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

Summary: 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 ¹⁵N‐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 (N₂O_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 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 (N₂O_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 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. Highlights: Examined N₂O production pathways in subtropical and temperate forest soils. Rate of N₂O emission in subtropical forest soil was larger than temperate forest soil. Heterotrophic nitrification was responsible for large N₂O production in subtropical soil. Soil pH and C:N ratio were identified as key factors affecting the N₂O production pathway. [ABSTRACT FROM AUTHOR]