Subtropical forest nitrous oxide emissions under nitrogen deposition and seasonal variations: new insights highlighting roles of soil nitrifiers and denitrifiers

Tropical and subtropical forests are considered sources of nitrous oxide, one of the greenhouse gases mainly produced via microbial nitrification and denitrification in soil. However, in recent decades, the emission of nitrous oxide has been influenced by increasing atmospheric nitrogen (N) deposition primarily due to frequent human activities. N deposition, particularly wet N deposition, depends heavily on precipitation. This makes predicting future change trends of nitrous oxide emissions more challenging, especially in tropical and subtropical forests in southern China due to high natural atmospheric N deposition and the distinct wet-dry seasonality. Therefore, it is necessary to compare the differences in nitrous oxide emissions not only among different levels of N deposition but also between wet and dry seasons. It is also necessary to further uncover the underlying mechanisms of soil microbes in regulating nitrous oxide emissions in the two different seasons.

To clarify the questions above, a field investigation experiment was conducted in a subtropical evergreen broadleaved forest in Dinghushan Biosphere Reserve, southern China. The forest received 4 levels of N addition treatments to simulate the increasing N deposition. Results revealed that nitrous oxide emissions were stimulated in the dry season but depressed in the wet season by N addition. By combining soil/microbial N contents and microbial nitrifying/denitrifying functional genes, researchers speculated possible mechanisms of soil functional microbes in regulating nitrous oxide emissions in the two seasons. After N addition, the relative active soil microbes (nitrifiers and denitrifiers) in the wet season facilitated autotrophic nitrification and nitrifier-denitrification, while nitrous oxide produced via these two pathways tended to be reduced because of the wetter soil environment, finally inducing the negative responses of nitrous oxide emission; the dry season soil produced nitrous oxide via autotrophic nitrification, autotrophic nitrification and denitrification, but the reduction of nitrous oxide was very weak, finally retaining nitrous oxide and leading to positive responses of nitrous oxide emission. Therefore, soil nitrifiers and denitrifiers acted as sensitive microbial factors in responding to N addition and played dominant roles in deciding the final fates of nitrous oxide.

This work was conducted by the team of Professor SHEN Weijun from South China Botanical Garden under Chinese Academy of Sciences. Relevant results were reported on the international journal Journal of Geophysical Research: Biogeosciences, with a title of “Seasonal variations in N₂O emissions in a subtropical forest with exogenous nitrogen enrichment are predominately influenced by the abundances of soil nitrifiers and denitrifiers”. The findings of the work have significant implications for predicting future change trends in the nitrous oxide emissions from subtropical/tropical forests, especially under the increasingly sharpened wet‐dry seasonality caused by altered regional precipitation patterns.

For further reading, please refer to: https://doi.org/10.1029/2019JG005477.