Diverse metabolism drives comammox in continental-scale agricultural streams: Important ammonia oxidation but low N 2 O production

Agriculture receives approximately 25 % of the annual global nitrogen input, 37 % of which subsequently runs off into adjacent low-order streams and surface water, where it may contribute to high nitrification and nitrous oxide (N 2 O). However, the mechanisms of nitrification and the pathways controlling N 2 O production in agricultural streams remain unknown. Here, we report that the third microbial ammonia oxidation process, complete ammonia oxidation (comammox), is widespread and contributes to important ammonia oxidation with low ammonia-N 2 O conversion in both basin- and continental-scale agricultural streams. The contribution of comammox to ammonia oxidation (21.5 ± 2.3 %) was between that of bacterial (68.6 ± 2.7 %) and archaeal (9.9 ± 1.8 %) ammonia oxidation. Interestingly, N 2 O production by comammox (18.5 ± 2.1 %) was higher than archaeal (10.5 ± 1.9 %) but significantly lower than bacterial (70.2 ± 2.6 %) ammonia oxidation. The first metagenome-assembled genome (MAG) of comammox bacteria from agricultural streams further revealed their potential extensive diverse and specific metabolism. Their wide habitats might be attributed to the diverse metabolism, i.e. harboring the functional gene of nitrate reduction to ammonia, while the lower N 2 O would be attributed to their lacking biological function to produce N 2 O. Our results highlight the importance of widespread comammox in agricultural streams, both for the fate of ammonia fertilizer and for climate change. However, it has not yet been routinely included in Earth system models and IPCC global assessments. Synopsis Widespread but overlooked comammox contributes to important ammonia oxidation but low N 2 O production, which were proved by the first comammox MAG found in agricultural streams.