Optimizing nitrogen fertilizer application to improve nitrogen use efficiency and grain yield of rainfed spring maize under ridge-furrow plastic film mulching planting

Efficient nitrogen (N) fertilizer management strategies are integral components of rational farmland cropping systems. Ridge-furrow plastic film mulching (RFPM) is a widely used micro-catchment planting system in the rain-fed farming area of the Loess Plateau in China, there are still problems of excessive N fertilizer application and unreasonable application method under this system. In this study, we conducted a two-year field experiment with traditional flat planting (FP) as a control, to determine the effect of N application rate (180 kg ha-1, 240 kg ha-1, and 300 kg ha-1) on soil nitrate N, total N, grain yield and N fertilizer partial factor productivity (NFPF) of spring maize under RFPM with three ridge-furrow ratios (RF40-70 = 40 cm:70 cm; RF55-55 = 55 cm:55 cm; RF70-40 = 70 cm:40 cm). In addition, under N application rate of 240 kg ha-1, a micro-plot experiment was conducted to determine the fate of fertilizer N by applying N15-labeled urea on ridges and in furrows under RFPM, respectively. The results showed that compared with flat planting (FP), RFPM reduced the residue of NO3-- N (0-200 cm soil layer) and accumulation total N (0-100 cm soil layer) in the soil under three N application rates, especially under RF70-40. Reducing the N application rate decreased the accumulation of NO3--N and total N under each ridge-furrow ratio. Compared with FP, RFPM increased the N accumulation, grain yield, NFPF for spring maize under all three N application rates. However, the N harvest index decreased as the N application rate increased under RFPM, thereby suggesting that the targeted N production could be improved for plants by optimizing the N application rate. By balancing the grain yield and NFPF, the optimal N application rates under FP, RF40-70, RF55-55, and RF70-40 were determined as 223 kg ha-1, 204 kg ha-1, 228 kg ha-1, and 207 kg ha-1, respectively, and these results also suggest that RFPM can lower the N input threshold compared with FP. In addition, N isotope tracing was used to clarify the fate of fertilizer N under RFPM, which showed that the contribution of fertilizer N to plants increased when N fertilizer was applied in furrows, and reduced the residual proportion of fertilizer N in soil. Therefore, our findings suggest that N fertilizer should be applied in the furrows after forming the ridges and furrows under RFPM.