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

[摘要] Subtropical forests in southern China have received chronically large amountsof atmogenic nitrogen (N), causing N saturation. Recent studies suggest thata significant proportion of the N input is returned to the atmosphere, inpart as nitrous oxide (N₂O). We measured N₂O emission fluxes byclosed chamber technique throughout two years in a Masson pine-dominatedheadwater catchment with acrisols (pH ~ 4) at Tieshanping(Chongqing, SW China) and assessed the spatial and temporal variability intwo 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 μgN₂O-N m⁻² h⁻¹ were recorded on the HS shortly after rainstorms during monsoonal summer, whereas emission fluxes during the drywinter season were generally low. Overall, N₂O emission was lower inGDZ than on HS, rendering the mesic HS the dominant source of N₂O inthis landscape. Temporal variability of N₂O emissions on HS was largelyexplained by soil temperature (ST) and moisture, pointing at denitrification as amajor process for N removal and N₂O production. The concentration ofnitrate (NO₃⁻) in pore water on HS was high even in the rainyseason, 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. Theextrapolated 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 fromsubtropical forests so far. Annual N₂O-N emissions amounted to 8–10%of the annual atmogenic N deposition, suggesting that forests on acid soilsin southern China are an important, hitherto overlooked component of theanthropogenic N₂O budget.

[学科分类] 地球化学与岩石