Improved soil composition promotes nutrient recovery during vegetation restoration in karst peak-cluster depressions

The relationship between the soil structure and nutrient content during vegetation restoration of karst peak-cluster depression remains uncertain. Via field investigations and laboratory experiments, the changes in soil aggregate composition and nutrient distribution characteristics and their relationship during four stages of vegetation restoration (grassland (GL), shrub grass (SG), bush forest (BF) and secondary forest (SF) stages) in a karst peak-cluster depression were evaluated. The results showed that (1) vegetation restoration enhanced the formation of > 0.25 mm aggregates, while the distribution of aggregates in each stage was dominated by > 2 mm aggregates, constituting>72.18 % of the total aggregates, while the content of the other aggregates decreased with decreasing size. (2) Vegetation restoration gradually increased the soil organic matter (SOM), total potassium (TK) and available phosphorus (AP) contents, which peaked at the SF stage; the total nitrogen (TN), total phosphorus (TP), alkali-hydrolyzable nitrogen (AN), rapidly available potassium (AK) and exchangeable calcium (ExCa2+) and magnesium ions (ExMg2+) contents peaked in the BF stage and then slightly decreased, indicating that soil nutrient storage thresholds existed in the karst area. (3) The TN, AP, and ExMg2+ contents increased with decreasing aggregate size, and the AN and AK contents first increased and then decreased with decreasing aggregate size. (4) Among the soil aggregates, the nutrient contribution rate was dominated by > 2 mm aggregates, accounting for 41.42 %∼89.01 % of the total aggregates, with a coefficient of determination ranging from 0.456 to 0.998 according to stepwise regression analysis, suggesting that soil nutrients are highly correlated with the nutrient contents in the > 2 and 2–1 mm aggregates and that the soil aggregate composition determines the aggregate nutrient contribution. Vegetation restoration improves the soil structure and nutrient retention capacity, and the aggregate composition is the decisive factor of the nutrient distribution.