Soil carbon and nitrogen accumulation following agricultural abandonment in a subtropical karst region

Vegetation restoration following agricultural abandonment is an effective method for soil organic carbon (SOC) sequestration. However, SOC dynamics during post-agriculture succession in different soil layers, and its regulation by soil nitrogen (N), are not well understood. We investigated SOC and soil N dynamics during post-agriculture succession at soil depths of 0-10, 10-20, 20-30, and 30-50 cm in a subtropical karst area in southwest China. The succession sequence included grassland (similar to 9 years), shrubland (similar to 25 years), secondary forest (similar to 52 years), and primary forest, with cropland as a reference. In the 0-50 cm soil profile, the SOC and soil N stocks will recover to the primary forest level in similar to 74 years at a rate of 112.35 g Cm-2 yr(-1) and in similar to 100 years at a rate of 12.07 g Nm(-2) yr(-1), respectively. The accumulation rate of SOC and soil N was higher in the topsoil (0-20 cm) than in the subsoil (20-50 cm). The estimated time for SOC stock to recover to the primary forest level in topsoil and subsoil was similar (72 and 76 years, respectively), whereas that for soil N stock require more time in topsoil than in subsoil (106 and 89 years, respectively). The higher rate of relative N stock change than that of SOC suggests that progressive N limitation does not occur in the long term following agricultural abandonment. Our results highlight the importance of soil depth and soil N in determining SOC change for secondary succession. Furthermore, the results suggest that, compared to that of previous studies, which considered only surface soil in the karst regions of southwest China, the recovery period of C and N in soil profile may be longer than that assumed previously.