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

Abstract

Long-term elevated atmogenic deposition (~5 g m−2 year−1) of reactive nitrogen (N) causes N saturation in forests of subtropical China which may lead to high nitrous oxide (N2O) emissions. Recently, we found high N2O emission rates (up to 1,730 μg N2O–N m−2 h−1) during summer on well-drained acidic acrisols (pH = 4.0) along a hill slope in the forested Tieshanping catchment, Chongqing, southwest China. Here, we present results from an in situ 15N–NO3 − labeling experiment to assess the contribution of nitrification and denitrification to N2O emissions in these soils. Two loads of 99 at.% K15NO3 (equivalent to 0.2 and 1.0 g N m−2) were applied as a single dose to replicated plots at two positions along the hill slope (at top and bottom, respectively) during monsoonal summer. During a 6-day period after label application, we found that 71–100 % of the emitted N2O was derived from the labeled NO3 − pool irrespective of slope position. Based on this, we assume that denitrification is the dominant process of N2O formation in these forest soils. Within 6 days after label addition, the fraction of the added 15N–NO3 − emitted as 15N–N2O was highest at the low-N addition plots (0.2 g N m−2), amounting to 1.3 % at the top position of the hill slope and to 3.2 % at the bottom position, respectively. Our data illustrate the large potential of acid forest soils in subtropical China to form N2O from excess NO3 − most likely through denitrification.