Spatial and temporal variability of N<sub>2</sub>O emissions in a subtropical forest catchment in China

Abstract

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.

The importance of denitrification for N2O emissions from an N-saturated forest in SW China: results from in situ 15N labeling experiments

“…4b) was considerably smaller than annual average values reported earlier for the sites (Zhu et al 2013a). Similar to NO 3 -in pore water, KCl-extractable NO 3 -in O/A horizons increased sharply upon the addition of 1 g N m -2 as KNO 3 .…”

“…4a) were slightly lower than average values typically observed in summer at the two sites (Zhu et al 2013a). The addition of KNO 3 caused an instantaneous increase in pore water NO 3 -concentrations, followed by a rapid decrease at both sites and N addition rates.…”

“…In a previous study, we observed high N 2 O emission rates in summer on the well drained hill slope (HS) of TSP with WFPS and temperature being the main drivers (Zhu et al 2013a). The studies at TSP indicate that denitrification could indeed be a significant sink for N deposited in the catchment, resulting in substantial N 2 O emissions that have to be taken into account in regional N budgets.…”

“…Each site included triplicate experimental plots. Major soil characteristics of the sites are summarized in Table 1 and detailed information can be found in Zhu et al (2013a).…”

“…Subtropical forests in South China receive increasing amounts of reactive nitrogen (N r ) by atmogenic deposition, including nitrogen oxides (NO x ) and ammonia (NH x ) (Liu et al 2011), which could result in N-saturation (Chen and Mulder 2007a) and may stimulate N 2 O emissions. Recently, high N 2 O emissions have been reported from acidic subtropical forests in South China (4.4 kg N ha -1 year -1 , Fang et al 2009;4.1-5.0 kg N ha -1 year -1 , Zhu et al 2013a), raising questions about the mechanisms involved in N 2 O production in these soils. N 2 O in soil is predominately produced via ammonium (NH 4 ? )…”