Effects of furrow corn straw mulch on the coupling of physical protection and chemical recalcitrance of soil organic carbon at sloping farmland

Severe soil erosion on sloping farmland accelerates aggregate breakdown and selective carbon transport, threatening sustainable agricultural development. While furrow corn straw mulch effectively mitigates soil degradation, the coupling mechanism between physical protection and chemical stability in the regulation of soil organic carbon (SOC) accumulation on hillslopes remains elusive. In this study, the SOC, dissolved organic carbon (DOC), humic substances ( HA , FA , and HM ), aggregate stability (MWD), and molecular structure (¹³C NMR) at the upper, middle, and lower hillslope positions were analysed on 3°, 6°, and 9° hillslopes of the Chinese Chernozem region after an 11-year long-term field experiment to compare them with those without furrow corn straw mulch (CK) and furrow corn straw mulch (SR). Under the CK treatment, the spatial distribution of SOC varied with slope gradient, it decreased from the upper to lower positions on the 3° and 6° hillslopes, but accumulated at the lower position on the 9° hillslope. In contrast, compared with the CK, the SR treatment significantly altered this spatial heterogeneity, increasing the SOC by 52.3–112.5% and the DOC by 36.8% at the lower positions of the 6° and 9° hillslopes. Topographical effects on carbon fractions were non-linear. DOC dynamics on the 3° gentle slope exhibited distinct patterns compared to steeper gradients, which is likely driven by shifts in slope-specific hydrological partitioning. Furthermore, solid-state ¹ ³C NMR spectroscopy demonstrated that SR altered the molecular composition of SOC, characterized by a significant accumulation of Alkyl C and a maximum 120.9% increase in humic acid at the lower position of the 6° hillslope. The concurrent enhancement in aggregate stability (MWD increased by 1.9%-58.5%) alongside these chemically recalcitrant fractions indicates a coupled physical-chemical stabilization process. These findings suggest that furrow corn straw mulch not only physically protects SOC through improved aggregation but also promotes the accumulation of more persistent organic carbon fractions, effectively counteracting carbon loss in erosion-prone landscapes.