Transport Of Organic Carbon Research Articles

Relationships of the hydraulic flow characteristics with the transport of soil organic carbon and sediment loss in the Loess Plateau

Erosive power is characterized by the hydraulic forces of moving water and determines the water transport capacity of soil organic carbon (SOC) and sediment. Understanding the relationships of hydraulic flow characteristics (flow velocity and depth, shear stress and stream power) with the transport of sediments and SOC will improve the understanding of the transport mechanism of sediment and SOC, which in turn will improve SOC prediction. To address this issue, two loess soils (one silt loam and one silty clay) were selected, and 32 simulated rainfall experiments were conducted in a 1m by 5m soil pan at a varying slopes (10°, 15°, 20°, and 25°) and two rainfall intensities (90mmh−1 and 120mmh−1). The results showed that the flow velocities had significant positive linear relationships with the SOC concentrations (P<0.001) under the rainfall intensity of 90mmh−1 and that the flow velocities also had close relationships with the suspension transport of clay particles in the fine-textured soil and the rolling transport of light large aggregates in the coarse-textured soil. Under the rainfall intensity of 120mmh−1, the runoff depth was positively correlated with the SOC concentrations due to the suspension transport of the clay particles and was negatively correlated with the sediment concentration. The slope had a greater effect on the sediment and SOC loss of the coarse-textured soil than those of the fine-textured soil. Additionally, the stream power was a better descriptor of the sediment loss in the loess soils than shear stress. Overall, both the soil texture and rainfall intensity changed the relationships of the hydraulic flow characteristics with the sediment and SOC loss. Finally, the results of our study will provide important knowledge for improving or building hydraulic-based SOC and sediment loss models.

A Random Forest approach to predict the spatial distribution of sediment pollution in an estuarine system.

Modeling the magnitude and distribution of sediment-bound pollutants in estuaries is often limited by incomplete knowledge of the site and inadequate sample density. To address these modeling limitations, a decision-support tool framework was conceived that predicts sediment contamination from the sub-estuary to broader estuary extent. For this study, a Random Forest (RF) model was implemented to predict the distribution of a model contaminant, triclosan (5-chloro-2-(2,4-dichlorophenoxy)phenol) (TCS), in Narragansett Bay, Rhode Island, USA. TCS is an unregulated contaminant used in many personal care products. The RF explanatory variables were associated with TCS transport and fate (proxies) and direct and indirect environmental entry. The continuous RF TCS concentration predictions were discretized into three levels of contamination (low, medium, and high) for three different quantile thresholds. The RF model explained 63% of the variance with a minimum number of variables. Total organic carbon (TOC) (transport and fate proxy) was a strong predictor of TCS contamination causing a mean squared error increase of 59% when compared to permutations of randomized values of TOC. Additionally, combined sewer overflow discharge (environmental entry) and sand (transport and fate proxy) were strong predictors. The discretization models identified a TCS area of greatest concern in the northern reach of Narragansett Bay (Providence River sub-estuary), which was validated with independent test samples. This decision-support tool performed well at the sub-estuary extent and provided the means to identify areas of concern and prioritize bay-wide sampling.

On the long-range offshore transport of organic carbon from the Canary Upwelling System to the open North Atlantic

Abstract. A compilation of measurements of net community production (NCP) in the upper waters of the eastern subtropical North Atlantic had suggested net heterotrophic conditions, purportedly supported by the lateral export of organic carbon from the adjacent, highly productive Canary Upwelling System (CanUS). Here, we quantify and assess this lateral export using the Regional Ocean Modeling System (ROMS) coupled to a nutrient, phytoplankton, zooplankton, and detritus (NPZD) ecosystem model. We employ a new Atlantic telescopic grid with a strong refinement towards the northwestern African shelf to combine an eddy-resolving resolution in the CanUS with a full Atlantic basin perspective. Our climatologically forced simulation reveals an intense offshore flux of organic carbon that transports about 19 Tg C yr−1 away from the nearshore 100 km over the whole CanUS, amounting to more than a third of the NCP in this region. The offshore transport extends beyond 1500 km into the subtropical North Atlantic, adding organic carbon along the way to the upper 100 m at rates of between 8 and 34 % of the alongshore average NCP as a function of offshore distance. Although the divergence of this lateral export of organic carbon enhances local respiration, the upper 100 m layer in our model remains net autotrophic in the entire eastern subtropical North Atlantic. However, the vertical export of this organic carbon and its subsequent remineralization at depth makes the vertically integrated NCP strongly negative throughout this region, with the exception of a narrow band along the northwestern African shelf. The magnitude and efficiency of the lateral export varies substantially between the different subregions. In particular, the central coast near Cape Blanc is particularly efficient in collecting organic carbon on the shelf and subsequently transporting it offshore. In this central subregion, the offshore transport adds as much organic carbon as nearly 60 % of the local NCP to the upper 100 m, giving rise to a sharp peak of offshore respiration that extends to the middle of the gyre. Our modeled offshore transport of organic carbon is likely a lower-bound estimate due to our lack of full consideration of the contribution of dissolved organic carbon and that of particulate organic carbon stemming from the resuspension of sediments. But even in the absence of these contributions, our results emphasize the fundamental role of the lateral redistribution of the organic carbon for the maintenance of the heterotrophic activity in the open sea.

Ocean acidification compromises a planktic calcifier with implications for global carbon cycling

Anthropogenically-forced changes in ocean chemistry at both the global and regional scale have the potential to negatively impact calcifying plankton, which play a key role in ecosystem functioning and marine carbon cycling. We cultured a globally important calcifying marine plankter (the foraminifer, Globigerina bulloides) under an ecologically relevant range of seawater pH (7.5 to 8.3 total scale). Multiple metrics of calcification and physiological performance varied with pH. At pH > 8.0, increased calcification occurred without a concomitant rise in respiration rates. However, as pH declined from 8.0 to 7.5, calcification and oxygen consumption both decreased, suggesting a reduced ability to precipitate shell material accompanied by metabolic depression. Repair of spines, important for both buoyancy and feeding, was also reduced at pH < 7.7. The dependence of calcification, respiration, and spine repair on seawater pH suggests that foraminifera will likely be challenged by future ocean conditions. Furthermore, the nature of these effects has the potential to actuate changes in vertical transport of organic and inorganic carbon, perturbing feedbacks to regional and global marine carbon cycling. The biological impacts of seawater pH have additional, important implications for the use of foraminifera as paleoceanographic indicators.

Zonal characteristics of sediment-bound organic carbon loss during water erosion: A case study of four typical loess soils in Shaanxi Province

The soils of the Loess Plateau of China exhibit zonal characteristics, with declining clay percentages from the southern to northern regions. However, these differences have been largely ignored in studies of soil erosion and the related organic carbon (OC) loss. The goal of this study was to investigate the effect of gradual changes in soil texture on the loss and selective transport of soil and soil organic carbon (SOC) on the Loess Plateau of China. Four typical soils with decreasing clay contents (26.3%, 21.2%, 15.6% and 12.1%) in Shaanxi Province were selected and studied. Rainfall-simulated experiments were performed in 1m (width)×5m (length) plots with a rainfall intensity of 120mmh−1 on three slope gradients (15°, 20° and 25°). The runoff and sediment yields, particle size distributions of the sediments, OC concentrations and enrichment ratios of OC in sediment (ERoc) during rainfall processes were measured and calculated. A comparison of the rainfall characteristics between rainfall events in different soils indicated that a decrease in the clay percentage increased the intensity of rill erosion. The runoff rates of different events decreased with the clay percentage, whereas no general pattern was observed in the sediment yield rate due to differences in the initial timing and intensity of rill erosion. Furthermore, the ERoc values in the sediments of all the events were between 0.64 and 1.44, with 90% of values between 0.80 and 1.20. Similarly, the effective particle size distribution of the sediments exhibited relatively constant values with rainfall duration. Non-significant correlations (P>0.05) between ERoc and dispersed and non-dispersed sediment particles suggested that the transport of sediment and OC was non-selective. Moreover, significant linear correlations were observed between the amount of SOC loss and sediments in all the events. However, there was no significant difference in total SOC loss between the rainfall events, except for in events with a soil clay percentage of 15.6%. SOC losses in soils with 26.3% and 21.2% clay percentages were mainly due to the high SOC concentrations of the original soils, whereas those in other soils were due to the large amount of lost sediment. However, the variations in soil loss and SOC loss were not associated with changes in the clay content of the soil. These results suggest that zonal soils with decreasing clay contents do not influence the selective transport of OC, but soil textures affect the sediment and SOC loss through complicated mechanisms.

Total organic carbon transport by the Alaknanda River, Garhwal Himalayas, India

Transport and fate of organic carbon by the fluvial system play a significant role in the global biogeochemical cycle of carbon. Previous studies show that the transportation of modern organic carbon from the Himalayan River system accounts for 10–20% of the total global flux to the oceans. Till date, no study has been published which dealt with the transport of organic carbon in the headwaters of the Ganga River. The Alaknanda River is a headwater stream of the Ganga, which flows in the Western Himalayas of India. Water and freshly deposited channel sediment samples were collected during the months of March 2014 and August 2014 and analysed for dissolved organic carbon (DOC), particulate organic carbon (POC) and channel organic carbon (COC). The observed variability of organic carbon concentration was correlated with factors such as discharge, physiography and suspended sediment concentration (SSC). The results show that seasonal erosivity in the basin influences its DOC concentration and physiography, thus acting as a key parameter which controls transportation, oxidation and residence time of the organic matter. The allochthonous input of sediments from the erosional activities is the major source of organic carbon. At Devprayag, Alaknanda contributes 66% of the total DOC flux carried by the Ganga River. The comparison with the previously published values indicate that due to differences in physiography and chemical weathering rate, the Ganga River transports organic carbon mainly as a dissolved load in its upstream and predominantly as POC down the Himalayan foothills.

Organic carbon transport in the Songhua River, NE China: Influence of land use

Carbon transported by rivers is an important component of the global carbon cycle. Here, we report on organic carbon transport along the third largest river in China, the Songhua River, and its major tributaries. Water samples were collected seasonally or more frequently to determine dissolved organic carbon (DOC) and particulate organic carbon (POC) concentrations and C/N and stable carbon isotopic ratios. Principal component analysis and multiple regression analysis of these data, in combination with hydrological records for the past 50 years, were used to determine the major factors influencing the riverine carbon fluxes. Results indicate that the organic carbon in the Songhua River basin is derived mainly from terrestrial sources. In the 2008–2009 hydrological year, the mean concentrations of DOC and POC were 5.87 and 2.36 mg/L, and the estimated fluxes of the DOC and POC were 0.30 and 0.14 t·km−2·year−1, respectively. The riverine POC and DOC concentrations were higher in subcatchments with more cropland, but the area-specific fluxes were lower, owing to decreased discharge. We found that hydrological characteristics and land-use type (whether forest or cropland) were the most important factors influencing carbon transport in this system. Agricultural activity, particularly irrigation, is the principal cause of changes in water discharge and carbon export. Over the last 50 years, the conversion of forest to cropland has reduced riverine carbon exports mainly through an associated decrease in discharge following increased extraction of water for irrigation.

Old before your time: Ancient carbon incorporation in contemporary aquatic foodwebs

AbstractThe current paradigm in trophic dynamic theory is that organisms will gain more energy from fresh, “young” organic materials because they contain the essential elements and chemical characteristics for enhancing growth, unlike more aged and thus conceptually degraded resources. However, a growing body of recent literature is raising exceptions to this resource‐age paradigm and highlights that organisms present in diverse aquatic settings in fact incorporate old organic carbon into their biomass. This shift in resource utilization has the potential to alter foodweb stability, structure, and dynamics, but has yet to be integrated into ecological studies and conceptual frameworks. In this review, we first outline a number of recent studies providing direct evidence of the transport and incorporation of old organic carbon in aquatic food webs, and then discuss the ecological implications of this changing view in resource utilization. We ultimately highlight future research avenues that will provide new insights into the response of aquatic foodwebs to the increased mobilization and delivery of old organic carbon to aquatic settings following anthropogenic perturbations and ongoing climatic change.

Distribution and sources of organic matter in size-fractionated nearshore sediments off the Barcelona city (NW Mediterranean)

Continental shelves are recognized to play a key role in the biogeochemical cycle of carbon, linking terrestrial and marine carbon reservoirs. In this study we investigate the physical and biogeochemical processes that control the source, transport and fate of organic carbon (OC) in the continental shelf off Barcelona city, in the NW Mediterranean Sea. Surface sediment samples were collected from depths of 10–40 m during late summer and autumn 2012. Grain size and biogeochemical parameters such as OC, its stable isotope δ13C, total nitrogen (TN) and OC/TN ratios were analysed in size-fractionated sediments. The influence of environmental factors over the study area was determined using hydrological and oceanographic time series, together with video images of the Barcelona coast line and nearshore region.We have found a wide range of OC contents, from 0.13 to 8.68%, depending on water depth and sediment particle size. The highest OC concentration was always found in the clay fraction (<4 μm), followed by the sand fraction (>63 μm) that contained terrestrial plant debris. Wave activity, discharge of the Besòs River and the “Espigó de Ginebra” outfall were the main mechanisms controlling the sorting of sediments by their grain size and thus the distribution of OC in the inner shelf off Barcelona. In addition, we observed that the organic matter in clay particles was progressively degraded seawards, probably because these particles remain suspended in the water column much more time compared to those that are heavier and, therefore, they are exposed for longer time periods to oxygenated conditions. Both OC/TN ratios and δ13C values found suggest that the organic matter preserved was predominantly land supplied.

Total organic carbon fluxes of the Red River system (Vietnam)

AbstractRiverine transport of organic carbon from terrestrial ecosystems to the oceans plays an important role in the global carbon cycle. The Red River is located in Southeast Asia where river discharge, sediment loads and fluxes of elements (carbon, nitrogen and phosphorus) associated with suspended solids have been dramatically altered over past decades as a result of reservoir impoundment and land use, population, and climate change. Dissolved organic carbon (DOC) and particulate organic carbon (POC) concentrations were measured monthly at four stations of the Red River system from January 2008 to December 2010. The results reveal that POC changed synchronically with total suspended solids (TSS) concentration and with the river discharge, whereas no clear trend was observed for DOC concentration. The mean value of total organic carbon (TOC = DOC + POC) flux in the delta of the Red River was 31.5 × 1013 ± 4.0 × 1013 MgC.yr−1 (range 27.9–35.8 × 1013 MgC.yr−1 which leads to a specific TOC flux of 2012 ± 255 kgC.km−2.yr−1 during this 2008–2010 period. About 80% of the TOC flux was transferred to the estuary during the rainy season as a consequence of the higher river water discharge. The high mean value of the POC:Chl‐a ratio (1585 ± 870 mgC.mgChl‐a−1) and the moderate C:N ratio (7.3 ± 0.1) in the water column system suggest that organic carbon in the Red River system is mainly derived from erosion and soil leaching in the basin. The effect of two new dam impoundments in the Red River was also observable with lower TOC fluxes in 2010 compared with 2008. Copyright © 2017 John Wiley &amp; Sons, Ltd.

Mesoscale ocean fronts enhance carbon export due to gravitational sinking and subduction

Enhanced vertical carbon transport (gravitational sinking and subduction) at mesoscale ocean fronts may explain the demonstrated imbalance of new production and sinking particle export in coastal upwelling ecosystems. Based on flux assessments from 238U:234Th disequilibrium and sediment traps, we found 2 to 3 times higher rates of gravitational particle export near a deep-water front (305 mg C⋅m-2⋅d-1) compared with adjacent water or to mean (nonfrontal) regional conditions. Elevated particle flux at the front was mechanistically linked to Fe-stressed diatoms and high mesozooplankton fecal pellet production. Using a data assimilative regional ocean model fit to measured conditions, we estimate that an additional ∼225 mg C⋅m-2⋅d-1 was exported as subduction of particle-rich water at the front, highlighting a transport mechanism that is not captured by sediment traps and is poorly quantified by most models and in situ measurements. Mesoscale fronts may be responsible for over a quarter of total organic carbon sequestration in the California Current and other coastal upwelling ecosystems.

Correction: Sources and Dynamics of Inorganic Carbon within the Upper Reaches of the Xi River Basin, Southwest China

[This corrects the article DOI: 10.1371/journal.pone.0160964.].

Contrasting composition of terrigenous organic matter in the dissolved, particulate and sedimentary organic carbon pools on the outer East Siberian Arctic Shelf

Abstract. Fluvial discharge and coastal erosion of the permafrost-dominated East Siberian Arctic delivers large quantities of terrigenous organic carbon (Terr-OC) to marine waters. The composition and fate of the remobilized Terr-OC needs to be better constrained as it impacts the potential for a climate–carbon feedback. In the present study, the bulk isotope (δ13C and Δ14C) and macromolecular (lignin-derived phenols) composition of the cross-shelf exported organic carbon (OC) in different marine pools is evaluated. For this purpose, as part of the SWERUS-C3 expedition (July–September 2014), sediment organic carbon (SOC) as well as water column (from surface and near-bottom seawater) dissolved organic carbon (DOC) and particulate organic carbon (POC) samples were collected along the outer shelves of the Kara Sea, Laptev Sea and East Siberian Sea. The results show that the Lena River and the DOC may have a preferential role in the transport of Terr-OC to the outer shelf. DOC concentrations (740–3600 µg L−1) were 1 order of magnitude higher than POC (20–360 µg L−1), with higher concentrations towards the Lena River plume. The δ13C signatures in the three carbon pools varied from −23.9 ± 1.9 ‰ in the SOC, −26.1 ± 1.2 ‰ in the DOC and −27.1 ± 1.9 ‰ in the POC. The Δ14C values ranged between −395 ± 83 (SOC), −226 ± 92 (DOC) and −113 ± 122 ‰ (POC). These stable and radiocarbon isotopes were also different between the Laptev Sea and the East Siberian Sea. Both DOC and POC showed a depleted and younger trend off the Lena River plume. Further, the Pacific inflow and the sea-ice coverage, which works as a barrier preventing the input of “young” DOC and POC, seem to have a strong influence in these carbon pools, presenting older and more enriched δ13C signatures under the sea-ice extent. Lignin phenols exhibited higher OC-normalized concentrations in the SOC (0.10–2.34 mg g−1 OC) and DOC (0.08–2.40 mg g−1 OC) than in the POC (0.03–1.14 mg g−1 OC). The good relationship between lignin and Δ14C signatures in the DOC suggests that a significant fraction of the outer-shelf DOC comes from “young” Terr-OC. By contrast, the slightly negative correlation between lignin phenols and Δ14C signatures in POC, with higher lignin concentrations in older POC from near-bottom waters, may reflect the off-shelf transport of OC from remobilized permafrost in the nepheloid layer. Syringyl ∕ vanillyl and cinnamyl ∕ vannillyl phenol ratios presented distinct clustering between DOC, POC and SOC, implying that those pools may be carrying different Terr-OC of partially different origin. Moreover, 3,5-dihydroxybenzoic acid to vanillyl phenol ratios and p-coumaric acid to ferulic acid ratios, used as a diagenetic indicators, enhanced in POC and SOC, suggesting more degradation within these pools. Overall, the key contrast between enhanced lignin yields both in the youngest DOC and the oldest POC samples reflects a significant decoupling of terrestrial OC sources and pathways.

Intracellular Nitrate of Marine Diatoms as a Driver of Anaerobic Nitrogen Cycling in Sinking Aggregates.

Diatom-bacteria aggregates are key for the vertical transport of organic carbon in the ocean. Sinking aggregates also represent pelagic microniches with intensified microbial activity, oxygen depletion in the center, and anaerobic nitrogen cycling. Since some of the aggregate-forming diatom species store nitrate intracellularly, we explored the fate of intracellular nitrate and its availability for microbial metabolism within anoxic diatom-bacteria aggregates. The ubiquitous nitrate-storing diatom Skeletonema marinoi was studied as both axenic cultures and laboratory-produced diatom-bacteria aggregates. Stable 15N isotope incubations under dark and anoxic conditions revealed that axenic S. marinoi is able to reduce intracellular nitrate to ammonium that is immediately excreted by the cells. When exposed to a light:dark cycle and oxic conditions, S. marinoi stored nitrate intracellularly in concentrations >60 mmol L-1 both as free-living cells and associated to aggregates. Intracellular nitrate concentrations exceeded extracellular concentrations by three orders of magnitude. Intracellular nitrate was used up within 2–3 days after shifting diatom-bacteria aggregates to dark and anoxic conditions. Thirty-one percent of the diatom-derived nitrate was converted to nitrogen gas, indicating that a substantial fraction of the intracellular nitrate pool of S. marinoi becomes available to the aggregate-associated bacterial community. Only 5% of the intracellular nitrate was reduced to ammonium, while 59% was recovered as nitrite. Hence, aggregate-associated diatoms accumulate nitrate from the surrounding water and sustain complex nitrogen transformations, including loss of fixed nitrogen, in anoxic, pelagic microniches. Additionally, it may be expected that intracellular nitrate not converted before the aggregates have settled onto the seafloor could fuel benthic nitrogen transformations.

Dissimilatory microbial iron reduction release DOC (dissolved organic carbon) from carbon-ferrihydrite association

Carbon-mineral association between iron oxides and dissolved organic carbon (DOC) occurs ubiquitously in natural environments, and is thus an important control on the transport and sequestration of organic carbon. However, the stability of C-mineral association under reducing conditions and its implication on carbon turnover and cycling is unclear. In this study, anaerobic incubation experiments were conducted to investigate the biodegradability of ferrihydrite (Fe-oxide)-adsorbed natural DOC with known Fe-reducing bacterium, Shewanella oneidensis MR-1. Two types of bulk DOC were used to form the carbon-ferrihydrite association –forest fresh leaf layer (LDOC) and forest humified layer (HDOC). Under controlled pH and redox condition, DOC concentrations increased by 11.2 (±0.4)% and 13.3 (±0.3)% for LDOC and HDOC after anaerobic microbial iron reduction. Coupled to the DOC release, 28% and 14% of Fe(III) were reduced with LDOC and HDOC, respectively. Our results demonstrated that under controlled pH conditions, the C release was driven by dissimilatory iron reduction. Likely microbial iron reduction decrease the carbon sequestration potential via reducing the Fe-oxides surface area and therefore release DOC to the solution and make it more susceptible to microbial degradation. Further, dissimilatory iron reduction also shifted DOC spectroscopic properties (UV and fluorescence) and pronounced increase in humification index (HIX) values was observed after the incubation. Our results suggest dissimilatory iron reduction is an important mechanism for Fe dissolution and C mobilization, which impacts the long-term carbon transformation, storage and turnover in soil environments.

Sources and Dynamics of Inorganic Carbon within the Upper Reaches of the Xi River Basin, Southwest China.

The carbon isotopic composition (δ13C) of dissolved and particulate inorganic carbon (DIC; PIC) was used to compare and analyze the origin, dynamics and evolution of inorganic carbon in two headwater tributaries of the Xi River, Southwest China. Carbonate dissolution and soil CO2 were regarded as the primary sources of DIC on the basis of δ13CDIC values which varied along the Nanpan and Beipan Rivers, from −13.9‰ to 8.1‰. Spatial trends in DIC differed between the two rivers (i.e., the tributaries), in part because factors controlling pCO2, which strongly affected carbonate dissolution, differed between the two river basins. Transport of soil CO2 and organic carbon through hydrologic conduits predominately controlled the levels of pCO2 in the Nanpan River. However, pCO2 along the upper reaches of the Nanpan River also was controlled by the extent of urbanization and industrialization relative to agriculture. DIC concentrations in the highly urbanized upper reaches of the Nanpan River were typical higher than in other carbonate-dominated areas of the upper Xi River. Within the Beipan River, the oxidation of organic carbon is the primary process that maintains pCO2 levels. The pCO2 within the Beipan River was more affected by sulfuric acid from coal industries, inputs from a scenic spot, and groundwater than along the Nanpan River. With regards to PIC, the contents and δ13C values in the Nanpan River were generally lower than those in the Beipan River, indicating that chemical and physical weathering contributes more marine carbonate detritus to the PIC along the Beipan River. The CO2 evasion flux from the Nanpan River was higher than that in the Beipan River, and generally higher than along the middle and lower reaches of the Xi River, demonstrating that the Nanpan River is an important net source of atmospheric CO2 in Southwest China.

Sources and controls of organic carbon in lakes across the subarctic treeline

Abundant northern lakes have an intrinsic role in the transport, sequestration, and mineralization of terrestrial organic carbon. The quantity and quality of this carbon control vital aquatic biogeochemical processes, and influence the metabolic balance of lakes with subsequent impact on the global carbon cycle. We measured concentrations and type of dissolved organic matter and elemental and stable isotopic composition of carbon and nitrogen in 31 subarctic lakes with varying catchment types across the treeline in northern Finland, integrating both the pelagic (lake water) and the benthic (surface sediments) carbon pools for a comprehensive understanding of landscape influence on aquatic carbon dynamics. Wetland cover was identified as the primary catchment control over the aquatic carbon pools, reflected particularly in the bio-optical properties of lake water. Landscape influence on sediment carbon content and composition, mirroring largely the structure and productivity of the aquatic communities, was primarily connected to allochthonous nutrient inputs fueling autotrophic production. Basin depth and benthic production were identified as important internal controls on the surface sediment geochemistry. Overall, our results suggest that shallow subarctic lakes will be particularly susceptible to climate-mediated changes in the export of terrestrial organic matter from wetlands. Whether the landscape influence will promote the channeling of terrestrial carbon into the atmosphere via aquatic ecosystems will strongly depend on the interplay between the biogeochemical characteristic of the allochthonous carbon inputs, terrestrial nutrient fluxes, and the depth of the recipient ecosystems.

Diverse origins and pre-depositional histories of organic matter in contemporary Chinese marginal sea sediments

Marginal seas are estimated to account for up to 90% of organic carbon (OC) burial in marine sediments, and thus play an important role in global carbon cycle. However, comprehensive assessments of carbon budgets for marginal sea systems are challenging due to their inherent complexity, with spatial and temporal variability in carbon inputs and dispersal processes. We examine the Bohai Sea and Yellow Sea (BS–YS) in order to further our understanding of sedimentary OC delivery, translocation and accumulation in a shallow marginal sea system. Bulk properties and the content and isotopic compositions (Δ14C, δ13C) of source-specific plant wax n-alkyl lipid biomarkers were determined for a suite of surficial sediment samples. Variable δ13C values (−25.1‰ to −28.5‰) and contemporary radiocarbon ages of short-chain n-fatty acids (FAs; C16, C18) reflect modern autochthonous marine and/or fresh terrestrial plant input. In contrast, extremely depleted Δ14C values (−932‰ to −979‰) of short-chain n-alkanes (C16, C18) suggest a predominant input from sedimentary rocks (petrogenic OC) or petroleum. Abundance-weighted average δ13C and Δ14C values of long-chain leaf wax lipids (C26+28+30n-FAs, C24+26+28n-alkanols, C27+29+31n-alkanes) are −29.1±1.1‰ to −30.2±0.3‰, and −286±150‰ to −442±119‰, respectively, illustrating that terrestrial OC delivery is dominated by pre-aged (∼3000–500014Cyrs) C3 vegetation sources.A coupled carbon-isotopic mixing model, based on the bulk and compound-specific biomarker δ13C and Δ14C values, is used to partition the BS–YS sedimentary OC into three components that reflect both origins and transport processes. For all sampling sites, 31–64% is modern/contemporary OC, 24–49% is pre-aged terrestrial OC, and 7–26% is fossil OC, the latter likely derived from both physical erosion of ancient sedimentary rocks and fossil fuel sources. Pre-aged soil OC is most prominent in front of the modern and old Huanghe (Yellow River) delta (48% and 49%), and fossil OC is most significant north of the old Huanghe mouth (26%). Significant pre-aged soil contributions (33%) are also evident for sites further offshore, where transport and deposition of eolian dust supply may be important. For the three major deposition areas of the BS–YS system (Bohai Basin, sub-aqueous Huanghe delta and central south YS basin), we estimate that about 3.02Mt/yr of refractory, plant-derived pre-aged soil OC and 0.98Mt/yr of 14C-depleted fossil OC accumulates in surface sediments, corresponding to 35% and 11% of sediment TOC, respectively. Compared with estimates for fluxes from corresponding sources, the burial efficiency is close to 100% for pre-aged soil OC and 70% for fossil OC, implying efficient OC burial in delta and shelf environments. Re-burial of these two pools of terrigenous OC only affects carbon cycling on millennial and longer timescales respectively, and exerts little influence on the modern carbon cycle (<100yr). Carbon isotopic compositions of source specific biomarkers are a useful tool not only for constraining OC sources and transport vectors, but also for delineating their impact on the contemporary carbon cycling in marginal sea systems.

Climate change increases riverine carbon outgassing, while export to the ocean remains uncertain

Abstract. Any regular interaction of land and river during flooding affects carbon pools within the terrestrial system, riverine carbon and carbon exported from the system. In the Amazon basin carbon fluxes are considerably influenced by annual flooding, during which terrigenous organic material is imported to the river. The Amazon basin therefore represents an excellent example of a tightly coupled terrestrial–riverine system. The processes of generation, conversion and transport of organic carbon in such a coupled terrigenous–riverine system strongly interact and are climate-sensitive, yet their functioning is rarely considered in Earth system models and their response to climate change is still largely unknown. To quantify regional and global carbon budgets and climate change effects on carbon pools and carbon fluxes, it is important to account for the coupling between the land, the river, the ocean and the atmosphere. We developed the RIVerine Carbon Model (RivCM), which is directly coupled to the well-established dynamic vegetation and hydrology model LPJmL, in order to account for this large-scale coupling. We evaluate RivCM with observational data and show that some of the values are reproduced quite well by the model, while we see large deviations for other variables. This is mainly caused by some simplifications we assumed. Our evaluation shows that it is possible to reproduce large-scale carbon transport across a river system but that this involves large uncertainties. Acknowledging these uncertainties, we estimate the potential changes in riverine carbon by applying RivCM for climate forcing from five climate models and three CO2 emission scenarios (Special Report on Emissions Scenarios, SRES). We find that climate change causes a doubling of riverine organic carbon in the southern and western basin while reducing it by 20 % in the eastern and northern parts. In contrast, the amount of riverine inorganic carbon shows a 2- to 3-fold increase in the entire basin, independent of the SRES scenario. The export of carbon to the atmosphere increases as well, with an average of about 30 %. In contrast, changes in future export of organic carbon to the Atlantic Ocean depend on the SRES scenario and are projected to either decrease by about 8.9 % (SRES A1B) or increase by about 9.1 % (SRES A2). Such changes in the terrigenous–riverine system could have local and regional impacts on the carbon budget of the whole Amazon basin and parts of the Atlantic Ocean. Changes in riverine carbon could lead to a shift in the riverine nutrient supply and pH, while changes in the exported carbon to the ocean lead to changes in the supply of organic material that acts as a food source in the Atlantic. On larger scales the increased outgassing of CO2 could turn the Amazon basin from a sink of carbon to a considerable source. Therefore, we propose that the coupling of terrestrial and riverine carbon budgets should be included in subsequent analysis of the future regional carbon budget.

Forms and Fluxes of Soil Organic Carbon Transport via Overland Flow, Interflow, and Soil Erosion

Core Ideas Interflow is another crucial route of soil organic carbon lateral transport. Overland flow, interflow, and sediment contributed 5, 24, and 71%, respectively, of annual loss loads of soil organic carbon lateral transport. Dissolved organic carbon is another important component for SOC lateral transport. The contributions of hydrological pathways (including overland flow, interflow, and soil erosion) to lateral soil organic carbon (SOC) transport have remained unclear until now. Hillslopes were monitored using free‐draining lysimeters (8 m by 4 m) to quantify dissolved organic carbon (DOC) losses due to overland flow and interflow and the total organic carbon (TOC), water extractable organic carbon (WEOC), particulate organic carbon (POC), and mineral‐associated organic carbon (MOC) fractions in sediments from sloping croplands containing Regosols in Southwest China. The average annual DOC losses due to overland flow and interflow were 158.8 ± 33.0 and 750.4 ± 79.3 mg C m–2, respectively, and the TOC lost with sediment was 2201.0 ± 429.2 mg C m‒2. Overland flow, interflow, and sediment accounted for 5, 24, and 71%, respectively, of the annual SOC losses. The average annual DOC, POC, and MOC loss fluxes were 918.6 ± 115.3, 375.2 ± 94.4, and 1816.4 ± 331.8 mg C m–2, respectively. The MOC contents in the sediments were positively correlated with rainfall, and the DOC concentrations in the interflow water were negatively correlated with rainfall. In conclusion, soil erosion is the dominant hydrological route for lateral SOC transport, and interflow is another crucial route that is usually underestimated. Soil organic C is mainly lost in the forms of MOC and DOC, which is an important component of water ecosystems. Therefore, the mitigation of SOC losses would be more effective if soil erosion and interflow conservation practices were adopted together, particularly for Regosols on hillslopes.