Advance in soil dinitrogen emission

The amount of applied nitrogen fertilizer has increased dramatically since the invention of the industrial ammonia synthesis in the early 20th century. In some countries or regions, the amount of nitrogen fertilizer input has exceeded crop nitrogen demand. This has led to a large amount of nitrogen losses to the environment, causing environmental pollution such as ammonia volatilization, nitrous oxide emission and groundwater contamination. Soil microbes can transform active nitrogen into inert dinitrogen and consequently remove superfluous nitrogen from soil via denitrification and anammox. Direct and precise measurement of soil denitrification has been a continuous challenge due to high background concentration of atmospheric dinitrogen, which has hindered progress in research on soil dinitrogen emissions. This paper reviewed the main pathways of soil dinitrogen emissiondenitrification, dissimilatory nitrate reduction to ammonium (DNRA) and co-denitrification and their contributions to soil dinitrogen emission. It also covered the methods of soil dinitrogen flux determination (acetylene inhibition technique, ¹⁵N tracing method, N₂/Ar membrane-inlet mass spectrometry, helium environment method and natural abundance ¹⁵N₂O isotopic method) and their advantages, disadvantages. The key factors regulating soil dinitrogen emission (oxygen, dissolved organic carbon, nitrate, microbial community structure and functional gene expression) and the related mechanisms were also summarized. In conclusion, it was essential to develop new methods for in situ dinitrogen flux determination in undisturbed soils. More studies were needed to quantify soil dinitrogen flux in typical ecosystems (such as dryland, farmland, forest, grassland and wetland), clarify microbial mechanism involved, and simulate and predict the responses of soil dinitrogen emission to global change.