Magnitude and drivers of CO2 and CH4 emissions from an arid/semiarid river catchment on the Chinese Loess Plateau

Abstract

River systems are major sources of greenhouse gases (GHG). However, simultaneous CO2 and CH4 emissions from arid/semiarid rivers remain poorly studied. Here we investigated CO2 and CH4 emissions (diffusion + ebullition) and their potential sources and drivers in an arid/semiarid catchment on the Chinese Loess Plateau. The river water CO2 partial pressure (pCO2) averaged 885 ± 415 µatm and did not show a significant change along the stream network due to strong carbonate buffering. Groundwater inputs played a dominant role in controlling pCO2 which exhibited a pronounced seasonal variation. In-stream decomposition of organic matter mobilized by soil erosion was also an important contributor. The streams were consistently oversaturated in CH4, ranging between 0.06 and 0.77 µmol L−1. Both gas emissions exhibited significant seasonal variations, which were collectively controlled by gas concentration gradient across the water–air interface and flow dynamics. The mean CO2 and CH4 efflux were 55 mmol m−2 d–1 and 346 µmol m−2 d–1, respectively, with the highest efflux in August (high flow conditions) for both gases. River impoundment resulted in greatly reduced pCO2 due to enhanced primary production but elevated CH4 concentrations because of anaerobic degradation of organic-rich sediments. Reservoirs acted as a smaller CO2 source but a stronger CH4 source than flowing rivers. Ebullition was an important pathway of CH4 emissions in both lotic and lentic ecosystems, contributing 36 ± 23% and 55 ± 19% to the total CH4 emissions from rivers and reservoirs, respectively. Urban wastewater discharges modified riverine carbon cycling and created local hotspots of GHG production and emissions, which will be globally significant given the increasing human impacts worldwide. Our results reinforce the urgency to account for arid/semiarid rivers in future GHG emission estimates. Future work to better understand the impacts of groundwater inflows and aquatic ecosystem processes in controlling GHG dynamics is also needed.