Increasing riverine export of dissolved organic carbon from China

Abstract

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.

The Greenhouse Gas Budget of Terrestrial Ecosystems in East Asia Since 2000

“…(2016) that based on the observations and empirical formula from previous studies, they estimated Fexport of nine Chinese exorheic rivers during 2006–2009. (c) We used a recent data‐driven model estimate (Yan et al., 2023) of DOC export from land to oceans, which applied machine learning methods and a comprehensive set of natural and anthropogenic drivers. Based on Yan et al.…”

“…Based on Yan et al. (2023)'s estimate and the mean DIC/DOC and POC/DOC ratios observed in the nine rivers from Jiang et al. (2016), we calculated the total carbon exported through the southeast boundaries of East Asia.…”

“…(b) We used the estimate from Jiang et al (2016) that based on the observations and empirical formula from previous studies, they estimated Fexport of nine Chinese exorheic rivers during 2006-2009. (c) We used a recent data-driven model estimate (Yan et al, 2023) of DOC export from land to oceans, which applied machine learning methods and a comprehensive set of natural and anthropogenic drivers. Based on Yan et al (2023)'s estimate and the mean DIC/DOC and POC/DOC ratios observed in the nine rivers from Jiang et al (2016), we calculated the total carbon exported through the southeast boundaries of East Asia.…”

Carbon Emissions From Chinese Inland Waters: Current Progress and Future Challenges

“…Nutrients (e.g., N, and P) are one of the most critical drivers of metabolism that regulates CO 2 and CH 4 production in aquatic ecosystems (Battin et al., 2023; Zhang, Tang, et al., 2022). Urban and agricultural wastewater rich in nutrients and OC can enhance GPP by alleviating N and P limitations on the growth of aquatic plants (e.g., algae), thereby reducing the dissolved CO 2 partial pressure ( p CO 2 ) (Le et al., 2010; Qu & Kroeze, 2010; Yan, Lauerwald, et al., 2023). For example, eutrophication in the Lake Ulansuhai, a farmland drainage‐fed lake in North China, has significantly decreased the water‐to‐air CO 2 fluxes (Sun et al., 2021).…”

“…Yet, the magnitude of the impact on riverine C transport varies depending on a multitude of environmental and human factors. A recent synthesis based on in situ monitoring suggests a significant increase in DOC flux from Chinese rivers (∼4.0 Tg C yr −1 ) but a relatively stable DOC concentration during the period 2001−2015 (Yan, Lauerwald, et al., 2023). Despite the insignificant temporal changes in the national average DOC concentration, pronounced spatial heterogeneity was diagnosed with the DOC concentrations significantly increasing in the Yangtze and Huai rivers (Figure 1), which is attributable to anthropogenic perturbations (e.g., land use change, sewage discharge, and groundwater depletion) (Liu et al., 2020).…”