Effects of long-term increased N deposition on tropical montane forest soil N2 and N2O emissions

Abstract Nitrogen (N) deposition is projected to substantially increase in the tropics over the coming decades, which is expected to lead to enhanced N saturation and gaseous N emissions from tropical forests (via NO, N 2 O, and N 2 ). However, it is unclear how N deposition in tropical forests influences both the magnitude of gaseous loss of nitrogen and its partitioning into the N 2 and N 2 O loss mechanisms. Here, for the first time, we employed the acetylene inhibition technique and the 15 N-nitrate labeling method to quantify N 2 and N 2 O emission rates for long-term experimentally N-enriched treatments in primary and secondary tropical montane forest. We found that during laboratory incubation under aerobic conditions long-term increased N addition of up to 100 kg N ha −1  yr −1  at Jianfengling forest, China, did not cause a significant increase in either N 2 O or N 2 emissions, or N 2 O/N 2 . However, under anaerobic conditions, N 2 O emissions decreased and N 2 emissions increased with increasing N addition in the secondary forest. These changes may be attributed to substantially greater N 2 O reduction to N 2 during denitrification, further supported by the decreased N 2 O/N 2 ratio with increasing N addition. No such effects were observed in the primary forest. In both forests, N addition decreased the contribution of denitrification while increasing the contribution of co-denitrification and heterotrophic nitrification to N 2 O production. Denitrification was the predominant pathway to N 2 production (98–100%) and its contribution was unaffected by N addition. Despite the changes in the contributions of denitrification to N 2 O gas emissions, we detected no change in the abundance of genes associated with denitrification. While the mechanisms for these different responses are not yet clear, our results indicate that the effects of N deposition on gaseous N loss were ecosystem-specific in tropical forests and that the microbial processes responsible for the production of N gases are sensitive to N inputs.