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.