Rapid recuperation of soil nitrogen following agricultural abandonment in a karst area, southwest China.

Soil nitrogen (N) dynamics are crucial for ecosystem development and carbon sequestration. However, N dynamics during post-agriculture succession have not been well understood. Here soil N dynamics during post-agriculture succession were investigated in a subtropical karst area, southwest China. The succession sequence includes grassland (~10 years), shrubland (~29 years), secondary forest (~59 years) and primary forest with cropland as reference. The soil is calcareous lithosols. Soil N stock (0-15 cm) in the cropland (335.2 ± 55.1 g N m) was not significantly different from that in the grassland, but was elevated by 120 to 221 % in the later three succession stages. Across the sequence, soil N accumulated with an average rate of 12.4 g N m yr. Soil N stock recovered to the primary forest level in about 67 years following agricultural abandonment. NO concentrations increased while NH concentrations decreased with years following agricultural abandonment. High NO concentrations (66.5-120.9 mg N kg) may imply N saturation in the shrubland, secondary forest and primary forest. Rates of net N mineralization and nitrification increased with succession. δN of soil, leaves and litter was significantly elevated in the later three succession stages relative to the grassland, indicating that the N cycle became more open with succession. N release from bedrock weathering was likely a potential N source in addition to atmospheric N deposition and biological N fixation. Our study suggests that N would not be the limiting nutrient for secondary succession and ecological restoration in the karst region of southwest China. [ABSTRACT FROM AUTHOR]