Low Molecular Weight Dissolved Organic Carbon Research Articles

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.

Volatile and Dissolved Organic Carbon Sources Have Distinct Effects on Microbial Activity, Nitrogen Content, and Bacterial Communities in Soil.

Variation in microbial use of soil carbon compounds is a major driver of biogeochemical processes and microbial community composition. Available carbon substrates in soil include both low molecular weight-dissolved organic carbon (LMW-DOC) and volatile organic compounds (VOCs). To compare the effects of LMW-DOC and VOCs on soil chemistry and microbial communities under different moisture regimes, we performed a microcosm experiment with five levels of soil water content (ranging from 25 to 70% water-holding capacity) and five levels of carbon amendment: a no carbon control, two dissolved compounds (glucose and oxalate), and two volatile compounds (methanol and α-pinene). Microbial activity was measured throughout as soil respiration; at the end of the experiment, we measured extractable soil organic carbon and total extractable nitrogen and characterized prokaryotic communities using amplicon sequencing. All C amendments increased microbial activity, and all except oxalate decreased total extractable nitrogen. Likewise, individual phyla responded to specific C amendments-e.g., Proteobacteria increased under addition of glucose, and both VOCs. Further, we observed an interaction between moisture and C amendment, where both VOC treatments had higher microbial activity than LMW-DOC treatments and controls at low moisture. Across moisture and C treatments, we identified that Chloroflexi, Nitrospirae, Proteobacteria, and Verrucomicrobia were strong predictors of microbial activity, while Actinobacteria, Bacteroidetes, and Thaumarcheota strongly predicted soil extractable nitrogen. These results indicate that the type of labile C source available to soil prokaryotes can influence both microbial diversity and ecosystem function and that VOCs may drive microbial functions and composition under low moisture conditions.

Utilization of low-molecular-weight organic compounds by the filterable fraction of a lotic microbiome.

ABSTRACTFilterable microorganisms participate in dissolved organic carbon (DOC) cycling in freshwater systems, however their exact functional role remains unknown. We determined the taxonomic identity and community dynamics of prokaryotic microbiomes in the 0.22 µm-filtered fraction and unfiltered freshwater from the Conwy River (North Wales, UK) in microcosms and, using targeted metabolomics and 14C-labelling, examined their role in the utilization of amino acids, organic acids and sugars spiked at environmentally-relevant (nanomolar) concentrations. To identify changes in community structure, we used 16S rRNA amplicon and shotgun sequencing. Unlike the unfiltered water samples where the consumption of DOC was rapid, the filtered fraction showed a 3-day lag phase before the consumption started. Analysis of functional categories of clusters of orthologous groups of proteins (COGs) showed that COGs associated with energy production increased in number in both fractions with substrate addition. The filtered fraction utilized low-molecular-weight (LMW) DOC at much slower rates than the whole community. Addition of nanomolar concentrations of LMW DOC did not measurably influence the composition of the microbial community nor the rate of consumption across all substrate types in either fraction. We conclude that due to their low activity, filterable microorganisms play a minor role in LMW DOC processing within a short residence time of lotic freshwater systems.

Low Molecular Weight Dissolved Organic Carbon: Aging, Compositional Changes, and Selective Utilization During Global Ocean Circulation

AbstractThe composition and cycling dynamics of marine dissolved organic carbon (DOC) have received increased interest in recent years; however, little research has focused on the refractory, low molecular weight (LMW) component that makes up the majority of this massive C pool. We measured stable isotopic (δ13C), radioisotopic (Δ14C), and compositional (C/N, 13C solid‐state NMR) properties of separately isolated high molecular weight (HMW) and LMW DOC fractions collected using a coupled ultrafiltration and solid phase extraction approach from throughout the water column in the North Central Pacific and Central North Atlantic. The selective isolation of LMW DOC material allowed the first investigation of the composition and cycling of a previously elusive fraction of the DOC pool. The structural composition of the LMW DOC material was homogeneous throughout the water column and closely matched carboxylic‐rich alicyclic material that has been proposed as a major component of the marine refractory DOC pool. Examination of offsets in the measured parameters between the deep waters of the two basins provides the first direct assessment of changes in the properties of this material with aging and utilization during ocean circulation. While our direct measurements largely confirm hypotheses regarding the relative recalcitrance of HMW and LMW DOC, we also demonstrate a number of novel observations regarding the removal and addition of DOC during global ocean circulation, including additions of fresh carbohydrate‐like HMW DOC to the deep ocean and large‐scale removal of both semilabile HMW and recalcitrant LMW DOC.

Effects of soil organic residue amendment on losses of dissolved organic carbon, P, Cu and Zn via surface runoff from arable soils

Dissolved organic carbon (DOC) constitutes the largest carbon pool in surface waters, with soil being its main source via surface and subsurface runoff. Amendment with organic residues can reduce soil erosion and surface runoff, but little is known how this affects the amount and composition of DOC losses via surface runoff and the associated transfer of nutrients and pollutants. In this study, artificial rainfall experiments were conducted on three pairs of soil runoff plots with contrasting organic matter management. A mixture of grass and wheat straw residues was incorporated into the topsoil of one plot in each pair (OI treatment) but not into the other (control). Artificial rainfall was applied onto both treatments at an intensity of 49.1 mm h−1, and surface runoff samples were collected at designated time steps for analysis of DOC, total dissolved phosphorus (TDP), copper (TDCu) and zinc (TDZn) concentrations, as well as for characterization of DOC by UV–vis absorbance (SUVA254 and E2:E3) and fluorescence spectroscopy (FI) indexes. The organic amendment reduced the loss rates of DOC, TDP, TDCu and TDZn by reducing surface runoff, although increasing the concentrations of DOC, TDP and TDCu. Regardless of the treatment, DOC, TDP and TDCu concentrations were always maximal at the onset of runoff and then continuously decreased after the initial flush. The observed relationships of TDP and TDCu concentrations with DOC indicate that the export of “dissolved” P and Cu primarily occurred in DOC-bound form, while the concentration of TDZn was not correlated with that of DOC. Surface runoff from the amended soils generally contained more hydrophilic and low molecular weight DOC (as indicated by low SUVA254 and high E2:E3 and FI values), particularly at high DOC concentrations. The ratio between TDCu and DOC concentration showed a positive relationship to DOC aromaticity (as indicated by SUVA254 values) in the OI treatment. The results indicate that organic amendment can reduce total rates of dissolved element losses with surface runoff, but attention should also be paid to the composition of lost DOC and the concentrations of DOC-associated solutes.

Microbial uptake kinetics of dissolved organic carbon (DOC) compound groups from river water and sediments

Dissolved organic matter (DOM) represents a key component of carbon (C) cycling in freshwater ecosystems. While the behaviour of bulk dissolved organic carbon (DOC) in aquatic ecosystems is well studied, comparatively little is known about the turnover of specific DOC compounds. The aim of this study was to investigate the persistence of 14C-labelled low molecular weight (LMW) DOC at a wide range of concentrations (0.1 µM to 10 mM), in sediments and waters from oligotrophic and mesotrophic rivers within the same catchment. Overall, rates of DOC loss varied between compound groups (amino acids > sugars = organic acids > phenolics). Sediment-based microbial communities contributed to higher DOC loss from river waters, which was attributed, in part, to its greater microbial biomass. At higher DOC compound concentrations, DOC loss was greater in mesotrophic rivers in comparison to oligotrophic headwaters. A lag-phase in substrate use within sediments provided evidence of microbial growth and adaptation, ascribed here to the lack of inorganic nutrient limitation on microbial C processing in mesotrophic communities. We conclude that the higher microbial biomass and available inorganic nutrients in sediments enables the rapid processing of LMW DOC, particularly during high C enrichment events and in N and P-rich mesotrophic environments.

Release of Carbon in Different Molecule Size Fractions from Decomposing Boreal Mor and Peat as Affected by Enchytraeid Worms

Terrestrial export of dissolved organic carbon (DOC) to watercourses has increased in boreal zone. Effect of decomposing material and soil food webs on the release rate and quality of DOC are poorly known. We quantified carbon (C) release in CO2, and DOC in different molecular weights from the most common organic soils in boreal zone; and explored the effect of soil type and enchytraeid worms on the release rates. Two types of mor and four types of peat were incubated in laboratory with and without enchytraeid worms for 154 days at + 15 °C. Carbon was mostly released as CO2; DOC contributed to 2–9% of C release. The share of DOC was higher in peat than in mor. The release rate of CO2 was three times higher in mor than in highly decomposed peat. Enchytraeids enhanced the release of CO2 by 31–43% and of DOC by 46–77% in mor. High molecular weight fraction dominated the DOC release. Upscaling the laboratory results into catchment level allowed us to conclude that peatlands are the main source of DOC, low molecular weight DOC originates close to watercourse, and that enchytraeids substantially influence DOC leaching to watercourse and ultimately to aquatic CO2 emissions.

Direct Visualization of Individual Aromatic Compound Structures in Low Molecular Weight Marine Dissolved Organic Carbon

AbstractDissolved organic carbon (DOC) is the largest pool of exchangeable organic carbon in the ocean. However, less than 10% of DOC has been molecularly characterized in the deep ocean to understand DOC's recalcitrance. Here we analyze the radiocarbon (14C) depleted, and presumably refractory, low molecular weight (LMW) DOC from the North Central Pacific using atomic force microscopy to produce the first atomic‐resolution images of individual LMW DOC molecules. We evaluate surface and deep LMW DOC chemical structures in the context of their relative persistence and recalcitrance. Atomic force microscopy resolved planar structures with features similar to polycyclic aromatic compounds and carboxylic‐rich alicyclic structures with less than five aromatic carbon rings. These compounds comprise 8% and 20% of the measurable molecules investigated in the surface and deep, respectively. Resolving the structures of individual DOC molecules represents a step forward in molecular characterization of DOC and in understanding its long‐term stability.

Spatial and Seasonal Variations of Organic Carbon-Based Nutrients in Setiu Wetland, Malaysia

The aim of this study was to assess spatial and seasonal variations of dissolved and particulate organic carbon (DOC and POC, respectively) in the Setiu Wetland, which is located in the southern region of the South China Sea coastal area. Surface water samples were collected monthly from October 2009-September 2010. In addition, the concentration of chlorophyll-a was determined. The DOC and POC mean concentration varied between 80-340 μM and 60-115 μM, respectively. The results showed that the concentrations of these parameters were mainly influenced by human activities and season. Higher concentrations of DOC (150-340 μM) and POC (75-115 μM) were found at stations near the populated area, aquaculture and palm oil plantation. In addition, higher concentrations of DOC (200-275 μM) and POC (105-150 μM) were also recorded during the monsoon season due to runoff from terrestrial sources and resuspension from bottom sediments. No significant correlation between the low molecular weight (LMW) fractions of DOC and chlorophyll-a (R2=0.0096, p>0.05) further supports the hypothesis that phytoplankton not playing key role in regulating the organic carbon distribution. Higher percentage of LMW DOC in monsoon season suggesting possible input of this fraction from land runoff to the wetland area.

An experimental use of floating treatment wetlands (FTWs) to reduce phytoplankton growth in freshwaters

Eutrophication and the formation of phytoplankton blooms in freshwaters can be detrimental to water quality and biological health and produce organic matter that can be difficult to remove during water treatment processes. With the frequency of phytoplankton blooms increasing, remediation solutions are becoming increasingly popular. This study investigated the use of a peat-based floating treatment wetland (FTW) for reducing phytoplankton growth in eutrophic waters. Over a four-week period, the FTWs were able to reduce chlorophyll a concentrations by 80%, through sequestration of nitrate and phosphate and possibly due to the direct inhibitory properties of phenolic compounds. Although there are concerns about the leaching of dissolved organic carbon (DOC) from the FTWs, this may be more than offset by the beneficial suppression of phytoplankton growth and the resulting reduced input of ‘untreatable’ low molecular weight DOC.

Freeze-thaw cycles have minimal effect on the mineralisation of low molecular weight, dissolved organic carbon in Arctic soils.

Warmer winters in Arctic regions may melt insulating snow cover and subject soils to more freeze–thaw cycles. The effect of freeze–thaw cycles on the microbial use of low molecular weight, dissolved organic carbon (LMW-DOC) is poorly understood. In this study, soils from the Arctic heath tundra, Arctic meadow tundra and a temperate grassland were frozen to −7.5 °C and thawed once and three times. Subsequently, the mineralisation of 3 LMW-DOC substrates types (sugars, amino acids and peptides) was measured over an 8-day period and compared to controls which had not been frozen. This allowed the comparison of freeze–thaw effects between Arctic and temperate soil and between different substrates. The results showed that freeze–thaw cycles had no significant effect on C mineralisation in the Arctic tundra soils. In contrast, for the same intensity freeze–thaw cycles, a significant effect on C mineralisation was observed for all substrate types in the temperate soil although the response was substrate specific. Peptide and amino acid mineralisation were similarly affected by FT, whilst glucose had a different response. Further work is required to fully understand microbial use of LMW-DOC after freeze–thaw, yet these results suggest that relatively short freeze–thaw cycles have little effect on microbial use of LMW-DOC in Arctic tundra soils after thaw.

Ancient low–molecular-weight organic acids in permafrost fuel rapid carbon dioxide production upon thaw

Northern permafrost soils store a vast reservoir of carbon, nearly twice that of the present atmosphere. Current and projected climate warming threatens widespread thaw of these frozen, organic carbon (OC)-rich soils. Upon thaw, mobilized permafrost OC in dissolved and particulate forms can enter streams and rivers, which are important processors of OC and conduits for carbon dioxide (CO2) to the atmosphere. Here, we demonstrate that ancient dissolved organic carbon (DOC) leached from 35,800 y B.P. permafrost soils is rapidly mineralized to CO2. During 200-h experiments in a novel high-temporal-resolution bioreactor, DOC concentration decreased by an average of 53%, fueling a more than sevenfold increase in dissolved inorganic carbon (DIC) concentration. Eighty-seven percent of the DOC loss to microbial uptake was derived from the low-molecular-weight (LMW) organic acids acetate and butyrate. To our knowledge, our study is the first to directly quantify high CO2 production rates from permafrost-derived LMW DOC mineralization. The observed DOC loss rates are among the highest reported for permafrost carbon and demonstrate the potential importance of LMW DOC in driving the rapid metabolism of Pleistocene-age permafrost carbon upon thaw and the outgassing of CO2 to the atmosphere by soils and nearby inland waters.

Greenland Ice Sheet exports labile organic carbon to the Arctic oceans

Abstract. Runoff from small glacier systems contains dissolved organic carbon (DOC) rich in protein-like, low molecular weight (LMW) compounds, designating glaciers as an important source of bioavailable carbon for downstream heterotrophic activity. Fluxes of DOC and particulate organic carbon (POC) exported from large Greenland catchments, however, remain unquantified, despite the Greenland Ice Sheet (GrIS) being the largest source of global glacial runoff (ca. 400 km3 yr−1). We report high and episodic fluxes of POC and DOC from a large (>600 km2) GrIS catchment during contrasting melt seasons. POC dominates organic carbon (OC) export (70–89% on average), is sourced from the ice sheet bed, and contains a significant bioreactive component (9% carbohydrates). A major source of the "bioavailable" (free carbohydrate) LMW–DOC fraction is microbial activity on the ice sheet surface, with some further addition of LMW–DOC to meltwaters by biogeochemical processes at the ice sheet bed. The bioavailability of the exported DOC (26–53%) to downstream marine microorganisms is similar to that reported from other glacial watersheds. Annual fluxes of DOC and free carbohydrates during two melt seasons were similar, despite the approximately two-fold difference in runoff fluxes, suggesting production-limited DOC sources. POC fluxes were also insensitive to an increase in seasonal runoff volumes, indicating a supply limitation in suspended sediment in runoff. Scaled to the GrIS, the combined DOC (0.13–0.17 Tg C yr−1 (±13%)) and POC fluxes (mean = 0.36–1.52 Tg C yr−1 (±14%)) are of a similar order of magnitude to a large Arctic river system, and hence may represent an important OC source to the near-coastal North Atlantic, Greenland and Labrador seas.

Does co-extracted dissolved organic carbon cause artefacts in cell-based bioassays?

Bioanalytical tools are increasingly being employed for water quality monitoring, with applications including samples that are rich in natural organic matter (or dissolved organic carbon, DOC), such as wastewater. While issues associated with co-extracted DOC have been identified for chemical analysis and for bioassays with isolated enzymes, little is known about its effect on cell-based bioassays. Using mixture experiments as diagnostic tools, this study aims to assess whether different molecular weight fractions of wastewater-derived DOC adversely affect cell-based bioassays, specifically the bioluminescence inhibition test with the bacteria Vibrio fischeri, the combined algae assay with Pseudokirchneriella subcapitata and the human cell line AREc32 assay for oxidative stress. DOC did not cause suppressive effects in mixtures with reference compounds. Binary mixtures further indicated that co-extracted DOC did not disturb cell-based bioassays, while slight deviations from toxicity predictions for low molecular weight fractions may be partially due to the availability of natural components to V. fischeri, in addition to organic micropollutants.

富营养化湖泊溶解性有机碳生物可利用性研究进展

PDF HTML阅读 XML下载 导出引用 引用提醒 富营养化湖泊溶解性有机碳生物可利用性研究进展 DOI: 10.5846/stxb201209241349 作者: 作者单位: 南通大学地理科学学院,中国科学院南京地理与湖泊研究所湖泊与环境国家重点实验室,中国科学院南京地理与湖泊研究所 作者简介: 通讯作者: 中图分类号: 基金项目: 国家自然科学基金资助项目（41201076，31270507，31070420）；湖泊与环境国家重点实验室开放基金资助项目（2012SKL006） The bioavailability of dissolved organic carbon in the eutrophic lakes Author: Affiliation: Geographical Sciences College,Nantong University,State key Laboratory of Lake Environment Science,Nanjing Institute of Geography and Limnology,Chinese Academy of Sciences,State key Laboratory of Lake Environment Science,Nanjing Institute of Geography and Limnology,Chinese Academy of Sciences Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:富营养化湖泊溶解性有机碳（DOC）包括内源和外源性碳源，不同来源碳源在物质化学结构组成和分子量级等方面具有显著差异，进而影响到对细菌的生物可利用性和碳素在食物网中的传递效率。根据国内外文献，综述了内外源DOC在碳稳定同位素值域上的显著差异，建议通过对DOC碳稳定同位素的分析来识别富营养化湖泊中DOC的主要来源；通过对比内外源DOC在碳水化合物、结合态中性糖和腐殖质含量上的差异，并结合细菌生长参数如细菌二级生产力、细菌呼吸作用及细菌生长效率来分析内外源DOC对细菌的生物可利用性。从富营养化湖泊DOC来源的角度探讨其生物可利用性和碳素传递效率，有助于了解富营养化湖泊食物网中碳素循环特征，加强对湖泊生态学的认识，为湖泊环境治理与保护提供科学依据。 Abstract:The dissolved organic carbon (DOC) pool is composed of both autochthonous and allochthonous DOC, and its concentration in lakes generally increases with the trophic status. Accumulation of the autochthonous DOC was observed in the eutrophic lake, and the allochthonous DOC was highest in the dystrophic lake. Carbohydrates constitute a large component of the DOC, the consumption of DOC by heterotrophic bacteria is one of the largest fluxes of carbon in most aquatic ecosystems, but the bioavailability and the efficiency of carbon transfer in lakes food web is affected by the distribution of molecular weight and chemical composition. The DOC can be separated into high and low molecular weight DOC fractions by cross-flow ultrafiltration, but which fraction is more bioreactive is still in dispute.Stable carbon isotope can be used to trace the origins of organic carbon, and the approach depends on the fact that DOC from different origins has different stable isotopic compositions. The riverine DOC has a δ13C value of -27‰, which is different from freshwater phytoplankton, with a range from -35‰ to -25‰. This paper reviewed the researches on the stable carbon isotope ratio of the autochthonous and allochthonous DOC, suggesting that the main sources of DOC in eutrophic lakes can be identified by using natural stable carbon isotope ratio of DOC; the difference on the total dissolved carbohydrates (TCHO) and dissolved combined neutral sugar (DCNS) concentrations, as well as the humic substances (HS) was compared between the autochthonous and allochthonous DOC. Net increases in TCHO and DCNS were observed in the autochthonous DOC during phytoplankton blooms, whereas the HS fraction was quantitatively important in the allochthonous DOC. Many studies have reported the bacterial availability of TCHO and DCNS, and the ratio of TCHO/DOC is used to characterize the bioavailability of DOC, whereas HS can also increase bacterial secondary production and support bacterial growth if labile substrates are abundant.Furthermore, to elucidate the bioavailability of the two sources of DOC, the bacterial secondary production, bacterial respiration and bacteria growth efficiency (BGE) was analyzed together. The DOC can be either transformed to bacterial secondary production or respired to inorganic carbon. BGE is the fraction of assimilated organic carbon that supports growth. The source of DOC and its chemical composition could be a key regulator of BGE. Traditionally, the autochthonous DOC has been considered to be the main source for bacterial as well as other secondary production, the allochthonous DOC was long considered relatively recalcitrant to bacterial degradation. However, in lakes which are both humic-rich and oligotrophic, the ecosystem respiration exceeds gross primary production, suggesting that the allochthonous DOC can be incorporated into the bacteria biomass and makes a significant carbon and energy subsidy for lakes food web, but little of the allochthonous carbon assimilated by bacteria is likely to reach higher consumers. Recent studies suggest that bacterial BGE increases with the concentration of the low molecular weight DOC in allochthonous DOC.Discussing the bioavailability and efficiency of carbon transfer in the food web from the sources of DOC, will be helpful to investigate the characterization of carbon cycling in the eutrophic lakes, enhance our understanding of the lake ecology and to provide scientific references for the lake management and protection. 参考文献 相似文献 引证文献

Dynamics of vegetation, nitrogen and carbon as indicators of the operation of peatland buffers

Forestry in peatlands may result in an increased release of inorganic nitrogen (N), phosphate phosphorus (PO4-P), dissolved organic matter (DOM) and solids. Peatland buffers with sedimentation ponds have been constructed in order to prevent leaching of solids and nutrients to the watercourses. Results of peatland buffer functionality have been contradictory probably because the potential nutrient retention capacity of the peat, the biomass of nutrient binding microorganisms and vegetation, and the amount of N liberated in a gaseous form to the atmosphere varies between buffers. There fore, a detailed knowledge of N cycling in peatland buffers is crucial. In addition, the possible impact of the quality of DOM on nitrous oxide (N2O) dynamics is unknown. Three buffers were constructed in eastern Finland: a spruce swamp and a brook margin meadow (constructed prior to our study in 1997) and a spruce-pine mire with lake margin fen during the research period in 2005. Fluxes of N2O were measured in buffers and a clearcut area in the catchment of the spruce-pine mire with lake margin fen and the rate of N2O accumulation was determined for peat profiles cored in the spruce swamp. In addition, the production capacity of N2O-N under different fertilization levels was measured from peat samples cored from the spruce swamp in the laboratory. Nutrient concentrations were measured and DOM characterized by means of DOC from the surface waters entering the buffer and from the vadose water. The quality of DOC was characterized in terms of molecular size fractions and aromaticity indexes. The dynamics of DOM characteristics were related to those of N2O by evaluating the significance of DOC quality on the N2O flux. Vegetation cover was monitored temporally and spatially over a three year period in order to evaluate possible vegetation change following buffer construction. In the buffers and the sedimentation pond, N2O emissions were low, although fluxes from the sedimentation pond were higher than those from N loaded humic lakes. The rate of N2O accumulation and N2O emissions after fertilization were high in the peat samples cored from the spruce swamp indicating a high capacity to produce N2O if free nitrate (NO3 -) enters the buffer. However, the observed concentrations of N inflow to the buffers were low; supporting low N2O emissions even near the water inflow. The presence of low molecular weight DOC seems to be a significant controller for N2O efflux between the soil and the atmosphere after forestry operations. Concentrations of PO4-P were occasionally higher in the water outflow than those in the inflow in the buffers indicating a possible nutrient leak. Thus, the changes in vegetation cover reflected an area of effective water flow paths within the buffer rather than eutrophication. Changes in above-ground biomass or its N content did not indicate an increased nutrient binding capacity in the buffer vegetation. Small buffers on organic soils may often have more importance in preventing the flow of solids rather than nutrients. The significance of buffers as a source of the greenhouse gas N2O was negligible, both because of the low flux rates, and the small area used for water protection.

Dynamics of rare earth elements in water–soil systems: The case study of the Pineta San Vitale (Ravenna, Italy)

The European Union Site of Community Importance Pineta San Vitale (Ravenna, Italy) is located in a complex lagoon system nearby the Adriatic Sea. This area is affected by relevant environmental pressures related to subsidence, groundwater salinization and industrial contamination. The occurrence of rare earth elements (REE), and the characterization of processes that control their migration in the water–soil system were performed for implementing the knowledge of the present environmental status in the area.Nine soil profiles were opened in the Pineta San Vitale and each horizon was characterized. In the parent material, dominant quartz, followed by plagioclase, mica, chlorite and calcite were recognized by XRD. Topsoil samples were characterized by neutral or sub-acid pH, relatively high organic carbon content (LOI up to 43wt.%; TOC: 38÷61g/kg), and low carbonate (11÷20g/kg CaCO3). Subsoil horizons were sub-alkaline or moderately alkaline, rich in carbonate (up to 114g/kg CaCO3), and depleted in organic matter. The electrical conductivity (EC: 0.2÷4.2 mS/cm) values along the soil profiles did depend on the water table fluctuations and the salinity of groundwater. Sequential extraction tests on two soil profiles showed the highest Y and ΣREE occurrence in the residual fraction of soils, with values generally increasing at depth, and total amounts similar to values estimated in the upper continental crust (UCC). Poor fractionation of REE was observed when the bulk soil horizon was considered, but different REE patterns occurred in the organic, oxide, and carbonate soil fractions, suggesting REE fractionation during weathering and pedogenic processes.Physical–chemical parameters and concentrations of total and dissolved elements were determined in 44 water samples. Waters showed near-neutral or slightly alkaline pH (6.8÷8.0) and total dissolved solids mostly in the range of 2 to 24g/L, with values increasing with increasing seawater influence. Concentrations of ΣREE determined in waters filtered through 0.4μm pore size filters varied from 0.1 to 189μg/L (0.7·10−9÷1.3·10−6mol/L ΣREE), with the highest values observed in soil waters. REE values were not related to salinity, nor to major ions, while were positively correlated to Al concentrations. Further filtration through 0.015μm pore size filters removed about 90% of REE compared to the 0.4μm filtered samples, i.e. the studied waters showed a small quantity of truly dissolved REE species. Speciation modeling results by Visual MINTEQ indicated that truly dissolved REE species are dominated by bi-dentate organic complexes in the soil waters rich in dissolved organic carbon (up to 74mg/L DOC); aqueous complex with the carbonate and bicarbonate ligands increased in waters having low DOC. UCC-normalized patterns of REE in the colloidal fraction of soil waters showed slight enrichments in the middle REE (MREE), likely due to preferential adsorption of the MREE onto <0.4μm particles. UCC-normalized patterns of REE truly dissolved in soil waters showed slight enrichments in the heavy REE (HREE), probably due to dominant low molecular weight DOC in the truly dissolved fraction.For evaluating the potential REE contribution from the atmosphere, 12 vegetation samples (ivy leaves) were submitted to leaching tests using acidulated water as a proxy of rain water. The REE leached from leaves were in the range of 0.5 to 10μg/kg ΣREE, suggesting a significant atmospheric contribution to the water–soil system. UCC-normalized patterns of REE leached from leaves were nearly flat, possibly indicating a dominant geogenic source of atmospheric REE.

UV irradiation of natural organic matter (NOM): impact on organic carbon and bacteria

UV-induced transformation of dissolved organic matter (DOC) is often accompanied by reduction of molecular weight and aromaticity and an increase of low-molecular weight (LMW) matter that can be utilized as a substrate by heterotrophic bacteria. Moreover, the generation of reactive transients and mineralization of DOC occurs. For a better understanding of the modification that starts after irradiation and to distinguish between possible chemical and biological modifications, we selected different natural organic matter (NOM) from Norway and Germany. The aqueous solutions were treated by UV irradiation and divided into two aliquot samples. NaN3 anti-bacterial treatment was applied to one sample, and high-pressure size-exclusion chromatography (HPSEC) analysis was used for both. In all samples, we found typical modifications of NOM after UV irradiation. Incubation (>7 days) of UV-irradiated NOM samples resulted in lower levels of LMW matter and increased aromaticity. Parallel to these changes of carbon fractions, an increase in bacterial cell numbers was observed. Addition of NaN3 to NOM, however, inhibited the reduction of LMW matter, indicating that microbial activity accounted for the observed changes in NOM. Analysis of the bacterial community composition by denaturing gradient gel electrophoresis (DGGE) of the amplified 16S rRNA genes revealed that bacterial communities of non-irradiated and UV-irradiated NOM were different and that UV selected for specific members of α-proteobacteria, β-proteobacteria, and Bacteriodetes. Our results imply that after UV-irradiation of NOM, specific bacterial members are well adapted to low pH, high LMW DOC concentrations, and oxidative stress, and therefore thrive well on UV-irradiated humic matter.

Aged black carbon identified in marine dissolved organic carbon

Produced on land by incomplete combustion of organic matter, black carbon (BC) enters the ocean by aerosol and river deposition. It has been postulated that BC resides in the marine dissolved organic carbon (DOC) pool before sedimentary deposition and may attribute to its great 14C age (1500–6500 14C years). Here we report the first radiocarbon measurements of BC in high molecular weight DOC (UDOM). BC exported from rivers is highly aromatic and &lt;500 14C years old, while open ocean samples contain less aromatic BC and have an age of 18,000 ± 3,000 14C years. The low abundance of BC in UDOM (0.5–3.5%) suggests that it is more labile than presently believed and/or the low molecular weight DOC contains a larger proportion of aged BC.

DOC and N2O dynamics in upland and peatland forest soils after clear-cutting and soil preparation

Forest clear-cutting followed by soil preparation means disturbance for soil microorganisms and disruption of N and C cycles. We measured fluxes of N2O and dissolved organic carbon (DOC) in upland soil (podzol) and adjacent peat within a clear-cut forest catchment. Both soil types behaved in a similar way, showing net uptake of N2O in the first year after the clear-cutting, and turning to net release in the second. The N2O flux dynamics were similar to those of N content in logging residues, as reported from a nearby site. As organic matter is used in the food web of the decomposers, we attempted to explain the dynamics of N2O uptake and release by measuring the concurrent dynamics of the low molecular weight (LMW) fraction and the aromaticity of DOC in a soil solution. The labile and most readily available LMW fractions of DOC were nearly absent in the year following the clear-cutting, but rose after two years. The more refractory high molecular weight (HMW) fraction of DOC decreased two years after the clear-cutting. The first year’s net uptake of N2O could be accounted for by the growth of decomposer biomass in the logging residues and detritus from the degenerating ground vegetation, resulting in immobilization of nitrogen. Simultaneously, the labile, LMW fraction of DOC became almost completely exhausted. The low availability of the LMW fraction could retard the growth and cause the accumulated decomposer biomass to collapse. During the following winter and summer the fraction of LMW clearly increased, followed by increased N2O emissions. The presence of LMW DOC fractions, not the concentration of DOC, seems to be an important controller for N2O liberation after a major disturbance such as clear-cutting and site preparation. The complex connection between DOC characteristics, nitrification or denitrification merits further studies.