Abstract:  

Dissolved organic carbon (DOC) plays an important role in ecosystem. Little is known about the dynamics of riverine DOC exports and their responses to climate change and permafrost degradation. To examine the seasonal variations of concentrations and exports of DOC and their responses to changes of air temperature and precipitation, we collected monthly river water samples from 8 catchments under alpine meadow in the Three Rivers’ headwater regions on the QinghaiTibet Plateau from August 2016 to July 2017. The discharges in these rivers were monitored, and DOC concentrations of water samples were analyzed in the laboratory. The results showed: (1) The annual average DOC concentrations of these streams varied from (4.05±1.20) mg·L-1 to (6.55±2.86) mg·L-1, with an mean value of 5.30 mg·L-1. The DOC concentrations were significantly negatively correlated with the coverage area of alpine swamp meadow (ASM), but significantly correlated with the coverage of alpine meadow (AM). The average DOC concentrations in the rivers in permafrost regions were significantly higher than those in nonpermafrost regions. (2) The DOC concentrations showed a great variations with seasonal changes of air temperature. From the spring to the beginning of summer, DOC concentrations increased sharply. During this period, the monthly mean air temperature increased from -8℃ to 2℃. The DOC concentrations decreased rapidly as the air temperature increased from 2℃ to 13℃. From summer to winter, the temperature decreased from 13℃ to -8℃. Meanwhile, the average DOC concentrations decreased gradually, and the lowest value was recorded in December. (3) The seasonal changes of DOC export varied greatly, ranging from (0.006±0.0005) kg·km-2·d-1 and (3.01±0.74) kg·km-2·d-1 among the rivers, with an average value of（1.12±0.81）kg·km-2·d-1. DOC export fluxes were positively correlated with the discharges. The highest DOC export was recorded in the spring snowmelt period and the rainy summer. Since the climate change may lead to a thicker active layer in permafrost regions, our results showed that the DOC export increased along with temperature, indicating that climate warming can cause a higher DOC export in alpine meadow in the permafrost regions on the QinghaiTibetan Plateau.