Increasing riverine export of dissolved organic carbon from China

Estimation of organic carbon source composition and riverine outflow using an integrated watershed hydrological-carbon modelling approach

Carbon source apportionment and outflow estimation are the primary scientific considerations for reducing carbon output from watershed ecosystems to ocean. However, carbon loss and transportation mechanisms from soil to river system driven by watershed hydrological cycle, remain unclear. Our study developed a process-based watershed organic carbon model that integrates soil biogeochemical processes, overland loss, riverine metabolism and transportation driven by hydrological processes, and estimates the sources, outflows and their spatial distributions. The proposed model was validated using long-term field observations of runoff and labile particulate, dissolved, and total organic carbon (LOC, DOC and TOC) loads across the Xiangxi Watershed in China. The biases within ±0.25 were for all runoff simulations and for 71.4 % (30/42) of carbon load simulations, and both Nash-Sutcliffe efficiency and correlation coefficient were over 0.60 for runoff simulation and for 83.3 % (35/42) of carbon load simulations. Annual average TOC load flowing into rivers was 11.3 ton.km-2.yr-1, with resistant particulate organic carbon (ROC) as the main form, accounting for 88.7 % of the TOC load. Atmospheric deposition was the primary TOC source with a contribution of 87.9 %, followed by soil loss. Annual average riverine TOC outflow was 3.8 ton.yr-1, with LOC and DOC accounting for 57.5 % and 40.0 %, respectively. This indicates that a majority of ROC decomposed into DOC and LOC via riverine metabolism and sedimentation. Our study provides insights into integration mechanisms of watershed hydrological and carbon cycles, and contributes to strategies for controlling water and carbon losses to strengthen terrestrial carbon sequestration.

The Tibetan Plateau acts as a net greenhouse gas sink

The greenhouse gas budget on the Tibetan Plateau remains unknown and the potential for methane (CH4) and nitrous oxide (N2O) emissions from an intensifying livestock system and expanding surface water in offsetting terrestrial carbon dioxide (CO2) sinks are both of great concerns and uncertainties, which compromise an accurate assessment of Tibetan Plateau contribution to China's ambitious climate goals by 2060s. Here we integrated greenhouse gas flux measurements at ∼500 sites in empirical modeling approaches, emissions from the livestock sector with process-based biogeochemistry modeling to estimate CH4 and N2O fluxes across terrestrial ecosystems and inland waters in 2000s and 2010s. We found that emissions from livestock and inland waters, predominantly contributed by CH4, compensated ∼21% and ∼13% of carbon sinks provided by forests and grasslands after adjusting carbon burial in sediments and riverine carbon export, respectively. The Tibetan Plateau then acted as an appreciable greenhouse gas sink that almost compensated for its contemporary anthropogenic emissions, making it nearly climate-neutral. The enhancement of terrestrial CO2 sinks in the 2060s under medium warming scenario would be counterbalanced by livestock CH4 emissions when the current overgrazing status continues. By transitioning to a livestock-forage balance and implementing mitigation initiatives to reduce livestock emission intensity, the greenhouse gas sink is projected to increase by more than 1.5 times. We suggested that a transition towards sustainable pastoralism illuminates the path to minimizing ecosystem greenhouse gas emissions and amplifying the role of the Tibetan Plateau in fulfilling China's climate ambition.

Carbon sinks associated with biological carbon pump in karst surface waters: Progress, challenges, and prospects

The biological carbon pump (BCP) associated with aquatic photosynthesis in karst surface waters converts dissolved inorganic carbon (DIC) into organic carbon. In the context of global climate change, BCP could be an important carbon sink mechanism, ultimately regulating atmospheric carbon dioxide (CO2) and mitigating climate change. Because of the high DIC and pH, and low dissolved CO2 [CO2 (aq)], the hydrochemical characteristics of karst surface water bodies cause C limitation in BCP efficiency. The effect of CO2 fertilization on water bodies can promote autochthonous production, thereby creating carbon sinks in such water bodies. The significant sink-enhancement potential of BCP in karst surface water bodies has attracted widespread attention. The stability of the autochthonous organic carbon (AOC) produced by BCP in karst aquatic ecosystems is key to the formation of long-term carbon sinks by carbonate weathering. In this review, we summarize recent progress in the carbonate weathering of carbon sinks in karst surface waters with coupled BCPs. Furthermore, we elucidated the possibility of using CO2 (aq) fertilization to achieve carbon sinks and its mechanism of action. On this basis, we propose three processes and mechanisms that could affect AOC stability and outline the challenge of accurately estimating carbonate weathering carbon sinks associated with BCP in karst surface waters. Our comprehensive analyses facilitated the identification of the role of karst surface aquatic ecosystems in the global carbon cycle by providing a reference and scientific basis.

Spatial and temporal distribution characteristics and source apportionment of biogenic elements using APCS-MLR model in the main inlet tributary of Danjiangkou Reservoir

Danjiangkou Reservoir has been widely concerned as the water source of the world's longest cross basin water transfer project. Biogenic elements are the foundation of material circulation and key factors affecting water quality. However, there is no comprehensive study on the biogenic elements in tributaries of Danjiangkou Reservoir, hindering a detailed understanding of geochemical cycling characteristics of biogenic elements in this region. Guanshan River, one of the main tributaries that directly enter the Danjiangkou Reservoir, was token as the research object. Spatiotemporal distribution characteristics of basic water quality parameters and biogenic elements were studied. Water quality was comprehensively evaluated through water quality index (WQI). Absolute principal component score-multiple linear regression (APCS-MLR) model was adopted to explore the main sources of biogenic elements. Results showed that, in terms of season, the concentrations of total nitrogen (TN), total phosphorus (TP), and dissolved organic carbon (DOC) were significantly higher in wet season than in dry season, while no significant differences were found for dissolved inorganic carbon (DIC) and dissolved silica (DSi). Spatially, the concentrations of dissolved carbon, DIC, TN, and TP in the middle and lower reaches were higher than that in the upstream. DOC concentration peaked in the middle reaches, while DSi showed higher concentrations in the upstream. WQI values indicated that the river water quality was between good and excellent, although the water quality in wet season was slightly worse than that in the dry season. PCA extracted five potential sources, which accounting for 84.12% of the total variance, including rock weathering, mixed source of sewage discharge and agricultural non-point source pollution, dissolved soil CO2, seasonal factor, and agricultural non-point source pollution. These sources contributed 38.96%, 12.33%, 13.54%, 23.95%, and 11.21% to river water quality parameters, respectively. Strengthening the monitoring of biogenic elements, controlling pollutant discharge, and exploring the relationship between biogenic elements and other pollutants are important for the water environment management in this basin.

Molecular composition limits the reaction kinetics of riverine dissolved organic matter decomposition

The bioavailability and degradation of riverine dissolved organic matter (DOM) play crucial roles in greenhouse gas emissions; however, studies on the kinetic decomposition of fluvial DOM remain scarce. In this study, the decomposition kinetics of dissolved organic carbon (DOC) were characterized using the reactivity continuum model through 28-day bio-incubation experiments with water samples from the Yangtze River. The relationship between DOM composition and decomposition kinetics was analyzed using optical and molecular characterization combined with apparent decay coefficients. Our results revealed that DOM compounds rich in nitrogen and sulfur were predominantly removed, exhibiting a transition from an unsaturated to a saturated state following microbial degradation. These heteroatomic compounds, which constituted 75.61 % of the DOM compounds positively correlated with the decay coefficient k0, underwent preferential degradation in the early stages of bio-incubation due to their higher bioavailability. Additionally, we observed that S-containing fractions with high molecular weight values (MW > 400 Da) may be associated with larger reactivity grades. This study underscored the complex interplay between DOM composition and its kinetic decomposition in river ecosystems, providing further support for the significance of molecular composition in large river DOM as crucial factors affecting decomposition.

Escalating Carbon Export from High-Elevation Rivers in a Warming Climate

High-elevation mountains have experienced disproportionately rapid warming, yet the effect of warming on the lateral export of terrestrial carbon to rivers remains poorly explored and understood in these regions. Here, we present a long-term data set of dissolved inorganic carbon (DIC) and a more detailed, short-term data set of DIC, δ13CDIC, and organic carbon from two major rivers of the Qinghai-Tibetan Plateau, the Jinsha River (JSR) and the Yalong River (YLR). In the higher-elevation JSR with ∼51% continuous permafrost coverage, warming (>3 °C) and increasing precipitation coincided with substantially increased DIC concentrations by 35% and fluxes by 110%. In the lower-elevation YLR with ∼14% continuous permafrost, such increases did not occur despite a comparable extent of warming. Riverine concentrations of dissolved and particulate organic carbon increased with discharge (mobilization) in both rivers. In the JSR, DIC concentrations transitioned from dilution (decreasing concentration with discharge) in earlier, colder years to chemostasis (relatively constant concentration) in later, warmer years. This changing pattern, together with lighter δ13CDIC under high discharge, suggests that permafrost thawing boosts DIC production and export via enhancing soil respiration and weathering. These findings reveal the predominant role of warming in altering carbon lateral export by escalating concentrations and fluxes and modifying export patterns.

Labile dissolved organic matter (DOM) and nitrogen inputs modified greenhouse gas dynamics: A source-to-estuary study of the Yangtze River

Although rivers are increasingly recognized as essential sources of greenhouse gases (GHG) to the atmosphere, few systematic efforts have been made to reveal the drivers of spatiotemporal variations of dissolved GHG (dGHG) in large rivers under increasing anthropogenic stress and intensified hydrological cycling. Here, through a source-to-estuary survey of the Yangtze River in March (spring) and October (autumn) of 2018, we revealed that labile dissolved organic matter (DOM) and nitrogen inputs remarkably modified the spatiotemporal distribution of dGHG. The average partial pressure of CO2 (pCO2), CH4 and N2O concentrations of all sampling sites in the Yangtze River were 1015 ± 225 μatm, and 87.5± 36.5 nmol L-1, and 20.3 ± 6.6 nmol L-1, respectively, significantly lower than the global average. In terms of longitudinal and seasonal variations, higher GHG concentrations were observed in the middle-lower reach in spring. The dominant drivers of spatiotemporal variations in dGHG were labile, protein-like DOM components and nitrogen level. Compared with the historical data of dGHG from published literature, we found a significant increase in N2O concentrations in the Yangtze River during 2004-2018, and the increasing trend was consistent with the rising riverine nitrogen concentrations. Our study emphasized the critical roles of labile DOM and nitrogen inputs in driving the spatial hotspots, seasonal variations and annual trends of dGHG. These findings can contribute to constraining the global GHG budget estimations and controls of GHG emission in large rivers in response to global change.