Farmland Abandonment Reshapes Surface Soil Organic Carbon Dynamics in the Hilly Red Soil Region of South China

Farmland abandonment is a widespread land-use transition that may reshape surface soil organic carbon (SOC), yet its effects in the hilly red-soil region of South China remain insufficiently understood. Here, 30 m-resolution CLCD land-cover data, 90 m-resolution SOC data, and environmental variables were integrated with trajectory tracking, paired-sample comparison, temporal gradient analysis, geostatistics, and geographically weighted regression (GWR) to identify abandoned farmland and assess the SOC responses. The results showed that farmland abandonment rates fluctuated between 0.7% and 6.6% during 2000–2020, with abandonment hotspots progressively shifting toward the 2015–2020 period. Approximately 87% of abandoned farmland occurred in low-slope areas (0–15°). At the regional scale, farmland abandonment did not produce a consistent enhancement of surface SOC, with a negligible mean difference between abandoned and control farmland (−0.020 g/kg). This weak regional mean response masked contrasting local changes, with 43.76% of samples showing SOC gains and 46.59% showing SOC losses, indicating that spatial heterogeneity rather than the net regional effect is central to interpreting abandonment-induced SOC responses. Abandonment duration exhibited pronounced temporal gradient effects, with the strongest SOC accumulation occurring during the 5–10-year stage, followed by gradual stabilization. The GWR results indicated that abandonment duration was negatively associated with SOC change rates, whereas the annual mean NDVI showed a positive association, reflecting the combined effects of vegetation recovery, topographic conditions, and human activity intensity. These findings support spatially differentiated carbon accounting and abandoned-farmland management in subtropical hilly agroecosystems.