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 (fCO(2)) 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 fCO(2) 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 fCO(2) 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 fCO(2) in rivers and reservoirs exhibited similar decreasing trend from south to north as a result of universal climate restraint, the averaged fCO(2) in Pearl River and Yangtze River basins showed much higher than that in Northeastern rivers. While, the averaged fCO(2) 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 fCO(2) in lakes. Meanwhile, the fCO(2) 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 fCO(2) levels either in rivers, lakes or in reservoirs. In lakes, the fCO(2) is closely linked to chlorophyll a (Chl-a) and dissolved oxygen (DO), whereas, the fCO(2) in rivers is primarily associated with organic carbon (OC) and total nitrogen (TN), highlighting the diverse controlling mechanisms of fCO(2) in various inland water types. In addition, we found that water body sizes play an important role in regulating the fCO(2) 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.