Effect of organic fertilizer substitution on soil fertility and yield in a continuous maize cropping system: evidence from an eight-year long-term field experiment

Introduction Long-term fertilization regimes strongly influence soil fertility and carbon–nutrient cycling in cropland ecosystems. However, in calcareous soils of arid irrigated regions in north-western China, evidence remains limited on how different fertilization practices affect canopy function and soil CO 2 exchange during maize grain filling. Methods Based on an eight-year field experiment at the National Agricultural Science Baiyin Observation and Experimental Station, this study evaluated the effects of different fertilization regimes on soil fertility, canopy function, soil net CO 2 exchange rate (NCER), and maize yield in a continuous maize cropping system. Four treatments with equivalent nutrient inputs were established: no fertilization (CK), conventional chemical fertilization (NP), one-third substitution of chemical fertilizer with commercial organic fertilizer (1/3M), and sole application of commercial organic fertilizer (M). Leaf photosynthetic traits, canopy structure, soil water content, and soil NCER were measured at the early, middle, and late grain-filling stages, while soil physicochemical properties, yield components, and plant nutrient contents were determined after harvest. Results Fertilization significantly altered the nutrient pool structure of the Sierozem. Compared with CK, NP markedly increased soil available phosphorus and total phosphorus, whereas M significantly increased soil organic matter, total nitrogen, and available potassium, but also raised soil electrical conductivity. By contrast, 1/3M achieved a more coordinated improvement in soil nutrient status while maintaining stable soil pH. Although canopy function gradually declined with grain-filling progression, NP and 1/3M maintained significantly higher photosynthetic capacity than CK during the middle and late stages. Soil NCER showed clear stage-dependent variation, peaking at the middle grain-filling stage, and was significantly enhanced by fertilization, with 1/3M increasing NCER by 107.3% at the middle stage and by 145.6% at the late stage relative to CK. Maize yield was highest under 1/3M, increasing by 143.0% relative to CK. Discussion Overall, canopy function during grain filling was closely coordinated with soil NCER and yield formation, and one-third substitution of chemical fertilizer with organic fertilizer was the most effective strategy for improving soil fertility, sustaining canopy function, stabilizing carbon processes, and increasing yield.