Crop Responses to Application of Optimum Nitrogen Fertilizers on Soils of Various Fertilities Formed from Long-Term Fertilization Regimes

How crops uptake synthetic nitrogen (N) applied to soils of differing fertilities formed from long-term fertilization regimes is a noteworthy scientific question that remains unclear. This is important for understanding nitrogen use efficiency (NUE) and N losses of synthetic N input to soils of different quality. Here, we investigated crop biomass, N uptake, root growth, and NUE in response to three synthetic N rates (including no N fertilizer, No-N; optimum N, Opt.-N; original N, Ori.-N) in long-term plots (since 2006), with various background fertilities induced by long-term fertilization regimes (nil N, optimum synthetic N, conventional synthetic N, and balanced synthetic N and manure) in a cropping system of winter wheat–summer maize rotation. When Opt.-N was applied, no significant differences in crop biomass between soils. However, N uptake by the crops on nil N soils was lower than that on N-fertilized soils. Crop biomass and N uptake in the nil N soils were significantly lower than those in the N-fertilized soils under No-N and Ori.-N supply, and no significant differences were observed between the N-fertilized soils. When Opt.-N was supplied to the nil N soils, grain yield and N uptake increased by 126–191% and 138–148%, respectively, compared with No-N. This change was associated with a large increase in root mass and root length density in the soils. In optimum synthetic N soils, the crop maintained a high yield (7.0–8.2 Mg ha-1) and N uptake (136–228 kg N ha-1) when they received continuous Opt.-N rate. Crop yield and N uptake significantly decreased when N fertilizer supply was ceased, indicating a low legacy effect; however, the long-term straw return alleviated the above decline. Conventional synthetic N soils with long-term excessive N application continued to maintain a comparable high yield (6.1–7.0 Mg ha-1) and N uptake (128–206 kg N ha-1) even after N fertilizer supply was stopped, indicating a high legacy effect. This effect could be applied with less N fertilizer to reduce N losses. Balanced synthetic N and manure could provide resistance to N fertilizer reduction and maintain high yield and soil fertility even after N fertilizer supply is ceased. Our results highlight the strong resilience of soils to crop productivity when resupplying N fertilizer to long-term nil N soils and the different legacy effects from historic fertilization regimes, while emphasizing that balanced synthetic N and manure is an ideal measure for sustainable N management.