Increasing riverine export of dissolved organic carbon from China

River transport of dissolved organic carbon (DOC) to the ocean is a crucial but poorly quantified regional carbon cycle component. Large uncertainties remaining on the riverine DOC export from China, as well as its trend and drivers of change, have challenged the reconciliation between atmosphere-based and land-based estimates of China's land carbon sink. Here, we harmonized a large database of riverine in-situ measurements and applied a random forest model, to quantify riverine DOC fluxes (F-DOC) and DOC concentrations (C-DOC) in rivers across China. This study proposes the first DOC modeling effort capable of reproducing well the magnitude of riverine C-DOC and F-DOC, as well as its trends, on a monthly scale and with a much wider spatial distribution over China compared to previous studies that mainly focused on annual-scale estimates and large rivers. Results show that over the period 2001-2015, the average C-DOC was 2.25 +/- 0.45 mg/L and average F-DOC was 4.04 +/- 1.02 Tg/year. Simultaneously, we found a significant increase in F-DOC (+0.044 Tg/year(2), p = .01), but little change in C-DOC (-0.001 mg/L/year, p > .10). Although the trend in C-DOC is not significant at the country scale, it is significantly increasing in the Yangtze River Basin and Huaihe River Basin (0.005 and 0.013 mg/L/year, p < .05) while significantly decreasing in the Yellow River Basin and Southwest Rivers Basin (-0.043 and -0.014 mg/L/year, p = .01). Changes in hydrology, play a stronger role than direct impacts of anthropogenic activities in determining the spatio-temporal variability of F-DOC and C-DOC across China. However, and in contrast with other basins, the significant increase in C-DOC in the Yangtze River Basin and Huaihe River Basin is attributable to direct anthropogenic activities. Given the dominance of hydrology in driving F-DOC, the increase in F-DOC is likely to continue under the projected increase in river discharge over China resulting from a future wetter climate.

Automated summary

This study presents a model capable of accurately simulating dissolved organic carbon (DOC) concentrations and fluxes in rivers across China. The model shows that while DOC concentrations in Chinese rivers have not changed significantly over the past 15 years, there has been a significant increase in DOC fluxes, particularly in the Yangtze River Basin and the Huaihe River Basin. These increases are attributed to anthropogenic activities, although hydrology still plays a dominant role in determining the variability of DOC. With projected increases in river discharge due to a wetter climate, it is likely that DOC fluxes will continue to increase in the future.