A systematic assessment in the 4.2 ha. forested TieShanPing catchment (Chongqing, PR China) lends support to our earlier hypothesis that nitrogen-saturated, sub-tropical forests in south China are regional hotspots for the emission of N2O, a potent GHG. TieShanPing receives about 40 kg ha-1 yr-1 of atmogenic reactive nitrogen, predominantly as ammonium derived from agriculture. Denitrification is the major sink removing about 80% of the input of reactive nitrogen. In wet years as much as one quarter of t his may be emitted as N2O. Recent research suggests that soil acidity, which is particularly high in many of the forest soils in south China, may be an important factor stimulating high N2O emission. If our findings in the TieShanPing catchment are repres entative for forest soils in southern China in general, these systems make an importantcontribution to China's national N2O budget. In addition, it would suggest that agriculture, through its contribution to atmogenic ammonia, which is subsequently deposi ted in forests, contributes far more to the national N2O budget than estimated from direct N2O emissions in croplands alone. The proposed research project has a two-step approach, where we1.assess if south China is a regional hot spot for the emission o f N2O due to long-term high deposition of reactive nitrogen. This will be done through synoptic measurements of atmogenic deposition and runoff of nitrogen as well as measurements of N2O emission fluxes from five selected small, nitrogen-saturated catchme nts characterized by hot-humid summers. Results will be compared with nitrogen depositions and N2O emissions from two catchments in northern China with drier summers and more neutral soils. 2.characterize the underlying microbial processes by state-of-t he-art molecular methods to understand how N-removal and N2O emissions are regulated by soil physical and chemical factors in forest soils in south China.