• exotic species;
  • N mineralization;
  • N leaching;
  • native species;
  • nitrification;
  • reforestation;
  • southern China

Nitrogen cycling is a critical component in plantations, yet the spatial and temporal variations of N transformations in different stages of reforestation are often poorly known. Here we report the seasonal variation of soil in situ net N mineralization, net nitrification and N leaching in five young plantations (two monocultures of exotic species: Eucalyptus urophylla and Acacia crassicarpa; a native species monoculture; a 10-species mixture and a 30-species mixture) and a shrubland (without experimental planting) in subtropical southern China. Our results show that net N mineralization and nitrification rates in the E. urophylla monoculture (13.5 and 9.98 kg N ha−1 yr−1, respectively) were the lowest among six planting treatments, less than 1/3 of those in the 10-species and 30-species mixtures. Two exotic fast-growing monocultures had 10–60% lower soil extractable nitrate concentrations than the native plantations and shrubland and had the lowest nitrogen leaching losses. The leguminous A. crassicarpa monoculture did not have higher soil N availability in comparison with non-leguminous species. Both N mineralization and nitrification varied seasonally; soil moisture seemed to be important in controlling these temporal variations. This study highlights that in the early stages of reforestaion, a better understanding of plant species effects on soil N cycling would be beneficial to forest management decisions and could provide a critical foundation for advancing restoration practices.