Quantification of microbially induced soil N2O emissions by an inhibitory cocktail in mountain forest ecosystems

Simultaneously quantifying soil N2O emissions contributed by different microbial groups and revealing the underlying mechanisms have long been challenging but fundamental to understanding terrestrial nitrogen cycling. Here, a novel inhibitory cocktail approach based on the use octyne, acetylene, antibiotics and different combinations was developed to explore different microbially sourced N2O emissions in mountain forest ecosystems. Our results indicated that the nitrification process was the pathway leading to major N2O emissions, with ammonia-oxidizing bacteria (AOB) playing a greater role than ammonia-oxidizing archaea (AOA) in soils with pH > 4.5, while AOA contributed more only in extremely acidic soils. Soil pH was identified as the key factor that regulated AOA and AOB abundances and their relative contributions. However, when the soil pH increased above 5.5, it was not the AOB abundance but the higher intrinsic N2O emission capability (INP) for AOB that could explain its dominant role. Moreover, there was an opposite pattern in the relationship between pH- intrinsic N2O emission capability from AOA and AOB, indicating its potential for assessing microbially sourced N2O emissions. Notably, we found, for the first time, that the denitrification process played nonnegligible roles in the mountain forest ecosystem, and during this process, denitrifying fungi were more powerful than denitrifying bacteria. This difference may be attributed to distinct enzymatic activities and metabolic pathways involved in N2O generation and consumption between the denitrifying fungi and bacteria. Overall, this study sheds new light on the mechanisms of microbially mediated soil N2O emissions in mountain forest ecosystems.