Numerical simulation of water- and heat-flow regimes of mulched soil in rain-fed soybean field in central Japan

Abstract Numerical simulations for predicting soil-water and heat-flow regimes beneath mulched soils are important to increase the opportunity for efficient use of mulching in agriculture. This study simulated water and heat flows by HYDRUS-1D model under rice-straw mulching and bare soil treatments for rain-fed soybean ( Glycine max ) cultivation at Gifu Prefecture in central Japan. Soil hydraulic and thermal parameters with the straw mulching were optimized by adding a hypothetical 3-cm straw layer. The optimized parameters were validated with field-measured soil-moisture and temperature regimes of two consecutive soybean-growing seasons of 2015 and 2016. HYDRUS-1D model performed fairly well in simulating heat and water movements at 5, 15 and 25 cm soil depths under the two treatments. The model simulated soil moisture and temperature with small Root-Mean Square Error (RMSE): 0.016–0.044 cm 3  cm –3 for soil moisture and 0.94–1.63 °C for soil temperature. The simulation results revealed that straw mulching increased soil moisture but reduced soil temperature at the three soil depths compared to the bare soil. The moisture content of the additional straw layer (3 cm) and heat flow through it showed a decreasing pattern compared to the soil layers. The straw mulching contributed 16–96% vapor to the total water flow and stored/released 0.0322 to 0.0295 MJ m –2 heat per day at 0–3 cm straw layer. Water balance in the soybean field revealed that the straw layer enhanced rain water infiltration by limiting evaporation from the soil surface. The simulation results can be useful for better management of soil hydro-thermal regimes under straw-mulched fields in dryland cultivations.