Interaction of Nitrogen and Phosphorus on Wheat Yield, N Use Efficiency and Soil Nitrate Nitrogen Distribution in the North China Plain

To establish the optimal fertilization rate to maintain high wheat yield and low environmental risk in the North China Plain, a two-year field experiment (2016–2018) was conducted with six combinations of two N rates (N0; N1: 240 kg ha−1) and three P rates (P0; P1: 150 kg ha−1; P2: 240 kg ha−1) to investigate the effects of these regimes on wheat grain yield, N assimilatory enzyme activity (nitrate reductase, NR and glutamine synthetase, GS), N use efficiency (NUE) and soil nitrate nitrogen distribution. Results showed that N application increased grain yield and grain protein concentration (GPC) significantly, in association with increased residual soil nitrate in both growing seasons, across the three P application rates. Only P application had no effect on these parameters. Significant interactions of N and P fertilizer were found on most of the tested parameters. Highest grain yield, GPC and NUE, and the lowest residual soil nitrate were observed under the treatment of N1P1. N accumulation in plant was highly correlated with grain yield and NUE, and N assimilatory enzyme activity at flowering was highly correlated with grain yield and GPC, which contributed to achieving high yield and NUE simultaneously. Regression analysis showed that the response of grain yield, NUE and residual soil nitrate to P rates fitted quadratic model, in which the maximum values of grain yield, NUE and the lowest residual soil nitrate were obtained at the extent of P rate 140 ~ 150 kg P ha−1. Hence, we recommended fertilizer rate of N 240 kg ha−1 and P 150 kg ha−1 as the optimal nitrogen-phosphorus regime in the North China Plain, which can maintain higher crop yield while lower environmental risk.