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 (similar to 10 years), shrubland (similar to 29 years), secondary forest (similar to 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(-2)) 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(-2) yr(-1). Soil N stock recovered to the primary forest level in about 67 years following agricultural abandonment. NO3 (-) concentrations increased while NH4 (+) concentrations decreased with years following agricultural abandonment. High NO3 (-) concentrations (66.5-120.9 mg N kg(-1)) may imply N saturation in the shrubland, secondary forest and primary forest. Rates of net N mineralization and nitrification increased with succession. delta N-15 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.