High contribution of ammonia-oxidizing archaea (AOA) to ammonia oxidation related to a potential active AOA species in various arable land soils

Purpose Ammonia oxidation is the limiting step in soil nitrification and critical in the global nitrogen cycle. The discovery of ammonia-oxidizing archaea (AOA) has improved our knowledge of microbial mechanisms for ammonia oxidation in complex soil environments. However, the relative contributions of AOA and ammonia-oxidizing bacteria (AOB) to ammonia oxidation remain unclear. Materials and methods In this study, through large geographical scale sampling in China, totally nine samples representing various types of arable land soils were selected for analyzing the ammonia oxidation activity. The AOA and AOB activities were separately determined by using the dicyandiamide and 1-octyne inhibition method. High-throughput pyrosequencing and DNA stable-isotope probing (DNA-SIP) analysis were applied to investigate the distribution and activity of Candidatus Nitrosocosmicus franklandus in the arable land soils. Results and discussion In this study, AOA abundance (3.2 × 10 7 –3.4 × 10 9 copies g −1 ) and activity (0.01–1.33 mg N kg −1 dry soil day −1 ) were evaluated for nine selected arable land soils and accounted for 4–100% of ammonia oxidation. By separately determining AOA and AOB rates, we observed that archaeal ammonia oxidation dominated the ammonia oxidation process in six soils, revealing a considerable contribution of AOA in ammonia oxidation in arable land soils. Based on high-throughput pyrosequencing analysis, the AOA species Ca. N. franklandus with relatively low abundance (0.6–13.5% in AOA) was ubiquitously distributed in all the tested samples. Moreover, according to the DNA-SIP analysis for Urumqi sample, the high activity and efficiency of Ca. N. franklandus in using CO 2 suggests that this species plays an important role in archaeal ammonia oxidation in arable land soils. Conclusions Through determining the AOA activity and analyzing the potential predominant functional AOA species, this study greatly improves our understanding of ammonia oxidation in arable land soils.