Increase In Dissolved Organic Carbon Research Articles

No long‐term trends in pCO2 despite increasing organic carbon concentrations in boreal lakes, streams, and rivers

AbstractConcentrations of dissolved organic carbon (DOC) from terrestrial sources have been increasing in freshwaters across large parts of the boreal region. According to results from large‐scale field and detailed laboratory studies, such a DOC increase could potentially stimulate carbon dioxide (CO2) production, subsequently increasing the partial pressure of CO2 (pCO2) in freshwaters. However, the response of pCO2 to the presently observed long‐term increase in DOC in freshwaters is still unknown. Here we tested whether the commonly found spatial DOC‐pCO2 relationship is also valid on a temporal scale. Analyzing time series of water chemical data from 71 lakes, 30 streams, and 4 river mouths distributed across all of Sweden over a 17 year period, we observed significant DOC concentration increases in 39 lakes, 15 streams, and 4 river mouths. Significant pCO2 increases were, however, only observed in six of these 58 waters, indicating that long‐term DOC increases in Swedish waters are disconnected from temporal pCO2 trends. We suggest that the uncoupling of trends in DOC concentration and pCO2 are a result of increased surface water runoff. When surface water runoff increases, there is likely less CO2 relative to DOC imported from soils into waters due to a changed balance between surface and groundwater flow. Additionally, increased surface water runoff causes faster water flushing through the landscape giving less time for in situ CO2 production in freshwaters. We conclude that pCO2 is presently not following DOC concentration trends, which has important implications for modeling future CO2 emissions from boreal waters.

Impact of anthropogenic disturbance on the chemistry of a small urban pond

Mirror Lake, one of the scenic locations on The Ohio State University's campus, experiences an intense bioturbation event as part of an annual tradition revolving around the rivalry football game against the University of Michigan. This tradition involves thousands of students jumping into the lake over one night in the week leading up to the football game.Water samples were collected from several locations in the lake before, during, and after the Mirror Lake Jump to determine the impact of this event on lake water chemistry. There were significant and systematic increases in the concentrations of Na+, K+, Cl−, total nitrogen, ammonia, and dissolved organic carbon (DOC) associated with the jump, especially in the eastern side of the lake where most of the students entered. Over the 3-h period from 10 p.m. to 1 a.m. on the eastern side of the lake, Na+, K+, and Cl− concentrations increased by about 2–4 ppm, 1.5–3 ppm, and 4–6 ppm, respectively. The total nitrogen concentration increased about five to six fold, from 450–500 ppb to 2300–2800 ppb over the height of the event on the eastern side of the lake. Similar increases were observed for DOC, increasing from 3.6 to 18 ppm. This DOC increase was coincident with a 5‰ shift in δ13C, from a mean of around −28‰ in the early hours of the evening to a maximum of −23‰, implying a large influx of isotopically heavy carbon into the lake. Ammonia concentrations varied substantially from year to year, but always showed a systematic increase in concentration during the event. Smaller changes in major ion and nutrient concentrations were observed in the middle and western side of the lake, where fewer students entered the lake.The changes in concentration and the timing and spatial distribution of these changes are primarily attributed to anthropogenic input from jumpers in the form of bodily fluids (e.g., evaporated sweat, sebum and urine). Over a single night, these anthropogenic event inputs represent roughly 10% of the annual nitrogen budget of the lake, emphasizing the direct impact humans can have on urban water bodies on short time scales.

Salinity, dissolved organic carbon, and interpopulation variability hardly influence the accumulation and effect of copper in Mytilus edulis.

To improve the ecological relevance of environmental risk assessment, an improved understanding is needed of 1) the influence of environmental conditions on the toxicity of pollutants, and 2) the effect of these factors in combination with possible interpopulation variability. The influences of salinity and dissolved organic carbon (DOC) on the accumulation and effect of copper (Cu) to settled mussels were investigated with mussels from a North Sea and a Baltic Sea population. We found that both populations were equally Cu-sensitive, even though the Baltic Sea population lives in suboptimal conditions. Baltic Sea mussels, however, accumulated more Cu. This suggests that these populations may have different ways of coping with excess Cu. The influence of salinity on Cu toxicity to settled mussels was limited for both populations. An increase in DOC did not decrease the Cu accumulation or effect in either population. This suggests that DOC-Cu complexes are bioavailable for settled mussels. These findings are in contrast with previous research which indicated that DOC decreased the toxicity and accumulation of Cu in the D-larvae life stage. As a consequence, the mussel larval stage is not the most Cu-sensitive life stage at high DOC concentrations. Furthermore, a DOC correction factor for Cu toxicity cannot be used for settled mussels. This should be accounted for in future marine Cu environmental risk assessment. Environ Toxicol Chem 2017;36:2074-2082. © 2017 SETAC.

Spatial and temporal patterns of dissolved organic matter quantity and quality in the Mississippi River Basin, 1997–2013

Recent studies have found insignificant or decreasing trends in time-series dissolved organic carbon (DOC) datasets, questioning the assumption that long-term DOC concentrations in surface waters are increasing in response to anthropogenic forcing, including climate change, land use, and atmospheric acid deposition. We used the weighted regressions on time, discharge, and season (WRTDS) model to estimate annual flow-normalized concentrations and fluxes to determine if changes in DOC quantity and quality signal anthropogenic forcing at 10 locations in the Mississippi River Basin. Despite increases in agriculture and urban development throughout the basin, net increases in DOC concentration and flux were significant at only 3 of 10 sites from 1997 to 2013 and ranged between −3.5% to +18% and −0.1 to 19%, respectively. Positive shifts in DOC quality, characterized by increasing specific ultraviolet absorbance at 254 nm, ranged between +8% and +45%, but only occurred at one of the sites with significant DOC quantity increases. Basinwide reductions in atmospheric sulfate deposition did not result in large increases in DOC either, likely because of the high buffering capacity of the soil. Hydroclimatic factors including annual discharge, precipitation, and temperature did not significantly change during the 17-year timespan of this study, which contrasts with results from previous studies showing significant increases in precipitation and discharge over a century time scale. Our study also contrasts with those from smaller catchments, which have shown stronger DOC responses to climate, land use, and acidic deposition. This temporal and spatial analysis indicated that there was a potential change in DOC sources in the Mississippi River Basin between 1997 and 2013. However, the overall magnitude of DOC trends was not large, and the pattern in quantity and quality increases for the 10 study sites was not consistent throughout the basin.

Paleolimnological evidence of the consequences of recent increased dissolved organic carbon (DOC) in lakes of the northeastern USA

As a result of reductions in sulfate deposition and changing climate, dissolved organic carbon (DOC) concentrations have increased in many lakes situated in forests of northeastern North America and northern Europe since the 1990s. Although this increase is well documented, the associated ecological implications remain unclear. In particular, DOC strongly influences the vertical temperature structure of lakes, with increasing DOC often leading to a shallower epilimnion. We investigated the effect of increased DOC concentrations on lake thermal structure using fossil diatom records from six remote Maine lakes. Sedimentary diatom profiles from three pairs of small (<0.5 km2) lakes were compared, with each pair containing one lake with a documented significant increase in DOC and the other experiencing no change in DOC since the early 1990s. Lake thermal structure was inferred from changes in the relative abundance of Discostella stelligera and Aulacoseira species, two diatom taxa that are associated with changes in thermal stratification. The three lakes without increasing DOC showed no change in diatom-inferred mixing depth over the past few decades. Of the lakes with documented increases in DOC, two showed the highest turnover in diatom community structure over time. Profiles from both of these lakes also indicated directional change in diatom-inferred mixing depth over the past 20 years, but the direction of change differed. This study demonstrates that recent increases in DOC have the potential to alter the physical and biological structure of lakes, but that these responses may differ across lakes.

Sustained Biogeochemical Impacts of Wildfire in a Mountain Lake Catchment

Wild and prescribed fires can cause severe deterioration in water quality, including increases in sediment, nutrients and dissolved organic carbon (DOC). Due to the unpredictability of wildfires, few studies have been able to employ before-after, control-intervention experimental designs, or to evaluate fire-induced water-quality changes in the context of long-term datasets. Here, we present data from a lake draining a moorland catchment in the United Kingdom, part of a 22-site, 25 year monitoring network, which experienced a major wildfire in 2011. The main water-quality response was a large, sustained increase in nitrate concentrations, sufficient to raise acidity and aluminium concentrations, effectively reversing over a decade of recovery from the effects of acid deposition. Concurrently, we observed a clear reduction in DOC concentrations, contrasting with prescribed fire studies from similar ecosystems (none based on before-after studies) that have suggested that burning causes DOC to increase. However, data from a downstream water supply reservoir do suggest a fire-induced change in DOC quality towards more soil-derived aromatic organic compounds, and lake sediment data suggest a large increase in particulate organic carbon. We conclude that the biogeochemical responses to wildfire in our moorland catchment were broadly similar to those observed in forest ecosystems elsewhere, but that historically high nitrogen deposition has made the ecosystem particularly susceptible to nitrate leaching and (re-)acidification. The observed reduction in DOC concentrations casts some doubt on the widely held view that prescribed burning in moorland systems has contributed to long-term DOC increases.

Humic dissolved organic carbon drives oxidative stress and severe fitness impairments in Daphnia

Increases in dissolved organic carbon (DOC) in the form of humic substances, causing browning of surface water, have been reported worldwide. Field surveys indicate that higher DOC levels can influence primary production and thus plankton composition. Experimental studies on the direct effects of humic DOC on aquatic organisms have shown varying results depending on concentration and additional environmental factors. Moreover, changes in life-histories and stress responses have usually been tested separately, rather than in combination. We experimentally tested the impact of a sudden increase in humic DOC on two species of the zooplankton cladoceran Daphnia, across several levels of biological organisation, from cellular to population responses. In D. magna, strong impacts on reproduction (delayed maturity and reduced number of offspring) were coupled with overall stress induction (increases in antioxidant capacity and oxidative damage, combined with a reduced amount of available energy). In D. longispina, increased mortality and lowered fecundity were observed. We conclude that a strong input of humic DOC into aquatic systems can have severe negative impacts on zooplankton species, and has the potential to alter zooplankton community structures.

Effects of ocean acidification on pelagic carbon fluxes in a mesocosm experiment

Abstract. About a quarter of anthropogenic CO2 emissions are currently taken up by the oceans, decreasing seawater pH. We performed a mesocosm experiment in the Baltic Sea in order to investigate the consequences of increasing CO2 levels on pelagic carbon fluxes. A gradient of different CO2 scenarios, ranging from ambient ( ∼ 370 µatm) to high ( ∼ 1200 µatm), were set up in mesocosm bags ( ∼ 55 m3). We determined standing stocks and temporal changes of total particulate carbon (TPC), dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), and particulate organic carbon (POC) of specific plankton groups. We also measured carbon flux via CO2 exchange with the atmosphere and sedimentation (export), and biological rate measurements of primary production, bacterial production, and total respiration. The experiment lasted for 44 days and was divided into three different phases (I: t0–t16; II: t17–t30; III: t31–t43). Pools of TPC, DOC, and DIC were approximately 420, 7200, and 25 200 mmol C m−2 at the start of the experiment, and the initial CO2 additions increased the DIC pool by ∼ 7 % in the highest CO2 treatment. Overall, there was a decrease in TPC and increase of DOC over the course of the experiment. The decrease in TPC was lower, and increase in DOC higher, in treatments with added CO2. During phase I the estimated gross primary production (GPP) was ∼ 100 mmol C m−2 day−1, from which 75–95 % was respired, ∼ 1 % ended up in the TPC (including export), and 5–25 % was added to the DOC pool. During phase II, the respiration loss increased to ∼ 100 % of GPP at the ambient CO2 concentration, whereas respiration was lower (85–95 % of GPP) in the highest CO2 treatment. Bacterial production was ∼ 30 % lower, on average, at the highest CO2 concentration than in the controls during phases II and III. This resulted in a higher accumulation of DOC and lower reduction in the TPC pool in the elevated CO2 treatments at the end of phase II extending throughout phase III. The “extra” organic carbon at high CO2 remained fixed in an increasing biomass of small-sized plankton and in the DOC pool, and did not transfer into large, sinking aggregates. Our results revealed a clear effect of increasing CO2 on the carbon budget and mineralization, in particular under nutrient limited conditions. Lower carbon loss processes (respiration and bacterial remineralization) at elevated CO2 levels resulted in higher TPC and DOC pools than ambient CO2 concentration. These results highlight the importance of addressing not only net changes in carbon standing stocks but also carbon fluxes and budgets to better disentangle the effects of ocean acidification.

Century‐long increasing trend and variability of dissolved organic carbon export from the Mississippi River basin driven by natural and anthropogenic forcing

AbstractThere has been considerable debate as to how natural forcing and anthropogenic activities alter the timing and magnitude of the delivery of dissolved organic carbon (DOC) to the coastal ocean, which has ramifications for the ocean carbon budget, land‐ocean interactions, and coastal life. Here we present an analysis of DOC export from the Mississippi River to the Gulf of Mexico during 1901–2010 as influenced by changes in climate, land use and management practices, atmospheric CO2, and nitrogen deposition, through the integration of observational data with a coupled hydrologic/biogeochemical land model. Model simulations show that DOC export in the 2000s increased more than 40% since the 1900s. For the recent three decades (1981–2010), however, our simulated DOC export did not show a significant increasing trend, which is consistent with observations by U.S. Geological Survey. Our factorial analyses suggest that land use and land cover change, including land management practices (LMPs: i.e., fertilization, irrigation, tillage, etc.), were the dominant contributors to the century‐scale trend of rising total riverine DOC export, followed by changes in atmospheric CO2, nitrogen deposition, and climate. Decadal and interannual variations of DOC export were largely attributed to year‐to‐year climatic variability and extreme flooding events, which have been exacerbated by human activity. LMPs show incremental contributions to DOC increase since the 1960s, indicating the importance of sustainable agricultural practices in coping with future environmental changes such as extreme flooding events. Compared to the observational‐based estimate, the modeled DOC export was 20% higher, while DOC concentrations were slightly lower. Further refinements in model structure and input data sets should enable reductions in uncertainties in our prediction of century‐long trends in DOC.

Rates of CO2 efflux and changes in DOC concentration resulting from the addition of POC to the fluvial system in peatlands

Fluvial systems are increasingly recognised as sites of carbon processing, particularly in peatlands, where high quantities of particulate organic carbon (POC) enter the stream network during storm events. However, the fate of this POC and potential transformations to dissolved and gaseous phases are poorly understood. Here, a series of mixing experiments are undertaken, using particulate organic matter (POM) and streamwater derived from an eroding peat catchment to simulate high quantities of material entering the fluvial system during storm event. Solutions were monitored for CO2 release and changes in dissolved organic carbon (DOC) concentration. The role of microbial versus physical processes, and the influence of the initial streamwater pH and DOC concentration were also investigated. Results show that the addition of POM to the system leads to high rates of CO2 release, particularly within the first 24 h. The key control on rates of CO2 release is the initial DOC concentration of the streamwater used, with highest rates of CO2 release from solutions with the lowest initial DOC concentration. Rates of DOC production were lower, with average increases in the region of 0.8 mg/l over 24 h. There is evidence to suggest that rates of DOC increase are also influenced by initial streamwater DOC. The rapid rates of CO2 release from the fluvial system indicate that significant carbon loss to the atmosphere is possible, representing ‘hot moments’ upon the addition of POC to the fluvial system, whereas locations where POC residence times are long will be ‘hot spots’ of carbon transformation.

Effect of algal flocculation on dissolved organic matters using cationic starch modified soils

Modified soils (MSs) are being increasingly used as geo-engineering materials for the sedimentation removal of cyanobacterial blooms. Cationic starch (CS) has been tested as an effective soil modifier, but little is known about its potential impacts on the treated water. This study investigated dissolved organic matters in the bloom water after algal removal using cationic starch modified soils (CS-MSs). Results showed that the dissolved organic carbon (DOC) could be decreased by CS-MS flocculation and the use of higher charge density CS yielded a greater DOC reduction. When CS with the charge density of 0.052, 0.102 and 0.293meq/g were used, DOC was decreased from 3.4 to 3.0, 2.3 and 1.7mg/L, respectively. The excitation–emission matrix fluorescence spectroscopy and UV254 analysis indicated that CS-MS exhibits an ability to remove some soluble organics, which contributed to the DOC reduction. However, the use of low charge density CS posed a potential risk of DOC increase due to the high CS loading for effective algal removal. When CS with the charge density of 0.044meq/g was used, DOC was increased from 3.4 to 3.9mg/L. This study suggested, when CS-MS is used for cyanobacterial bloom removal, the content of dissolved organic matters in the treated water can be controlled by optimizing the charge density of CS. For the settled organic matters, other measures (e.g., capping treatments using oxygen loaded materials) should be jointly applied after algal flocculation.

Fine-scale temporal characterization of trends in soil water dissolved organic carbon and potential drivers

Long-term monitoring of surface water quality has shown increasing concentrations of dissolved organic carbon (DOC) across a large part of the Northern Hemisphere. Several drivers have been implicated including climate change, land management change, nitrogen and sulphur deposition and CO2 enrichment. Analysis of stream water data, supported by evidence from laboratory studies, indicates that an effect of declining sulphur deposition on catchment soil chemistry is likely to be the primary mechanism, but there are relatively few long term soil water chemistry records in the UK with which to investigate this, and other, hypotheses directly. In this paper, we assess temporal relationships between soil solution chemistry and parameters that have been argued to regulate DOC production and, using a unique set of co-located measurements of weather and bulk deposition and soil solution chemistry provided by the UK Environmental Change Network and the Intensive Forest Monitoring Level II Network. We used statistical non-linear trend analysis to investigate these relationships at 5 forested and 4 non-forested sites from 1993 to 2011. Most trends in soil solution DOC concentration were found to be non-linear. Significant increases in DOC occurred mostly prior to 2005. The magnitude and sign of the trends was associated qualitatively with changes in acid deposition, the presence/absence of a forest canopy, soil depth and soil properties. The strongest increases in DOC were seen in acidic forest soils and were most clearly linked to declining anthropogenic acid deposition, while DOC trends at some sites with westerly locations appeared to have been influenced by shorter-term hydrological variation. The results indicate that widespread DOC increases in surface waters observed elsewhere, are most likely dominated by enhanced mobilization of DOC in surficial organic horizons, rather than changes in the soil water chemistry of deeper horizons. While trends in DOC concentrations in surface horizons have flattened out in recent years, further increases may be expected as soil chemistry continues to adjust to declining inputs of acidity.

A decade of weekly dissolved organic carbon values in the coastal upwelling of the NW Spain (Atlantic Galician Rías)

Dissolved organic carbon (DOC) was measured weekly in five coastal upwelling Galician rías (Ría de Vigo, Ría de Pontevedra, Ría de Arousa, Ría de Muros and Ría de Ares–Betanzos) and the first decadal DOC series was examined in these rías. The temporal variability of DOC was assessed by considering biweekly values at 10 oceanographic stations for the decade 2002–2011. Seasonal variability in surface DOC concentrations was observed, with higher values (101±8μmolL−1; average±STD) during the accumulation period (DOCb; May–October) in all inner and outer stations. During the rest of the year (DOCc; November–April) surface DOC concentrations decreased to 88±6μmolL−1. The temporal distribution of DOC and temperature were comparable, with a marked seasonal variability, suggesting that the temperature was the main proxy for DOC. The average decadal seasonal increase in surface DOC (ΔDOCs=DOCb−DOCc) was between 7 and 18μmolL−1 for all stations. The interannual variability of surface DOC ranged between 0 and 18μmolL−1; 0 and 11% of the average DOC of each station. Decadal DOC trends were slightly positive in middle-inner areas of the Rías Baixas (Ría de Vigo, Ría de Pontevedra, Ría de Arousa and Ría de Muros) in agreement with positive temperature trends. The outer area of the Rías Baixas showed positive DOC trends in the stations of Ría de Vigo and Ría de Arousa and negative DOC trends in the stations of Ría de Pontevedra and Ría de Muros, although temperature presented positive trends in these four stations. The Ría de Ares–Betanzos exhibited negative trends in DOC and temperature.

Extreme weather years drive episodic changes in lake chemistry: implications for recovery from sulfate deposition and long-term trends in dissolved organic carbon

Interannual climate variability is expected to increase over the next century, but the extent to which hydroclimatic variability influences biogeochemical processes is unclear. To determine the effects of extreme weather on surface water chemistry, a 30-year record of surface water geochemistry for 84 lakes in the northeastern U.S. was combined with landscape data and watershed-specific weather data. With these data, responses in sulfate (SO4 2−) and dissolved organic carbon (DOC) concentrations were characterized during an extreme wet year and an extreme dry year across the region. Redundancy analysis was used to model lake chemical response to extreme weather as a function of watershed features. A response was observed in DOC and SO4 2− concentration in response to extreme wet and dry years in lakes across the northeastern U.S. Acidification was observed during drought and brownification was observed during wet years. Lake chemical response was related to landscape characteristics in different ways depending on the type of extreme year. A linear relationship between wetland coverage and DOC and SO4 2− deviations was observed during extreme wet years. The results presented here help to clarify the variability observed in long-term recovery from acidification and regional increases in DOC. Understanding the chemical response to weather variability is becoming increasingly important as temporal variation in precipitation is likely to intensify with continued atmospheric warming.

Influences of artificial root exudate components on the behaviors of BDE-28 and BDE-47 in soils: desorption, availability, and biodegradation.

Behaviors of BDE-28 and BDE-47 in two distinct soils (Phaeozem and Acrisol) as affected by the separate addition of root exudate components (i.e., oxalic acid, glycine, and fructose) were investigated by a soil microcosm incubation experiment. The results showed that root exudate components promoted the desorption of BDE-28 (57.6-235.0%) and BDE-47 (56.9-223.7%) from the soils due to the enhancement of their water solubilities. The addition of root exudate components increased the n-butanol extractability of BDE-28 and BDE-47 by 20.3-72.5 and 48.6-169.2%, respectively, which had a positive correlation with the concentrations of dissolved organic carbon (DOC) in the soils (p < 0.01), suggesting that the increase of DOC in the soils by root exudate components was the major factor to enhance the extractability. Fructose and oxalic acid promoted the desorption and increased the availability of BDE-28 and BDE-47 in the soils more efficiently than glycine. The addition of different root exudate components resulted in distinct shifts in soil microbial community structure (p < 0.05). Oxalic acid caused the greatest impacts on the soil bacterial communities and increased the degradation rates of BDE-28 and BDE-47 most obviously. The findings of this study clarified the roles of root exudate components in affecting the behaviors of polybrominated diphenyl ethers (PBDEs) in soils.

Dissolved organic carbon and dissolved organic nitrogen concentrations and exports upstream and downstream of the Dallas–Fort Worth metropolis, Texas, USA

Concentrations and export of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) from terrestrial landscapes to near-coastal zones vary with land use. Information on (DOC) and (DON) concentrations and exports from urban ecosystems is sparse; thus, their source from within urbanised watersheds such as soil or vegetation or from permitted sewage discharge is unknown. We examined DOC and DON concentrations and exports in four gauged subwatersheds in the humid subtropical, upper Trinity River basin, upstream and downstream of the Dallas–Fort Worth metropolis in Texas, USA. Annual average DOC concentrations ranged from 5.7±0.4 to 6.4±0.8mgL–1 and DON concentrations ranged from 0.31±0.05 to 0.33±0.14mgL–1. Dissolved organic carbon exports, which included permitted sewage discharge, ranged from 522kgkm–2 year–1 above Dallas–Fort Worth to 3637kgkm–2 year–1 below Dallas–Fort Worth. Permitted effluent discharge contributed between 1 and 35% of DOC loading above and below the Dallas–Fort Worth metropolis. DON exports ranged from 27 to 179kgkm–2 year–1 above and below Dallas–Fort Worth respectively. There was difficulty apportioning permitted effluent-discharge contribution to DON because of the transformations among nitrogen-species. A moderate but significant relationship was found between DOC and sodium concentrations (R2=0.45; P&lt;0.0001; n=40) and between DOC and potassium concentrations (R2=0.45; P&lt;0.0001; n=40). Dissolved organic nitrogen also displayed a significant relationship with sodium (R2=0.33; P&lt;0.001; n=40) and potassium (R2=0.59; P&lt;0.001; n=40), suggesting that increases in these cations to aquatic ecosystems may induce increases in DOC and DON concentrations. Although DOC export was significantly correlated with medium-density urban land use (r=0.96; P&lt;0.05: n=4), DON export was not (r=0.93; P&gt;0.05; n=4), suggesting that land-management practices and permitted point-source discharges have a significant effect on aquatic DOC and DON concentrations and exports derived from urban watersheds.

Drying effects on selenium and copper in 0.01 M calcium chloride soil extractions

The study aimed to understand the effects of drying and rewetting the soils on soluble selenium (Se) and copper (Cu) concentrations in 0.01M CaCl2 soil extraction, and the mechanisms leading to these changes. Soluble Se and Cu concentrations were measured in the extractions of 58 soil samples taken from grassland and arable land in The Netherlands before and after oven drying at 40°C. Selenium speciation and dissolved organic matter (DOM) fractionation were performed using Donnan membrane technique (DMT) and DAX-8 resin. Drying and rewetting the soils increased soluble Se and Cu concentrations in CaCl2 extraction. The increase of both elements was associated with the increase of dissolved organic carbon (DOC). Compared to the magnitude of DOC increase (1.3 to 11 fold), the increase of soluble Se (1.3 to 34 fold) was more pronounced, whereas the increase of soluble Cu (1.1 to 4.6 fold) was less strong. The DMT analysis confirmed the dominance of colloidal sized DOM-associated Se both in the moist and dried soil extractions, showing that the colloidal sized organic Se is mainly responsible for the increase of total soluble Se upon drying. The Se to DOC ratio increased upon drying in most of the soils, indicating enrichment of Se in DOM after drying. No consistent difference in Se to DOC ratio was found between the fulvic acids (FA) and hydrophilic (Hy) fraction of DOM after drying. Fractionation of DOM showed that Hy and FA fraction accounted for respectively 74% and 26% of total DOC in the dried soils, which corresponds to the Se fraction in Hy and FA of DOM. The decrease of SUVA (0.3 to 1.0 fold) after drying implied a decrease of FA:Hy ratio in DOM and a decrease of average DOM aromaticity. Upon drying, a positive correlation between the change of Cu loading on DOC (Cu:DOC ratio) and the change of aromaticity (SUVA) of DOM was found, showing that the increase of Cu complexed with (mainly) aromatic DOM explains soluble Cu increase upon drying. The decrease of Cu loading on DOC upon drying was attributed to a decrease of the aromaticity and affinity to Cu of the DOM. Colloidal sized organic Se and Cu–DOM complex are the major species of Se and Cu, respectively, in the extractions of moist and dried soils, which may not directly be available for plant uptake.

Changes in dissolved organic carbon and total dissolved nitrogen fluxes across subtropical forest ecosystems at different successional stages

AbstractLateral transports of carbon and nitrogen are important processes linking terrestrial ecosystems and aquatic systems. Most previous studies made in temperate forests found that fluxes of carbon and nitrogen by runoff water varied in different forests, but few studies have been made in subtropical forests. This study was to investigate dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) fluxes at the catchment scale along a subtropical forest succession gradient from pine forest (pioneer) to coniferous and broadleaved mixed forest (transitional) to broadleaved forest (mature). Our results showed that DOC concentration significantly decreased (p&lt;0.001) while TDN concentration significantly increased (p&lt;0.001) in runoff water from pioneer to mature forests, which in turn resulted in a decrease in DOC flux and an increase in TDN flux, as mean annual runoff did not vary significantly among three succession forest catchments. The mean (±standard deviation) annual DOC flux was 118.1±43.6, 88.3±16.7 and 77.2±11.7 kg ha−1 yr−1for pioneer, transitional and mature forest catchments, respectively; and the mean annual TDN flux was 9.9 ±2.7, 18.2±3.0 and 21.2 ±4.5 kg ha−1 yr−1for pioneer, transitional and mature forest catchments, respectively. The mature forest reduced DOC flux by increased soil chemical adsorption and physical protection. An increase in TDN flux from pioneer to mature forests was consistent with the previous finding that mature forest was nitrogen saturated while pioneer forest was nitrogen limited. Therefore large‐scale conversion of pioneer forests to transitional or mature forests in subtropical China will reduce DOC concentration and increase TDN concentration in the down‐stream water, which may have significant impact on its water quality and aquatic biological activities.

Effects of photodemethylation on the methylmercury budget of boreal Norwegian lakes.

Methylmercury (MeHg) concentrations in freshwater fish from southeastern Norway continue to increase, highlighting the need for a comprehensive understanding of MeHg sources, cycling, and degradation in the aquatic environment. The authors assessed the importance of photodemethylation in the MeHg budget of 4 Norwegian lakes. Photodemethylation rates were determined using incubation experiments with MeHg-spiked natural lake water. The authors determined full-spectrum exposure rates at all study sites and waveband-specific rates (photosynthetically active radiation, ultraviolet-A radiation, and ultraviolet-B radiation) at 1 clear-water (Sognsvann) and 1 humic (Langtjern) site. No significant differences in photodemethylation rates between the sites were found, and the authors' observed rates agreed with available literature for lake and wetland waters. The authors paired experimentally derived photodemethylation rates with lake-specific incident irradiation, light attenuation, and MeHg concentrations to estimate MeHg loss through photodemethylation for the study sites. For Langtjern, losses through photodemethylation equalled 27% of total annual inputs, highlighting the importance of photodemethylation in the MeHg budget. Furthermore, the authors assessed how changes in terrestrial dissolved organic carbon (DOC) exported to freshwaters and climate-driven reductions in ice cover duration may affect MeHg losses through photodemethylation. Results suggest that future increases in DOC may lead to higher aqueous MeHg concentrations in boreal lakes due to increased DOC-associated MeHg inputs paired with significant decreases in the loss of MeHg through photodemethylation due to increased light attenuation.

Spatial controls on dissolved organic carbon in upland waters inferred from a simple statistical model

Dissolved organic carbon (DOC) concentrations in upland surface waters in many northern hemisphere industrialised regions are at their highest in living memory, provoking debate over their “naturalness”. Because of the implications for drinking water treatment and supply there is increasing interest in the potential for mitigation through local land management, and for forecasting the likely impact of environmental change. However, the dominant controls on DOC production remain unresolved, hindering the establishment of appropriate reference levels for specific locations. Here we demonstrate that spatial variation in long-term average DOC levels draining upland UK catchments is highly predictable using a simple multiple logistic regression model comprising variables representing wetland soil cover, rainfall, altitude, catchment sensitivity to acidification and current acid deposition. A negative relationship was observed between DOC concentration and altitude that, for catchments dominated by organo-mineral soils, is plausibly explained by the combined effects of changing net primary production and temperature-dependent decomposition. However, the magnitude of the altitude effect was considerably greater for catchments with a high proportion of wetland cover, suggesting that additional controls influence these sites such as impeded respiratory loss of carbon in wet soils and/or an increased susceptibility to water level drawdown at lower altitudes. The model suggests (1) that continuing reductions in sulphur deposition on acid sensitive organo-mineral soils, will drive further significant increases in DOC and, (2) given the differences in the magnitude of the observed altitude-DOC relationships, that DOC production from catchments with peat-dominated soils may be more sensitive to climate change than those dominated by mineral soils. However, given that mechanisms remain unclear, the latter warrants further investigation.