Rapid Soil Fertility Improvement Enhances Maize Productivity and Resilience in Eutric Regosols: Evidence from a Four-Year Field Experiment

Eutric Regosols are globally important but low-fertility soils with poor nutrient retention, limiting crop productivity and increasing environmental risks. This study evaluated whether combining a moderate reduction in synthetic nitrogen (N) fertilizer with organic manure application could rapidly improve soil fertility, sustain maize yield, enhance nitrogen use efficiency (NUE), and increase yield resilience in these soils. A four-year field experiment was conducted on a purple soil (Eutric Regosol) with five treatments: no N (CK), conventional synthetic N (CN), a 20% synthetic N reduction (OP), and the OP treatment combined with 3000 (OPM1) or 6000 (OPM2) kg ha−1 of organic fertilizer. Maize yield, yield components, NUE indices, soil properties, and net economic benefits (NEB) were measured. OP alone reduced yield by 7.57% compared to CN. OPM2 progressively increased yield, surpassing CN by 12.36% after four years, and indicated greater yield resilience during a high-rainfall year. OPM2 also significantly improved topsoil organic matter (+12.9%), total N (+46.3%), and NUE indices over time. Although initial NEB was lower for organic-amended treatments, OPM2 achieved higher economic returns than CN in the latter two years. Integrating a 20% synthetic N reduction with 6000 kg ha−1 of organic manure is an effective strategy for rapid fertility improvement in Eutric Regosols. This approach compensates for yield reductions from less synthetic N, progressively enhances yield and NUE, improves soil health, increases economic returns, and strengthens buffering capacity against high-rainfall events.