Carbon emissions from rivers, lakes, and reservoirs comprise a large proportion of the global carbon cycle. Recent studies on carbon emission from inland waters in China have been focused on single lakes or rivers, and the underlying factors driving CO₂ emissions remain unclear. 

Recently, a research group led by Prof. ZHANG Yunlin from the Nanjing Institute of Geography and Limnology of the Chinese Academy of Sciences compiled meta data from literatures with measured data on efflux of CO₂ from inland waters and obtained a total of 1,405 measurements, including 658 river sites, 625 lake sites, and 122 reservoir sites sampled primarily between 2010 and 2020, and investigated the effluxes of CO₂ and drivers across inland waters in China. 

This work was published in Global Change Biology on Oct. 10.

Based on several data sharing platforms, the researchers obtained a series of data products including land use and land cover, gross primary productivity (GPP) and net primary productivity (NPP) at spatial resolutions of 30 m and 1 km. Then, they extracted data on catchment urban and agricultural land use, gross domestic product (GDP), population density, NPP, topsoil organic carbon, topsoil pH, and precipitation. 

"Through analyzing these data products, we unravel the driven mechanisms of CO₂ efflux from lakes, reservoirs and rivers in China," said Prof. ZHANG. 

The researchers found notably higher CO₂ efflux from rivers than from lakes and reservoirs. The effluxes of CO₂ from rivers and lakes increased significantly with the increasing catchment urban land use, and CO₂ effluxes of lakes and reservoirs increased significantly with increasing catchment agricultural land use. 

The effluxes of CO₂ from lakes, reservoirs, and rivers increased with increasing catchment GDP and population density. In comparison, no significant relationships were found between the CO₂ efflux and catchment annual NPP, topsoil organic carbon concentration, pH of topsoil, or catchment precipitation.   

Previous studies have found that compared to less populated regions, the increase in population density results primarily in eutrophication of inland waters caused by the discharge of agricultural, industrial, and residential effluents, as well as nonpoint sources of organic carbon. In eutrophic waters, primary production is commonly increased with high amounts of bio-labile organic matter, favoring microbial degradation and thereby strongly enhancing the CO₂ production and emission from inland waters.

These results suggest that anthropogenic disturbances in relation to urbanization and agricultural land use can influence CO₂ emissions from inland waters more than catchment productivity, which previously has been identified as the main driver for CO₂ emissions from inland waters in less populated regions. 

This work demonstrated that the presence of anthropogenic disturbances in catchments, represented by urban and agricultural land use, GDP, and population density, were positively related to the emission of CO₂. 

It highlighted the importance of in-situ production of CO₂ via the degradation of household effluents, nonpoint source- and algal-organic carbon in catchments draining densely populated areas compared with CO₂ being directly delivered through inflowing catchment streams.