Effects of Long-Term Fertilization Regimes on Crop Yield Stability and Grain Quality in Maize and Winter Wheat Rotation of Northern China

The long-term effects of organic and inorganic fertilizer application on crop yield stability and grain quality were investigated in a maize–winter wheat rotation system in Tianjin, northern China, based on a continuous field experiment initiated in 1979. Forty-five years of data (1979–2023) were analyzed across six fertilizer treatments: an unfertilized control (CK); nitrogen only (N); nitrogen + phosphorus (NP); nitrogen + phosphorus + potassium (NPK); farmyard manure only (M); and nitrogen combined with high-rate manure (NM). The results indicated that the NM treatment yielded the highest crop productivity for both maize and winter wheat, with grain yields increasing by 81.6% and 162.6%, respectively, relative to the N treatment. Manure application significantly improved yield stability: the coefficient of variation (CV) of the winter wheat grain yield was the lowest under the M treatment, whereas maize grain yield exhibited the highest stability under the NM treatment. Grain quality analyses revealed that the N treatment significantly increased the wet gluten and protein content in winter wheat grain by 40.43% and 13.6%, respectively, relative to CK; the sedimentation value followed a similar trend. However, starch content remained statistically unchanged across all treatments. Collectively, the long-term combined application of nitrogen fertilizer and manure can steadily increase crop yield, mitigate inter-annual yield variability, and have no adverse effects on grain quality. These findings indicate that integrated N + manure fertilization is a more robust and sustainable alternative to sole chemical or sole organic fertilization for achieving high, stable yields and maintaining grain quality in intensive cereal production systems.