Ridge-furrow plastic mulching and dense planting with reduced nitrogen improve soil hydrothermal conditions, rainfed soybean yield and economic return in a semi-humid drought-prone region of China

Abstract

Soybean (Glycine max L. Merr) is an important crop around the world. Adaptive soil and crop managements are crucial for improving rainfed soybean productivity and resource use efficiency under changing climate. The ridge-furrow plastic mulching system was applied for the first time to rainfed soybean, and its combined effects with dense planting and reduced nitrogen (N) on soil water and temperature, soybean growth, yield, water use efficiency (WUE) and economic return were explored in 2019 and 2020 in a semi-humid drought-prone region of China. There were two planting modes: flat cultivation with no mulching and ridge-furrow cultivation with plastic film mulching on the ridge, and four crop management practices: sparse planting (160,000 plants ha−1) with reduced N rate (30 kg N ha−1), sparse planting with high N rate (60 kg N ha−1), dense planting (320,000 plants ha−1) with reduced N rate (30 kg N ha−1) and dense planting with high N rate (60 kg N ha−1). Compared with flat cultivation with no mulching, ridge-furrow plastic mulching significantly increased average surface soil water (0–20 cm) by 6.7% and soil temperature (by 0.5–1.5 ℃ at 10–25 cm depths), which promoted leaf area index (LAI) by 9.0%, total chlorophyll content (Chl) by 10.4%, net photosynthesis rate (Pn) by 1.6%, resulting in increases in soybean yield, WUE and net income (NI) by 11.1%, 10.7% and 8.6%, respectively. Compared with sparse planting, dense planting decreased average Pn by 7.5%, aboveground biomass per plant by 15.7%, pods per plant by 29.9% and hundred-grain weight by 3.8%; however, it improved LAI by 15.7%, soybean yield by 32.3%, WUE by 28.8% and NI by 44.3%. Compared with reduced N rate, high N rate improved average LAI by 14.0%, Chl by 11.1% and Pn by 4.2%, resulting in increases in soybean yield by 10.1%, WUE by 9.7% but NI by only 2.4%. Compared with the traditional flat cultivation with no mulching and sparse planting with high N rate, ridge-furrow plastic mulching and dense planting with reduced or high N rate significantly improved soybean yield (by 25.6% or 38.8%) and WUE (by 23.8% or 38.0%) (P < 0.05), but the former produced greater net income (12,749.7 CNY ha−1) than the latter (12,433.4 CNY ha−1) with 50% less N input. In conclusion, ridge-furrow plastic mulching and dense planting with reduced N rate is a more promising practice to trade off yield, water productivity, economic return and environmental benefit of rainfed soybean.