Increasing SOC sequestration and closing N cycle during post-agricultural restoration in karst region, Southwest China

Purpose Post-agricultural restoration affects soil organic carbon (SOC) sequestration and ecosystem nitrogen (N) cycle. However, the control mechanism of SOC sequestration and alteration of ecosystem N status following post-agricultural restoration are not well understood in karst regions. Methods Croplands, abandoned croplands, and native vegetation forests were selected to represent three stages following post-agricultural restoration using a space for time substitution approach in a karst critical zone in Guizhou province, Southwest China. The variations of soil aggregate associated SOC and relationships between soil Ca and SOC were analyzed to identify SOC sequestration potential. Foliar δ15N composition and soil to plant 15N enrichment factor (EF = δ15Nlitter − δ15Nsoil) were analyzed to determine ecosystem N status. Results Macro-aggregate proportions and their SOC concentrations significantly increased following post-agricultural restoration. Soil Ca concentrations non-linearly increased with increasing SOC concentrations of bulk soils and aggregates. Foliar δ15N values and EF values significantly decreased following post-agricultural restoration, mainly attributed to the increasing plant uptake of 15N-depleted inorganic N, which was produced from soil organic nitrogen (SON) mineralization and nitrification. During post-agricultural restoration, the increasing plant biomass and slow SON mineralization led to more inorganic N uptake and less N loss, i.e., a more closed N cycle. Conclusion Soil aggregates and Ca play important roles in promoting SOC sequestration, and ecosystem N cycles are towards closed during post-agricultural restoration in the karst ecosystem.