Comammox ammonia oxidization and N₂O production in vadose and saturated zone soils

Riparian zones are biogeochemical hotspots of N₂O emission. The important role of ammonia oxidation in N₂O production has been reported in rivers, even higher than heterotrophic denitrification, however, the mechanisms controlling riparian N₂O production remain unknown. Here, we found that complete ammonia oxidation (comammox) process significantly contributes to ammonia oxidation and N₂O production in subsurface riparian zone. The contribution of comammox to ammonia oxidation in surface soils (38.7 ± 6.5 %) is equivalent to that of bacterial ammonia oxidation (49.4 ± 6.9 %), and both processes are significantly more important than archaeal ammonia oxidation (11.9 ± 4.1 %). Moreover, comammox dominates ammonia oxidation in both vadose and saturated zone soils, especially in summer with the average contributions of over 67.9 %. However, as a biotic N₂O production pathway, comammox produces more N₂O in vadose (up to 72 %) and saturated zone (up to 100.0 %) soils, significantly higher than that produced by archaeal ammonia oxidation (32.0 ± 4.0 %) and bacterial ammonia oxidation (18.0 ± 5.7 %). We obtained the first metagenome-assembled genomes (MAGs) of comammox bacteria in a riparian profile, and metagenomic binning analysis revealed that the wide niche of comammox may attribute to its diverse metabolic pathways, which not only perform ammonia oxidation but also nitrate reduction to ammonia. This research enhances the understanding of the biogeochemical ammonia cycle in riparian zone systems and provides new insights for the control of nitrogen pollution in riparian zones.