Soil organic carbon enhancement via small macroaggregate stabilization under long-term nitrogen and short-term organic amendments

Nitrogen fertilizer is essential for maintaining crop yields. However, excessive application degrades soil structure, accelerates soil nutrient loss, and threatens sustainable agriculture. Organic amendments can alleviate in alleviating soil degradation, but the effects of different organic amendments on aggregate stability and soil organic carbon stock (SOCs) under long-term nitrogen fertilization remain poorly understood. This study was conducted during the winter wheat season within a winter wheat–summer maize rotation system in the North China Plain. In this season, irrigation is frequent, agricultural inputs are intensive, and carbon loss is considerable. Enhancing SOCs in this season is therefore crucial for maintaining soil fertility and productivity. A two-year experiment was conducted under long-term nitrogen fertilization using a split-plot design. The main plots consisted of two nitrogen levels: N0 (0 kg ha −1 ) and N240 (240 kg ha −1 ). Four types of OA as the subplots: straw return (S), chicken manure (M), mushroom residue (MR), and biochar (B). Results showed that nitrogen fertilizer and organic amendments significantly increased SOCs, with N240-B showing the highest SOCs, 21.61 % higher than that of the S ( P<0.05 ). In > 5 mm aggregates, the SOCs of N0-B were the highest, 100.59 % and 22.27 % greater than those of N0-S and N240-B, respectively. In the 0.25–0.5 mm aggregates, the SOC of N240-S was significantly higher than other treatments, 192.57 % and 44.57 % higher than N240-B and N0-S, respectively. Meanwhile, nitrogen fertilizer reduced aggregate stability, whereas organic amendments increased the proportion of macroaggregates and the stability of aggregates. The MWD of N0-B was the highest, 22.79 % greater than that of N0-S. Path analysis and random forest modeling indicated that SOCs in the 0.25–0.5 mm fraction contributed significantly to SOCs. These findings demonstrate that organic amendments—particularly biochar—can effectively improve soil structure and enhance SOCs in intensively managed wheat systems under long-term nitrogen input. • Organic amendments increased SOC in >2 mm and <2 mm aggregates under N0 and N240. • Organic amendments improved soil aggregate stability. • SOCs in 0.25–0.5 mm aggregates contributed most to total SOC.