Rural River Reaches are Emission Hotspots for Greenhouse Gases

Abstract

Rivers play a pivotal role in the global carbon and nitrogen biogeochemical cycles, contributing disproportionately to the global budget of greenhouse gases (GHGs) relative to their areas. Rural river reaches in particular have been influenced strongly by human activities and are potential hotspots for GHG emissions. However, the mechanism responsible for that production remains poorly understood. Here, we used metagenomic sequencing and microbial taxonomic annotation to explore GHGs production mechanism in a subtropical river receiving agricultural return flow and decentralized rural sewage treatment tailwater. The global warming potential (GWP) of rural river reaches, at both the 20- and 100-year scale, surpasses that other river reaches by 283.6% and 298.9%, respectively. The input of tailwater from decentralized sewage treatment facilities and agricultural return flow significantly increased the nutrient loading of rural reaches and changed the abundance of functional genes and composition of microbial communities in the water column. Adequate denitrification substrates and incomplete denitrification driven by them dominated the production of N2O. In tandem, less photosynthesis, aerobic CO2 fixation, and strong aerobic respiration resulted in high CO2 production from rural river reaches. Despite substantial aerobic CH4 oxidation in the water column, the prevalence of sufficient reaction substrates and alternative CH4 oxidation substitutes means that rural river reaches can still function as an aquatic hotspot for CH4 production, this controlled by methylotrophic and hydrogenotrophic methanogenesis during the wet and dry periods, respectively. This study’s findings emphasize the need to improve the collection rate and treatment efficiency of rural domestic sewage and strengthen the control of agricultural non-point source pollution, so as to better mitigate the indirect greenhouse effect of sewage and fertilization in rural regions.

Keywords: agricultural return flow, carbon cycle, greenhouse gases, incomplete denitrification, rural domestic sewage