Characteristics and influencing factors of CO2 emission from inland waters in China

Inland water bodies, being the most active biogeochemical cycle reactors, play a pivotal role in the global carbon cycle and CO₂ budget. This study integrates existing observation dataset of CO₂ flux (fCO₂) in rivers, lakes and reservoirs in China, to elucidate their spatial-temporal patterns and key influencing factors and to reappraise the significance for regional carbon balance. It showed that, the fCO₂ in rivers, lakes and reservoirs in China presented significant variability with large range of −379.3–4947.6, −160.1–785.0 and −74.0–1603.1 mg CO₂ m⁻² h⁻¹, respectively. The median of the fCO₂ in rivers was of 228.5 mg CO₂ m⁻² h⁻¹, observably higher than these in lakes and reservoirs (26.0 and 28.3 mg CO₂ m⁻² h⁻¹, respectively). The fCO₂ in rivers and reservoirs exhibited similar decreasing trend from south to north as a result of universal climate restraint, the averaged fCO₂ in Pearl River and Yangtze River basins showed much higher than that in Northeastern rivers. While, the averaged fCO₂ in the Mongolia-Xinjiang lake district and the Northeast lake district were higher than other lake districts, followed by the Qinghai-Xizang lake district, and the Eastern and Yungui lake district were generally low, contradicting the climatic restriction. The water primary production enhancement resulting from human activities was the main driver of spatial variation in the fCO₂ in lakes. Meanwhile, the fCO₂ in rivers presented seasonal pattern with higher wet season than dry season, while opposite patterns were found in lakes and reservoirs. Seasonal temperature, precipitation and water primary production were main factors. Furthermore, it showed pH was a key factor indicating the variability of the fCO₂ levels either in rivers, lakes or in reservoirs. In lakes, the fCO₂ is closely linked to chlorophyll a (Chl-a) and dissolved oxygen (DO), whereas, the fCO₂ in rivers is primarily associated with organic carbon (OC) and total nitrogen (TN), highlighting the diverse controlling mechanisms of fCO₂ in various inland water types. In addition, we found that water body sizes play an important role in regulating the fCO₂ levels, and small waters act as hotspots of CO₂ flux. Additionally, widespread urbanization and agricultural activities may enhance CO₂ emissions from rivers but potentially mitigating that from lakes. Nevertheless, the comprehensive impact of these factors on CO₂ emissions in inland water requires further evaluation. Based on the extrapolation method, we re-estimated that the total CO₂ emission of inland waters in China is approximately 117.3 Tg yr⁻¹, which could offset 4.6%–12.8% of the total land carbon sink in China.