Organic Carbon Degradation Research Articles

Straw additive enhances manure compost quality by promoting diverse aerobic bacteria and unitary thermophilic fungi

Shortening the initial activation time and extending the duration of the thermophilic phase are key to improving compost product quality in cold-climate regions. This study set up three treatments that cattle manure (CM), manure with rice straw (MR), and manure with maize straw (MM) in the field with ambient temperature ranging from –6 to 7 ℃. Compared with traditional manure composting, composting with straw performed more effectively, and the effect of the addition of maize straw surpassed that of the addition of rice straw. Straw additives markedly increased the compost pile temperature and extended the thermophilic phase duration. In addition, straw addition improved compost product maturity, as indicated by the humic-like substance content, absorbance ratio, and germination index. To further illustrate this result, the microbial community structure during the composting process was studied. During the thermophilic phase, straw additives, especially maize straw, improved the formation of a diverse aerobic bacterial community and a unitary thermophilic fungal community, and promoted a stronger relationship between the bacterial and fungal communities, as revealed by co-inertia analysis. The abundance of functional genes indicated that straw addition increased the activities of organic carbon degradation and transformation. This study demonstrated the necessity of enhancing the interaction between thermophilic–aerobic bacteria and thermophilic fungi to improve compost product quality.

Organic Carbon Reaction Kinetics in Bioturbated Sediments

AbstractMost organic carbon delivered to the seafloor is degraded within the bioturbated layer. Theory and empirical evidence have shown that organic carbon reactivity relates to the age of a particle. However, due to particle mixing, the age‐depth linear relation induced by sediment accretion is obfuscated. Here we combine a Lagrangian particle tracking model that resolves the age distribution of particles in the bioturbated zone and couple it to age‐dependent organic carbon degradation. Depth profiles for organic carbon concentration, reactivity and degradation rate are presented for sediments receiving low and high reactivity organic carbon in coastal, continental slope and deep‐sea environments. Our results show that a simple first‐order kinetics model suffices for well‐mixed sediments and systems receiving pre‐processed materials. A reactive continuum approach is needed for poorly mixed sediments receiving highly reactive organic carbon and for sediments below the bioturbated layer.

Environmental impact of potentially toxic elements accumulated in surface sediments of the Erikli Lagoon, Black Sea coast (Türkiye)

This study aims to (a) determine the concentration and distribution of elements in the surface sediments of the Erikli Lagoon, which are under natural and anthropogenic pressure, (b) determine possible effects on the environment using ecological indices, and (c) reveal possible sources. Multiple elements, total organic carbon and chlorophyll degradation products were analyzed in sediment samples. Enrichment factor (EF), contamination factor (CF), and geoaccumulation index (Igeo) were calculated to determine the sources of the elements. Modified hazard quotient (mHQ), ecological contamination index (ECI), contamination severity index (CSI) and potential ecological risk index (PERI) were calculated to determine ecological risks. It was found that Mn, Hg, As, Fe and Cd entering the lake are of anthropogenic origin. These elements pose a low to moderate ecological threat to the lake. Agricultural and domestic discharges and atmospheric deposition are the main sources of these elements. A moderate ecological risk with an average value of 194.89 was determined in the lagoon based on PERI and contamination levels of metals. The elements that pose this risk are Hg and Cd, due to their high toxicity. According to ECI and CSI, the ecological risk is low, with average values of 0.99 and 0.30, respectively.

Drivers of microbial carbon biomass variability in two oceanic regions of the Gulf of Mexico

The microbial plankton community is an integral part of the pelagic ecosystem. It hosts essential functional groups that play a vital role in organic carbon production, release, uptake, and degradation within open-ocean ecosystems. Given its significance, carbon biomass estimates are urgently needed, especially in oligotrophic regions, to provide and enhance our knowledge of biogenic carbon pools. They also aid in validating biogeochemical models that characterize the functioning of these extensive marine ecosystems within the global carbon cycle. This study addresses the temporal variability of microbial community biomass in two oceanic zones: the west-central (Perdido) and southern (Coatzacoalcos) areas of the Gulf of Mexico. During three seasonally contrasting periods (nortes, rainy, and dry seasons), seawater samples were collected from the euphotic zone in both regions to estimate the carbon biomass of different pico- (<2–3 µm), nano-, and microplankton groups (>3–200 µm). Carbon biomass assessments for the microbial groups were based on their abundance and carbon conversion factors. Overall, we found a significant contribution of pico-prokaryotic components (heterotrophic bacteria, Prochloroccocus, and Synechoccocus) to the total microbial carbon stock of the euphotic zone (84–89 % global estimates). The finding suggests these microorganisms are key functional groups that drive carbon production and fate in the Gulf of Mexico ecosystem. Pico-cyanobacteria, especially Prochloroccocus, were the dominant primary producers (68–82 % total autotrophic carbon), mainly in the upper layer of the oligotrophic euphotic zone. This vertical pattern implies that the deep chlorophyll-a maximum (DCM) depth level was unrelated to a net increase in phytoplankton biomass in the three study periods. The distribution of microbial carbon biomass exhibited striking differences associated with winter mixing (the nortes season), high river discharge accompanied by cross-shelf transport (the rainy season), and the dynamics of mesoscale structures. Ecological aspects, such as the habitat preference of the organisms and the seasonal complementary development of mixotrophic and heterotrophic grazers and their prey, were also essential drivers in regulating the microbial carbon pool of both oceanic regions. The microbial carbon assessments conducted in this study contribute to identifying and quantifying key planktonic functional groups involved in the biogeochemical carbon cycle in the Gulf of Mexico open-ocean ecosystem.

Self-circulating pulse alternating ventilation composting technology: Biotic and abiotic effects of mature compost on humification during food waste composting

Traditional composting technology relies on external additives such as rice husks to optimize the moisture content and permeability of the pile, which increases transportation and economic costs. This study developed a self-circulating pulse alternating ventilation composting technology, which used mature compost as an additive and explored the optimal ratio and strengthening mechanism. Six composting experiments were set up with mature compost as additive and rice husk as control based on the self-made alternating ventilation composting reactor. The results showed that adding 10 % mature compost (wet weight) significantly extended the high-temperature phase (15 days). The total organic carbon degradation degree was 1.10 to 2.11 times higher, and the degree of directional humification was 1.56 to 3.39 times higher than those of the other groups. This may be due to the biological and non-biological effects of the mature compost. On one hand, adding mature compost increased the relative abundance of lignocellulose-degrading functional microbes, which enhanced the metabolic levels of xylanolysis and cellulolysis. On the other hand, it accelerated the dissolution and release of humus precursors, increasing the content of aromatic compounds such as phenyl-β-D-glucoside and indolizine, and thereby accelerating the condensation process. These results demonstrate that self-circulating pulse alternating ventilation composting technology is a low-cost green composting technology with the potential to achieve efficient treatment of food waste.

Spatial confined synthesis of submicron Fe-MoS2 with abundant surface cationic groups and sulfur vacancies for enhanced peroxymonosulfate activation

ABSTRACT The widespread use of emerging refractory organic contaminants poses a significant threat to human health, prompting the need for a cost-effective and efficient removal strategy. While the iron ions/PMS system effectively removes organic pollutants, slow Fe3+ to Fe2+ transformation hampers its efficiency, and the homogeneous distribution of iron ions complicates separation, resulting in secondary sludge pollution. Herein, we developed a novel submicron Fe-MoS2 (S-Fe-MoS2) catalyst with abundant surface cationic groups and sulfur vacancy through a cationic polyacrylamide aerogels (CPAMA) confined hydrothermal synthesis strategy. These features promote active site exposure, enhance reactant adsorption, and accelerate electron transfer between Mo and Fe sites, improving catalytic kinetics and promoting Fe3+/Fe2+ cycle for PMS activation. As a result, the S-Fe-MoS2/PMS system exhibited a high catalytic rate constant (k obs) of 0.32 min−1, in the degradation of 4-chlorophenol (4-CP), 1.5 times higher than that of the conventional Fe-MoS2/PMS system. It also achieved 82.9% total organic carbon (TOC) degradation within 60 min. Additionally, it possessed similar degradation performance for various organic pollutants, along with remarkable reusability (four cycles) and broad pH adaptability (2–8), indicating significant potential for widespread application. This study provided a new way for developing advanced heterogeneous catalysts with high efficiency for water treatment.

Climate-driven succession in marine microbiome biodiversity and biogeochemical function.

Seasonal and El Niño-Southern Oscillation (ENSO) warming result in similar ocean changes as predicted with climate change. Climate-driven environmental cycles have strong impacts on microbiome diversity, but impacts on microbiome function are poorly understood. We quantified changes in microbial genomic diversity and functioning over 11 years covering seasonal and ENSO cycles at a coastal site in the southern California Current. We observed seasonal oscillations between large genome lineages during cold, nutrient rich conditions in winter and spring versus small genome lineages, including Prochlorococcus and Pelagibacter , in summer and fall. Parallel interannual changes separated communities depending on ENSO condition. Biodiversity shifts translated into clear oscillations in microbiome functional potential. Ocean warming induced an ecosystem with less iron but more macronutrient stress genes, depressed organic carbon degradation potential and biomass, and elevated carbon-to-nutrient biomass ratios. The consistent microbial response observed across time-scales points towards large climate-driven changes in marine ecosystems and biogeochemical cycles.

Globally distributed marine Gemmatimonadota have unique genomic potentials

BackgroundGemmatimonadota bacteria are widely distributed in nature, but their metabolic potential and ecological roles in marine environments are poorly understood.ResultsHere, we obtained 495 metagenome-assembled genomes (MAGs), and associated viruses, from coastal to deep-sea sediments around the world. We used this expanded genomic catalog to compare the protein composition and update the phylogeny of these bacteria. The marine Gemmatimonadota are phylogenetically different from those previously reported from terrestrial environments. Functional analyses of these genomes revealed these marine genotypes are capable of degradation of complex organic carbon, denitrification, sulfate reduction, and oxidizing sulfide and sulfite. Interestingly, there is widespread genetic potential for secondary metabolite biosynthesis across Gemmatimonadota, which may represent an unexplored source of novel natural products. Furthermore, viruses associated with Gemmatimonadota have the potential to “hijack” and manipulate host metabolism, including the assembly of the lipopolysaccharide in their hosts.ConclusionsThis expanded genomic diversity advances our understanding of these globally distributed bacteria across a variety of ecosystems and reveals genetic distinctions between those in terrestrial and marine communities.-srP_8yBftBuaKffV-GvkTVideo

Microbial communities and functions are structured by vertical geochemical zones in a northern peatland

Northern peatlands are important carbon pools; however, differences in the structure and function of microbiomes inhabiting contrasting geochemical zones within these peatlands have rarely been emphasized. Using 16S rRNA gene sequencing, metagenomic profiling, and detailed geochemical analyses, we investigated the taxonomic composition and genetic potential across various geochemical zones of a typical northern peatland profile in the Changbai Mountains region (Northeastern China). Specifically, we focused on elucidating the turnover of organic carbon, sulfur (S), nitrogen (N), and methane (CH4). Three geochemical zones were identified and characterized according to porewater and solid-phase analyses: the redox interface (<10 cm), shallow peat (10–100 cm), and deep peat (>100 cm). The redox interface and upper shallow peat demonstrated a high availability of labile carbon, which decreased toward deeper peat. In deep peat, anaerobic respiration and methanogenesis were likely constrained by thermodynamics, rather than solely driven by available carbon, as the acetate concentrations reached 90 μmol·L−1. Both the microbial community composition and metabolic potentials were significantly different (p < 0.05) among the redox interface, shallow peat, and deep peat. The redox interface demonstrated a close interaction between N, S, and CH4 cycling, mainly driven by Thermodesulfovibrionia, Bradyrhizobium, and Syntrophorhabdia metagenome-assembled genomes (MAGs). The archaeal Bathyarchaeia were indicated to play a significant role in the organic carbon, N, and S cycling in shallow peat. Although constrained by anaerobic respiration and methanogenesis, deep peat exhibited a higher metabolic potential for organic carbon degradation, primarily mediated by Acidobacteriota. In terms of CH4 turnover, subsurface peat (10–20 cm) was a CH4 production hotspot, with a net turnover rate of ∼2.9 nmol·cm−3·d−1, while the acetoclastic, hydrogenotrophic, and methylotrophic methanogenic pathways all potentially contributed to CH4 production. The results of this study improve our understanding of biogeochemical cycles and CH4 turnover along peatland profiles.

Microbial Communities Degrade Ancient Permafrost‐Derived Organic Matter in Arctic Seawater

AbstractThe Arctic is experiencing rapid warming, which among other processes results in increasing erosion of coastal permafrost and the release of ancient organic carbon (OC) into the Arctic Ocean, which in turn might result in greenhouse gas emissions following its decomposition. Supply of terrigenous organic matter to the ocean affects near‐shore nutrient concentrations and the composition of microbial communities—highlighting the need to understand the fate of permafrost‐derived carbon in this fragile ecosystem. We incubated material from coastal Yedoma permafrost for 85 days in seawater collected during the Arctic Century expedition. During this experiment, 2.8 ± 1.4% of OC from coastal Yedoma was respired to CO2. Radiocarbon analysis revealed that 88 ± 15% of the released CO2 originated from ancient material (∼40,000 years), indicating that degradation of permafrost OC reintroduces old carbon into the short‐term carbon cycle. Hence, the permafrost climate feedback may be enhanced in the coming decades when coastal erosion accelerates. Additionally, 0.9 ± 0.3% of Yedoma OC was leached as dissolved OC. The observed net production of inorganic nitrogen during the incubation could potentially provide a negative feedback by stimulating primary production. Bacterial community analysis showed a succession of primary responders to biolabile substrates (e.g., Psychrobacter and Colwellia) followed by secondary consumers of less biolabile substrates (e.g., Maribacter and Pseudohongiella), plus a potential establishment of permafrost associated‐bacteria on particles. Overall, our data show that OC input from thawing permafrost stimulates bacterial dynamics, with likely implications for regional biogeochemical cycles and the Earth's climate.

Interactions between iron and carbon in permafrost thaw ponds

Thawing permafrost forms “thaw ponds” that accumulate and transport organic carbon (OC), redox-active iron (Fe), and other elements. Although Fe has been shown to act as a control on the microbial degradation of OC in permafrost soils, the role of iron in carbon cycling in thaw ponds remains poorly understood. Here, we investigated Fe-OC interactions in thaw ponds in partially and fully thawed soils (“bog” and “fen” thaw ponds, respectively) in a permafrost peatland complex in Abisko, Sweden, using size separation (large particulate fraction (LPF), small particulate fraction (SPF), and dissolved fraction (DF)), acid extractions, scanning electron microscopy (SEM), Fe K-edge X-ray absorption spectroscopy (XAS), and Fourier Transform Infrared (FTIR) spectroscopy. The bulk total Fe (total suspended Fe) in the bogs ranged from 135 mg/L (mean = 13 mg/L) whereas the fens exhibited higher total Fe (1.5 to 212 mg/L, mean = 30 mg/L). The concentration of bulk total OC (TOC) in the bog thaw ponds ranged from 50 to 352 mg/L (mean = 170 mg/L), higher than the TOC concentration in the fen thaw ponds (8.5 to 268 mg/L, mean = 17 mg/L). The concentration of 1 M HCl-extractable Fe in the bog ponds was slightly lower than that in the fens (93 ± 1.2 and 137 ± 3.5 mg/L Fe, respectively) with Fe predominantly (>75 %) in the DF in both thaw stages. Fe K-edge XAS analysis showed that while Fe(II) was the predominant species in LPF, Fe(III) was more abundant in the DF, indicating that the stage of thawing and particle size may control Fe redox state. Furthermore, Fe(II) and Fe(III) were partially complexed with natural organic matter (NOM, 8 to 80 %) in both thaw ponds. Results of our work suggest that Fe and OC released during permafrost thaw into thaw ponds (re-)associate, potentially protecting OC from microbial decomposition while also stabilizing the redox state of Fe.

Cumulative Effects of Watershed Disturbances and Run-of-river Dams on Mercury Cycling: Case Study and Recommendations for Environmental Managers.

Run-of-river power plants (ROR) represent the majority of hydroelectric plants worldwide. Their environmental impacts are not well documented and are believed to be limited, particularly regarding the contamination of food webs by methylmercury (MeHg), a neurotoxin. RORs are typically installed in small rivers where combined effects of watershed disturbances with dam construction can complicate environmental management. We report a multi-year case study on the Saint-Maurice River (Canada) where an unpredicted temporary increase in MeHg accumulation in predator fish was observed after the construction of two ROR plants. The associated pondages acted as sedimentation basins for mercury (Hg) and organic matter from a watershed disturbed by a forest fire and by logging. This fresh organic carbon likely fueled microbial MeHg production. Hg methylation was more associated with environmental conditions than to the presence of Hg, and main methylating microbial groups were identified. A constructed wetland was a site of significant Hg methylation but was not the main source of the fish Hg increase. Organic carbon degradation was the main driver of MeHg accumulation at the base of the food chain whereas trophic levels explained the variations at the top of the food chain. Overall, carbon cycling was a key driver of Hg dynamics in this system, and ROR plants can cause temporary (ca. 12 years) Hg increase in food webs when developed in disturbed watersheds, although this increase is smaller than for large reservoirs. Recommendations for future ROR construction are to establish a good environmental monitoring plan with initial high temporal resolution and to consider recent and potential watershed disturbances in the plan.

Enhanced heterogeneous Electro-Fenton catalysis using CNT/MoS2/FeCo-LDH cathode membranes for efficient phenol removal from wastewater

Herein, we innovatively crafted a novel CNT/MoS2/FeCo-LDH cathode membrane using a facile vacuum filtration method. Benefiting from the unique heterostructure between FeCo-LDH and MoS2, the prepared CNT/MoS2/FeCo-LDH cathode membranes catalyzed the production of high-concentration H2O2 (7.59 mg/L) at a low current density of 1.50 mA/cm2. The peak degradation efficiency of phenol (98.80%) was reached in only 60 min. Under optimized operating conditions, the phenol removal rate of the CNT/MoS2/FeCo-LDH cathode membrane remained above 80.00% within a wide pH range (3.0–9.0), demonstrating excellent adaptability. The synergistic incorporation of Mo, Fe, and Co ions facilitated additional chemical reaction pathways for Fenton-like reactions, thereby significantly enhancing hydrogen peroxide production. Notably, our approach exhibited moderate total organic carbon degradation efficiency, even for the actual pretreatment of coking wastewater, presenting a promising strategy for addressing recalcitrant industrial wastewater.

Bubble-less photocatalytic ozonation with ceramic membranes and static mixers enhances the total organic carbon removal and degradation of iodinated X-ray contrast agents

Conventional bubble-based ozonation of wastewater creates several issues, e.g., the low degradation of certain micropollutants like X-ray contrast agents, mass transfer limitations due to inhomogeneous mixing, and the waste of undissolved ozone. In contrast, this work uses tubular ceramic membranes for bubble-free and bubble-less ozonation. Static mixers are placed around the membrane to enhance the radial mixing of ozone, creating homogeneous reaction conditions throughout the membrane contactor. Additionally, the ozone mass transfer significantly increases with static mixers. Furthermore, the membrane is made of a photocatalyst (TiO2) on the shell-side to improve the micropollutant degradation by photocatalytic ozonation. An ozone-stable hydrophobic treatment method is developed to use ceramic membranes as a membrane contactor for the ozonation of water. After hydrophobic treatment, the ozone mass transfer coefficient is in the order of 1.3 ⋅ 10−6 ms−1 using bubble-less operation resulting in an ozone concentration up to 4.0 mg L−1. Overall, bubble-less operation significantly increases the ozone mass transfer compared to bubble-free operation. Photocatalytic ozonation in a membrane contactor with static mixers increases the degradation of diatrizoate by 37% compared to sole ozonation in the same membrane contactor. Furthermore, this process increases the removal of total organic carbon in experiments with a water matrix consisting of four different micropollutants by 115% compared to sole ozonation.

Strong linkage between benthic oxygen uptake and bacterial tetraether lipids in deep-sea trench regions

Oxygen in marine sediments regulates many key biogeochemical processes, playing a crucial role in shaping Earth’s climate and benthic ecosystems. In this context, branched glycerol dialkyl glycerol tetraethers (brGDGTs), essential biomarkers in paleoenvironmental research, exhibit an as-yet-unresolved association with sediment oxygen conditions. Here, we investigated brGDGTs in sediments from three deep-sea regions (4045 to 10,100 m water depth) dominated by three respective trench systems and integrated the results with in situ oxygen microprofile data. Our results demonstrate robust correlations between diffusive oxygen uptake (DOU) obtained from microprofiles and brGDGT methylation and isomerization degrees, indicating their primary production within sediments and their strong linkage with microbial diagenetic activity. We establish a quantitative relationship between the Isomerization and Methylation index of Branched Tetraethers (IMBT) and DOU, suggesting its potential validity across deep-sea environments. Increased brGDGT methylation and isomerization likely enhance the fitness of source organisms in deep-sea habitats. Our study positions brGDGTs as a promising tool for quantifying benthic DOU in deep-sea settings, where DOU is a key metric for assessing sedimentary organic carbon degradation and microbial activity.

Integrated AI-driven optimization of Fenton process for the treatment of antibiotic sulfamethoxazole: Insights into mechanistic approach

Antibiotics, as a class of environmental pollutants, pose a significant challenge due to their persistent nature and resistance to easy degradation. This study delves into modeling and optimizing conventional Fenton degradation of antibiotic sulfamethoxazole (SMX) and total organic carbon (TOC) under varying levels of H2O2, Fe2+ concentration, pH, and temperature using statistical and artificial intelligence techniques including Multiple Regression Analysis (MRA), Support Vector Regression (SVR) and Artificial Neural Network (ANN). In statistical metrics, the ANN model demonstrated superior predictive accuracy compared to its counterparts, with lowest RMSE values of 0.986 and 1.173 for SMX and TOC removal, respectively. Sensitivity showcased H2O2/Fe2+ ratio, time and pH as pivotal for SMX degradation, while in simultaneous SMX and TOC reduction, fine tuning the time, pH, and temperature was essential. Leveraging a Hybrid Genetic Algorithm-Desirability Optimization approach, the trained ANN model revealed an optimal desirability of 0.941 out of 1000 solutions which yielded a 91.18% SMX degradation and 87.90% TOC removal under following specific conditions: treatment time of 48.5 min, Fe2+: 7.05 mg L−1, H2O2: 128.82 mg L−1, pH: 5.1, initial SMX: 97.6 mg L−1, and a temperature: 29.8 °C. LC/MS analysis reveals multiple intermediates with higher m/z (242, 270 and 288) and lower m/z (98, 108, 156 and 173) values identified, however no aliphatic hydrocarbon was isolated, because of the low mineralization performance of Fenton process. Furthermore, some inorganic fragments like NH4+ and NO3− were also determined in solution. This comprehensive research enriches AI modeling for intricate Fenton-based contaminant degradation, advancing sustainable antibiotic removal strategies.

Effect of nitrogen retention composite additives Ca(H2PO4)2 and MgSO4 on the degradation of lignocellulose, compost maturation, and fungal communities in compost.

This study investigated the effects of the nitrogen retention composite additives Ca(H2PO4)2 and MgSO4 on lignocellulose degradation, maturation, and fungal communities in composts. The study included control (C, without Ca(H2PO4)2 and MgSO4), 1% Ca(H2PO4)2 + 2% MgSO4 (CaPM1), 1.5% Ca(H2PO4)2 + 3% MgSO4 (CaPM2). The results showed that Ca(H2PO4)2 and MgSO4 enhanced the degradation of total organic carbon (TOC) and promoted the degradation of lignocellulose in compost, with CaPM2 showing the highest TOC and lignocellulose degradation. Changes in the three-dimensional excitation-emission matrix fluorescence spectroscopy (3D-EEM) of dissolved organic matter (DOM) components in compost indicated that the treatment group with the addition of Ca(H2PO4)2 and MgSO4 promoted the production of humic acids (HAs) and increased the degree of compost decomposition, with CaPM2 demonstrating the highest degree of decomposition. The addition of Ca(H2PO4)2 and MgSO4 modified the composition of the fungal community. Ca(H2PO4)2 and MgSO4 increased the relative abundance of Ascomycota, decreased unclassified_Fungi, and Glomeromycota, and activated the fungal genera Thermomyces and Aspergillus, which can degrade lignin and cellulose during the thermophilic stage of composting. Ca(H2PO4)2 and MgSO4 also increased the abundance of Saprotroph, particularly undefined Saprotroph. In conclusion, the addition of Ca(H2PO4)2 and MgSO4 in composting activated fungal communities involved in lignocellulose degradation, promoted the degradation of lignocellulose, and enhanced the maturation degree of compost.

Burial of Organic Carbon in Swedish Fjord Sediments: Highlighting the Importance of Sediment Accumulation Rate in Relation to Fjord Redox Conditions

AbstractFjords are net carbon sinks with high organic carbon (OC) burial rates; however, the key drivers of OC burial in these systems are not well constrained. To study the role of water column redox condition and OC composition on OC preservation in fjord sediments, we determined OC accumulation rates (OCAR), OC source, and OC degradation in three Swedish fjords with variable redox conditions (long‐term oxic, seasonally hypoxic, and long‐term anoxic). Average OCARs were variable between and within the fjords studied (2–122 g OC m−2 yr−1), but highest rates were at the mouth for each fjord. Based on a δ13C mixing model, Swedish fjords bury predominantly marine‐derived OC (∼83% of the total OC burial) likely because of relatively gentle slopes, low riverine discharge, and high marine inflow. Using a multi‐biomarker approach (lignin, photosynthetic pigments, and total hydrolyzable amino acids) we found, terrestrially‐ and marine‐derived OC were moderately degraded under the various redox conditions sampled, suggesting water column redox and OC source are not primary drivers of OC burial in these fjords. Rather, high sediment accumulation rates, common to fjords globally, lead to low oxygen exposure times, thus promoting efficient burial of OC regardless of its chemical composition.

Quality and Biodegradation Process of Dissolved Organic Carbon in Typical Fresh-Leaf Leachate in the Wuhan Urban Forest Park

The study investigated the leaching and biodegradation of dissolved organic carbon in leaf leachates from typical fresh leaves in the Wuhan Urban Forest Park, Central China. The fresh leaf-leached dissolved organic carbon quality and biodegradability, as well as their potential determinants, were examined for 12 major tree species, including deciduous trees and shrubs. A 28-day indoor incubation was conducted at two temperature conditions of 20 °C and 30 °C. Sampling was conducted within the planned time frame for experimental measurements, and a first-order kinetic model for dissolved organic carbon degradation was fitted. The utilization of the fir tree as the predominant deciduous species and cuckoo as the primary shrubs provided advantages in increasing the carbon sequestration capacity of urban forests. There was no significant difference in the degradation rate of the leaching solution at different temperatures, but the k value of the first-order kinetic model was different. At 20 °C, the dissolved organic carbon degradation rate was positively correlated with electrical conductivity and total dissolved nitrogen, while it was negatively correlated with the humification index and ratio of dissolved organic carbon to total dissolved nitrogen. At 30 °C, the degradation rate of dissolved organic carbon showed a positive correlation with total dissolved phosphorus and total dissolved nitrogen, while it was negatively correlated with the humification index, ratio of dissolved organic carbon to total dissolved nitrogen and ratio of dissolved organic carbon to total dissolved phosphorus.

Reactive transport modeling of organic carbon degradation in marine methane hydrate systems

Natural methane hydrate has often been observed in sand layers that contain no particulate organic carbon (POC), but are surrounded by organic-rich, fine-grained marine muds. In this paper, we develop a reactive transport model (RTM) of a microbially-mediated set of POC degradation reactions, including hydrolysis of POC driven by extracellular enzymes, fermentation of the resulting high-molecular weight dissolved organic carbon (HMW-DOC), and methanogenesis that consumes low-molecular weight dissolved organic carbon (LMW-DOC). These processes are mediated by two groups of microbes, fermenters and methanogens that are heterogeneously distributed in different lithologies, with the largest numbers of microbes in the large pores of coarse-grained layers. We find that the RTM can reproduce methane hydrate occurrences observed in two different geological environments, at Walker Ridge Site 313-H (Gulf of Mexico) and IODP Site U1325 (Cascadia Margin). We also find that microbes can degrade POC even if they are physically separated, as extracellular enzymes and DOC can diffuse away from where they are produced by microbes. Microbial activity is highest at relatively early times after burial at shallow depths and near lithological boundaries, where concentration gradients transport solutes to intervals that contain the most microbes.