Effects of plant density, nitrogen rate and supplemental irrigation on photosynthesis, root growth, seed yield and water-nitrogen use efficiency of soybean under ridge-furrow plastic mulching

The ridge-furrow plastic mulching system has been reported to increase crop yield and water-nitrogen use efficiency in semi-arid and semi-humid regions, but the effects of plant density, nitrogen (N) rate and supplemental irrigation on chlorophyll, photosynthesis, root growth and seed yield of soybean under ridge-furrow plastic mulching are poorly understood. A two-year field experiment (2019 and 2020) was conducted on soybean in a split-split plot design, with two plant densities (D16: 160,000 plants ha−1; D32: 320,000 plants ha−1) as main plots, two N rates (N30: 30 kg N ha−1; N60: 60 kg N ha−1) as sub plots, and two irrigation levels (W0: rainfed; W30: supplemental irrigation of 30 mm at the flowering-podding stage) as sub-sub plots. The results showed that plant density, N rate and supplemental irrigation had significant effects on leaf chlorophyll content, chlorophyll fluorescence parameters, photosynthesis, aboveground/root growth and seed yield. Increasing plant density enhanced leaf area index (LAI) by 31.9% and aboveground biomass (AGB) by 59.4%, seed yield by 27.4%, water use efficiency of seed yield (WUEs) by 27.9% and nitrogen partial factor productivity (NPFP) by 26.6%. Increasing N rate increased leaf chlorophyll content by 11.9% and photosynthetic rate by 5.5%, thereby increasing LAI by 11.5%, AGB by 10.3%, seed yield by 8.9% and WUEs by 8.2%. Supplemental irrigation increased chlorophyll content by 7.6% and photosynthetic rate by 3.7%, thereby increasing LAI by 4.7%, AGB by 11.0%, seed yield by 4.7% and NPFP by 4.0%, while supplemental irrigation only increased WUEs under D32. Compared with D16N30W0, D32N60W30 enhanced seed yield by 44.0% and WUEs by 37.0%, but reduced NPFP by 28.8%. Compared with D32N30W30, D32N60W30 increased both seed yield and WUEs by 8.5%, but it reduced NPFP by 45.5% with 100% higher N input. LAI, AGB, WUEs, NPFP and some leaf physiological (EXC, Chla/Chlb, EWTleaf and Ls) and root (TRLD, TRVD and TRSD) indexes were closely related to soybean yield. This study provides a better understanding how plant density, N rate and supplemental irrigation improved soybean productivity and water-nitrogen use efficiency by regulating chlorophyll, photosynthesis and aboveground/root growth.