Heterotrophic nitrification is responsible for large rates of N ₂ O emission from subtropical acid forest soil in China

Abstract

Summary Subtropical acidic forest soil is an important source of global nitrous oxide (N 2 O). However, the rates of autotrophic nitrification and denitrification in such soil are less than in temperate forest soil. We hypothesized that this difference is related to different N 2 O production pathways. We carried out paired 15 N‐labelling experiments under aerobic conditions (60% water‐holding capacity) in the laboratory to compare N 2 O production pathways between subtropical and temperate forest soils to determine the differences in N 2 O production based on controlling factors. Our results showed that the contributions of NH 4 + oxidation to NO 3 − to total N 2 O production (C NH4 ) were small in all soils studied (21–30%). Contributions of denitrification to total N 2 O production (C NO3 ) in subtropical forest soil (34%) were significantly less than those in temperate forest soil (54%). However, rates of N 2 O emission from denitrification (N 2 O d ) were similar between subtropical and temperate forest soils, indicating that denitrification was probably not the predominant process causing the difference between the rates of N 2 O production in these soils. The average contribution of heterotrophic nitrification to total N 2 O production (C ON ) was significantly larger in subtropical forest soil (45%) than in temperate forest soil (15%). Soil pH and the C:N ratio were identified as key factors, with C ON negatively correlated with soil pH ( r = −0.60, P < 0.05) and positively correlated with soil C:N ratio ( r = 0.78, P < 0.01), although the significance was probably weakened when forest types were taken into consideration. The rate of N 2 O production by heterotrophic nitrification (N 2 O h ) (average, 3.0 μg N kg −1 day −1 ) was also significantly larger in subtropical forest soil than in temperate forest soil (average, 0.7 μg N kg −1 day −1 ). Therefore, based on this study, the heterotrophic N 2 O pathway seems responsible for the larger N 2 O production in subtropical acid forest soil than in temperate forest soil. This study is among the first to elucidate in detail contributions to the processes of N 2 O production and to account for its large rates of production in subtropical acidic forest soil in China, which has implications for the prediction of N 2 O production in forest soil using ecosystem modelling. Highlights Examined N 2 O production pathways in subtropical and temperate forest soils. Rate of N 2 O emission in subtropical forest soil was larger than temperate forest soil. Heterotrophic nitrification was responsible for large N 2 O production in subtropical soil. Soil pH and C:N ratio were identified as key factors affecting the N 2 O production pathway.