Contribution and fate of maize residue-(15)N and urea-(15)N as affected by N fertilization regime

Increasing amounts of crop residues are being returned to croplands. Understanding nitrogen (N) availability in crop residues under various N fertilization regimes is important in optimizing N management. Pot experiments were conducted to investigate the contribution, fate and residual effects of urea and maize residue-N using a (15)N isotope technique. Four N regimes were tested: three basal–topdressing ratios of N applied as urea (100–0, 75–25, 50–50) and one basal application of 75% N as urea and 25% N as manure (75U+25M). An average of 31.4% wheat N uptake was derived from urea, 9.2% from maize residue, and 59.5% from the soil in the first season. During the growing stages of wheat, maize residue contributed 0.3–4.8% and 3.1–13.2% to soil mineral and microbial biomass N, respectively, and those originated from urea was 1.0–4.2% and 4.6–16.8%, respectively. Regarding the fate of urea and residue-N after the first season, 35.9% and 16.9% of urea-(15)N and residue-(15)N were recovered by wheat, 28.1% and 46.9% remained in the soil, and 36.0% and 36.2% were lost. The contribution of urea to crop N uptake and N recovery efficiency increased, while that of residue-N decreased with increasing proportion of topdressing N. Substituting 25% mineral N with manure decreased the urea-(15)N loss without negative effects on crop dry matter and N uptake. Residual urea-(15)N and maize residue-(15)N from the previous season contributed 3.9% and 3.0% to summer maize N uptake. Additionally, 29.3% of residue-(15)N remained in the soil after the second season, while only 18.6% of urea-(15)N remained. Our study suggests that fertilizer and crop residue are actively involved in soil N transformation and plant N nutrition, emphasizing the capacities of organic residues to sustainably supply nutrients. Considering the utilization of both N fertilizer and maize residue, we may suggest a 75–25 split in N fertilizer application, but more appropriate options need to be further assessed under different cropping systems.