Soil carbon–nitrogen coupled accumulation following the natural vegetation restoration of abandoned farmlands in a karst rocky desertification region

Abstract

Vegetation restoration on abandoned farmlands is one of the typical methods for accumulating soil organic carbon (OC) and total nitrogen (TN). However, the coupling of soil OC and N accumulation following vegetation restoration remain poorly known in karst rocky desertification regions in Southwest China. Four natural vegetation restoration lands, namely, grassland, shrubland, secondary forest I, and secondary forest II, were chosen with farmland as a reference. Soil samples were collected to determine the soil aggregates, soil OC and TN accumulation, soil inorganic N content, and soil N transformation rates. The amount of different soil aggregates fractions and stability of soil aggregates, levels of soil OC and TN in whole soil and aggregates, levels of soil inorganic N, and soil N transformation rates were considerably increased by vegetation restoration following farmland abandonment. The increases in total soil OC and TN stocks mainly contributed to the increase of OC and TN stocks in large aggregates. The soil C:N ratios and amount of exchangeable calcium were the important factors associated with soil OC and TN accumulation. Soil net N mineralization and nitrification rates were similar to the patterns of soil OC or TN concentrations in grassland to secondary forest. Soil substrate controlled soil N transformation rates along vegetation restoration. Our results suggested that abandoning farmlands for natural vegetation restoration can improve the soil structure, facilitate the coupled accumulation of soil C and N, and enhance soil N transformation rates and N availability, but the coupling accumulation of soil C and N may be limited by soil N in the early stages of vegetation restoration.