Dissolved Organic Carbon Pool Research Articles

Controls on the Respiration of Ancient Carbon Draining From Permafrost Soils Into Sunlit Arctic Surface Waters

AbstractThe thawing of ancient organic carbon stored in arctic permafrost soils, and its oxidation to carbon dioxide (CO2, a greenhouse gas), is predicted to amplify global warming. However, the extent to which organic carbon in thawing permafrost soils will be released as CO2 is uncertain. A critical unknown is the extent to which dissolved organic carbon (DOC) from thawing permafrost soils is respired to CO2 by microbes upon export of freshly thawed DOC to both dark bottom waters and sunlit surface waters. In this study, we quantified the radiocarbon age and 13C composition of CO2 produced by microbial respiration of DOC that was leached from permafrost soils and either kept in the dark or exposed to ultraviolet and visible wavelengths of light. We show that permafrost DOC most labile to microbial respiration was as old or older (ages 4,000–11,000 a BP) and more 13C‐depleted than the bulk DOC in both dark and light‐exposed treatments, likely indicating respiration of old, 13C‐depleted lignin and lipid fractions of the permafrost DOC pool. Light exposure either increased, decreased, or had no effect on the magnitude of microbial respiration of old permafrost DOC relative to respiration in the dark, depending on both the extent of DOC oxidation during exposure to light and the wavelength of light. Together, these findings suggest that photochemical changes affecting the lability of permafrost DOC during sunlight exposure are an important control on the magnitude of microbial respiration of permafrost DOC in arctic surface waters.

Variable aging and storage of dissolved black carbon in the ocean

During wildfires and fossil fuel combustion, biomass is converted to black carbon (BC) via incomplete combustion. BC enters the ocean by rivers and atmospheric deposition contributing to the marine dissolved organic carbon (DOC) pool. The fate of BC is considered to reside in the marine DOC pool, where the oldest BC 14C ages have been measured (>20,000 14C y), implying long-term storage. DOC is the largest exchangeable pool of organic carbon in the oceans, yet most DOC (>80%) remains molecularly uncharacterized. Here, we report 14C measurements on size-fractionated dissolved BC (DBC) obtained using benzene polycarboxylic acids as molecular tracers to constrain the sources and cycling of DBC and its contributions to refractory DOC (RDOC) in a site in the North Pacific Ocean. Our results reveal that the cycling of DBC is more dynamic and heterogeneous than previously believed though it does not comprise a single, uniformly "old" 14C age. Instead, both semilabile and refractory DBC components are distributed among size fractions of DOC. We report that DBC cycles within DOC as a component of RDOC, exhibiting turnover in the ocean on millennia timescales. DBC within the low-molecular-weight DOC pool is large, environmentally persistent and constitutes the size fraction that is responsible for long-term DBC storage. We speculate that sea surface processes, including bacterial remineralization (via the coupling of photooxidation of surface DBC and bacterial co-metabolism), sorption onto sinking particles and surface photochemical oxidation, modify DBC composition and turnover, ultimately controlling the fate of DBC and RDOC in the ocean.

Rapid removal and replacement of dissolved organic matter during circulation through ultramafic crust

Large volumes of seawater have passed through the rocky subseafloor throughout Earth's history. The scale of circulation is sufficiently large to impact the cycling of marine dissolved organic carbon (DOC), one of the largest pools of reduced carbon on Earth whose sources and sinks remain enigmatic, and to sequester carbon over geologic timescales. While the fate of DOC in numerous mafic systems has been examined, no previous reports are available on the less studied but still abundant ultramafic systems. We analyzed the concentration and composition of DOC from the Lost City hydrothermal field (30°N, Mid-Atlantic Ridge), a long-lived ultramafic system with minimal magmatic input. We show that per liter of seawater, more DOC is removed and a rate >650 times faster rate than in mafic ridge flank systems. Simultaneously, newly synthesized 14C-free organics are exported into the water column, adding a pre-aged component to the deep DOC pool. The sequestration of oceanic organic molecules onto minerals could partially account for the substantial total organic carbon present in ultramafic rocks, which is currently interpreted as evidence of chemoautotrophy or abiotic synthesis.

Responses of soil hexapod communities to increasing nitrogen in a subarctic grassland

The warming of boreal ecosystems accelerates decomposition and increases nitrogen (N) availability. The impact of increased N on subarctic soil fauna communities, however, remains poorly understood. We investigated the response of soil hexapods to a N addition experiment in a subarctic grassland. We characterized the soil hexapod communities using environmental DNA metabarcoding and analyzed the levels of dissolved organic carbon (DOC), dissolved organic nitrogen (DON), microbial carbon (Cmic), and microbial nitrogen (Nmic). N addition increased DON and Nmic, while DOC and Cmic pools remained unchanged. Furthermore, N addition caused shifts in soil hexapod community compositional diversity between control and N plots in herbivore and microbivore taxa. The levels of DON and Nmic strongly correlated with these shifts, explaining 54% and 45% of the compositional variability, respectively. This study demonstrates a clear link between N availability and shifts in soil hexapod communities, associated to changes in microbial and dissolved N pools in subarctic grasslands.

Radiolytically reworked Archean organic matter in a habitable deep ancient high-temperature brine

Investigations of abiotic and biotic contributions to dissolved organic carbon (DOC) are required to constrain microbial habitability in continental subsurface fluids. Here we investigate a large (101–283 mg C/L) DOC pool in an ancient (>1Ga), high temperature (45–55 °C), low biomass (102−104 cells/mL), and deep (3.2 km) brine from an uranium-enriched South African gold mine. Excitation-emission matrices (EEMs), negative electrospray ionization (–ESI) 21 tesla Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS), and amino acid analyses suggest the brine DOC is primarily radiolytically oxidized kerogen-rich shales or reefs, methane and ethane, with trace amounts of C3–C6 hydrocarbons and organic sulfides. δ2H and δ13C of C1–C3 hydrocarbons are consistent with abiotic origins. These findings suggest water-rock processes control redox and C cycling, helping support a meagre, slow biosphere over geologic time. A radiolytic-driven, habitable brine may signal similar settings are good targets in the search for life beyond Earth.

Dynamics of the Marine Dissolved Organic Carbon Reservoir in Glacial Climate Simulations: The Importance of Biological Production

AbstractThe marine dissolved organic carbon (DOC) reservoir rivals the atmospheric carbon inventory in size. Recent work has suggested that the size of the DOC reservoir may respond to variations in sea temperature and global overturning circulation strength. Moreover, mobilization of marine DOC has been implicated in paleoclimate events including Cryogenian glaciation and Eocene hyperthermals. Despite these suggestions, the dynamics of the marine DOC reservoir are poorly understood, and previous carbon cycle modeling has generally assumed this reservoir to be static. In this study, we utilize an Earth system model of intermediate complexity to assess the response of the marine DOC reservoir to various glacial boundary conditions. Our results indicate that the marine DOC reservoir is responsive to glacial perturbations and may shrink or expand on the order of 10–100 Pg C. In contrast to recent studies that emphasize the importance of DOC degradation in driving the mobility of DOC reservoir, our study indicates the importance of DOC production. In the experiment under full glacial boundary conditions, for example, a 19% drop in net primary production leads to an 81 Pg C reduction in the DOC pool, without which the atmospheric CO2 concentration would have been lower by approximately 38 ppm by dissolved inorganic carbon changes alone. Thus, DOC reservoir variability is necessary to fully account for the simulated changes in atmospheric CO2 concentration. Our findings based on glacial experiments are corroborated in a different set of simulations using freshwater flux to induce weakening of the Atlantic meridional overturning circulation.

Microorganisms, their residues, and soil carbon storage under a continuous maize cropping system with eight years of variable residue retention

Understanding the effects of agricultural management on microbial community dynamics and their relationships with soil carbon (C) properties is crucial to designing better management strategies (e.g., crop residue and tillage management) to enhance soil organic carbon (SOC) storage and other ecosystem services.In a no-till (NT) continuous maize production system in Northeast China, we combined living and dead microbial biomarker analyses, mid-infrared spectroscopy, and multivariate statistics to examine the effect of variable rates (0 %, 33 %, 67 % and 100 %) of maize stover retention for eight years on soil microbial community structure and the related change in soil C properties. We observed increases in the relative abundance of saprotrophic fungi (SF%) and the fungal to bacterial biomass ratio in the 33 % (NT33) and 67 % (NT67) retention treatments relative to complete stover removal (NT0). However, these increases were absent in the 100 % (NT100) stover return treatment suggesting a non-linear microbial community response to retention rate.Structural equation modeling revealed a negative relationship between dissolved organic carbon (DOC) concentration and the microbial stress indicator, as determined by ratios of cyclopropane phospholipid fatty acids to their precursors. Such negative relationship had a negative impact on SF%, subsequently influencing the fungal to bacterial necromass ratio and the aromaticity of soil C. A radar diagram area-based Soil Quality Index (SQI) approach showed that NT33 was associated with the highest SQI area, suggesting that low rates (33 %) of residue return had the greatest SOC sustainability through the enrichment of the labile DOC pool, efficient microbial growth and maintaining fungal necromass C persistence.This study adds to the mechanistic understanding of cellular trait-based microbial strategies with implications for soil C dynamics, and provides guidance for future management strategies in conservation agriculture.

Carbon on the Ocean Surface: Temporal and Geographical Investigation

The sea surface nanolayer (SSnL) is enriched with lipids and proteins; working together, these macromolecules create a unique region between the atmosphere and ocean. Lipids and proteins affect the ocean’s surface chemistry, which causes surface tension suppression. Carbon enrichment is ultimately reflected in organic sea spray aerosols and wave damping mechanisms that impact micrometeorology. In the present work, we calculate and examine carbon in the SSnL with a global analysis of carbon that incorporates organic surfactant dynamics. Our carbon estimates are based on chlorophyll distributions deduced from Energy Exascale Earth System Model output data. We assume that chlorophyll can approximate phytoplankton and that phytoplankton are primary contributors to the dissolved organic carbon (DOC) pool. The model calculates the carbon concentration from phytoplankton and zooplankton concentrations, fits a Langmuir isotherm to determine fractional surface coverage, and converts from the fractional coverage to a mass using surface excess to estimate carbon in the SSnL. Organic geochemical diversity of the ocean surface is evidenced by monthly variability and global differences among carbon averages. The model is an estimation of carbon because of the limited data for carbohydrate co-adsorption and molecular behavior of the DOC and particulate organic carbon (POC) pool, but our results can be used as a benchmark throughout future works. Our results reveal large seasonal shifts across the major biomes and among regional interfacial totals. However, the total mass of ∼10–4 gigatons does not change significantly throughout the year. We propose based on our results that adsorptive equilibria control the organic content of the nanolayer. To the best of our knowledge, these calculations constitute the first example of SSnL organic carbon integrations performed on a planetary scale.

Carbonate�Corganic decoupling during the first Neoproterozoic carbon isotope excursion

<p><b>The Neoproterozoic Era (1000�C539 Ma) experienced repeated negative carbon isotope excursions, representing the largest negative excursions in Earth history and their origins remain enigmatic. One salient observation is that unlike younger Phanerozoic carbon isotope excursions that almost exclusively exhibit covariation in the carbon isotopes of carbonate and organic carbon, about half of Neoproterozoic excursions are expressed in carbonate carbon isotopes (��</b><sup><b>13</b></sup><b>C</b><sub><b>carb</b></sub><b>) but their organic carbon isotopes (��</b><sup><b>13</b></sup><b>C</b><sub><b>org</b></sub><b>) lack the same anomaly. Documenting which excursions exhibit carbonate�Corganic coupling or decoupling may provide a possible clue as to the enigmatic dynamics of the anomalous Neoproterozoic carbon cycle. We report here that a newly recognized and the first Neoproterozoic carbon isotope anomaly also exhibits decoupling. Thus, the ca. 940 Ma Majiatun anomaly now represents the oldest-known decoupled Neoproterozoic excursion��300 m.y. earlier than previously thought. Ruling out altogether or mostly merely localized artifacts such as diagenesis, changes in water depth, and contamination of detrital organic matter, we interpret this new evidence of carbonate�Corganic ��</b><sup><b>13</b></sup><b>C decoupling in the context of other anomalies through time. Although the remineralization of a large pool of dissolved organic carbon (DOC) was proposed to explain late Neoproterozoic ��</b><sup><b>13</b></sup><b>C decoupled anomalies, our earlier evidence of decoupling suggests this pool was much more ancient. Our new data also require the temporal oscillation between decoupling and coupling of the Neoproterozoic carbon cycle, implying a dynamic diminution of the large Precambrian DOC pool reflective of the rising but equally variable oxygenation of the oceans and rise of animal life at this time.</b></p>

Tracing terrestrial versus marine sources of dissolved organic carbon in the largest monsoonal Godavari estuary in India using stable carbon isotopes

To examine the sources of dissolved organic carbon (DOC) and its modifications within the estuary, time-series sampling was conducted at 10 stations covering the entire salinity range in the Godavari estuary. The salinity of the upper estuary was close to zero and increased to 32 at the lower estuary during the dry period whereas the entire estuary was filled with fresh water during the wet period. The estuary received a high concentration of suspended particulate matter (SPM) during the wet compared to the dry period. The concentration of DOC and DON was 1.5–3 times higher during the wet than dry period with DOC: DON ratios of 5–20. The depleted isotopic composition of DOC (δ13CDOC) was observed during the dry period in the upper estuary and it was enriched towards the lower estuary suggesting that terrestrial organic matter (mainly C3 plant) is the source in the upper and marine sources in the lower estuary. Highly enriched δ13CDOC values were noticed during July–August, which was associated with peak discharge, suggesting that soil organic matter may be a potential source. Enhanced phytoplankton biomass was observed during October–November associated with depleted δ13CDOC from that peak discharge period suggesting DOC released by phytoplankton bloom by phytoplankton bloom (Merismopedia sp.) by freshwater algae may be contributed significantly to the DOC pool. Based on the deviation from the simple mixing line between the upper and lower estuary, the removal of DOC seems to be occurring through microbial decomposition and photo-oxidation during the dry period. The modification of DOC within the estuary may be minimal due to the low residence of water (<1 day) during the wet period. As a result, a significant amount of terrestrial carbon is transported to the coastal ocean during the wet period and its impact is reported to be seen up to 15–25 km from the coast in the coastal Bay of Bengal.

Rare earth element behaviour in seawater under the influence of organic matter cycling during a phytoplankton spring bloom – A mesocosm study

Rare earth elements (REEs) are used as powerful proxies for a variety of oceanic processes. The understanding of their biogeochemical behaviour in the marine environment is therefore essential. While the influence of OM-cycling on REE patterns in seawater is considered as insignificant, it has been shown that algae and bacteria provide good sorption surfaces for REEs and that components of the dissolved OM pool are able to complex REEs, thus potentially altering their behaviour. To investigate the impact of bio-associated processes on REEs in the bio-productive marine environment, we conducted an indoor mesocosm experiment that mimicked a phytoplankton spring bloom in the neritic coastal North Sea. The incubation period of 38 days covered two distinct phytoplankton bloom phases (diatoms followed by Phaeocystis sp.) and an interjacent bacterioplankton maximum. All dissolved REEs (dREEs) except samarium showed similar temporal concentration patterns, which were closely connected to the bloom succession. The concentration patterns were shaped by the ‘phytoplankton-shuttle’, which summarizes adsorption processes on phytoplankton-derived particulate OM (POM) and resulted in decreasing dREE concentrations alongside chlorophyll-a and POM maxima. The ‘heterotrophic-shuttle’ resulted in increasing dREE concentrations likely linked to heterotrophically mediated regeneration of POM and associated desorption processes. The effect of these processes on dREEs resulted in enhanced fractionation of light REEs (LREEs) relative to heavy REEs (HREEs) during adsorption processes and decreased fractionation as a result of desorption. At times of high dissolved organic carbon (DOC) concentrations, we observed a stabilization of especially dHREEs likely in organic complexes. To test the potential influence of DOC on dREEs, we used a PHREEQC model approach that revealed dREE complexation with components of the DOC pool and an increase in complexation with atomic mass of the REEs. That is, at high DOC concentrations OM-dREE complexation leads to an effective and preferential buffering of dHREE against adsorption. Our findings reveal that OM-cycling influences concentration patterns of dREEs via ad- and desorption processes as well as organic complexation with parts of the OM pool, suggesting these processes can have a significant impact on dREE concentrations in the natural marine environment under high OM conditions.

Effects of Temperature on Transparent Exopolymer Particle Production and Organic Carbon Allocation of Four Marine Phytoplankton Species.

Simple SummaryTEP is the bridge mediating DOC and POC conversion in the ocean, which is a key component of the marine carbon cycle. TEP accelerates the accumulation or deposition rate of organic carbon at the sea surface and promotes the short- and long-term sequestration of carbon, counteracting atmospheric CO2 increase and global warming. Temperature, as the main driving factor of carbon fixation during phytoplankton photosynthesis, is closely related to TEP production. However, little is known about the effects of temperature on TEP the production, DOC secretion, and carbon pool allocation of phytoplankton. This study analyzed the effect of temperature on the carbon pool allocation of phytoplankton and the significance of TEP in the marine carbon pool. Our results suggest that increased temperature affects carbon pool allocation in phytoplankton cells by promoting DOC exudation and extracellular TEP formation. This study provides an important basis for understanding the contribution of TEP to the allocation of POC and DOC and will benefit the prediction of phytoplankton TEP production and the marine carbon cycle under the background of global warming.Transparent exopolymer particles (TEP) are sticky polymeric substances that are commonly found in the periphery of microbial cells or colonies. They can naturally flocculate smaller suspended particles into larger aggregates and thus play a crucial role in the biological pump and the global carbon cycle. Phytoplankton are the major contributors to marine TEP production, whereas the way TEP production interacts with abiotic factors at the species level is generally unknown but critical for estimating carbon fluxes. In this study, the effects of temperature on TEP production and carbon allocation were studied in two representative diatom species (Nitzschia closterium and Chaetoceros affinis) and two model dinoflagellate species (Prorocentrum micans and Scrippisella trichoidea). The results showed that temperature had a significant impact on TEP production in all species. First, increased temperature promoted the TEP production of all four species. Second, elevated temperature affected the carbon pool allocation, with enhanced dissolved organic carbon (DOC) exudation in the form of TEP in all species. The TEP-C/DOC percentages of N. closterium and P. micans were 93.42 ± 5.88% and 82.03 ± 21.36% at the highest temperature (24 °C), respectively, which was approximately two to five times higher than those percentages at 16 °C. In contrast, TEP’s contribution to the POC pool is lower than that to the DOC pool, ranging from 6.74 ± 0.79% to 28.31 ± 1.79% for all species. Moreover, phytoplankton TEP production may be related to cellular size and physiology. The TEP content produced by the smallest N. closterium (218.96 ± 15.04 fg Xeq./μm3) was ~5 times higher compared to P. micans, S. trichoidea, or C. affinis. In conclusion, TEP production is temperature sensitive and species specific, which should be taken into consideration the regarding TEP-mediated oceanic carbon cycle, particularly in the context of global warming.

Dissolved pyrogenic carbon leaching in soil: Effects of soil depth and pyrolysis temperature

Pyrogenic organic carbon (PyC) is found in soils as a heterogeneous mixture of thermally altered plant residues that range in their susceptibility to losses via mineralization, leaching, and erosion. Leaching of PyC as DOC (DPyC) within the soil profile is likely influenced by the chemical composition of solid PyC and the dominant soil processes and properties at a particular depth. Here we report the results of a 2-year laboratory decomposition and leaching study designed to investigate the interactive effects of pyrolysis temperature levels (no pyrolysis, 300 °C, and 450 °C) and soil depth (surface soil, 0–10 cm depth; subsurface soil, 50–70 cm depth) on losses of 13C-labeled jack pine (Pinus banksiana) wood, wood pyrolyzed at 300 °C (PyC300) and wood pyrolyzed at 450 °C (PyC450) as dissolved organic carbon (DOC). Losses of wood and PyC in the 13C- DOC pool were measured in leachates drained from soils once a month over the course of 1 year, and at one single leaching event after 2 years. We found that pyrolysis temperature levels interacted with time and soil depth to affect losses of wood and PyC as DOC throughout the 1-year incubation and leaching study, with greater DOC losses from wood in surface soil (0.73 ± 0.076% of added C) than in subsurface soil (0.40 ± 0.063% of added C) averaged across sampling time. Monthly DOC losses from PyC were not affected by soil depth. Cumulative data indicated a small contribution of wood (2.86 ± 0.07%), PyC300 (0.40 ± 0.04%), and PyC450 (0.16 ± 0.01%) to total DOC leached from soils. DOC losses from wood and PyC300 (as a proportion of added C) were greater in surface soil than in subsurface soil, whereas DOC losses from PyC450 were unaffected by differences between surface and subsurface soil. Losses of DOC were greater from PyC300 than from PyC450 in surface soil, with no significant differences in DOC losses between PyC300 and PyC450 in subsurface soil. One single leaching event at the end of the 2-year decomposition study resulted in higher DOC losses from wood in subsurface soil than in surface soil likely due to desorption, and no differences in DOC losses between PyC300 and PyC450 regardless of soil depth. Our results suggest strong interactions between the initial physicochemical composition of organic C inputs and soil properties (soil depth as a proxy) that control the mobility and transport of PyC and should be better represented in Earth system models.

Tracing the evolution of dissolved organic carbon (DOC) pool in the Ediacaran ocean by Germanium/silica (Ge/Si) ratios of diagenetic chert nodules from the Doushantuo Formation, South China

The Ediacaran Period (ca. 635–538 Ma) was a critical interval in Earth’s history, bridging the Proterozoic and the Phanerozoic Earth systems. It witnessed the diversification of eukaryotic phytoplanktons in the early stage, the evolution of macroscopic multicellular lives in the late stage, and frequent oscillations of carbonate carbon isotopes of large magnitudes, presumably reflecting large perturbations of the global carbon cycle. It has been proposed that a large dissolved organic carbon (DOC) pool, hundreds to thousands times larger than the present one, was developed in the Ediacaran deep ocean. The wax and wane of the marine DOC pool might be responsible for the secular variation of carbon isotopes of marine dissolved inorganic carbon (DIC) pool, which was recorded in the carbonate carbon isotope chemostratigraphy. However, neither the extent nor the evolutionary history of marine DOC pool in the Ediacaran ocean has been well constrained. In this study, we use Germanium/silica (Ge/Si) ratios of chert nodules from the Ediacaran Doushantuo Formation in the Yangtze Gorges area, South China, to trace the evolution of marine DOC pool. These chert nodules yield abundant microfossils, i.e. acanthomorphic acritarchs, and were precipitated in the early diagenetic replacement of calcareous silty/muddy sediments. Most chert nodules show positive correlations between Al contents and Ge/Si ratios, which can be resolved by a binary mixing between a silica (quartz) endmember and an Al-rich silicate (clay) endmember. On the one hand, the Ge/Si ratios of silica component ((Ge/Si)SiO2) are comparable to the modern seawater composition, suggesting the normal seawater rather than hydrothermal fluids was the major source of Si. On the other, the Ge/Si ratios of the clay component ((Ge/Si)clay) are one to two orders of magnitudes higher than that of normal marine clays. The Ge-enrichment of the clay component could be attributed to the extensive chelation of Ge by organic ligands and massive formation of organic-Ge-Clay complex at high DOC concentration. This observation supports the presence of a large DOC pool in the Ediacaran ocean. In addition, there is an increase of (Ge/Si)clay from the Member II to Member III of the Doushantuo Formation, suggesting an increase of DOC concentration. The expansion of marine DOC pool is coincident with the reduction of the sizes of the upper acanthomorphic acritarch assemblage in the Member III. We suggest that the increase of marine DOC pool might be attributed to the reduction of sizes of particulate organic matter, which favored DOC generation and accumulation in the Ediacaran seawater.

The Ocean's labile DOC supply chain

AbstractMicrobes of the surface ocean release, consume, and exchange labile metabolites at time scales of minutes to days. The details of this important step in the global carbon cycle remain poorly resolved, largely due to the methodological challenges of studying a diverse pool of metabolites that are produced and consumed nearly simultaneously. In this perspective, a new compilation of published data builds on previous studies to obtain an updated estimate of the fraction of marine net primary production that passes through the labile dissolved organic carbon (DOC) pool. In agreement with previous studies, our data mining and modeling approaches hypothesize that about half of ocean net primary production is processed through the labile DOC pool. The fractional contributions from three major sources are estimated at 0.4 for living phytoplankton, 0.4 for dead and dying phytoplankton, and 0.2 for heterotrophic microbes and mesoplankton.

Using CDOM spectral shape information to improve the estimation of DOC concentration in inland waters: A case study of Andean Patagonian Lakes

For the last two decades different scientific disciplines have focused on lacustrine dissolved organic matter (DOM) given its importance in the biogeochemistry of carbon and in ecosystem functioning. New satellites supply the appropriate resolutions to evaluate chromophoric dissolved organic matter (CDOM) in inland waters, opening the possibility to estimate DOM at appropriate spatiotemporal scales. This requires, however, a robust relationship between CDOM and dissolved organic carbon (DOC). In this work, we evaluated the use of CDOM as a proxy of DOC in 7 Andean Patagonian lakes. Considering the entire data set, CDOM absorption coefficients (a355 and a440) were linearly related with DOC. Shallow lakes, however, drove this relationship showing a moderate relationship, whereas, deep lakes with lower colour presented a weaker relationship. Therefore, we assessed the use of CDOM spectral shape information to improve DOC estimates regardless of observed DOM differences due to climatic seasonality and lakes' morphometry. The use of well-known CDOM spectral shape metrics (i.e., S275–295 and a250:a365 ratio) significantly improved DOC estimation. Particularly, using a Gaussian decomposition approach we found that much of the variation in the spectral shape, associated with the variability of CDOM:DOC ratio, was explained by differences in two dynamic regions centred at 270 and 320 nm. A strong nonlinear relationship was found between the a270:a320 ratio and the DOC-specific absorption coefficients a*355 and a*440. This was translated into a further improvement in DOC estimation yielding the higher R2 and lower mean absolute differences (MAPD < 16%), either considering the entire data set or shallow and deep lakes separately. Our results highlight that incorporating the CDOM spectral shape information improves the characterization of the DOC pool of inland waters, which is particularly relevant for remote and/or inaccessible sites and has significant implications for the environmental management, biogeochemical studies and future remote sensing applications.

Sensitivity of Steady State, Deep Ocean Dissolved Organic Carbon to Surface Boundary Conditions

AbstractWe use the transport matrices of a data‐constrained circulation model to efficiently compute the steady state distribution of the deep ocean dissolved organic carbon (DOC) at a 1° horizontal resolution by propagating the surface DOC boundary conditions into the ocean interior. An equivalent simulation in the traditional forward modeling approach would be prohibitively computationally expensive. Our model simulates the total DOC as the sum of two DOC pools, the refractory and the semi‐labile. The model is able to simulate the large‐scale features of the deep ocean DOC without local sources or sinks of DOC in the ocean interior. The deep ocean DOC in the model is sensitive to the preformed DOC concentrations in the formation sites of deep and bottom waters, where observations are lacking. Furthermore, our model experiments indicate that the deep Atlantic DOC gradient is sensitive to the mixing of deep waters with different concentrations of preformed refractory DOC, the transport of semi‐labile DOC from the surface North Atlantic, and the decay rate of semi‐labile DOC. These, combined with the observation that much of the deep ocean DOC gradient is in the Atlantic, suggests that the semi‐labile DOC may be an important component of the deep Atlantic DOC. Finally, we show that DOC export depends substantially on the depth level where it is evaluated.

Production of Labile Protein-Like Dissolved Organic Carbon Associated With Anaerobic Methane Oxidization in the Haima Cold Seeps, South China Sea

Cold seeps where methane-rich fluids escape from the seafloor generally support enormous biomass of chemosynthetic organisms and associated fauna. In addition to transporting a great amount of methane toward the seafloor, cold seeps also contribute to the aged, dissolved organic carbon (DOC) pool in the deep ocean. Here, two sediment cores from the “Haima cold seeps,” northern South China Sea and a nearby reference core were analyzed for pore-water sulfate and DOC concentrations, δ13C of DOC, and optical properties of dissolved organic matter (DOM). High DOC concentrations (0.9–3.7 mM) accompanied by extremely low δ13C values (−43.9 to −76.2‰) suggest the conversion of methane into sedimentary DOC pool in the seep sediments. Parallel factor analysis (PARAFAC) of the fluorescence excitation-emission matrices shows higher fluorescent intensities of labile protein-like components (C2 and C4) and lower fluorescent intensities of refractory humic-like components (C1 and C3) in the seep cores compared to the reference core. The intensity of C2 is positively correlated with DOC concentrations and δ13C-DOC in the seep sediments, suggesting that the labile protein-like DOM was produced by the anaerobic oxidation of methane (AOM). Moreover, low humification index (HIX) and high biological index (BIX) values also indicate intensified production of relatively labile DOM with lower degradation degree in the seep cores compared to the reference core. Hence, we highlight that methane-derived DOC may serve as important carbon and energy sources for heterotrophic microbial communities due to its relatively labile nature.

Molecular transformation of dissolved organic carbon of rhizosphere soil induced by flooding and copper pollution

Dissolved organic carbon (DOC) in rhizosphere soil is critical for multiple geochemical processes. Soils that suffer from heavy metal pollution accompanied by flooding are present worldwide, while little research has explored their effects on rhizosphere DOC structures, especially at the molecular level. Therefore, this study was designed with the aim of improving the understanding of rhizosphere DOC fate in copper (Cu) amended and/or flooded soils. A 180-day laboratory incubation including soils with different Cu levels with and without flooding was conducted. Fourier transform ion cyclotron resonance mass spectrometry analysis showed that the number of formulas detected only in the rhizosphere DOC of flooded and Cu-treated soil was as high as 528–1375. Flooding and Cu pollution reduced molecular weight, aromaticity and compounds containing CHON2 and CHON3 of rhizosphere DOC, but increased its oxidation degree. Furthermore, Cu pollution resulted in DOC with more abundant lignin-, carbohydrate- and protein-like components but less condensed aromatic-like components. Overall, due to Cu pollution and flooding, the rhizosphere DOC pool contained more chemically and biologically labile components. Structural equation modeling indicated that this change in DOC structure was partly explained by soil enzyme activities (decrease in invertase, cellulase, urease activity and the increase in polyphenol oxidase and peroxidase activity). The findings highlighted the ecological risk of Cu pollution and flooding to soil health from the perspective of rhizosphere carbon.

Interspecific differences in root exudation for three tropical seagrasses and sediment pore-water dissolved organic carbon beneath them

Seagrass beds act as blue carbon sinks globally; however, little attention has been given to carbon dynamics in the seagrass rhizosphere. Hence, in this study, the quantity and characteristics of dissolved organic carbon (DOC) from root exudation of the three dominant tropical seagrasses (Thalassia hemprichii, Enhalus acoroides, and Cymodocea rotundata) and sediment pore water beneath them were compared, to examine their interspecific differences, and to establish a connection between seagrass root exudation and sediment carbon. The rate of root-exuded DOC from T. hemprichii (2.15 ± 1.06 mg g DW root-1 h-1) was significantly higher (p < 0.05) than that from E. acoroides (0.72 ± 0.39 mg g DW root-1 h-1) and C. rotundata (0.46 ± 0.25 mg g DW root-1 h-1). Root exudation rates were more affected by root hair density and root hair length than by root carbon, nitrogen, and soluble sugar content. Simultaneously, DOC concentrations of the sediment pore water beneath T. hemprichii, E. acoroides and C. rotundata were 22.05 ± 11.61 mg l-1, 15.55 ± 2. 66 mg l-1, and 14.32 ± 1.82 mg l-1, respectively. The corresponding absorption coefficients at 254 nm (a254) were 30.53 ± 18.00, 17.31 ± 2.24, and 14.07 ± 2.03, respectively, while the relevant specific ultraviolet absorbances at 254 nm (SUVA254) were 1.38 ± 0.29, 1.19 ± 0.26 and 1.03 ± 0.28, respectively. Therefore, the roots of T. hemprichii exuded DOC at a higher rate, leading to a higher pore-water DOC pool in the sediment. This suggests that T. hemprichii played a greater role in the sediment carbon pool through root exudation. Thus, it can be considered as the priority species for transplantation to promote the carbon sink function of seagrass beds.