Decrease In Dissolved Organic Carbon Research Articles

Ca/Na concentration-constrained variations of dissolved organic matter leaching from groundwater-irrigation area soil in North China Plain.

This study investigates the quantity and quality variations of dissolved organic matter (DOM) leaching from the soil in groundwater irrigation area of the North China Plain, constrained by the concentration of Ca/Na. Soil samples with dominant humic-like (HLC) and protein-like (PLC) components were paired with parallel concentration gradients of Ca/Na extractants for equilibrium experiments. Fluorescence-PARAFAC, UV-visible spectroscopy, and multiple statistical analyses were combined for data analysis and interpretation. The results reveal that the primary DOM components remained dominant for specific soil sample, with a higher relative abundance of PLC (HLC) in Ca (Na) extract. HLC preferentially binds to soil phase in all extractions, while PLC is readily released into the solution. However, Ca inhibits HLC desorption and promotes PLC release more significantly than Na, as indicated by stronger ion/proton reaction (IPR) and electrostatic effect (ESE). The strongest IPR and ESE are seen in the HLC-dominated DOM extracted with Ca, suggesting a condition where Ca bridges to HLC and forms total dissolved organic carbon (DOC) that decreases. In contrast, Na extraction exhibits only a weaker ESE that is offset by soil-contained HLC and exchangeable Ca, resulting in subtle DOC decrease. The trends in leaching of HLC and PLC are self-dependent, and the level of variation in either component correlates with the increasing concentration of specific cations present. These findings underscore the crucial role of soil organic matter (SOM) composition and its interaction with leaching cations in soil management in large-scale groundwater irrigation areas, where SOM quality and groundwater chemistry vary.

Stimulated or Inhibited Response of Methane Flux to Nitrogen Addition Depends on Nitrogen Levels

AbstractUpland forest soil is significant atmospheric methane (CH4) sink that is suggested to be affected by increased nitrogen (N) availability, likely from N deposition or addition. However, the effect of N addition on upland soil CH4 uptake and the underlying mechanisms remain unclear. Here, in a montane forest, frequent in situ sampling over three years revealed that this forest soil was a sink of atmospheric CH4 with an uptake rate of 304.9 ± 26.9 mg CH4/m2/yr under ambient treatment. High N addition suppressed CH4 uptake by 23.2% compared to ambient conditions, whereas low N addition stimulated CH4 uptake by 35.6%. N addition did not change the monthly pattern of CH4 flux, which was quite similar to the pattern of leachate dissolved organic carbon (DOC) concentrations from the organic layer. The linear relationship between the CH4 flux and DOC concentration indicated increased CH4 uptake with an increase in DOC. The induced decrease in CH4 uptake under high N addition was partly related to the high soil N availability and partly contributed to a decrease in DOC due to high N addition. In contrast, the increased CH4 uptake under temporarily low N addition was most likely correlated with a release in N limitation to CH4‐oxidizing bacteria. These results indicated that the effect of N addition on CH4 uptake was dependent on the level of N, and DOC could regulate CH4 uptake in upland soil, which should be investigated in depth when understanding the responses of dynamic CH4 fluxes to N additions.

Drivers of Dissolved Organic Carbon Mobilization From Forested Headwater Catchments: A Multi Scaled Approach

Understanding and predicting catchment responses to a regional disturbance is difficult because catchments are spatially heterogeneous systems that exhibit unique moderating characteristics. Changes in precipitation composition in the Northeastern U.S. is one prominent example, where reduction in wet and dry deposition is hypothesized to have caused increased dissolved organic carbon (DOC) export from many northern hemisphere forested catchments; however, findings from different locations contradict each other. Using shifts in acid deposition as a test case, we illustrate an iterative “process and pattern” approach to investigate the role of catchment characteristics in modulating the steam DOC response. We use a novel dataset that integrates regional and catchment-scale atmospheric deposition data, catchment characteristics and co-located stream Q and stream chemistry data. We use these data to investigate opportunities and limitations of a pattern-to-process approach where we explore regional patterns of reduced acid deposition, catchment characteristics and stream DOC response and specific soil processes at select locations. For pattern investigation, we quantify long-term trends of flow-adjusted DOC concentrations in stream water, along with wet deposition trends in sulfate, for USGS headwater catchments using Seasonal Kendall tests and then compare trend results to catchment attributes. Our investigation of climatic, topographic, and hydrologic catchment attributes vs. directionality of DOC trends suggests soil depth and catchment connectivity as possible modulating factors for DOC concentrations. This informed our process-to-pattern investigation, in which we experimentally simulated increased and decreased acid deposition on soil cores from catchments of contrasting long-term DOC response [Sleepers River Research Watershed (SRRW) for long-term increases in DOC and the Susquehanna Shale Hills Critical Zone Observatory (SSHCZO) for long-term decreases in DOC]. SRRW soils generally released more DOC than SSHCZO soils and losses into recovery solutions were higher. Scanning electron microscope imaging indicates a significant DOC contribution from destabilizing soil aggregates mostly from hydrologically disconnected landscape positions. Results from this work illustrate the value of an iterative process and pattern approach to understand catchment-scale response to regional disturbance and suggest opportunities for further investigations.

Effects of soil amendments on fractions and stability of soil organic matter in saline-alkaline paddy

Soil amelioration is an effective practice to alleviate the adverse effects of soil salinization. However, increasing the fertility of salt-affected soils has been challenging, particularly in coastal saline-alkaline paddy soils. Here, we carried out a 45-day incubation experiment to evaluate the impacts of soil amendments on fractions and stability of soil organic matter (SOM) in a saline-alkaline paddy. The experiment simulates the flooding-draining practice and consists of CaCO3, gypsum and biochar amendments using different fertility soils. We measured dissolved organic carbon (DOC) and nitrogen (DON) in supernatant liquids, water-soluble cations, water extractable organic carbon (WEOC) and nitrogen (WEON), and microbial biomass carbon (MBC) and nitrogen (MBN) in soils after the incubation. Results showed that water soluble sodium (Na+) was significantly decreased under all amendments (by 17%–32%), except in high fertility soil. We found a significant decrease in DOC (by 36%–47%) under gypsum treatment, but in DON (by 18%–59%) under biochar treatment. However, there was no significant effect on DOC or DON under CaCO3 treatment. Gypsum treatment led to decreased WEOC content (by 0.067%–5.4%), but increased MBC (by 0.16%–44%) and MBN (by 8.3%–37%) in all soils. Biochar treatment caused a decrease in the ratios of WEOC to soil organic carbon (SOC) and WEON to total nitrogen (TN), and an increase in MBC:SOC and MBN:TN ratios. These results suggest that gypsum and biochar amendments can enhance SOM stability in the saline-alkaline paddy. However, SOM stability was not enhanced under CaCO3 treatment, probably due to the presence of a large amount of Na+ in these soils. Our study highlights that soil amelioration has different effects on soil carbon and nitrogen cycles in the saline-alkaline paddy soils, which is associated with water-logged condition.

DOM Biological Lability in an Estuarine System in Two Contrasting Periods

Estuarine processes play a key role in determining the amount and quality of land-derived dissolved organic matter (DOM) reaching the oceans. Microbial-mediated reactions can affect the concentration, quality, and bioavailability of DOM within an estuary. In this study, we investigated biological DOM removal in a small estuary and its variability in two contrasting seasons (spring and autumn) characterized by natural differences in the concentration and quality of the riverine DOM. Two incubation experiments were carried out using natural DOM and heterotrophic prokaryotes community collected at the estuary in March and September. Dissolved organic carbon (DOC) concentration, DOM fluorescence, and the heterotrophic prokaryotes abundance (HPA) showed marked differences between the two seasons. These parameters were followed through time for up to two months. Despite the marked differences in the initial conditions, the DOC removal rates were surprisingly similar in the two periods (16 µM DOC month−1 in March and 18 µM DOC month−1 in September), with the biggest removal in the first 48 h. The trend of fluorescent DOM (FDOM) during the incubation showed marked differences between the two periods. In March, the net removal of all the FDOM components was observed consistently with the decrease in DOC; whereas, in September, the net production of humic-like substances was observed.

氮添加对亚热带常绿阔叶林土壤溶液化学特性的影响

土壤溶液被称作"土壤的血液"，是土壤中各种生物化学反应的中介物质，在外界环境发生变化时，土壤溶液化学成分能在其他土壤指标尚无变化之前对环境变化做出迅速响应。为了探索持续增加的大气氮（N）沉降对森林生态系统的影响，以华西雨屏区亚热带常绿阔叶林为对象，设置对照（CK，0 g m^-2 a^-1），低N（LN，5 g m^-2 a^-1），高N（HN，15 g m^-2 a^-1）三种N处理，通过人工施加硝酸铵（2017年9月起改施硝酸钠及氯化铵）的方法模拟N沉降增加情景，N处理42个月后，使用负压土壤溶液采样器定位收集A层（37-45 cm）及B层（52-60 cm）土壤溶液，并进行分析（每月1次，为期1 a）。结果表明：对照处理中A、B两层土壤溶液NO₃^-浓度达（3.94±0.77）mg/L、（4.27±1.13）mg/L，N添加显著提高两层土壤溶液NO₃^-浓度和B层NH₄⁺浓度；N添加显著降低土壤溶液pH，且显著增加Al³⁺浓度，Ca²⁺和Mg²⁺含量有增加趋势，但影响不显著；N处理使A层土壤溶液可溶性有机碳（DOC）浓度显著降低，对两层土壤溶液芳香化指数（AI）无显著影响；两层土壤溶液电导率（EC）及氧化还原电位（Eh）显著增加；此外，两层土壤溶液中许多化学成分均呈现极显著相关，特别是NO₃^-与EC相关系数达到了0.855。本研究中，对照处理极高的NO₃^-含量以及B层土壤溶液硝酸盐浓度高于A层，表明该亚热带常绿阔叶林N的有效性超过植物和微生物的总营养需求而发生淋溶，该生态系统已达到氮饱和状态，此外N添加会显著促进土壤酸化和铝离子活化，表层土壤溶液DOC的降低一定程度反映了凋落物分解受N添加的抑制作用，显著升高的氧化还原电位可能导致土壤中某些金属元素的迁移率降低。;Soil solution is called soil blood, which is the medium of various biochemical reactions in the soil. When the external environment changes, the chemical composition of the soil solution can quickly respond to the environmental changes before other soil indicators have changed. In order to explore the impact of increasing atmospheric nitrogen (N) deposition on forest ecosystems, we set up three N treatments including the control (CK, 0 g m^-2 a^-1), low N (LN, 5 g m^-2 a^-1), and high N (HN, 15 g m^-2 a^-1) in a subtropical evergreen broad-leaved forest in the raining area of west China, artificially applied ammonium nitrate to simulate the increase of N deposition (replaced with sodium nitrate and ammonium chloride since September 2017). After 42 months of N treatment, soil solution from two soil layers (layer A:37-45 cm, layer B:52-60 cm) was collected using negative pressure soil solution sampler and analyzed chemical composition once a month for a year. In CK, soil solution NO₃^- concentration reached (3.94±0.77) mg/L(layer A) and (4.27±1.13) mg/L(layer B). Nitrogen additions significantly increased soil solution NO₃^- and NH₄⁺ concentration in layer B. Nitrogen additions significantly reduced soil solution pH value and increased Al³⁺ concentration, and the concent of Ca²⁺ and Mg²⁺ increased with no significant in statistics. Nitrogen treatment significantly reduced the dissolved organic carbon (DOC) concentration in layer A, and had no significant effect on the aromatic index (AI) of soil solution in the two layers. The electrical conductivity (EC) and the redox potential (Eh) of soil solution increased significantly in two soil layers. In addition, many chemical components in the soil solution of two layers had extremely significant correlation, especially the correlation coefficient between NO₃^- and EC reached 0.855. In this study, the extremely high NO₃^- concentration in CK and higher NO₃^- concentration in layer B than in layer A indicated that the nitrogen availability of the subtropical evergreen broad-leaved forest exceeded the total nutrient requirements of plants and microorganisms, and the ecosystem had reached the state of nitrogen saturation. In addition, nitrogen addition significantly promoted soil acidification and aluminum activation. The decrease of DOC in surface soil solution to some extent reflected the inhibition of litter decomposition by nitrogen addition. The significant increase of redox potential may lead to the decrease of the mobility of some metal elements in the soil.

Using Carbon-14 of dissolved organic carbon to determine groundwater ages and travel times in aquifers with low organic carbon

This study used dissolved organic carbon (DOC) carbon-14 (14C) to determine groundwater ages that were then used to calculate groundwater travel times in southern Nevada aquifers that have low organic content. These travel times are compared with the more standard dissolved inorganic (DIC) 14C method for determining groundwater ages and travel times. Groundwater travel times for aquifers in southern Nevada using DOC 14C are thousands of years shorter than DIC 14C travel times for groundwater flow outside of recharge areas along four of the five flow paths evaluated for this study. The DOC 14C travel times range from 2,300 to 2,900 years (yrs) as compared to DIC 14C travel times that range from 8,200 to 22,000 yrs (uncorrected ages) and 4,700 to 19,000 yrs (corrected ages). The DOC 14C groundwater travel times in carbonate-rock and volcanic-rock aquifers of southern Nevada are similar to travel times determined from: (1) hydrogeologic data; (2) observations of rapid high-level tritium transport at the Nevada National Security Site; and (3) 550,000-yr δ18O and δ13C global climate records for calcite precipitated in Devils Hole, Nevada at the end of one of the flow paths. DOC 14C travel time calculations need to account for fewer processes than DIC 14C for aquifers that contain little organic matter and do not have redox reactions. DOC 14C travel-times can be calculated directly from the DOC 14C data in these aquifers without corrections if dissolution of organic carbon and sorption and matrix diffusion of DOC 14C onto or into the aquifer matrix is minimal. Laboratory experiments showed that little organic carbon was being leached from aquifer rocks, sorption of organic carbon ranged from 4.3% sorbed in carbonate rocks to 0.5% sorbed in volcanic rocks, and matrix diffusion coefficients were slower in lower porosity carbonate rocks (1.7 × 10−7 cm2/s) than in higher porosity volcanic rocks (2.9 × 10−7 cm2/s). The lack of dissolution of organic carbon in study area aquifers is also supported by the decrease in DOC along flow paths and the DOC composition of groundwater changing little as groundwater flows from recharge areas into the adjacent valleys. In contrast, DIC 14C groundwater travel-time calculations must be corrected for complex chemical reactions and physical processes, including mineral/gas dissolution, mineral/gas precipitation/exsolution, cation exchange, and carbon isotopic exchange that can significantly change the amount of DIC 14C in groundwater along flow paths by processes other than radioactive decay. DIC 14C can also be affected by sorption and matrix diffusion, but these processes may, or may not, be captured in geochemical modeling of precipitation/dissolution and carbon isotopic exchange reactions. Corrected DIC 14C ages, therefore, represent maximum ages.

Potential utilization of terrestrially derived dissolved organic matter by aquatic microbial communities in saline lakes.

Lakes receive large amounts of terrestrially derived dissolved organic matter (tDOM). However, little is known about how aquatic microbial communities interact with tDOM in lakes. Here, by performing microcosm experiments we investigated how microbial community responded to tDOM influx in six Tibetan lakes of different salinities (ranging from 1 to 358 g/l). In response to tDOM addition, microbial biomass increased while dissolved organic carbon (DOC) decreased. The amount of DOC decrease did not show any significant correlation with salinity. However, salinity influenced tDOM transformation, i.e., microbial communities from higher salinity lakes exhibited a stronger ability to utilize tDOM of high carbon numbers than those from lower salinity. Abundant taxa and copiotrophs were actively involved in tDOM transformation, suggesting their vital roles in lacustrine carbon cycle. Network analysis indicated that 66 operational taxonomic units (OTUs, affiliated with Alphaproteobacteria, Actinobacteria, Bacteroidia, Bacilli, Gammaproteobacteria, Halobacteria, Planctomycetacia, Rhodothermia, and Verrucomicrobiae) were associated with degradation of CHO compounds, while four bacterial OTUs (affiliated with Actinobacteria, Alphaproteobacteria, Bacteroidia and Gammaproteobacteria) were highly associated with the degradation of CHOS compounds. Network analysis further revealed that tDOM transformation may be a synergestic process, involving cooperation among multiple species. In summary, our study provides new insights into a microbial role in transforming tDOM in saline lakes and has important implications for understanding the carbon cycle in aquatic environments.

The photochemical release of dissolved organic matter from resuspended sediments: Insights from fluorescence spectroscopy

Sediments exposed to sunlight can serve as an important source of dissolved organic carbon (DOC) and nutrients to overlying waters. However, the photochemical release processes of dissolved organic matter (DOM) from resuspended sediments and the characteristics of photoreleased DOM are not fully understood. In this study, excitation-emission matrix fluorescence combined with parallel factor analysis (EEMs-PARAFAC) was used to study the photochemical release of DOM qualitatively and quantitatively. The EEMs-PARAFAC demonstrated that the photoreleased DOM is dominated by humic-like substances, and the photorelease process could be consist of the photoproduction and photodegradation of DOM. The concurrent photodegradation may result in the underestimation of photoreleased DOM. Moreover, the significant increases in DOC content and fluorescence intensity of humic-like components along with increasing nitrate and ferric ion indicated that nitrate and ferric ion could facilitate the photoproduction of DOM through the photochemical produced hydroxyl radical. However, the decreases in DOC and fluorescence intensity were also observed at high concentration of nitrate and ferric ion, owing to the photodegradation of DOM by redundant hydroxyl radical. All of these results suggest that EEMs-PARAFAC is an effective and sensitive analytical technique for evaluating DOM photoreleased from suspended lake sediments and previous studies may underestimate photochemical release of DOM from sediments due to the overlook of the subsequently photodegradation of these released DOM. Thus, the photochemical release of DOM and its associated pollutants from suspended particles in shallow and eutrophic lakes should be more significant then should be paid more attention.

Disturbance legacies increase and synchronize nutrient concentrations and bacterial productivity in coastal ecosystems.

Long‐term ecological research can resolve effects of disturbance on ecosystem dynamics by capturing the scale of disturbance and interactions with environmental changes. To quantify how disturbances interact with long‐term directional changes (sea‐level rise, freshwater restoration), we studied 17 yr of monthly dissolved organic carbon (DOC), total nitrogen (TN), and phosphorus (TP) concentrations and bacterioplankton productivity across freshwater‐to‐marine estuary gradients exposed to multiple disturbance events (e.g., droughts, fire, hurricanes, and low‐temperature anomalies) and long‐term increases in water levels. By studying two neighboring drainages that differ in hydrologic connectivity, we additionally tested how disturbance legacies are shaped by hydrologic connectivity. We predicted that disturbance events would interact with long‐term increases in water levels in freshwater and marine ecosystems to increase spatiotemporal similarity (i.e., synchrony) of organic matter, nutrients, and microbial activities. Wetlands along the larger, deeper, and tidally influenced Shark River Slough (SRS) drainage had higher and more variable DOC, TN, and TP concentrations than wetlands along the smaller, shallower, tidally restricted Taylor River Slough/Panhandle (TS/Ph) drainage. Along SRS, DOC concentrations declined with proximity to coast, and increased in magnitude and variability following drought and flooding in 2015 and a hurricane in 2017. Along TS/Ph, DOC concentrations varied by site (higher in marine than freshwater wetlands) but not year. In both drainages, increases in TN from upstream freshwater marshes occurred following fire in 2008 and droughts in 2010 and 2015, whereas downstream increases in TP occurred with coastal storm surge from hurricanes in 2005 and 2017. Decreases in DOC:TN and DOC:TP were explained by increased TN and TP. Increases in bacterioplankton productivity occurred throughout both drainages following low‐temperature events (2010 and 2011) and a hurricane (2017). Long‐term TN and TP concentrations and bacterioplankton productivity were correlated (r > 0.5) across a range of sampling distances (1–50 km), indicating spatiotemporal synchrony. DOC concentrations were not synchronized across space or time. Our study advances disturbance ecology theory by illustrating how disturbance events interact with long‐term environmental changes and hydrologic connectivity to determine the magnitude and extent of disturbance legacies. Understanding disturbance legacies will enhance prediction and enable more effective management of rapidly changing ecosystems.

Controls on Dissolved Organic Carbon Bioreactivity in River Systems

Inland waters transport, transform and retain significant amounts of dissolved organic carbon (DOC) that may be biologically reactive (bioreactive) and thus potentially degraded into atmospheric CO2. Despite its global importance, relatively little is known about environmental controls on bioreactivity of DOC as it moves through river systems with varying water residence time (WRT). Here we determined the influence of WRT and landscape properties on DOC bioreactivity in 15 Swedish catchments spanning a large geographical and environmental gradient. We found that the short-term bioreactive pools (0–6 d of decay experiments) were linked to high aquatic primary productivity that, in turn, was stimulated by phosphorus loading from forested, agricultural and urban areas. Unexpectedly, the percentage of long-term bioreactive DOC (determined in 1-year experiments) increased with WRT, possibly due to photo-transformation of recalcitrant DOC from terrestrial sources into long-term bioreactive DOC with relatively lower aromaticity. Thus, despite overall decreases in DOC during water transit through the inland water continuum, DOC becomes relatively more bioreactive on a long time-scale. This increase in DOC bioreactivity with increasing WRT along the freshwater continuum has previously been overlooked. Further studies are needed to explain the processes and mechanisms behind this pattern on a molecular level.

Reused Cultivation Water Accumulates Dissolved Organic Carbon and Uniquely Influences Different Marine Microalgae.

Reusing growth medium (water supplemented with nutrients) for microalgae cultivation is required for economical and environmentally sustainable production of algae bioproducts (fuels, feed, and food). However, reused medium often contains microbes and dissolved organic matter that may affect algae growth. While the accumulation of dissolved organic carbon (DOC) in reused medium has been demonstrated, it is unclear whether DOC concentrations affect algae growth or subsequent rates of algal DOC release. To address these questions, lab-scale experiments were conducted with three marine microalgae strains, Navicula sp. SFP, Staurosira sp. C323, and Chlorella sp. D046, grown in medium reused up to four times. Navicula sp. and Chlorella sp. grew similarly in reused medium as in fresh medium, while Staurosira sp. became completely inhibited in reused medium. Across the three algae, there was no broad trend between initial DOC concentration in reused medium and algae growth response. Navicula sp. released less DOC overall in reused medium than in fresh medium, but DOC release rates did not decrease proportionally with increased DOC concentrations. Net DOC accumulation was much lower than gross DOC released by Navicula sp. and Staurosira sp., indicating the majority of released DOC was degraded. Additionally, biodegradation experiments with reused media showed no further net decrease in DOC, suggesting the accumulated DOC was recalcitrant to the associated bacteria. Overall, these results suggest that taxa-specific factors may be responsible for algae growth response in reused medium, and that DOC release and accumulation are insensitive to prior cultivation rounds. Choosing an algae strain that is uninhibited by accumulated DOC is therefore critical to ensure successful water reuse in the algae industry.

Dissolved organic carbon and major and trace elements in peat porewater of sporadic, discontinuous, and continuous permafrost zones of western Siberia

Abstract. Mobilization of dissolved organic carbon (DOC) and related trace elements (TEs) from the frozen peat to surface waters in the permafrost zone is expected to enhance under ongoing permafrost thaw and active layer thickness (ALT) deepening in high-latitude regions. The interstitial soil solutions are efficient tracers of ongoing bio-geochemical processes in the critical zone and can help to decipher the intensity of carbon and metals migration from the soil to the rivers and further to the ocean. To this end, we collected, across a 640 km latitudinal transect of the sporadic to continuous permafrost zone of western Siberia peatlands, soil porewaters from 30 cm depth using suction cups and we analyzed DOC, dissolved inorganic carbon (DIC), and 40 major elements and TEs in 0.45 µm filtered fraction of 80 soil porewaters. Despite an expected decrease in the intensity of DOC and TE mobilization from the soil and vegetation litter to the interstitial fluids with the increase in the permafrost coverage and a decrease in the annual temperature and ALT, the DOC and many major and trace elements did not exhibit any distinct decrease in concentration along the latitudinal transect from 62.2 to 67.4° N. The DOC demonstrated a maximum of concentration at 66° N, on the border of the discontinuous/continuous permafrost zone, whereas the DOC concentration in peat soil solutions from the continuous permafrost zone was equal to or higher than that in the sporadic/discontinuous permafrost zone. Moreover, a number of major (Ca, Mg) and trace (Al, Ti, Sr, Ga, rare earth elements (REEs), Zr, Hf, Th) elements exhibited an increasing, not decreasing, northward concentration trend. We hypothesize that the effects of temperature and thickness of the ALT are of secondary importance relative to the leaching capacity of peat, which is in turn controlled by the water saturation of the peat core. The water residence time in peat pores also plays a role in enriching the fluids in some elements: the DOC, V, Cu, Pb, REEs, and Th were a factor of 1.5 to 2.0 higher in mounds relative to hollows. As such, it is possible that the time of reaction between the peat and downward infiltrating waters essentially controls the degree of peat porewater enrichments in DOC and other solutes. A 2° northward shift in the position of the permafrost boundaries may bring about a factor of 1.3 ± 0.2 decrease in Ca, Mg, Sr, Al, Fe, Ti, Mn, Ni, Co, V, Zr, Hf, Th, and REE porewater concentration in continuous and discontinuous permafrost zones, and a possible decrease in DOC, specific ultraviolet absorbency (SUVA), Ca, Mg, Fe, and Sr will not exceed 20 % of their current values. The projected increase in ALT and vegetation density, northward migration of the permafrost boundary, or the change of hydrological regime is unlikely to modify chemical composition of peat porewater fluids larger than their natural variations within different micro-landscapes, i.e., within a factor of 2. The decrease in DOC and metal delivery to small rivers and lakes by peat soil leachate may also decrease the overall export of dissolved components from the continuous permafrost zone to the Arctic Ocean. This challenges the current paradigm on the increase in DOC export from the land to the ocean under climate warming in high latitudes.

Permeate flux recovery of ceramic membrane using TiO 2 with catalytic ozonation

Abstract In the present study, ozonation and catalytic ozonation were applied in ceramic membrane ultrafiltration to investigate their effects on membrane fouling. The humic acid solution and titanium dioxide (TiO 2 ) were used as tested water and catalyst, respectively. Controlled operating parameters include ozone gas flow rate, catalyst dosage and ozone dosage. The results reveal that the permeate flux of membrane could be recovered up to 98% of the initial permeate flux when ozone gas was injected into the membrane module at the time of permeate flux decreased to 60% of the initial permeate flux. Two monitored water quality parameters, dissolved organic carbon (DOC) and UV absorbance at 254 nm (UV 254 ) were both removed up to 97% under tested experimental conditions. When TiO 2 was used as the catalyst in catalytic ozonation, the permeate flux of membrane was only recovered up to 93% of the initial permeate flux when catalysts and ozone gas were introduced into the membrane module at the time of permeate flux decreased to 60% of the initial permeate flux. However, the decrease of DOC and UV 254 achieved up to 97% and 100%, respectively. This indicates that the application of catalytic ozonation in the ceramic membrane ultrafiltration system could obtain higher permeate water quality than using ozonation alone. Both ozonation and catalytic ozonation could effectively reduce membrane fouling and maintain stable permeate flux of the tested ceramic membrane system.

Changes in soil organic carbon and its active fractions in different desertification stages of alpine-cold grassland in the eastern Qinghai–Tibet Plateau

In recent years, the desertification of alpine-cold grasslands has become increasingly serious in the Qinghai–Tibet Plateau in China, but it has not received the same amount of attention as has desertification in (semi)arid areas. Little is thus known about the change in soil organic carbon (SOC) during alpine-cold grassland desertification. To quantify the impacts of desertification on vegetation, SOC and its active fractions in alpine-cold grasslands, areas of light desertified grassland, medium desertified grassland, heavy desertified grassland, serious desertified grassland, and nondesertified grassland were selected as experimental sites in the eastern Qinghai–Tibet Plateau in China. The species number, height and coverage of vegetation were surveyed, and the soil particle fractions, SOC and SOC active fractions (including dissolved organic carbon (DOC), microbial biomass carbon (MBC), and labile organic carbon (LOC) were measured to a depth of 0–100 cm. The results showed that alpine-cold grassland desertification resulted in a significant reduction in vegetation cover, plant biomass, fine soil particles, SOC, DOC, LOC and MBC. The decreases in DOC, LOC and MBC were more rapid and apparent than were those in SOC, and the decrease in MBC was the most obvious among them. The rates of reduction in SOC concentrations accelerated as desertification progressed; most of the SOC was lost in the middle and later desertification stages, with lower losses during early desertification. The results indicate that active SOC fractions, particularly MBC, are more sensitive to desertification and can be used as sensitive indicators of desertification. Efforts to limit desertification and reduce SOC loss in alpine-cold grasslands should focus on early desertification stages and adopt strategies to prevent overgrazing and control the erosion of soil by wind.

Sediment resuspension effects on dissolved organic carbon fluxes and microbial metabolic potentials in reservoirs

Sediment resuspension can affect water quality in lakes and reservoirs. We investigated the effect of sediment resuspension on benthic fluxes of dissolved organic carbon (DOC), metals (Fe, Mn), and nutrients (N, P) in three drinking water reservoirs using sediment core incubations. Measurement of Fe and Mn fluxes, and of microbial potentials to degrade organic substrates (Biolog EcoPlates™) were employed to understand mechanisms regulating DOC exchange after sediment resuspension. Single sediment resuspension events resulted in DOC fluxes [−104 (into sediment) to 46 (release) mmol m−2 event−1] equal to 9–17 days of diffusive fluxes, making them a relevant process. Shallow reservoir sites were more likely to immobilize DOC after resuspension than deep sites. Sediment resuspension under anoxia always led to increases of DOC and metals in the overlying water. Resuspension did not necessarily mobilize nitrate or phosphorus even under anoxia, while ammonium was released after resuspension. Sediment resuspension increased hypolimnetic microbial potentials to utilize organic substrates in both spring and summer. However microbial cells counts and biomass either remained constant or decreased in summer. Adsorption to Fe minerals seemed to play a role in DOC immobilization as evidenced by a decrease in DOC:Fe molar ratios after resuspension in Fe limited sites and constant ratios in Fe rich sites. The results demonstrate a potential for DOC immobilization mainly by Fe minerals and to some extent by benthic microbes. Therefore, sediment resuspension can be beneficial for water quality in low nutrient, iron rich systems.

Photoproduction of dissolved organic carbon and inorganic nutrients from resuspended lake sediments.

Sediments exposed to simulated solar radiation can serve as an important source of dissolved organic carbon (DOC) to surrounding waters. However, it is still unclear if dissolved nutrients can be photoproduced from lake sedimentary organic matter. In this study, a series of laboratory-based experiments was conducted to address the photoproduction of dissolved inorganic nutrients and DOC from resuspended Taihu Lake sediments. Dissolved inorganic nutrients and DOC were photoproduced after 8-h irradiation. The released NH4+, NOx-, and DOC levels ranged from 3.57 to 12.14, 1.43 to 6.43, and 24.17 to 69.17μmolL-1, respectively. The variation in the amount released indicated that sediment source had an effect on DOC and nutrient photorelease. More DOC and nutrients were released from higher concentration suspensions. However, due to the light absorption by suspended sediment, less DOC and nutrients were released from per gram of suspended sediment in high concentration suspensions. The decrease in DOC and increase in dissolved inorganic nitrogen during the last 2-h irradiation indicated that the photoproduction of inorganic nutrients proceeded via direct photodissolution of suspended sediments and subsequent photodegradation of the produced dissolved organic matter. Our results demonstrated that the photoproduction flux of NH4+ and NOx- accounts for 12.3 and 6.5% of wet deposition, respectively, which suggest that the photodissolution of suspended sediment could be a potential source of DOC and dissolved nutrients in shallow water ecosystems.

Interaction between iron and dissolved organic matter in a marine shallow hydrothermal system off Dominica Island (Lesser Antilles)

Shallow submarine hydrothermal systems are extreme environments with unique biogeochemical conditions, originating from the interaction of hot, reduced fluids and cold, oxygenated seawater. Hydrothermal fluids represent one of the marine sources of essential trace elements like iron, the flux of which is controlled by dissolved organic matter (DOM) in and between terrestrial and marine ecosystems. However, the selectivity of the DOM with iron in marine environments is not well understood. Here we characterized at a molecular level the marine hydrothermal iron–DOM interaction using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). Our study site was a shallow-water hydrothermal system off Dominica Island (Lesser Antilles, Caribbean Sea), where reduced fluids were strongly enriched in dissolved Fe and Si as compared to surface seawater. The dissolved organic carbon (DOC) concentration was lower in the hydrothermal fluids, most likely due to low-DOC meteoric water influence, but also suggesting biotic or abiotic processes consuming or depleting DOC when seawater is re-circulated through the sediment. The formation of hydrous ferric oxides upon aeration of the hydrothermal fluids for 10h leads to an 8% decrease in DOC, indicating co-precipitation of iron and DOM. Re-solubilization of iron precipitates revealed increased relative abundance of aromatic compounds in co-precipitated DOM, which is in accordance with iron-coagulation observed in terrestrial environments. In conclusion, we provide evidence of co-precipitation of DOM with iron at hydrothermal systems as a selective process, which characteristically alters the molecular composition of DOM released with hydrothermal fluids.

Soil Fauna Affects Dissolved Carbon and Nitrogen in Foliar Litter in Alpine Forest and Alpine Meadow.

Dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) are generally considered important active biogeochemical pools of total carbon and nitrogen. Many studies have documented the contributions of soil fauna to litter decomposition, but the effects of the soil fauna on labile substances (i.e., DOC and TDN) in litter during early decomposition are not completely clear. Therefore, a field litterbag experiment was carried out from 13th November 2013 to 23rd October 2014 in an alpine forest and an alpine meadow located on the eastern Tibetan Plateau. Litterbags with different mesh sizes were used to provide access to or prohibit the access of the soil fauna, and the concentrations of DOC and TDN in the foliar litter were measured during the winter (the onset of freezing, deep freezing and thawing stage) and the growing season (early and late). After one year of field incubation, the concentration of DOC in the litter significantly decreased, whereas the TDN concentration in the litter increased. Similar dynamic patterns were detected under the effects of the soil fauna on both DOC and TDN in the litter between the alpine forest and the alpine meadow. The soil fauna showed greater positive effects on decreasing DOC concentration in the litter in the winter than in the growing season. In contrast, the dynamics of TND in the litter were related to seasonal changes in environmental factors, rather than the soil fauna. In addition, the soil fauna promoted a decrease in litter DOC/TDN ratio in both the alpine forest and the alpine meadow throughout the first year of decomposition, except for in the late growing season. These results suggest that the soil fauna can promote decreases in DOC and TDN concentrations in litter, contributing to early litter decomposition in these cold biomes.

Degradation of dibromophenols by UV irradiation

We examined the degradation of dibromophenols (DBPs), i.e. 2,4-DBP, 2,6-DBP and 3,5-DBP by ultraviolet (UV) irradiation and estimated the relationship between degradability and molecular orbital properties of each dibromophenol. The removal of DBPs under a UV lamp system was successfully performed in an aqueous solution. After 5 min of irradiation, the initial DBPs concentration of 20 mg/L was decreased to below 1 mg/L, and about 60% of bromide ion was released. A decrease in the concentration of dissolved organic carbon (DOC) suggested the mineralization of DBPs. The mineralization may occur after release of bromide ions because the decrease of DOC was slower than the release of bromide ions. The degradability of 3,5-DBP was slightly lower than 2,6-DBP and 2,4-DBP. Molecular orbital calculation suggested that the electrophilic frontier density and the highest occupied molecular orbital (HOMO) energy may be related to the degradability of DBPs.