospheric boundary condition, where the potential evaporation was calculated by Penman-Monteith model. Besides, the lower boundary was zero-flux boundary at 1.00 m depth where was impermeable bedrock. The statistical analysis results of soil hydraulic parameters showed the most significant difference between top and deep layer in agriculture-forestry-grass soil with the lowest viscosity and highest permeability, i.e., the largest saturated hydraulic conductivity and reciprocal of air entry pressure among the 4 plots. Correlation analysis of parameters showed that absolute of correlation coefficient between saturated hydraulic conductivity and bulk density and that between reciprocal of air entry pressure and bulk density could reach to 0.96. The distribution of soil water content of each profile was simulated, and soil water storage was calculated by the simulated soil water content. The simulation results showed the order of soil water storage under the 4 treatments from high to low as followed: agriculture-forestry-grass soil, protective farm-forest soil or common farm-forest soil and pure forest soil. The simulation results during specified periods indicted the mechanism of effects of top-layer soil hydraulic characteristics on soil water storage. For agriculture-forestry-grass soil, the top-layer soil cut off the upward movement channel of soil moisture to reduce the water consumption of the deep soil during the evaporation period, while the top-layer soil increased the downward movement to enhance the water storage of deep soil. In conclusion, agriculture-forestry-grass is worth of development and promotion in red soil slopes in south China.