Soil aggregates, carbon and nitrogen content, and water retention across land uses in the Reshui River Catchment

Accumulation and stability of soil organic carbon (SOC) and total nitrogen (TN) as well as soil water retention (SWR) are critical for soil sustainability, especially in fragile ecosystems such as dry–warm valleys. However, land-use effects on soil stability, SOC and TN content of soil aggregates, and SWR properties, i.e., field capacity (FC), permanent wilting point (PWP), and available water capacity (AWC) across valley slopes and bottoms in the Reshui River Catchment remain largely unclear. Soil samples within different soil depths from valley slopes (grassland, shrubland, forestland) and bottoms (bare land, shrubland, cropland: maize-fallow and maize-vegetable fields) were collected in October 2019 in this region. Results showed that valley bottoms exhibited lower clay content, aggregate stability, SOC, TN, and SWR compared to valley slopes. On slopes, grassland outperformed shrubland and forestland in terms of stabilizing soil structure and holding soil water in the upper soil layer, especially at 0–30 cm depth, soils in grassland had highest SOC and TN concentrations within different aggregate fractions. At valley bottoms, cropland increased SOC and TN contents due to fertilizer input, and maize-fallow cultivated soils had higher proportion of > 2 mm aggregates, aggregates stability, FC and AWC than shrubland and maize-vegetable cultivated soils. Grassland on the valley slope and maize-fallow cropland at the valley bottom might be the optimum choices for stabilizing aggregates, boosting soil carbon and nitrogen sequestration, and holding water. This study provides a theoretical basis for the ecological restoration of southwest dry and warm valley.