Effects of forest and grass restoration on soil aggregate stability, and organic carbon and nitrogen characteristics in an eroded area of the Reshui River

Land use affects the stability of soil aggregates and the distribution of carbon and nitrogen, thereby changing the process of soil carbon and nitrogen cycling. In this study, soil aggregates, carbon, nitrogen, and other physical and chemical properties in 0 − 100 cm soil profiles of grassland, shrubland, and forestland were systematically studied in a small watershed of the Reshui River under three different forest and grass restoration measures. This was done to explore suitable forest and grass restoration measures. The results showed that the distribution, stability, and carbon and nitrogen characteristics of soil aggregates were significantly affected by different forest and grass restoration measures. In the 0 − 10 cm, 10 − 30 cm, 30 − 60 cm soil layer, the aggregate content (> 2 mm) in the grassland was significantly higher than that in the shrubland and forestland (P < 0.05). In the 60 − 100 cm soil layer, the performance followed the trend of shrubland > forestland > grassland (P < 0.05). The distribution trends of soil aggregate mean weight diameter (MWD) and geometric mean diameter (GMD) were similar to the distribution trend of the ratio of > 0.25 mm aggregate (R_0.25). In the 0 − 10 cm and 10 − 30 cm soil layers, the total nitrogen and organic carbon contents of the grassland were significantly higher than those of the shrubland and forestland (P < 0.05). However, with the deepening of the soil profile, the distribution of carbon and nitrogen contents in the deep soil layer of grassland decreased. In the 30 − 60 cm and 60 − 100 cm soil layers, the distribution trend of total nitrogen and organic carbon contents of the soil was in the order of shrubland > forestland > grassland (P < 0.05). In addition, the carbon and nitrogen stoichiometric ratio of the grassland was always at a low level among the three land uses. Soil aggregate stability, and carbon and nitrogen contents were closely related to soil pH, soil particle composition, and other soil properties. The MWD was significantly and positively correlated with pH (P < 0.01), GMD was significantly and positively correlated with pH and silt contents (P < 0.05), and the correlation between R_0.25 and soil particle composition was extremely significant (P < 0.01). Additionally, the correlation between the total nitrogen and organic carbon contents was significant (P < 0.05). Soil total nitrogen and organic carbon contents were both significantly and negatively correlated with pH and silt content (P < 0.01), and positively correlated with sand content (P < 0.01). The results of this study showed that grassland had a better role in stabilizing soil structure, and stabilizing carbon and nitrogen in the soil surface. However, in the deeper soil layer, the stability of aggregates in shrubland and forestland was better than that in the grassland, and the contents of carbon and nitrogen were also higher. This study provides data to support ecological restoration in dry-hot valley areas.