Divergent responses of soil aggregate stability to long-term post-agricultural natural restoration across a climatic gradient

Natural vegetation restoration represents an effective approach for promoting soil organic carbon (SOC) accumulation and improving aggregate stability. However, systematic multi-site studies quantifying how long-term natural restoration modulates SOC accumulation and aggregate stability across broad climatic gradients remain scarce. To address this deficiency, the objective of this study was to investigate effects of 27-year post-agricultural natural restoration on aggregate stability, SOC in bulk soil and four size aggregates, and bulk soil organic matter (SOM) molecular composition across three representative zonal soils in China. Soil aggregates were fractionated into four size classes (> 2 mm, 0.25–2 mm, 0.053–0.25 mm, and < 0.053 mm) via wet sieving, and aggregate stability was evaluated using mean weight diameter (MWD), geometric mean diameter (GMD), and the percentage of water-stable aggregates (WSA). We found that compared with cropping, natural restoration significantly enhanced bulk SOC, total nitrogen (TN), MWD, GMD and WSA at Gongzhuling by 70%, 62%, 119%, 173%, and 31%, at Qiyang by 74%, 38%, 9%, 35%, and 10%, respectively, but not at Zhengzhou, which was mainly attributed to soil texture. Our results demonstrate that the positive effects of natural restoration on SOC accumulation and aggregate stability are more pronounced in fine-textured soils, such as loamy clay at Gongzhuling and Qiyang, relative to coarse-textured soils, such as sandy loam at Zhengzhou. Consequently, natural restoration is a viable strategy for ameliorating structural degradation in loamy clay soils.