Insight into the microbial nitrogen cycle in riparian soils in an agricultural region

Riparian zones are considered as an effective measure on preventing agricultural non-point source nitrogen (N) pollution. However, the mechanism underlying microbial N removal and the characteristics of N-cycle in riparian soils remain elusive. In this study, we systematically monitored the soil potential nitrification rate (PNR), denitrification potential (DP), as well as net N₂O production rate, and further used metagenomic sequencing to elucidate the mechanism underlying microbial N removal. As a whole, the riparian soil had a very strong denitrification, with the DP 3.17 times higher than the PNR and 13.82 times higher than the net N₂O production rate. This was closely related to the high soil NO₃^--N content. In different profiles, due to the influence of extensive agricultural activities, the soil DP, PNR, and net N₂O production rate near the farmland edge were relatively low. In terms of N-cycling microbial community composition, the taxa of denitrification, dissimilatory nitrate reduction, and assimilatory nitrate reduction accounted for a large proportion, all related to NO₃^--N reduction. The N-cycling microbial community in waterside zone showed obvious differences to the landside zone. The abundances of N-fixation and anammox genes were significantly higher in the waterside zone, while the abundances of nitrification (amoA&B&C) and urease genes were significantly higher in the landside zone. Furthermore, the groundwater table was an important biogeochemical hotspot in the waterside zone, the abundance of N-cycle genes near the groundwater table was at a relative higher level. In addition, compared to different soil depths, greater variation in N-cycling microbial community composition was observed between different profiles. These results reveal some characteristics of the soil microbial N-cycle in the riparian zone in an agricultural region and are helpful for restoration and management of the riparian zone.