DOC Pool Research Articles

Hydrogeochemical and isotopic signatures elucidate deep subsurface hypersaline brine formation through radiolysis driven water-rock interaction

Geochemical and isotopic fluid signatures from a 2.9–3.2 km deep, 45–55 °C temperature, hypersaline brine from Moab Khotsong gold and uranium mine in the Witwatersrand Basin of South Africa were combined with radiolytic and water–rock isotopic exchange models to delineate brine evolution over geologic time, and to explore brine conditions for habitability. The Moab Khotsong brines were hypersaline (Ca-Na-Cl) with 215–246 g/L TDS, and Cl− concentrations up to 4 mol/L suggesting their position as a hypersaline end-member significantly more saline than any previously sampled Witwatersrand Basin fluids. The brines revealed low DIC (∼0.266–∼1.07 mmol/L) with high (∼8.49–∼23.6 mmol/L) DOC pools, and several reduced gaseous species (up to 46 % by volume H2) despite microoxic conditions (Eh = 135–161 mV). Alpha particle radiolysis of water to H2, H2O2, and O2 along with anhydrous-silicate-to-clay alteration reactions predicted 4 mol/L Cl− brine concentration and deuterium enrichment in the fracture waters over a period > 1.00 Ga, consistent with previously reported 40Ar noble gas-derived residence times of 1.20 Ga for this system. In addition, radiolytic production of 7–26 nmol/(L × yr) H2, 3–11 nmol/(L × yr) O2, and 1–8 nmol/(L × yr) H2O2 was predicted for 1–100μ g/g 238U dosage scenarios, supporting radiolysis as a significant source of H2 and oxidant species to deep brines over time that are available to a low biomass system (102–103 cells/mL). The host rock lithology was predominately Archaean quartzite, with minerals exposed on fracture surfaces that included calcite, pyrite, and chlorite. Signatures of δ18Ocalcite, δ13Ccalcite, Δ33Spyrite,δ34Spyrite and 87Sr/86Sr obtained from secondary ion mass spectrometry (SIMS) microanalyses suggest several discrete fluid events as the basin cooled from peak greenschist conditions to equilibrium with present-day brine temperatures. The brine physiochemistry, geochemistry, and cellular abundances were significantly different from those of a younger, shallower, low salinity dolomitic fluid in the same mine, and both were different from the mine service water. These results indicate the discovery of one of few long-isolated systems that supports subsurface brine formation via extended water–rock interaction, and an example of a subsurface brine system where abiotic geochemistry may support a low biomass microbial community.

The importance of the dissolved organic matter pool for the carbon sequestration potential of artificial upwelling

In the face of climate change there is a need to reduce atmospheric CO2 concentrations. Artificial upwelling of nutrient-rich deep waters has been proposed as a method to enhance the biological carbon pump in oligotrophic oceanic regions in order to increase carbon sequestration. Here we examine the effect of different artificial upwelling intensities and modes (single pulse versus recurring pulses) on the dynamics of the dissolved organic matter pool (DOM). We introduced nutrient-rich deep water to large scale mesocosms (~44 m3) in the oligotrophic subtropical North Atlantic and found that artificial upwelling strongly increased DOM concentrations and changed its characteristics. The magnitude of the observed changes was related to the upwelling intensity: more intense treatments led to higher accumulation of dissolved organic carbon (>70 μM of excess DOC over ambient waters for the most intense) and to comparatively stronger changes in DOM characteristics (increased proportions of chromophoric DOM (CDOM) and humic-like fluorescent DOM), suggesting a transformation of the DOM pool at the molecular level. Moreover, the single upwelling pulse resulted in higher CDOM quantities with higher molecular weight than the recurring upwelling mode. Together, our results indicate that under artificial upwelling, large DOM pools may accumulate in the surface ocean without being remineralized in the short-term. Possible reasons for this persistence could be a combination of the molecular diversification of DOM due to microbial reworking, nutrient limitation and reduced metabolic capabilities of the prokaryotic communities within the mesocosms. Our study demonstrates the importance of the DOC pool when assessing the carbon sequestration potential of artificial upwelling.

Ediacaran carbon cycling and Shuram excursion recorded in the Tarim Block, northwestern China

The largest perturbation of the carbon cycle in geological history, named as the ‘Shuram Carbon Isotope Excursion (SCIE)’, has been widely reported in the Ediacaran Period in different continents over the world. In the Tarim Block, a complete Ediacaran carbonate succession was deposited; however, whether it recorded the SCIE or not still remains controversial, hindering establishment of a reliable stratigraphic framework and understanding of environmental perturbations and evolution during the Ediacaran Period. Here, we present two high-resolution carbon isotopic stratigraphy profiles from the upper Sugetbrak through the Qigebrak formations in the northwestern Tarim Block. Two negative excursions (TN1, TN2), and two positive excursions (TP1, TP2) are identified from both sections and supposed to be of the primary origin. Of these, the TN1 was recorded in a storm-dominated inner-to-middle ramp beginning with the marine transgression. It marks the largest negative excursion with a nadir down to ca. −12‰ which can be correlated to the SCIE of late Ediacaran in terms of its similar magnitude, variation pattern, time constraints and depositional characteristics. Other carbon isotope anomalies can also be correlated with coeval anomalies elsewhere around the world. The temporal coincidence of TN1 (or SCIE) with onset of transgressive, storm-dominated successions suggests that enhanced oceanic upwelling and storm agitations together may have greatly increased the depth of ventilated seafloor and extent/volume of oxygenated DOC pool in shallow marine realms, leading to the extremely δ13C excursion of carbonates. Our new carbon isotopic chemostratigraphic profiles of upper Ediacaran from the Tarim Block thus not only provide a more reliable time stratigraphic framework helpful for documenting depositional history and environment-life evolution, but also promote comparisons with other continental blocks globally.

Seasonal variability in particulate organic carbon degradation in the Kolyma River, Siberia

Major Arctic rivers are undergoing changes due to climate warming with higher discharge and increased amounts of solutes and organic carbon (OC) draining into rivers and coastal seas. Permafrost thaw mobilizes previously frozen OC to the fluvial network where it can be degraded into greenhouse gases and emitted to the atmosphere. Degradation of OC during downstream transport, especially of the particulate OC (POC), is however poorly characterized. Here, we quantified POC degradation in the Kolyma River, the largest river system underlain with continuous permafrost, during 9–15 d whole-water incubations (containing POC and dissolved OC—DOC) during two seasons: spring freshet (early June) and late summer (end of July). Furthermore, we examined interactions between dissolved and particulate phases using parallel incubations of filtered water (only DOC). We measured OC concentrations and carbon isotopes (δ13C, Δ14C) to define carbon losses and to characterize OC composition, respectively. We found that both POC composition and biodegradability differs greatly between seasons. During summer, POC was predominantly autochthonous (47%–95%) and degraded rapidly (∼33% loss) whereas freshet POC was largely of allochthonous origin (77%–96%) and less degradable. Gains in POC concentrations (up to 31%) were observed in freshet waters that could be attributed to flocculation and adsorption of DOC to particles. The demonstrated DOC flocculation and adsorption to POC indicates that the fate and dynamics of the substantially-sized DOC pool may shift from degradation to settling, depending on season and POC concentrations—the latter potentially acting to attenuate greenhouse gas emissions from fluvial systems. We finally note that DOC incubations without POC present may yield degradation estimates that do not reflect degradation in the in situ river conditions, and that interaction between dissolved and particulate phases may be important to consider when determining fluvial carbon dynamics and feedbacks under a changing climate.

Exploring changes in bacterioplankton community structure in response to tannic acid, a major component of mangrove litterfall of Sundarbans mangrove ecosystem: a laboratory mesocosm approach.

Tannic acid is a secondary compound produced by vascular plants and is a major component of mangrove litterfall. Tannic acid is water soluble, leaches out from mangrove litterfall and contributes to DOC and DON pools in adjacent estuaries. About 50% of the litterfall may be degraded and channelized into the marine microbial loop. The influence of tannic acid on bacterioplankton community structure was tested by setting up laboratory-based barrel experiments. Estuarine water from Stn3 of Sundarbans Biological Observatory Time Series(SBOTS) was enriched with tannic acid, and the change in concentration of dissolved nutrients was determined on a daily basis over a span of 15 days. Concentrations of tannic acid, gallic acid and other dissolved nutrients such as nitrate and ortho-phosphate were determined using a UV-Vis spectrophotometer. Tannic acid significantly affected the concentrations of gallic acid and dissolved nitrate in the barrels. Degradation of tannic acid was tracked by a decrease in concentration of tannic acid and generation of gallic acid. The influence of tannic acid on bacterioplankton community structure was analysed on the start (day 0), intermediate (day 3, day 5, day 7 and day 9) and end (day 15) of the experiment. Bacterioplankton community structure was elucidated by sequencing the V3-V4 region of 16S ribosomal RNA on an Illumina MiSeq platform. Proteobacteria was found to be the most dominant bacterial phylum in control and tannic acid-enriched barrels (barrels 1 and 2) on day 0. With the progression of experiment, the abundance of Proteobacteria altered significantly in the control barrel indicating the possible role of this phylum in the breakdown of tannic acid within estuarine mangroves. The abundance of Proteobacteria in the tannic acid-enriched barrels remained high, indicating that members of Proteobacteria may be capable of using tannic acid as a source of carbon and nitrogen. Tannic acid appeared to inhibit most of the other bacterioplankton phyla including Actinobacteria, Acidobacteria and Verrucomicrobia that existed in large abundance in the control barrel on day 15 but were almost absent in the tannic acid-enriched barrels. At class level, Bacteroides was found to be present in highest abundance in the tannic acid-enriched barrels. Tannic acid appeared to strongly influence the abundant bacterioplankton phyla and families as indicated by Pearson's correlation coefficient and non-metric multidimensional scaling ordination plots. Gallic acid is one of the final products of tannic acid degradation. Breakdown of tannic acid could influence the marine nitrogen and carbon cycling by releasing DON and DOC, respectively, into the adjacent estuaries. Information of breakdown and remineralization of components of litterfall such as tannic acid would also be important for calculation of carbon and nitrogen budgets of coastal ecosystems including in mangroves.

Agricultural land use changes stream dissolved organic matter via altering soil inputs to streams

Agricultural land use leads to significant changes in both the quality (e.g., sources and compositions) and quantity of dissolved organic matter (DOM) exported from terrestrial to aquatic ecosystems. However, the effect of agricultural activities often interacts with those of hydroclimatic drivers, making it difficult to delineate agriculture-induced changes and identify associated mechanisms. Using partial least square path modeling (PLS-PM), we examined the relative importance of agricultural land use, stream order, precipitation, and temperature in mediating allochthonous versus autochthonous sources and pathways that influenced stream DOM quality and quantity. We analyzed stream water DOM from 15 small streams draining watersheds across a gradient of agricultural land use in Southeast USA for about one year. For DOM quantity, agricultural land use increased the export of DOC and various DOM pools (terrestrial humic, microbial humic, and protein-like DOM) from land to streams, and for DOM quality, agricultural streams showed greater proportions of microbial humic compounds than forested streams. The PLS-PM model for DOM quantity accounted for 75.5% of total variance and identified that agricultural land use increased stream water DOM quantity primarily through increasing allochthonous inputs, which can be attributed to shallower flow paths in agricultural watersheds that enabled the export of organic materials from the upper, organic-rich soil horizon. PLS-PM models for DOM quality only explained ~13% of the total variance, highlighting the complex dynamics between environmental drivers and stream water DOM. Relative to commonly used multivariate statistic modeling (e.g., redundancy analysis (RDA)), PLS-PM models offer the advantages of identifying the primary pathway by which agricultural lands alter freshwater DOM and quantifying the relative importance of interactive effects of agriculture and hydroclimatic drivers. Therefore, structural equation modeling is a powerful tool that should be more widely adopted to distinguish among multiple drivers and mechanisms regulating freshwater biogeochemistry.

Sources of soil carbon loss during soil density fractionation: Laboratory loss or seasonally variable soluble pools?

To model global C budgets more accurately, we must better understand the dynamics and turnover of the many functional C pools that exist in soil. While soil density fractionation (SDF) is widely used to separate soil C pools based on the degree of stabilization by soil minerals, several studies have noted substantial losses of soil mass and soil C following SDF. As the source of these losses is unknown, they may lead to erroneous conclusions about SOM dynamics and inaccuracies in models of soil processes and global C cycling. For example, lab handling techniques such as air-drying soil could have an impact on soluble C losses. Alternatively, the observed C losses could represent pools of potentially soluble C that can vary by ecosystem and season, and thus might represent ephemeral, not well stabilized, and generally overlooked pools of soil C that are missing from SOM budgets and models. For example, in summer-dry climates such as the Pacific Northwest, soluble organic products of decomposition could accumulate over the summer when temperatures and microbial activities are high and precipitation is low, and subsequently leach during the fall and winter with precipitation. To address these divergent possibilities, soils were collected seasonally from a summer-dry forest in Oregon, and subsamples were subjected to 4 different laboratory handling procedures prior to fractionation: 1) air-drying and 2) oven-drying to simulate common laboratory drying techniques; 3) leaching to evaluate if potentially soluble C represented a pool that could be removed with simulated precipitation; and 4) immediate fractionation of fresh soil to determine if drying of soil caused artifacts in pools of potentially soluble soil C. Contrary to initial hypotheses, there were no seasonal trends to soluble DOC pools or total C loss during fractionation. Average mass loss during fractionation was 6% of initial dry weight and total C loss was 9% of total soil C. Soluble losses represented only 9% of total soil C loss. Particulate, or non-soluble mass loss was dominated by the high C:N free particulate organic matter, or LF. Thus, most C loss in this system was due to laboratory losses and that loss was biased towards an underestimation of LF matter in soil. DOC was a small enough component of loss that soil preparation prior to SDF was not significant. These results imply that in temperate soils, even in seasonally extreme ecosystems, relative proportions of mineral-associated organic matter and free particulate organic matter are seasonally robust, air drying of soil does not introduce error, and that immediate fractionation after field collection is not critical.

Enriched dissolved organic carbon export from a residential stormwater pond

In urban watersheds, stormwater retention ponds are intermediate junctions that capture, store, and discharge stormwater, and provide an organic-rich environment that transforms and retains nutrients and other constituents. This study investigated the concentrations and loads of dissolved and particulate organic carbon (DOC and POC) in discharges from a stormwater retention pond that receives runoff from a residential catchment. We installed an autosampler, a flowmeter, and a rain gauge at the outlet (weir) of the stormwater retention pond and collected samples from 13 storm events during the 2016 wet season (May–September). Results showed the dominance of DOC (11.2 mg L−1) over POC (0.6 mg L−1) in the pond discharges. The elevated DOC levels in the pond were close to eutrophic lakes and ponds (~10.3 mg L−1), but not statistically different from urban runoff at a nearby site. High-frequency monitoring of pond discharge waters showed that DOC concentrations peaked at the beginning of storm events due to initial surface runoff following a rainstorm (first-flush effect). Rainfall samples analysis suggested that precipitation accounted for a small fraction of DOC pool, but carbon to nitrogen (C/N) ratios supported that in situ aquatic sources could dominate DOC inputs in some storms. Relative to DOC, the first-flush effect was even more apparent for POC, and POC inputs from in situ aquatic sources were more common based on C/N ratios. The calculated export of total organic C (TOC = DOC + POC) was 22.5 kg ha−1 over the observed events, and the estimated export was 33.8 kg ha−1 over the 2016 wet season. Our data suggest that reducing high DOC export from residential stormwater ponds warrant controls on both inputs from the watershed and in situ aquatic sources.

Contrasting Patterns of Labile and Semilabile Dissolved Organic Carbon From Continental Waters to the Open Ocean

AbstractMarine and freshwater prokaryotes feed primarily on bioavailable labile dissolved organic carbon (BDOCL), as well as the bioavailable fraction of the semilabile DOC (BDOCSL) pool. These fractions are operationally defined here as the DOC consumed within a month and greater than a month to a year and a half, respectively. Organic matter from these different pools comes from various autochthonous and allochthonous sources, but their relative bioavailability is unknown across aquatic ecosystems. To fill this gap, we compiled literature information that included 653 batch culture DOC biodegradation experiments across eight aquatic ecosystem types over the past 20 years. We show that the proportion of BDOCL across all aquatic ecosystems was surprisingly consistent (6.1%) despite a 2 order of magnitude variation in initial DOC concentrations, suggesting an overall tight balance between carbon supply and consumption. A higher proportion of BDOCL, 16.3% on average, was observed in high productivity ecosystems. BDOCSL, on the other hand, gradually decreased from 16.0% in lakes to 7.2% in estuaries to undetectable in the open ocean, suggesting that terrestrial connectivity regulates BDOCSL across the continuum. Our results support that recent primary production fuels short‐term prokaryotic DOC needs with an increasing reliance on the abundant BDOCSL pool as ecosystems approach the land‐water interface. Batch culture experiments show that BDOCSL is metabolizable in freshwater and coastal environments but not in the open ocean. We estimate that BDOCSL can sustain 62% of total prokaryotic biomass in inland waters and coasts and an estimated total of 16.7% across aquatic biomes.

Variations of redox conditions in the atmosphere and Yangtze Platform during the Ediacaran-Cambrian transition: Constraints from Cr isotopes and Ce anomalies

The Ediacaran-Cambrian transition was characterized by the evolution from Ediacaran soft-body to Cambrian skeletal organisms, and fluctuations of the marine carbon cycle. Marine redox changes should have played an essential role in both ecological and environmental evolutions; however, temporary changes and spatial distribution of redox conditions in the Ediacaran-Cambrian oceans remain elusive and highly debated. In this study, we analyzed chromium (Cr) isotope and cerium (Ce) anomaly profiles in carbonates from the Upper Ediacaran-Lower Cambrian Dengying and Zhujiaqing formations at the Xiaotan section, South China, in an attempt to provide new insights helping unravel Earth's surface redox changes during the Ediacaran-Cambrian transition. Our results show that these carbonates record generally positive δ53Crauth values (up to 1.15‰) with fluctuations and prominent negative Ce anomalies (Ce/Ce⁎ values down to 0.35), implying a broad increase in oxygenation in the atmosphere and the inner shelf of the Yangtze Platform during the Ediacaran-Cambrian transition. The first appearance and subsequent diversification of the Cambrian animals may have benefited from this oxygenation event. The pronounced fluctuations of δ53Crauth and δ13Ccarb values can be explained by episodic oxidation of a deep-water DOC pool in a stratified water column across the chemocline through upwelling, releasing light carbon isotopes and organic-matter-bound isotopically unfractionated Cr (III) into shallow waters.

Sargassum sp. Act as a Large Regional Source of Marine Dissolved Organic Carbon and Polyphenols

AbstractDissolved organic carbon (DOC) plays critical roles in marine carbon cycling, but its sources and sinks remain uncertain. In this study, we monitored DOC exudation rates of Sargassum natans under visible light (λ > 390 nm) and solar radiation. DOC release rates ranged from 7 to 10 μg C g−1biomass hr−1 (wet weight) under visible light, but increased to 23 to 41 μg C g−1biomass hr−1 when exposed to natural sunlight. Results indicate that DOC released by Sargassum could amount to 0.3 to 1.2 Tg C/year, potentially contributing significantly to the marine DOC pool in the Gulf of Mexico and Western North Atlantic. We employed the Folin‐Ciocalteu phenolic content method, nuclear magnetic resonance (NMR) spectroscopy, and ultrahigh resolution Fourier transform ion cyclotron resonance mass spectrometry (FT‐ICR MS) to characterize the diverse pool of organic compounds exuded from Sargassum. Results from these complementary methods showed that Sargassum release large quantities of phlorotannins, a class of polyphenols that have very similar properties to terrestrial DOC. These phlorotannins and their oxygenated phenolic derivatives exhibit a high hydrogen deficiency and functionalization (i.e., 4 to 6 oxygen atoms per aromatic ring), representing 5 to 18% of the released DOC isolated by solid phase extraction. Thus, Sargassum is the largest biological source of open ocean polyphenols recorded to date. The amount of polyphenolic DOC released by Sargassum challenges previous beliefs that all polyphenols found within the oceans are remnants of terrestrial organic matter, although the stability of phlorotannins and their derivatives needs to be further evaluated.

Soil parent material and stand development stage effects on labile soil C and N pools in Chinese fir plantations

Plantation development stage (forest age sequence) influences environmental conditions and litter inputs, which can modify labile C and N pools. Changes in labile C and N pools during stand development could vary considerably with soil parent material. However, empirical evidence is scarce regarding the extent of interactive effects between parent material and stand development stage on labile C and N pools in plantations of Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.), the dominant plantation tree in southern China. We examined the concentrations of soil labile C and N (dissolved organic carbon, DOC, dissolved organic nitrogen, DON, dissolved inorganic nitrogen, DIN, microbial biomass carbon, Cmic, microbial biomass nitrogen, Nmic) and basic soil properties within a soil depth of 100 cm for three stand development stages (young, middle-aged and mature) of Chinese fir plantations grown on soils derived from granite or siltstone. The parent material influenced the responses of the DON, DIN and Nmic pools to stand development stage, whereas no interaction effect of parent material and stand on the Cmic and DOC pools was observed. The parent material had a clear effect on the standing pools of labile C and N. Siltstone soils tended to have greater pool sizes of DOC, DON, DIN, and Nmic and greater ratios of Cmic/total carbon (TC) and Nmic/total nitrogen (TN). The stand development stage did not affect the DOC, Cmic and Cmic/TC ratio. The mineral N and Nmic pools increased significantly and the DON and DON/TN ratio declined sharply from young to middle-aged stands in both soil type. The changes in the DON, DIN and Nmic pools and the Nmic/TN ratio from middle-aged to mature stands depend on the parent material. The concentrations of DOC, DON, DIN, Cmic and Nmic were positively correlated with multiple soil parameters, such as the TC, TN, C/N ratio, water-stable aggregates (WSA), and water content, but negatively correlated with the bulk density (BD). DOC, DON, Cmic and Nmic significantly decreased with soil depth, but the extent of the decrease in DOC and DON was greater in course-textured granite soils. Overall, the turnover rates of the labile C pools are higher in the siltstone soils where internal N cycling during a traditional rotation ~25 years maintain the supply of mineral N and improve N incorporation into microbial biomass. These results highlight the need to identify soil physicochemical properties before afforestation. The rotation length and N fertilizer application in Chinese fir plantations should be adjusted based on the soil parent material.

Mobile Colloidal Organic Carbon: An Underestimated Carbon Pool in Global Carbon Cycles?

Mobile colloids, 1-1000 nm particles, are ubiquitous in every ecosystem. They have small size, large specific surface area and high mobility in the subsurface, and thus can regulate the fate and transport of sorbing constituents such as nutrients, contaminants and organic carbon (OC). The movement of colloids and colloidal OC (COC) through soils is an important process in mass and energy transport (including carbon) both within and between ecosystems, e.g. from terrestrial to aquatic ecosystems, and likely contribute to the ecosystem- or global-scale carbon balance given their ubiquitous distribution and unique environmental functions. However, despite their importance for terrestrial and aquatic carbon transport and balance, colloids and COC have not been adequately accounted for because of the current operational definition uses 0.45 µm as the cutoff size for colloids. In this study, we quantified and characterized loadings of colloids and COC in aqueous samples collected from agricultural, forestry, freshwater wetland and estuary ecosystems. Results reveal that, in all samples regardless of the different sampling sources, the total colloidal loads were underestimated by ≥ 50% and considered as “dissolved” solutes when the operational definition (e.g., using a cutoff size of 0.45 m) was used. Together with a large number of data from the literature, our results further demonstrate that colloids are quantitatively substantial, carbon-dense and that as much as 8-19% of operationally defined DOC is in fact COC. Conservatively, this suggests that the COC pool potentially accounts for 13.6 TgC year-1 as a riverine flux and 530 ± 25 TgC of global DOC pool in the ocean in global carbon cycles. In addition, freshwater wetland was found to be a hotspot, which released more colloids and COC compared to the other sampled ecosystems. These findings clearly demonstrate the limitations of using the current operational definition for colloids and DOM and highlight the need for improving quantification and characterization of size-dependent colloidal and OC loads. Such effort will allow direct fundamental research into questions towards microbial access to ‘protected’ carbon by minerals and more accurate assessment of global carbon cycles.

Spatial and seasonal variations, partitioning and fluxes of dissolved and particulate nutrients in Jiaozhou Bay

The spatial and temporal dynamics of both dissolved and particulate nutrients were studied during 2015–2016 in the Jiaozhou Bay (JZB), a deeply anthropogenic impact ecosystem in China. The dissolved inorganic nutrients was mainly from the wastewater and riverine inputs which is demonstrated from the negative correlation between nutrients with salinity and their higher levels distribution observed in the northeastern and/or northwestern JZB, where mainly received industrial and agricultural source run-off, respectively. The high levels of dissolved and particulate organic pools were also observed in the same area but have autochthonous sources besides terrestrial input, which lead to their higher concentrations in autumn during phytoplankton bloom. The composition of DIN has been changed with the major component from NH4+ to NO3-, which probably due to the controlling load of NH4+ but no limitation of NO3- by human intervention. The average annual DIN/DIP ratio increased by one time but SiO32-/DIN ratio has no obvious variation over the past decade, which indicated that the potential limiting nutrient for phytoplankton growth has been converted from silicon to phosphorus in JZB. Additional, more prone to partition into POC but not DOC pools of organic carbon were found during the phytoplankton bloom in autumn due to the nutrient-rich, diatom dominated condition and both DOP and POP may be the potential P source and can relieve P limitation to some extent during the same time. Nutrient budgets calculation also demonstrated that riverine input and wastewater discharge are major sources of dissolved nitrogen and phosphorus, while silicate were mainly came from submarine fresh groundwater discharge. The mixing flow transported large amounts of nutrients from JZB to the Yellow Sea. Meanwhile, the net ecosystem metabolism and nitrogen metabolism calculation showed the whole ecosystem was heterotrophic and a net nitrogen fixation in JZB.

Understanding Dissolved Organic Matter Reactivity and Composition in Lakes and Streams Using Proton-Transfer-Reaction Mass Spectrometry (PTR-MS)

Here, we present a novel approach for investigating dissolved organic matter (DOM) composition using thermal desorption proton-transfer-reaction mass spectrometry (PTR-MS), a technique that provides insight into the molecular composition of DOM < m/z 500 (termed “PTR-DOC”). The applicability of PTR-MS for understanding the relationship between DOM composition and reactivity has yet to be explored. We present results from a synoptic sampling campaign of streams and lakes in a Swedish forest catchment where we measured DOM composition using PTR-MS and traditional optical methods and conducted DOM biodegradability assays. PTR-DOC comprised ≤12% of the total DOC pool. We found significant relationships between PTR-DOC and DOM degradability; reduced chemodiversity and low concentrations of PTR-DOC were both associated with the total DOM pool being more susceptible to microbial degradation. Furthermore, molecular differences were apparent among headwater lakes, headwater streams, and lakes further down the catc...

Effects of the interaction between temperature and revegetation on the microbial degradation of soil dissolved organic matter (DOM) – A DOM incubation experiment

DOM is the most bioavailable organic pool in the soil. The restoration of vegetation on abandoned cropland has a major impact on the concentration and composition of the DOM and thus affects the biodegradability of the soil DOM. Understanding the response of the microbial degradation of the DOM to temperature is important to maintain soil bioavailable organic matter in the field. We conducted a laboratory DOM solution incubation experiment to examine the temporal dynamics of DOM concentrations at temperatures of 4 °C (low), 20 °C (medium), and 35 °C (high) for four types of land uses: sloped cropland, grassland, shrub land, and woodland. Ultraviolet–visible and fluorescence spectroscopy were used to determine the structural complexity of the DOM. The conversion of the sloped cropland to shrub land and woodland significantly increased contents of DOC, DON, and recalcitrant substances in DOM soilution, such as humic-like material and fulvic acid, and stabilised the DOC pool, and reduced the decomposition of the DOC at 20 °C and 35 °C. The conversion of the sloped cropland to woodland dramatically reduced TDN decomposition. The DON loss after 60-day incubation significantly correlated with the initial content of tryptophan-like material. The biodegradability of the DON was higher and more sensitive to temperature than that of the DOC. Rising temperature initially promotes the decomposition of tryptophan-like material, and later promotes the degradation of more recalcitrant substances, such as humic-like material and fulvic acid, which enhanced the decomposition of the DOC and DON. The results suggest that the conversion of sloped cropland to shrubland and woodland not only promoted the accumulation of DOC, TDN, and recalcitrant substances in DOM solution, and decreased their biodegradability but also decreased the temperature sensitivity of the decomposition of the DOC and DON. Therefore, shrubland and woodland were the optimal choices for revegetation in the Loess Plateau of China.

Seasonal and habitat-wise variations of creek water particulate and dissolved organic carbon in arid mangrove (the Persian Gulf)

The present study examines the effect of mangrove vegetation and different seasons on organic carbon pools, distributions, and source compositions under the stressed hydro-climatological settings of the Iranian coast (Persian Gulf). Significant seasonal and spatial differences were detected only for the δ13POC (particulate organic carbon) owing to the fluctuation of its sources. Five end-member particulate organic carbon sources (POC) (mangrove leaf, planktonic particles, zooplankton, microphytobenthos (MPB), and sediment) contributed to the POC pool at different levels depending on their seasonal and site-specific abundance. Variations in topographic features such as, the elevations of mangrove and non-mangrove creeks, appeared to play an important role in regulating POC concentrations but not DOC concentrations. Planktonic particles contributed to the POC pool (maximum 10–65%) at the mangrove sites mostly in the summer whereas the contributions of sediment and MPB (5–35%) increased in winter. Iranian mangroves are weak exporters of carbon to the Persian Gulf where mangrove plant materials (leaf, litter) had little contributions to the POC and DOC pool (5–25% and 8–15%, respectively). It is most likely that OC export in these arid regions are limited by low rainfall and river input. Finally, seasonality and site-specific activity largely control OC dynamics in these relatively understudied arid mangroves.

Quantities and Fluxes of Dissolved and Particulate Black Carbon in the Changjiang and Huanghe Rivers, China

Recent studies have suggested that large rivers play important roles in mobilizing and transporting black carbon (BC) from land to the ocean. However, the influence of the Changjiang and Huanghe, the two largest rivers in China, on the fate of BC has not been determined. In this paper, we present measurements of BC in both the dissolved and particulate phases in the Changjiang and Huanghe Rivers and in the coastal waters of the East China Sea (ECS). Our results show that dissolved black carbon (DBC) accounted for 3.0 ± 0.4 % and 4.8 ± 3.6 % of the total DOC pool in the Changjiang and Huanghe Rivers and 3.4 ± 0.6 % of the DOC pool in the coast of the ECS. In addition, particulate black carbon (PBC) accounted for 13 ± 0.9 % and 22 ± 11 % of the POC pool in the Changjiang and Huanghe Rivers, respectively. We calculate that the Changjiang and Huanghe transported 4.7 × 1010 gC and 1.7 × 109 gC of DBC, and 2.0 × 1011 gC and 1.2 × 1010 gC of PBC to the ECS and Bohai Sea in 2015. The large amounts of BC transported by the two rivers represent a major sink of anthropogenically derived organic carbon and could have a significant impact on the ecosystem and carbon cycling in China’s marginal seas.

Changing Dynamics of Dissolved Organic Matter Fluorescence in the Northern Gulf of Mexico Following the Deepwater Horizon Oil Spill.

The characteristics of fluorescent components of dissolved organic matter (DOM) were examined using excitation emission matrix (EEM) fluorescence spectroscopy combined with parallel-factor analysis (PARAFAC) for seawater samples obtained from the northern Gulf of Mexico (NGoM) before, during, and after the 2010 Deepwater Horizon (DwH) oil spill. An EEMs PARAFAC modeling of samples collected within 16 km of the wellhead during the oil spill in May 2010, which included one typical subsurface sample with a PAH concentration of 1.09 μg/L, identified two humic-like and two previously reported oil-like components. Compared to prespill levels, however, there were order-of-magnitude higher fluorescence intensities associated with these components that are consistent with an oil-spill source. The spectral decomposition of the EEMs data using individual and combined data sets from coastal and offshore waters impacted by the DwH spill further revealed the changing nature of fluorescent DOM composition. Although the PAHs concentrations were at prespill conditions after the spill in 2012 and 2013 near the DwH site, the variable and anomalous levels of fluorescence intensities and DOC concentrations three years after the spill suggest the potential long-term persistence of the oil in the DOC pool in the NGoM.

Characterising changes in fluorescence properties of dissolved organic matter and links to N cycling in agricultural floodplains

Sand and gravel aquifer systems are common features below and adjacent to river networks and are important in providing a hydrological link between terrestrial and aquatic ecosystems. They are often used for intensive agriculture and therefore provide a conduit for the transport of nutrients to aquatic systems. Understanding the biogeochemical cycles of C and N in such systems is essential in efforts to improve water quality, with a major link being the transfer and properties of DOM which drives microbial processing of nutrients. In this work undertaken in the Trent Valley, U.K., the differences in dissolved organic matter (DOM) properties in the soil, aquifer ground water and river water are examined using excitation–emission matrix fluorescence spectroscopy (EEM), in addition to chemical analysis. In the aquifer system studied the water table is ∼1.5m below the land surface, suggesting that DOM and nutrient transfer would occur between the soil and groundwater. Nitrate levels in the ground water were ∼50mgL−1 NO3-N. Soil DOM properties were measured in KCl extracts used to extract inorganic and organic N so that further information could be obtained regarding possible sources and properties of the DON and the biodegradability of the DOC pool. Within the soils tested, fulvic-like properties and the humification index (HI) were significantly (P<0.05) higher in sandy soils suggesting that microbial accessibility to organic matter was greater. The applicability of assessing DOM characteristics in the different matrices were assessed and quantitative or qualitative comparisons made. Properties of DOM in both the soils and aquifer waters all showed a typical terrestrial source with principal component analysis (PCA) showing strong correlations between DOC, fulvic-like properties, the HI and tryptophan-like properties. A major finding was that the HI was generally higher in the groundwater, suggesting continued processing of DOM within the aquifer whilst qualitative comparisons showed the groundwater possessed a higher fluorescence index (FI). Whilst DON and the tyrosine-like proteins were correlated in the soil, the tryptophan-like proteins were strongly correlated to fulvic-like substances in both the soil and groundwater. This suggested that phenolic compounds were also being identified in this part of the EEM matrix, emphasising the need for additional compound analysis to fully understand the nature of DOM in these systems.