Spatial and temporal variability of N₂O emissions in a subtropical forest catchment in China

Abstract. 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 (N₂O). We measured N₂O 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 N₂O-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, N₂O emission was lower in GDZ than on HS, rendering the mesic HS the dominant source of N₂O in this landscape. Temporal variability of N₂O emissions on HS was largely explained by soil temperature (ST) and moisture, pointing at denitrification as a major process for N removal and N₂O production. The concentration of nitrate (NO₃⁻) in pore water on HS was high even in the rainy season, apparently never limiting denitrification and N₂O production. The concentration of NO₃⁻ decreased along the terraced GDZ, indicating efficient N removal, but with moderate N₂O-N loss. The extrapolated annual N₂O fluxes from soils on HS (0.54 and 0.43 g N₂O-N m⁻² yr⁻¹ for a year with a wet and a dry summer, respectively) are among the highest N₂O fluxes reported from subtropical forests so far. Annual N₂O-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 N₂O budget.