Carbon emissions from rivers, lakes, and reservoirs comprise a large proportion of the global carbon cycle and have attracted extensive attention from scholars worldwide. In recent years, studies on the carbon emission from inland waters in China have been focused on single lakes or rivers, and the influences of anthropogenic disturbances on inland water carbon emissions have received continuous attention. Several studies have investigated the emission fluxes of carbon dioxide (CO₂) from inland waters in China, but to date none of them have unraveled the underlying factors driving CO₂ emissions.

Recently, the research group led by Prof. Yunlin Zhang from the Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences compiled meta data from the literatures with measured data on efflux of CO₂ from inland waters and obtained a total of 1405 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 on Global Change Biology.

Based on a series of data sharing platforms, including the Resource and Environmental Science and Data Center, Chinese Academy of Sciences, and the National Earth Science Data Center of China, etc, the scientists 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. The scientists then extracted data on catchment %urban and agricultural land use, gross domestic product (GDP), population density, NPP, topsoil organic carbon, topsoil pH, and precipitation. The driven mechanisms of CO₂  efflux from lakes, reservoirs and rivers in China were then unraveled.

Notably higher CO₂ efflux from rivers than from lakes and reservoirs was found and altogether a total of 61.9 ± 55.3 TgC was emitted from Chinese inland waters annually. 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, an 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.

This work highlighted the high 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.

In the foreseeable future, more land will be transformed from natural forest and grassland into residential areas or agricultural land use, especially in developing countries. Such land use alterations will not only change the terrestrial carbon cycle but as shown here, also the inland water carbon cycle.

 Relationships between the efflux of CO₂  and the mean catchment %urban land use (b), %agricultural land use (c), gross domestic production (GDP, d), population density (e), net primary productivity (NPP, f), topsoil organic carbon (SOC, g), topsoil pH (h), and precipitation (i) of each sampling site collected from lakes, reservoirs, and rivers.

Link at https://doi.org/10.1111/gcb.16475.