Impacts of human disturbance on the biogeochemical nitrogen cycle in a subtropical river system revealed by nitrifier and denitrifier genes

Human activities have largely modified nitrogen (N) sources supply, cycling and export from land to ocean. Nitrification and denitrification are vital processes alleviating N pollution in aquatic ecosystems but the diverse responses and niche of microbial N retention to human disturbance are still understudied. Here we investigated the changes in N species and functional genes in the urban, agriculture and reservoir river sections of the Jiulong River (southeast China). Our results show that ammonia-oxidizing bacteria (AOB) (Nitrosomonas) were dominant in the urban river section receiving ammonium-rich sewage that enhanced nitrification and subsequent denitrification, while ammonia-oxidizing archaea (AOA) was more abundant than AOB in the river section flowing through areas of pomelo (Citrus maxima) agriculture with low pH, low ammonium and very high nitrate input. Warm temperatures and large total suspended matter (TSM) in the wet season promoted growth of nitrifiers and denitrifiers, which were mostly particle-attached. The potential river N retention through gaseous N removal (PRN2O and PRN2) in the agriculture section with huge N loading was among the lowest. Strong nitrification and denitrification were suspected to occur in the agricultural acid soil system rather than in the river network. In addition, the decreased TSM and N concentration promoted free-living microbes in the reservoir. The highest PRN2 and N2 production observed in the reservoir in the dry season implied that denitrification and anammox occurring in sediments was likely to increase N retention. This study suggests the diverse factors involved in processing of N pollution among diverse landscapes.