Comparative study on nitrogen mineralization of soil in woodland and cropland in karst and clasolite regions

In this study, the 15N tracing technique and N transformation model were used to investigate the mineralization of labile organic nitrogen (as N) (MNrec), recalcitrant organic N (MNlab) and organic N (MNorg) to NH4+ in calcareous and red soils of woodland and cropland in karst and clasolite regions, respectively. The results show that soil N mineralization rates are significantly affected by soil types and land uses. MNorgin calcareous soil (3.71 mg N/kg/d) of woodland is considerably lower than that of red soil (5.57 mg N/kg/d). There is no significant difference between MNlab(1.81 mg N/kg/d) and MNrec(1.90 mg N/kg/d) observed in calcareous soil of woodland. However, the MNlab(4.60 mg N/kg/d) is much higher than MNrec(0.96 mg N/kg/d) in red soil of woodland. After converting woodland into cropland, the MNorglevel significantly increases to 4.21 mg N/kg/d in calcareous soil, but it decreases to 2.93 mg N/kg/d in red soil. In contrast to woodland, MNlabincreases by approximately 72.5% and MNrec decreases by approximately 33.7% in calcareous soil of cropland, respectively. CaO and MgO concentrations in calcareous soils are related positively with MNlab but negatively with MNrec, suggesting that soil CaO and MgO are the important factors affecting N mineralization rates in karst regions.