Dynamics of soil nitrogen availability during post-agricultural succession in a karst region, southwest China

Soil nitrogen (N) availability is crucial in governing the dynamics of ecosystem structure and function. However, the variation pattern of soil N availability during post-agricultural succession has been poorly understood for karst regions. Here, soil N availability variables, including concentrations of soil total N (TN), dissolved inorganic N (DIN = ammonium (NH4+) + nitrate (NO3−)), dissolved organic N (DON), rates of net N mineralization and nitrification at different successional stages, i.e. cropland, grassland, shrubland and secondary forest were investigated at a regional scale, in order to get a general pattern of soil N availability dynamics following agricultural abandonment in the karst region of southwest China. Concentrations of soil TN, soil NO3−, DIN, net N mineralization and nitrification rates increased from grassland to forest. Soil NO3− concentration in secondary forest (37.71 ± 5.01 mg N kg− 1) reached the level in cropland (31.01 ± 2.93 mg N kg− 1). Soil NO3−:NH4+ ratio in grassland was not significantly different from 1, but increased directly and reached 13.02 ± 1.16 in secondary forest, equivalent to the level in cropland (15.15 ± 1.98). The dynamics of soil DIN:DON ratio followed the same pattern as soil NO3−:NH4+ ratio. Our study suggests that soil N availability increases and soil N cycling becomes progressively open following agricultural abandonment in the karst region, southwest China.