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 gas (GHG) relative to their areas. Rural river reaches are particularly affected by anthropogenic activities and may serve as significant GHG emission hotspots, while previous studies have focused primarily on GHG emissions from urban sewage treatment and agricultural systems, the effects of decentralized sewage treatment tailwater and agricultural return flow on riverine GHG dynamics remains poorly understood. In this study, we used metagenomic sequencing and microbial taxonomic annotation to investigate GHG production mechanism in a subtropical river receiving agricultural return flow and decentralized sewage treatment tailwater. We found that the global warming potential of rural river reaches surpassed that of other river reaches by 283.6 % and 298.9 % on 20- and 100-year time scales, respectively. These elevated GHG levels were linked to increased nutrient and organic matter loading, along with pronounced shifts in the abundance of functional genes related to GHG production and consumption in the rural river reaches. N₂O production was primarily driven by incomplete denitrification facilitated by abundant denitrification substrates. Concurrently, reduced photosynthesis and aerobic CO₂ fixation, coupled with strong aerobic respiration, led to high CO₂ production. Despite substantial aerobic CH₄ oxidation, the continuous availability of methanogenic substrates and alternative CH₄ oxidation substitutes sustained CH₄ production. These were controlled by methylotrophic and hydrogenotrophic methanogenesis during the wet and dry periods, respectively. These 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.