Nitrogen threshold under straw returning: optimizing nitrogen application to synergize the yield and nutritional...

Straw returning has the potential to reduce nitrogen (N) input by enhancing soil fertility; however, the optimal N application rate may vary under different crop residue management practices.

Based on a long-term field experiment initiated in 2011, this study investigated the effects of two residue management methods, namely, no tillage with straw mulch (SM, full straw return) and plow tillage incorporating straw mulch to a depth of approximately 15 cm (SP, full straw return), in combination with five nitrogen application rates (0, 90, 150, 210, and 270 kg ha^-1) on maize grain yield, nutritional quality, and the partial factor productivity of nitrogen fertilizer (PFPN).

The results indicated that the 1000-kernel weight and kernel number under SP were 7.09% and 6.26% higher than those under SM, respectively, resulting in a 9.24% higher yield in SP. Furthermore, PFPN was significantly greater under SP, by 13.88% compared with SM. This difference was more evident when the N application rate was below 150 kg ha^-1. Additionally, the comprehensive nutritional quality index (Q value), which integrates crude protein, crude fat, starch, and amino acid contents via the entropy weight method to avoid limitations of single-trait analysis, reached a higher linear plateau under SP than under SM, as SP enhanced the crude fat (by 6.68%) and starch (by 1.90%) contents in the grains.

In conclusion, SP demonstrated greater potential for N fertilizer savings while achieving high grain yield and superior nutritional quality. The optimal N application rates were 113.82-129.53 kg ha^-1 for SP and 129.90-135.81 kg ha^-1 for SM, ensuring the coordinated improvement of yield, nutritional quality, and PFPN.