Soil labile organic carbon (LOC) is a valuable and sensitive parameter of the changes in soil carbon (C) pools and further affects soil structural stability. However, the influences of soil-aggregate stability on LOC fractions under erosion conditions are still unclear, especially under the collapsing gully area of south China. Soils of five positions of collapsing gully erosion, including the upper catchment (UC), collapsing wall (CW), colluvial deposit (CD), scour channel (SC) and alluvial fan (AF) from Acric Ferralsols were investigated and sampled. Soil aggregate stability and LOC fractions were measured and analyzed. Soil water-stable aggregate and passive C (passive-C) contents significantly increased by 67–76% and 8.7–13.0% at the UC, CW, CD and SC positions, respectively, while soil labile C (labile-C) content was lower at these positions as compared to the AF position (p < 0.05). Moreover, the UC position’s soil C pool management index (CPMI) significantly increased by 37–40% compared to CW, CD, SC and AF soils, indicating that the soil of the UC position had a more stable C pool due to its stronger structural stability. SOC, silt, and amorphous iron oxide (Fea) contents significantly contributed to aggregate stability. We demonstrated that the depletion of soil aggregate stability could result in the decreases in soil LOC fractions, while soil properties of the OC but not the LOC pool regulated aggregate stability and thus affected soil structure across different collapsing gully erosion positions in the subtropical Acric Ferralsols region of south China. This study contributes to developing strategies to prevent soil erosion and improve global C cycle and soil quality, which could be beneficial to strengthen soil and water conservation, and improve soil fertility (e.g., SOC) and vegetation recovery, such as tea and tobacco.

The sustainability of wheat-maize rotation systems in the North China Plain is challenged by the over-reliance on chemical fertilizers, which leads to the decline of soil organic matter and structural degradation, particularly in the unique Shajiang black soil (Vertisol). While straw return is widely recommended to mitigate these issues, the synergistic mechanisms of its long-term combination with chemical fertilizers on soil nutrient stoichiometry and aggregate stability remain inadequately quantified. A long-term field experiment was conducted with the five fertilization treatments including: (1) no fertilizer or straw (CK), (2) chemical fertilizer alone (NPK), (3) straw return chemical fertilizer (NPKS), (4) straw return with 10% straw-decomposing microbial inoculant combined with chemical fertilizer (10%NPKS), and (5) straw return with 20% straw-decomposing microbial inoculant combined with chemical fertilizer (20%NPKS) in the Shajiang black soil (Vertisol) region to investigate the effects of straw return combined with chemical fertilizers on soil organic carbon (SOC), total nitrogen (TN) and total phosphorus (TP) stoichiometry, aggregate stability, and crop yield in winter wheat-summer maize rotation systems of North China Plain. Our study demonstrated that the co-application of straw with a straw-decomposing microbial inoculant is a highly effective strategy for enhancing soil health and crop productivity, with its efficacy being critically dose-dependent. Our results identified the 10%NPKS treatment as the optimal practice. It most effectively improved soil physical structure by significantly increasing the content of large macroaggregates (>0.5 mm) and key stability indices (MWD, GMD, WA), while concurrently enhancing nutrient cycling, as evidenced by elevated SOC, TN, and shifted C/P and N/P stoichiometry. Multivariate analyses confirmed strong positive correlations among these soil properties, indicating a synergistic improvement in soil quality. Crucially, these enhancements translated into significant yield gains, with a notable crop-specific response: maize yield was maximized under the 10%NPKS treatment, whereas wheat yield benefited sufficiently from NPKS treatment. A key mechanistic insight was that 20%NPKS treatment, despite leading to the highest SOC and TN, induced a relative phosphorus limitation and likely caused transient nutrient immobilization, thereby attenuating its benefits for soil structure and yield. We conclude that co-applying straw with a 10% microbial inoculant combined with chemical fertilizer represents the superior strategy, offering a sustainable pathway to synergistically improve soil structure, nutrient availability, and crop productivity, particularly in maize-dominated systems.

Long-term fertilization affects soil nutrient levels and aggregate stability, eventually altering crop yield. However, their responses to organic fertilizer application and straw returning are still unclear, particularly as the contributions of soil nutrient levels and aggregate stability on crop yields remain poorly quantified. Therefore, topsoil samples (0–20 cm) were collected from six fertilization treatments in a long-term (13-year) Shajiang black soil field experiment with no fertilization (CK), chemical fertilization (NPK), 50% NPK plus pig manure (50%NPKP), 50% NPK plus cattle manure (50%NPKC), 70% NPK plus pig manure with straw return (70%NPKPS), and 70% NPK plus cattle manure with straw return (70%NPKCS). We examined the characteristics of crop yield, soil nutrient levels, and soil aggregate stability parameters, including under different long-term fertilization treatments. The results show that long-term fertilization significantly influenced the distribution of soil nutrients and soil aggregates in Shajiang black soil. Compared to CK, organic fertilizers and straw returning significantly increased the soil organic matter (SOM), total nitrogen (TN), and total phosphorus (TP) contents but decreased soil pH, respectively, indicating the best strategies for improving soil fertility. Compared to the CK and NPK treatments, long-term organic fertilization and straw returning significantly increased the mean weight diameter (MWD) and geometric mean diameter (GMD) values and significantly decreased the fractal dimension (Dm) and mean weight-specific surface area (MWSSA) values, with the 70%NPKCS treatment showing the most pronounced effect of improving aggregate stability. A redundancy analysis revealed that SOM and TN exert significant effects on aggregate stability. Furthermore, a stepwise regression analysis showed that SOM and TN were positive factors affecting the yields of wheat and maize, while MWD and pH were negative factors affecting wheat yield, demonstrating that high crop yields are derived from soils with limited stability and high fertility. Thus, our findings indicate that the integrated application of cattle manure with straw returning was the most effective strategy to promote soil nutrient accumulation, improve aggregate stability, and enhance crop yield, albeit with the potential risk of soil acidification, which requires management in the Shajiang black soil (Vertisol) region of Northern China.