Soil carbon and nitrogen accumulation during long-term natural vegetation restoration following agricultural abandonment in Qingling Mountains

Agricultural land abandonment has been proposed as a strategy for reducing greenhouse gas emissions, increasing soil carbon (C) sequestration, and enhancing soil nitrogen (N) levels in degraded ecosystems. However, the dynamic patterns of soil C and N pools and fractions following long-term agricultural abandonment are not well understood. In this study, we investigated the dynamics of soil organic C (SOC), recalcitrant organic C (RC), total N (TN), and recalcitrant organic N (RN) in 0-40 cm soils along a natural vegetation restoration gradient (i.e., croplands -> grasslands -> shrublands -> forests) following agricultural abandonment in China's Qinling Mountains. Our results showed that the concentrations of SOC, RC, TN, and RN increased significantly along with vegetation restoration, but the SOC and TN stocks only increased in the 0-10 and 0-20 cm layers, respectively. The highest growth rates of new SOC (28.96 g m(-2) yr(-1)) and new RC (9.87 g m(-2) yr(-1)) were observed in shrublands, followed by the grasslands (new SOC, 14.80 g m(-2) yr(-1); new RC, 5.02 g m(-2) yr(-1)) and forests (new SOC, 9.06 g m(-2) yr(-1); new RC, 3.74 g m(-2) yr(-1)). The delta N-15 values of litter, fine root, and soil decreased significantly with stand age, indicating that the N cycling in abandoned ecosystems gradually became less open than that of croplands during vegetation restoration processes. The ratios of RN/TN decreased significantly with stand age across all soil layers, while the ratios of RC/SOC at 0-40 cm soils did not change with stand age and the differences among croplands, grasslands, shrublands, and forests were not significant. Our study reveals that soil C and N stocks only accumulate in topsoils during similar to 110 years of natural vegetation restoration and the recalcitrancy indices of soil C and N displayed different dynamic patterns, and also indicates that agricultural abandonment is an effective approach to increasing soil C and N stocks and mitigating the negative effects of agriculture on soil C and N pools.