Spatial and temporal variability of N 2O emissions in a subtropical forest catchment in China
Subtropical forests in southern China have received chronically large amounts of atmogenic nitrogen (N), causing N saturation. Recent studies suggest that a significant proportion of the N input is returned to the atmosphere, in part as nitrous oxide (N 2O). We measured N 2O emission fluxes by closed chamber technique throughout two years in a Masson pine-dominated headwater catchment with acrisols (pH ~ 4) at Tieshanping (Chongqing, SW China) and assessed the spatial and temporal variability in two landscape elements typical for this region: a mesic forested hillslope (HS) and a hydrologically connected, terraced groundwater discharge zone (GDZ) in the valley bottom. High emission rates of up to 1800 μg N 2O-N m −2 h −1 were recorded on the HS shortly after rain storms during monsoonal summer, whereas emission fluxes during the dry winter season were generally low. Overall, N 2O emission was lower in GDZ than on HS, rendering the mesic HS the dominant source of N 2O in this landscape. Temporal variability of N 2O emissions on HS was largely explained by soil temperature (ST) and moisture, pointing at denitrification as a major process for N removal and N 2O production. The concentration of nitrate (NO 3−) in pore water on HS was high even in the rainy season, apparently never limiting denitrification and N 2O production. The concentration of NO 3− decreased along the terraced GDZ, indicating efficient N removal, but with moderate N 2O-N loss. The extrapolated annual N 2O fluxes from soils on HS (0.54 and 0.43 g N 2O-N m −2 yr −1 for a year with a wet and a dry summer, respectively) are among the highest N 2O fluxes reported from subtropical forests so far. Annual N 2O-N emissions amounted to 8–10% of the annual atmogenic N deposition, suggesting that forests on acid soils in southern China are an important, hitherto overlooked component of the anthropogenic N 2O budget.