Heterotrophic nitrification is responsible for large rates of N2O emission from subtropical acid forest soil in China

Subtropical acidic forest soil is an important source of global nitrous oxide (N2O). 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 N2O production pathways. We carried out paired N-15-labelling experiments under aerobic conditions (60% water-holding capacity) in the laboratory to compare N2O production pathways between subtropical and temperate forest soils to determine the differences in N2O production based on controlling factors. Our results showed that the contributions of NH4+ oxidation to NO3- to total N2O production (C-NH4) were small in all soils studied (21-30%). Contributions of denitrification to total N2O production (C-NO3) in subtropical forest soil (34%) were significantly less than those in temperate forest soil (54%). However, rates of N2O emission from denitrification (N2Od) were similar between subtropical and temperate forest soils, indicating that denitrification was probably not the predominant process causing the difference between the rates of N2O production in these soils. The average contribution of heterotrophic nitrification to total N2O 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 N2O production by heterotrophic nitrification (N2Oh) (average, 3.0gNkg(-1)day(-1)) was also significantly larger in subtropical forest soil than in temperate forest soil (average, 0.7gNkg(-1)day(-1)). Therefore, based on this study, the heterotrophic N2O pathway seems responsible for the larger N2O 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 N2O production and to account for its large rates of production in subtropical acidic forest soil in China, which has implications for the prediction of N2O production in forest soil using ecosystem modelling.