Nodule Formation and Nitrogen Use Efficiency Are Important for Soybean to Adapt to Water and P Deficit Conditions

Drought stress and phosphorus (P) deficit decrease soybean P and nitrogen (N) accumulation, which limits soybean productivity. Therefore, soybean traits related to N and P uptake and/or their efficient utilization are important for soybean adaptation to P- and water-deficit conditions. We hypothesize that increasing soybean nodulation to enhance N and P uptake, and/or improving N and P use efficiency (PUE and NUE) are important for the adaptation of soybean to drought and low P conditions. To test this hypothesis, we selected four genotypes with different nodule dry weight (DW) and yield performance for a pot experiment under two water treatments [well-watered (WW) and cycle water stress (WS)] and three P levels [0 (P0, low), 60 (P60, mid), and 120 (P120, high) mg P kg−1 dry soil on top 40 cm]. Our study showed that P deficit and water stress significantly decreased soybean P and N accumulation, which limited seed yield under both WS and WW conditions. P addition increased soybean nodule dry weight (DW), thus increasing N and P uptake. Increasing nodule DW required high water use, and while there was no relationship found between nodule DW and yield under WS, a positive relationship under WW was shown. Partitioning more dry matter to seed could improve NUE and PUE. P addition did not change soybean NUE, which is important to yield determination under WS and P0 but has no effect on yield under WW. We conclude that increasing nodule formation improved soybean N and P uptake, which diminished the yield loss under WS and improved yield performance under WW. While high NUE reflects efficient utilization of N, which can improve yield under drought stress and low P availability, and does not impair the yield under WW. We propose that NUE and nodules are important traits for breeders to improve the tolerance to water- and P-deficit conditions.