Relative importance between nitrification and denitrification to N 2 O from a global perspective.

Nitrous oxide (N 2 O) is a potent greenhouse gas, and its mitigation is a pressing task in the coming decade. However, it remains unclear which specific process between concurrent nitrification and denitrification dominates worldwide N 2 O emission. We snagged an opportunity to ascertain whence the N 2 O came and which were the controlling factors on the basis of 1315 soil N 2 O observations from 74 peer-reviewed articles. The average N 2 O emission derived from nitrification (N 2 O n ) was higher than that from denitrification (N 2 O d ) worldwide. The ratios of nitrification-derived N 2 O to denitrification-derived N 2 O, hereof N 2 O n :N 2 O d , exhibited large variations across terrestrial ecosystems. Although soil carbon and nitrogen content, pH, moisture, and clay content accounted for a part of the geographical variations in the N 2 O n :N 2 O d ratio, ammonia-oxidizing microorganisms (AOM):denitrifier ratio was the pivotal driver for the N 2 O n :N 2 O d ratios, since the AOM:denitrfier ratio accounted for 53.7% of geographical variations in N 2 O n :N 2 O d ratios. Compared with natural ecosystems, soil pH exerted a more remarkable role to dictate the N 2 O n :N 2 O d ratio in croplands. This study emphasizes the vital role of functional soil microorganisms in geographical variations of N 2 O n :N 2 O d ratio and lays the foundation for the incorporation of soil AOM:denitrfier ratio into models to better predict N 2 O n :N 2 O d ratio. Identifying soil N 2 O derivation will provide a global potential benchmark for N 2 O mitigation by manipulating the nitrification or denitrification.