Phosphorus (P) availability can affect nitrogen (N) dynamics in forest soil, and this effect might depend largely on the soil N status of forest ecosystems. So far, however, this view has not been well tested among forests with contrasting N status. Here, we used a 6-year experiment with additions of N and P to evaluate the effects of P availability and its interaction with N availability on soil N dynamics in one N -saturated and two N -limited tropical forests in southern C hina. Soil inorganic N concentrations and rates of N mineralization, nitrification, nitrous oxide (N2O) emission and nitrate leaching were measured. Our results showed that addition of P alone changed soil N dynamics in the N -saturated forest only; it accelerated rates of soil N transformation and decreased rates of N2O emission and nitrate leaching, but had no significant effects on N dynamics in the two N -limited forests. Furthermore, compared with the addition of N alone, addition of both N and P caused significant increases in the rates of net N mineralization and nitrification and a significant decrease in N2O emission in the two N -limited forests. Our results suggest that P availability stimulates soil N dynamics only when the ecosystem is saturated with N or there is considerable N deposition.