Accumulation Of Dissolved Organic Carbon Research Articles

A self-sustaining effect induced by iron sulfide generation and reuse in pyrite-woodchip mixotrophic bioretention systems: An experimental and modeling study

Dual electron donor bioretention systems have emerged as a popular strategy to enhance dissolved nitrogen removal from stormwater runoff. Pyrite-woodchip mixotrophic bioretention systems showed a promoted and stabilized removal of dissolved nutrients under complex rainfall conditions, but the sulfate reduction process that can induce iron sulfide generation and reuse was overlooked. In this study, experiments and models were applied to investigate the effects of filler configuration and dissolved organic carbon (DOC) dissolution rate on treatment performance and iron sulfide generation in pyrite-woodchip bioretention systems. Key parameters govern that DOC dissolution and microbe-mediated processes were obtained by experiments. The water quality models that integrate one-dimensional constant flow, sorption and microbial transformation kinetics were used to predict the performance of bioretention systems. Results showed that the mixotrophic bioretention system with woodchip mixed in the vadose zone and pyrite in the saturated zone achieves a better performance in both nitrogen removal efficiency and by-product control. Comparably, woodchip and pyrite mixed in the saturated zone could encounter a high secondary pollution risk. The sensitivity coefficients of oxic/anoxic DOC dissolution rates to total nitrogen removal are 0.36 and -2.43 respectively. Iron sulfide generation was affected by DOC distribution and the competition between heterotrophic denitrifiers, autotrophic denitrifiers, and sulfate-reducing bacteria (SRB). DOC accumulation has an antagonistic effect on iron production and sulfate reduction. Extra DOC accumulation favors sulfate reduction while high DOC concentration inhibits pyrite-based denitrification and reduces Fe(III) production. The recycling of iron sulfide can improve the robustness and sustainability of bioretention systems.

Dissolved oxygen fluctuation alters dissolved organic carbon stability and can accumulate resistant fraction

Recurrent dissolved oxygen (DO) depletion is common in many estuaries and coastal areas worldwide. However, its impact on the accumulation of resistant dissolved organic carbon (DOC) remains controversial. To address how recurrent DO depletion affects the degradation and stability of marine DOC, a series of DOC degradation incubations in a particulate-dissolved coexisting system in April 2021 were conducted, followed by a field investigation in the East China Sea (ECS) in October. In the ECS, we found that DO ranged from 4.71 to 7.40 mg/L in the near-bottom waters, while the degradation index (DI), an indicator of amino acids molecules, increased from −0.78–2.84, suggesting an accumulation of labile DOC over DO depletion. This is further supported by the bacteria community composition that showed an increase in anaerobic metabolism families when DO <6.29 mg/L. Laboratory incubation revealed strong labile DOC release (∼ 243 ± 38 μmol/L) from particulate organic carbon (POC) over oxygen depletion, followed by significant DOC loss in either anoxic-oxic or oxic-oxic groups. The degraded original DOC was slightly less in the oxygen depletion-treated group compared to the oxic group, and the resistant proportion was estimated as 1% ∼ 8% after 1–14 days of anoxic treatments. The resistant DOC accumulation after DO alleviation for 30 days was much less in amount when compared to the big release of labile DOC over DO depletion (48 vs. 243 μmol/L) and we believed that the lack of observed resistant DOC accumulation in the field observation was due to the overlap of multiple anoxic-oxic cycling processes and the masking effect of newly labile DOC released from the POC.

Effects of a nanobiofilm-covered echelon oxygen-controlled composting process on carbon and nitrogen conversion and emission reduction efficiency

To address the defects of traditional composting products, such as slow fertilizer efficiency, high dosage and high emissions of gas pollutants, we propose a new composting process from the perspective of increasing the dissolved organic carbon (DOC) content in compost. The results showed that nanobiofilm-covered echelon oxygen-controlled composting could effectively reduce the emissions of CH4, CO2 and NH3 and increase the DOC content in the composting products. Among the methods, using nanobiofilm covering and applying moderate ventilation during the warming phase and high ventilation during the high-temperature phase, along with low ventilation during the cooling and maturation phases (T3), proved to be the most effective in promoting the conversion of carbon to dissolved organic carbon and in reducing carbon and nitrogen losses. Compared to the control, this treatment resulted in a 36.16% reduction in total carbon loss, a 21.14% reduction in total nitrogen loss, and a 57.22% reduction in greenhouse gas emissions while also increasing the DOC content by 82.64%. This is because treatment T3 initially maintained the composting process in an aerobic state, followed by a semianaerobic state, which is conducive to the accumulation of DOC. The research results provide a new technical path for improving the quality, efficiency and emission reduction of organic fertilizer.

Sunlight-Driven Photochemical Removal of Polypropylene Microplastics from Surface Waters Follows Linear Kinetics and Does Not Result in Fragmentation.

Floating microplastics are susceptible to sunlight-driven photodegradation, which can convert plastic carbon to dissolved organic carbon (DOC) and can facilitate microplastic fragmentation by mechanical forces. To understand the photochemical fate of sub-millimeter buoyant plastics, ∼0.6 mm polypropylene microplastics were photodegraded while tracking plastic mass, carbon, and particle size distributions. Plastic mass loss and carbon loss followed linear kinetics. At most time points DOC accumulation accounted for under 50% of the total plastic carbon lost. DOC accumulation followed sigmoidal kinetics, not the exponential kinetics previously reported for shorter irradiations. Thus, we suggest that estimates of plastic lifespan based on exponential DOC accumulation are inaccurate. Instead, linear plastic-C mass and plastic mass loss kinetics should be used, and these methods result in longer estimates of photochemical lifetimes for plastics in surface waters. Scanning electron microscopy revealed that photoirradiation produced two distinct patterns of cracking on the particles. However, size distribution analyses indicated that fragmentation was minimal. Instead, the initial population of microplastics shrank in size during irradiations, indicating photoirradiation in tranquil waters (i.e., without mechanical forcing) dissolved sub-millimeter plastics without fragmentation.

Nitrogen availability regulates the effects of a simulated marine heatwave on carbon sequestration and phycosphere bacteria of a marine crop

AbstractGreat hope has been pinned on seaweed cultivation as being a potent way of removing CO2 to reduce rates of sea surface warming and acidification. Marine heatwaves and nitrogen pollution in coastal ecosystems are serious current issues that need to be better understood to inform decision making and policy. Here, we investigated the effects of a simulated heatwave and nitrogen pollution on carbon sequestration by an important seaweed crop species and its phycosphere bacteria. Gracilaria lemaneiformis was grown in ambient and high nitrogen conditions (14 and 200 μM L−1). Photosynthetic rate, seaweed biomass and particulate organic carbon accumulation were significantly increased in “high nitrogen‐no heatwave” conditions. In “ambient nitrogen heatwave” conditions, the expression of genes related to photosynthesis was down regulated and the seaweeds lost more dissolved organic carbon (DOC) to the surrounding water, resulting in more refractory dissolved organic carbon (RDOC). In “high nitrogen heatwave” conditions, photosynthetic gene expression was upregulated; bacterial abundance was also increased that can explain the reduced DOC and RDOC accumulation. The simulated heatwave reduced bacterial diversity while high nitrogen alleviated this effect. These findings suggest that the economically important alga G. lemaneiformis may lose more DOC and RDOC to nearshore waters during marine heatwave events, enhancing carbon sequestration, while nitrogen enrichment has a counteractive effect.

The hidden role of dissolved organic carbon in the biogeochemical cycle of carbon in modern redox-stratified lakes

Abstract. The dissolved organic carbon (DOC) reservoir plays a critical role in the C cycle of marine and freshwater environments because of its size and implication in many biogeochemical reactions. Although it is poorly constrained, its importance in ancient Earth's C cycles is also commonly invoked. Yet DOC is rarely quantified and characterized in modern stratified analogues. In this study, we investigated the DOC reservoirs of four redox-stratified alkaline crater lakes in Mexico. We analyzed the concentrations and isotopic compositions of DOC throughout the four water columns and compared them with existing data on dissolved inorganic and particulate organic C reservoirs (DIC and POC). The four lakes have high DOC concentrations with great variability between and within the lakes (averaging 2 ± 4 mM; 1 SD, n=28; i.e., from ∼ 15 to 160 times the amount of POC). The δ13CDOC signatures also span a broad range of values from −29.3 ‰ to −8.7 ‰ (with as much as 12.5 ‰ variation within a single lake). The prominent DOC peaks (up to 21 mM), together with their associated isotopic variability, are interpreted as reflecting oxygenic and/or anoxygenic primary productivity through the release of excess fixed carbon in three of the lakes (Alberca de los Espinos, La Preciosa, and Atexcac). By contrast, the variability of [DOC] and δ13CDOC in the case of Lake Alchichica is mainly explained by the partial degradation of organic matter and the accumulation of DOC in anoxic waters. The DOC records detailed metabolic functions such as active DIC-uptake and DIC-concentrating mechanisms, which cannot be inferred from DIC and POC analyses alone but which are critical to the understanding of carbon fluxes from the environment to the biomass. Extrapolating our results to the geological record, we suggest that anaerobic oxidation of DOC may have caused the very negative C isotope excursions in the Neoproterozoic. It is, however, unlikely that a large oceanic DOC reservoir could overweigh the entire oceanic DIC reservoir. This study demonstrates how the analysis of DOC in modern systems deepens our understanding of the C cycle in stratified environments and helps to set boundary conditions for the Earth's past oceans.

Linkages between the molecular composition of dissolved organic matter and soil microbial community in a boreal forest during freeze-thaw cycles.

Soil dissolved organic matter (DOM) plays a vital role in biogeochemical processes. Global warming leads to increased freeze-thaw cycles (FTCs) in boreal forest soils, which can change DOM production and consumption. However, the interactions between the chemical composition of DOM molecules and the microbial communities that drive C decomposition in the context of freeze-thaw are poorly understood. Here, a FTCs incubation experiment was conducted. Combined with pyrolysis gas chromatography-mass spectrometry and high-throughput sequencing techniques, the relationships between DOM chemodiversity and microbial community structure were assessed. Results indicated that both low-frequency (2FTCs) and high-frequency freeze-thaw cycles (6FTCs) significantly increased soil dissolved organic carbon (DOC) contents in the surface (0-10 cm) and subsurface (50-60 cm) soil layers. In the topsoil, FTCs significantly reduced the relative abundance of aromatic compounds, but increased the relative proportions of alkanes, phenols, fatty acid methyl esters (Me) and polysaccharides in the DOM. In the subsuface soil layer, only the relative abundance of Me in the 6FTCs treatment increased significantly. The response of bacterial communities to FTCs was more sensitive than that of fungi, among which only the relative abundance of Gammaproteobacteria increased by FTCs. Moreover, the relative abundance of these taxa was positively correlated with the increment of DOC. Co-occurrence networks confirmed DOM-bacterial interactions, implying that specific microorganisms degrade specific substrates. At class level, Gammaproteobacteria were significantly positively correlated with labile C (polysaccharides and alkanes), whereas other bacterial classes such as Actinobacteria, Alphaproteobacteria, and Thermoleophilia were significantly positively correlated with aromatic compounds in the topsoil. Collectively, FTCs tended to activate DOM and enhance its biodegradability of DOM, potentially hampering DOC accumulation and C sequestration. These findings highlight the potential of DOM molecular mechanisms to regulate the functional states of soil bacterial communities under increased FTCs.

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.

Linkages Among Dissolved Organic Matter Export, Dissolved Metabolites, and Associated Microbial Community Structure Response in the Northwestern Sargasso Sea on a Seasonal Scale.

Deep convective mixing of dissolved and suspended organic matter from the surface to depth can represent an important export pathway of the biological carbon pump. The seasonally oligotrophic Sargasso Sea experiences annual winter convective mixing to as deep as 300 m, providing a unique model system to examine dissolved organic matter (DOM) export and its subsequent compositional transformation by microbial oxidation. We analyzed biogeochemical and microbial parameters collected from the northwestern Sargasso Sea, including bulk dissolved organic carbon (DOC), total dissolved amino acids (TDAA), dissolved metabolites, bacterial abundance and production, and bacterial community structure, to assess the fate and compositional transformation of DOM by microbes on a seasonal time-scale in 2016–2017. DOM dynamics at the Bermuda Atlantic Time-series Study site followed a general annual trend of DOC accumulation in the surface during stratified periods followed by downward flux during winter convective mixing. Changes in the amino acid concentrations and compositions provide useful indices of diagenetic alteration of DOM. TDAA concentrations and degradation indices increased in the mesopelagic zone during mixing, indicating the export of a relatively less diagenetically altered (i.e., more labile) DOM. During periods of deep mixing, a unique subset of dissolved metabolites, such as amino acids, vitamins, and benzoic acids, was produced or lost. DOM export and compositional change were accompanied by mesopelagic bacterial growth and response of specific bacterial lineages in the SAR11, SAR202, and SAR86 clades, Acidimicrobiales, and Flavobacteria, during and shortly following deep mixing. Complementary DOM biogeochemistry and microbial measurements revealed seasonal changes in DOM composition and diagenetic state, highlighting microbial alteration of the quantity and quality of DOM in the ocean.

Dissolved Trace Metals and Organic Matter Distribution in the Northern Adriatic, an Increasingly Oligotrophic Shallow Sea

We report a monthly distribution of Zn, Cd, Pb, Cu, Ni and Co, biologically relevant trace metals (TMs), within one year, in the productive surface layer at two stations with different trophic characters in the northern Adriatic (NA). The TM data was accompanied by a multivariable dataset, including dissolved organic carbon (DOC), surface-active organic substances (SAS), nitrogen-containing polymeric organic material (N-POM), nutrients, pH, dissolved O2, chlorophyll a (Chl a), seawater temperature, and the Po River discharge rate. At the eu- to mesotrophic station 108, the concentrations of dissolved TMs were 5–116 nM for Zn, 0.04–0.18 nM for Cd, 0.05–0.63 nM for Pb, 3–17 nM for Cu, 4–11 nM for Ni, and 0.2–1.2 nM for Co, while at the oligotrophic station 107, they were 6–224 nM for Zn, 0.03–0.16 nM for Cd, 0.05–1.25 nM for Pb, 3–17 nM for Cu, 4–19 nM for Ni, and 0.1–0.7 nM for Co. The characterization of organic matter (OM) in conjunction with the analysis of correlations with TMs indicated that the OM–TMs interactions differed between the two stations; namely, the freshly produced OM detected at station 108 was probably involved in the complexation of Cu, Co, Cd, and Ni, whereas at station 107, such complexation processes were not present. Accumulation of DOC was observed during the summer months at both stations. Our results present a significant contribution to biogeochemical studies in the NA by focusing on the complexity of TM–OM interactions, which is a prerequisite for interpretation of their responses to local and global changes.

青藏高原泥炭地水位下降促进土壤碳积累的影响机制

酚类物质作为泥炭地重要的碳分解抑制剂, 植被作为泥炭地关键的碳输入来源, 它们在土壤碳(可溶性有机碳(DOC)等)周转过程中都发挥着重要作用。然而, 目前关于植被群落结构、酚类物质以及DOC含量对水位波动的响应存在较大争议。因此, 为明确泥炭地水位下降对植被群落结构、酚类物质以及DOC含量的影响并探明三者间的潜在联系, 以若尔盖高原泥炭地作为研究对象, 选取红原县日干乔地区3处不同地下水位泥炭地(水位由高到低依次为S1(-1.9 cm)、S2(-10 cm)、S3(-19 cm)样地), 调查不同水位条件下植被群落结构特征, 并探究酚类物质及土壤碳含量对水位波动的响应。结果表明：(1)从S1到S3样地水位下降促进土壤DOC显著增加(P < 0.05), 土壤总碳从S1到S2显著增加(P < 0.05), 而从S2到S3无显著差异；(2)泥炭地水位下降促使禾本科(发草Deschampsia cespitosa)、莎草科(木里薹草Carex muliensis、乌拉草Carex meyeriana)植物大量出现, 植被群落高度显著增加(P < 0.05)。植被群落地上生物量由153.67 g/m²增加至649.22 g/m², 地下生物量由1067.52 g/m²增加至3424.78 g/m², 植物体地上地下部总酚分别增加34.58%和13.17%(P < 0.05)。这将有可能促进植物输入更多碳进入土壤系统；(3)随水位下降, 土壤总酚、水溶性酚以及多元酚分别增加29.06%、542.87%和110.56%(P < 0.05)。一元酚呈先增加后减少, 但总体仍表现为增加, 二元酚无明显变化；(4)结构方程模型分析发现, 一元酚和多元酚能显著促进DOC和总碳积累(P < 0.05), 二元酚对DOC和总碳的积累影响不显著。以往的研究认为水位下降会加速碳流失, 但本研究发现这样的流失似乎被植物以及酚酸的作用所抵消。故此, 对泥炭地碳循环的研究应关注土壤－植物整个系统并考虑土壤中酚类物质的影响。;Phenolic substances, as important carbon decomposition inhibitors in peatlands, and vegetation, as a key carbon input source in peatlands, play an important role in soil carbon (Dissolved organic carbon (DOC)) cycle. However, the response of vegetation community structure, phenolic substances and DOC content to water level fluctuations has been controversial. Therefore, in order to elucidate the effects of water table drawdown on vegetation community structure, phenolic substances and DOC content, and explore the potential relationship among them in peatland. Taking the Zoige Plateau peatland as the research object, three peatlands with different groundwater levels in Riganqiao of Hongyuan County were selected (The order of water table from high to low: S1 (-1.9 cm), S2 (-10 cm), S3 (-19 cm) sample plot). To explore the characteristics of vegetation community structure under different water tables and to explore the response of phenolic substances and soil carbon content to water table fluctuations. The results showed that: (1) The decrease of water table from S1 to S3 promoted the soil DOC to increase significantly (P < 0.05), and soil total carbon was significantly increased from S1 to S2 (P < 0.05), but had no significant difference from S2 to S3.; (2) The decrease of water table in peatland has led to the emergence of large amounts of Gramineae (Deschampsia cespitosa) and Cyperaceae (Carex muliensis and Carex meyeriana), and the height of vegetation community was significantly increased (P < 0.05). Aboveground biomass increased from 153.67 g/m² to 649.22 g/m² (P < 0.05), and underground biomass increased from 1067.52 g/m² to 3424.78 g/m² (P < 0.05). Total phenols in aboveground and underground parts of plants increased by 34.58% and 13.17%, respectively (P < 0.05). And this may promoted more plant carbon to enter the soil system. (3) With the decrease of water table, soil total phenols, water-soluble phenols and polyphenols increased by 29.06%, 542.87% and 110.56%, respectively (P < 0.05). The content of monophenol first increased and then decreased, but the overall content still showed an increasing trend, and the change of diphenol content was not obvious; (4) Structural equation model analysis shows that monophenol and polyphenols can significantly promote DOC and total carbon accumulation (P < 0.05), while diphenol had no significant effects on soil DOC and total carbon accumulation. Previous studies have shown that falling water table accelerate carbon losses, but this study found that this loss appears to be offset by the effects of plants and phenolic acids. Therefore, the study of carbon cycle in peatland should focus on the whole soil-plant system and consider the effect of phenolic substances in soil.

The Seasonal Flux and Fate of Dissolved Organic Carbon Through Bacterioplankton in the Western North Atlantic.

The oceans teem with heterotrophic bacterioplankton that play an appreciable role in the uptake of dissolved organic carbon (DOC) derived from phytoplankton net primary production (NPP). As such, bacterioplankton carbon demand (BCD), or gross heterotrophic production, represents a major carbon pathway that influences the seasonal accumulation of DOC in the surface ocean and, subsequently, the potential vertical or horizontal export of seasonally accumulated DOC. Here, we examine the contributions of bacterioplankton and DOM to ecological and biogeochemical carbon flow pathways, including those of the microbial loop and the biological carbon pump, in the Western North Atlantic Ocean (∼39–54°N along ∼40°W) over a composite annual phytoplankton bloom cycle. Combining field observations with data collected from corresponding DOC remineralization experiments, we estimate the efficiency at which bacterioplankton utilize DOC, demonstrate seasonality in the fraction of NPP that supports BCD, and provide evidence for shifts in the bioavailability and persistence of the seasonally accumulated DOC. Our results indicate that while the portion of DOC flux through bacterioplankton relative to NPP increased as seasons transitioned from high to low productivity, there was a fraction of the DOM production that accumulated and persisted. This persistent DOM is potentially an important pool of organic carbon available for export to the deep ocean via convective mixing, thus representing an important export term of the biological carbon pump.

Accumulation of DOC in the South Pacific Subtropical Gyre from a molecular perspective

The subtropical South Pacific Gyre (SPG) encompasses the largest oligotrophic region of the global ocean. In these remote waters dissolved organic matter (DOM) accumulates in the surface waters, though constituting a potential source of nutrients and energy to sustain microbial life. On a zonal transect across the SPG, we quantified bulk dissolved organic carbon (DOC) and assessed the DOM composition via ultrahigh resolution mass spectrometry (UHR-MS) of solid-phase extracted DOC (SPE-DOC) to elucidate the molecular-level reasons behind the apparent recalcitrance of the DOM prevailing in the SPG. We included a comparison between two individual formula assignment approaches to UHR-MS data in absorption and magnitude mode which yielded consistent results.DOC concentrations exceeding 100 μmol C L−1 in the warm and saline waters of the central gyre were higher than in the surface waters of the adjacent western South Pacific. Along the transect, concentrations of SPE-DOC were less variable than bulk DOC. Nevertheless, molecular-level investigation revealed that the composition of the DOM accumulated in the central SPG generally conformed to characteristics of surface ocean DOM, but all assessed properties were more pronounced. We found high abundances of potentially labile unsaturated aliphatic molecular formulas and a low calculated degradation index for the DOM of likely marine microbial origin. Markedly decreased molar N/C ratios in the central gyre indicated preferential microbial utilization of nitrogen-containing DOM. A distinct imprint of extensive photochemical reworking was manifested in the low aromaticity of the DOM in the photic layer. Over the whole water column, ageing of DOM was evident through the small, but significant contribution of SPE-DOC to apparent oxygen utilization as well as on molecular level. Our findings demonstrate that SPE-DOC captures carbon fractions relevant on timescales of seasons to timescales covering ocean circulation and biogeochemical processes in stable gyre systems are imprinted in the DOM molecular composition.

Intercropping and N application enhance soil dissolved organic carbon concentration with complicated chemical composition

Soil dissolved organic carbon (DOC) plays a critical role in migration and transformation of soil organic carbon. For revealing effects of intercropping and N application on dynamics of DOC concentration and chemical structure, a multi-year located field experiment with cropping patterns (maize monocropping and maize-potato intercropping) subjected to four N levels (0 kg ha−1, 125 kg ha−1, 250 kg ha−1, 375 kg ha−1) was conducted. The results showed that N application raised DOC concentration while DOC accumulation, had a quadratic relationship with N application rate during the entire growth period of maize. At the same N level, intercropping increased the concentration and accumulation of DOC compared with monocropping. Meanwhile N application promoted the contents of amines and aromatic compounds and average molecular weight of DOC molecules, which reached the highest value at rate of 125 kg N ha-1. Compared with monocropping at the same N level, there was a tendency to increase the content of amines and aromatic compounds and the average molecular weight of DOC in intercropping, thus improving the degree of humification and aromatization of soil DOC. Soil temperature, water content and NH4+-N were the main factors mediating DOC concentration, while DOC concentration and soil temperature significantly affected the chemical structure of DOC. The structural equation model showed intercropping and N application elevated soil DOC concentration by increasing soil temperature, litter and root biomass, and the improvement of DOC concentration and soil temperature further enhanced the average molecular weight of DOC, resulting in accumulation of DOC.

Nitrogen fertilization overweighs intercropping in promotion of dissolved organic carbon concentration and complexity in potato-cropped soil

Intercropping promoted land productivity and be performed worldwide. This study aimed to assess the effect of intercropping on soil dissolved organic carbon (DOC) and its nitrogen (N) regulation. The potato was monocropped and intercropped with maize under four N levels (0, 62.5, 125 and 187.5 kg ha−1 for N0, N1, N2 and N3, respectively) in a fixed plot experiment from 2014 in subtropical China. Soil DOC concentration was determined and seasonal and interannual dynamics were analyzed from 2016 to 2018; Absorption values of specified wavelength were selected to characterize the annul DOC composition. Finally, the driving factors of DOC variation were explored using structural equation modeling (SEM). Soil DOC concentration and accumulation under N-applied treatments were higher than N0 and the increase peaked at N1. Compared with monocropping, intercropping significantly increased DOC accumulation by 20.29% only at N0. N application significantly improved the contents of amines and aromatic compounds and the average molecular weight of DOC, thus improving the degree of humification and aromatization of soil DOC. The SEM showed that the chemical composition of DOC was complicated with increasing soil temperature, aboveground and root biomass, while DOC concentration and chemical composition significantly affected the accumulation of DOC. Agricultural diversified cropping affects the dynamics and composition of soil DOC, which is intensively regulated by N fertilization. This study expands the insight into the variable DOC concentration and composition and its role in soil organic carbon turnover under the varying fertilization and cropping patterns.

Bacterioplankton response to nitrogen and dissolved organic matter produced from salmon mucus.

Aquaculture releases organic matter to the water column through excretion, fecal pellets, and uneaten food, but also by the continuous release of fish epithelium mucus. The effect of the latter on natural bacterial assemblages was determined using ammonium amended experiments at Puyuhuapi fjord in Chilean Patagonia. Mucus was added to seawater coming from 2 and 100 m depth and ammonium, nitrite and nitrate, dissolved organic carbon (DOC), picoplankton abundance, and active composition (i‐tag 16S rRNA) were followed for 24 h. The results showed a significant response from the microbial community but only at surface depth after 2 and 6 h of incubation. A reduction of DOC and ammonium concentration and accumulation of nitrite and nitrate over time was observed, mainly at 100 m. Changes in the composition of active bacteria between treatments were observed at different taxonomic levels, associated with Alphaproteobacteria (Clade SAR11), Bacteroidetes (Polaribacter) and Gammaproteobacteria (Colwellia, Oceaniserpentilla) and other bacteria such as Nitrospina sp, a nitrite‐oxidizing bacteria at some hours during the incubation. Fish pathogens, such as Vibrio and Piscirickettsia were rare (<0.02%). Overall, our study suggests that fish mucus can cause rapid modifications in microbial assemblages and stimulate organic matter and nutrient cycling, including heterotrophic and autotrophic (nitrification) in areas influenced by aquaculture.

Maintaining microbial diversity mitigates membrane fouling of an anoxic/oxic membrane bioreactor under starvation condition

The aim of this study was to evaluate the contribution of dissolved organic carbon (DOC) and microbial community dynamics to membrane fouling development in membrane bioreactor (MBR). We operated laboratory-scale anoxic/oxic-MBRs under prolonged starvation conditions in different seasons and the dynamics and diversity of the microbial communities were investigated. Although fouled-MBRs showed DOC accumulation in the activated sludge (AS), the fouling-mitigated MBR suggested that dissolved oxygen was consumed and DOC of the sludge supernatant was degraded. 16S rRNA genes analysis of AS in the MBRs revealed that Chitinophagaceae and Candidatus Promineofilum specifically increased in the fouling-mitigated MBR, suggesting that they played important roles in membrane fouling mitigation; high microbial diversity in the reactor also contributed to fouling mitigation. In the fouled reactor, enrichment of Xanthomonadaceae might be related to fouling causing substances formation leading to membrane fouling development; lower microbial diversity also contributed to fouling development in the fouled MBR.

Net Community Production, Dissolved Organic Carbon Accumulation, and Vertical Export in the Western North Atlantic

The annual North Atlantic phytoplankton bloom represents a hot spot of biological activity during which a significant fraction of net community production (NCP) can be partitioned into dissolved organic carbon (DOC). The fraction of seasonal NCP that is not respired by the heterotrophic bacterial community and accumulates as seasonal surplus DOC (∆DOC) in the surface layer represents DOC export to the upper mesopelagic zone, and in the North Atlantic this is facilitated by winter convective mixing that can extend to depths >400 m. However, estimates of ∆DOC and vertical DOC export for the western North Atlantic remain ill-constrained and the influence of phytoplankton community structure on the partitioning of seasonal NCP as ∆DOC is unresolved. Here, we couple hydrographic properties from autonomous in situ sensors (ARGO floats) with biogeochemical data from two meridional ship transects in the late spring (~ 44 – 56˚N along ~ -41˚W) and early autumn (~ 42 – 53˚N along ~ -41˚W) as part of the North Atlantic Aerosols and Marine Ecosystems Study (NAAMES). We estimate that 4 – 35% of seasonal NCP is partitioned as ∆DOC and that annual vertical DOC export ranges between 0.34 – 1.15 mol C m-2 in the temperate and subpolar North Atlantic. Two lines of evidence reveal that non-siliceous picophytoplankton, like Prochlorococcus, are indicator species of the conditions that control the accumulation of DOC and the partitioning of NCP as ∆DOC.

Impacts of redox conditions on dissolved organic matter (DOM) quality in marine sediments off the River Rhône, Western Mediterranean Sea

Sedimentary dissolved organic matter (DOM) is an important pool of intermediates produced and consumed during early diagenesis of organic matter in the anoxic subseafloor. Rapid degradation of organic matter in the coastal sediment results in stratification of redox zones. However, to date little is known about the selectivity with respect to organic matter of the initial microbially mediated anaerobic degradation processes under contrasting redox conditions and how these affect the composition of DOM. In order to study the effect of sulfate reducing vs. methanogenic conditions on DOM quality and degradation, sediments (0–18 cm) from the Rhône River Delta were incubated, with redox conditions being controlled by sulfate amendment. The progress of incubation was monitored by H2, CH4, sulfate, DIC, DOC and acetate production. DOM composition was determined by 3D Fluorescence Spectroscopy, i.e., Excitation Emission Matrix Spectroscopy (EEMs), and ultra-high-resolution Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR-MS). Parallel factor analysis (PARAFAC) of EEMs was used to distinguish different groups of DOM (humic-like and protein-like compounds) to evaluate composition, conjugation and size of DOM. Prior to incubation, humic-like sedimentary DOM predominated; nearly half of the molecular formulae (>5000) identified by FT-ICR-MS were CHO and one third were CHNO compounds. During incubation, protein-like DOM and CHNO formulae with 3 and 4 N atoms formulae were rare under both sulfate reducing and methanogenic conditions. Incubation under sulfate reducing conditions resulted in rapid release and net accumulation of dissolved organic carbon (DOC). Protein-like DOM was rapidly cycled and humic-like DOM accumulated. Consistently, CHNO formulae with 3 and 4 N atoms decreased faster, whereas formulae with one N and oxygen-rich unsaturated compounds became more concentrated. In contrast, during incubation under methanogenic conditions, there was no net accumulation of DOC; blue-shift of humic-like peaks suggest the transformation possibly associated with loss of oxygen-bearing functional groups in conjugated structures of humic substances. This interpretation is consistent with the relative decrease of oxygenation and carbon number in the pool of aromatic and highly unsaturated compounds observed by FT-ICR-MS analysis. Approximately 90% of molecular formulae that were lost under methanogenic conditions were accumulated under sulfate reducing conditions. Our results suggest that under sulfate reducing conditions degradation of organic matter results in the accumulation of highly oxidized DOM, while protein-like compounds are selectively consumed. When the redox regime changes to methanogenic conditions, microbes apparently utilize the humic-like and oxygen-rich compounds of the oxidized DOM pool that accumulated under sulfate reducing conditions. Consequently, redox regimes and the associated biogeochemical processes influence rate and fractions of DOM released by and consumed in the deep biosphere, which could ultimately shape the composition of the preserved sedimentary organic matter and the DOM released to the ocean.

Chronic nitrogen addition promotes dissolved organic carbon accumulation in a temperate freshwater wetland

Temperate wetlands have been undergoing increased nitrogen (N) inputs in the past decades, yet its influence on dissolved organic carbon (DOC) dynamics is still elusive in these ecosystems. Here, using a field multi-level N addition (0, 6, 12, and 24 g N m−2 year−1) experiment, we investigated the changes in aboveground plant biomass, DOC production from plant litters, DOC biodegradation, and DOC concentration in surface water and soil pore water (0–15 cm depth) following 10 years of N addition in a freshwater marsh of Northeast China. We observed that, irrespective of N addition levels, N addition caused an increase in DOC production from plant litters under both non-flooded and flooded conditions. Conversely, DOC biodegradation was inhibited by N addition in both surface water and soil pore water. Because of enhanced DOC production from plant litters and declined DOC biodegradation, N addition elevated DOC concentration in surface water and soil pore water across the growing season. In addition, long-term N addition increased aboveground plant biomass, but decreased species richness. Our results suggest that long-term N enrichment promotes DOC accumulation through the contrasting effects on litter-derived DOC production and microbial decomposition of DOC in temperate wetlands.