Soil Ammonia Oxidizing Bacteria (AOB) and Ammonia Oxidizing Archaea (AOA) Responses to Management Intensity and Season.

Nitrification is the process that converts ammonia to nitrate with N₂O released as a byproduct. In most soils, ammonia oxidation is carried out by ammonia oxidizing bacteria (AOB) and ammonia oxidizing archaea (AOA), but the relative contribution of each is poorly known. In this research we evaluated the seasonal contributions of AOA and AOB to nitrification from seven ecosystems on the same soil series. Soil samples were taken in summer and winter of 2016 along a management intensity gradient: two annual crop ecosystems (conventionally fertilized wheat and biologically fertilized wheat), one perennial crop ecosystem (poplar) and four native successional ecosystems (early succession, fertilized early succession, deciduous forest, and fertilized deciduous forest).  Our results showed that 1) overall nitrification rates in the two annual crop ecosystems were significantly higher than in perennial crop and native successional ecosystems (p<0.05), and in annual and early successional ecosystems nitrification was largely AOB derived (p<0.05). However, in poplar, deciduous forest and fertilized deciduous forest ecosystems, the contribution of AOA and AOB were similar to one another (p>0.05). 2) Nitrifier-derived N₂O production was higher in the two annual crop ecosystems than in perennial crop and native successional ecosystems (p<0.05). AOB were the major contributor in annual crop, perennial crop and early successional ecosystems (p<0.05). Interestingly, we did not observe seasonal effects except for conventionally fertilized wheat and poplar ecosystems, where the contribution of AOB to N₂O was significantly different between winter and summer (p<0.05). Our research demonstrates that AOB were the more important source of nitrate in annual and early successional ecosystems, and more important sources of nitrification-derived N₂O in all ecosystems except the late successional forest (p<0.05) where AOA contributed equally.