A Synthetic Model to Quantify Dissolved Organic Carbon Transport in the Changjiang River System: Model Structure and Spatiotemporal Patterns

Abstract

Riverine dissolved organic carbon (DOC) is an important carbon pool in the global biogeochemical cycle. DOC transport in river networks involves three processes: DOC input (including DOC release from terrestrial ecosystems and in situ production in river networks), in-river removal, and export through watershed outlets or estuaries. DOC transport in large river networks is gaining attention due to its important role in carbon and nutrient supply and CO 2 emission, for example. However, quantifying DOC input to rivers and in-river removal is still not properly understood. This study developed a synthesis model to quantify DOC transport in the Changjiang River Network (CRN) by coupling spiraling theory with Strahler river order. Our study suggested that the wetlands proportion and soil organic matter are valid parameters in a DOC model and our simulations showed that approximately 2.65-4.86 Tg C/year entered rivers and 1.16-2.12 Tg C/year was exported to the estuary as DOC, with a removal proportion of over 50% throughout the CRN in 1980-2015. The subbasins of the Dongting Lake, the Yichang-Jiujiang section of the mainstream, and the Min-Tuo River were primary contributors of DOC load, accounting for approximately 45% of the bulk DOC load of the CRN. The subbasins of Jinsha River and Yalong River in the upper reaches of the CRN and the subbasin of Wu River contributed less than 10% of the DOC load. DOC export by the CRN accounted for 0.3-1.2% of the global DOC flux from land to sea. Plain Language SummaryThe transport of dissolved organic carbon (DOC) in river networks is a key section of the global carbon biogeochemical cycle. DOC transport in river networks involves three processes: DOC input to rivers, in-river removal, and export through estuaries, and these processes can be altered by human activity and climate change. We have developed a synthesis model to individually simulate these three processes of riverine DOC transport by coupling hydrological and biogeochemical processes and have applied our model to the Changjiang River Network. Our study is helpful in comprehensively understanding riverine DOC transport from land to estuaries in large river networks. Key Points:• A synthesis model was developed to quantify DOC input to rivers, in-river removal, and export to the estuary in the CRN • The DOC removal proportion slightly decreases with increasing river order and is 33.6-60.1% at the subbasin and entire CRN scales • About 1.47-2.71 t C·km −2 ·year −1 entered the rivers, and 0.64-1.18 t C·km −2 ·year −1 was exported by the CRN to the estuary as DOC Supporting Information:• Supporting Information S1• Figure S1 • Figure S2 • Figure S3 • Table S1 . A synthetic model to quantify dissolved organic carbon transport in the Changjiang River system: Model structure and spatiotemporal patterns.

Exploring Spatially Explicit Changes in Carbon Budgets of Global River Basins during the 20th Century

“…For the five rivers analyzed, the role of discharge (Q) is important for almost all C fluxes as it influences the flow velocity and time available for transformations, with a commonly negative influence of increasing discharge on CO 2 efflux and C burial and a positive influence (although limited in some cases) on the export of the various C forms (because there is less burial and less CO 2 efflux), which is consistent with the findings in other studies. 36 , 44 , 46 , 48 , 76 , 77 An increase in temperature positively influences CO 2 efflux (SRC of 0.28 for the Amazon to 0.39 in the Yangtze and 0.65 in the Nile) and lowers C burial (not important in the Lena because of low temperatures and low production rates, and SRC values of −0.43 to −0.83 in the other rivers); it also negatively impacts the export of primarily DOC (high negative values of −0.43 for the Lena and −0.73 for the Amazon) and POC (−0.22 for the Nile to −0.73 for the Amazon). The parameters related to phytoplankton growth (e.g., solar radiation and maximum rates of phytoplankton growth or mortality) are important in most of the rivers for the export of POC and algal biomass (SRC values of 0.22–0.30, Tables SI5a–e ), but they also exert an influence on DOC export, C burial, and CO 2 emission in rivers heavily controlled by dams and reservoirs, such as the Mississippi 17 , 78 and Yangtze.…”

“… 46 , 47 The regional Model of Organic matter Removal and Export for Dissolved Organic Carbon (MORE-DOC) simulates riverine DOC processes in the Yangtze River for the period 1980–2015, but it relies on numerous observational data for calibration and does not include other C forms and relevant processes. 48 The regional process-based model National Integrated Catchment-based Ecohydrology (NICE)-BGC was recently applied to simulate the in-stream processing for DIC, DOC, and POC in global 153 river basins for the period 1980–2015 with a 1° × 1° spatial resolution, but the land-use change before the year 1992, impacts of dams and reservoirs, and processes relevant to sediment and C burial were not considered. 49 , 50 Since these process-based models do not fully consider multiple C forms and their associated in-stream dynamic processes, including C production, consumption, transformation, lateral transport, and interaction at the water-sediment and water-air interfaces, they have limited capabilities of hindcasting the historical spatially-explicit riverine fluxes and concentrations of multiple C forms on the global scale.…”

Global Controls on DOC Reaction Versus Export in Watersheds: A Damköhler Number Analysis

“…Using 0.1 as the cutoff, which corresponds to a removal efficiency of ∼10% instead, only the smallest (e.g., first order) river networks would be classified as export‐dominated systems. Furthermore, Da and river network removals are highly sensitive to the magnitude of $${v}_{f}$$ (Bertuzzo et al., 2017; Lv et al., 2019; Wollheim et al., 2018). Therefore, export‐dominated river networks defined in terms of DOC uptake may be highly reactive systems in terms of the cycling of other constituents with orders of magnitude higher uptake (e.g., nitrogen or phosphorous) (Wollheim et al., 2018).…”

“…We suggest that simple molecules or organic leachates represent a highly reactive fraction of the DOM reactivity continuum and overestimate actual DOC uptake in river networks (Mineau et al., 2016). DOC $${v}_{f}$$ derived from solute addition experiments is thus not suitable to scale DOC uptake at actual watershed scales (Lv et al., 2019).…”

The importance of organic carbon as a coadjutant in the transport of pollutants

“…Moreover, DOC quantification has also been carried out by mathematical modeling and satellite imagery. The use of both tools is dependent on organic carbon concentrations in periods of time that represent different conditions in the water body resulting from point source and non-point source inputs (Lv et al 2019;Vitale & Guardo 2019;Chen et al 2020;Cao & Tzortziou 2021;Rouhani et al 2021). For example, the Soil and Water Assessment Tool (SWAT), a hydrosedimentological model, has recently had state variable equations added to its codes to calculate DOC transport in rivers.…”