Functional traits of denitrification in a subtropical forest catchment in China with high atmogenic N deposition

Subtropical forests in SW China, receiving high nitrogen (N) deposition, have been reported to show high N retention. N removal by denitrification may be one important process favored by warm and wet soils during monsoonal summer when most of the atmogenic N input occurs. Due to low pH soils which are common in this region, N removal by denitrification may entail substantial N2O emissions. In this study, we explored intrinsic denitrification characteristics of soils from different landscape elements of a forested headwater catchment in SW China (Tieshanping, Chongqing). Laboratory incubations were used to measure instantaneous denitrification rates, apparent specific denitrifier growth rates, denitrification product stoichiometries (N2O/(N2O + N2)) and response to carbon-addition as a function of soil depth and landscape position. The results revealed that potential denitrification and denitrifier growth rates were highest in top soils along a hill slope, and decreased strongly with soil depth due to C-limitation. A hydrologically connected, colluvial groundwater discharge zone showed equally high instantaneous denitrification rates which were more equally distributed with depth. Denitrifying communities in the top horizons of the hill slope were less efficient in expressing N2O reductase in response to anoxia resulting in higher N2O/(N2 + N2O) product ratios than found in soils of the groundwater discharge zone, suggesting that a significant share of the deposited N can be lost as N2O from the hill slopes. Differences in denitrification traits appeared to be linked to eco-hydrological conditions in the two landscape elements and to substrate availability along the hydrological flowpath. Our study supports the notion that denitrification plays an important role for observed N removal in SW Chinese forest ecosystems and illustrates how habitat functions constrain the amount of N2O emitted during N removal.