Dissolved Organic Carbon Production Research Articles

Algorithm development and validation for satellite‐derived distributions of DOC and CDOM in the U.S. Middle Atlantic Bight

Oceanographic cruises were conducted within the U.S. Middle Atlantic Bight (MAB) to collect field measurements to develop algorithms to retrieve surface ocean colored dissolved organic matter (CDOM) and dissolved organic carbon (DOC) from NASA's MODIS‐Aqua and SeaWiFS satellite sensors and to investigate the processes that influence the distributions of CDOM and DOC. In order to develop empirical algorithms for CDOM and DOC, the CDOM absorption coefficient (aCDOM) was correlated with in situ remote sensing reflectance band ratios, and DOC was then derived from aCDOM through the aCDOM to DOC relationships. Our validation analyses demonstrate successful retrieval of DOC and CDOM using MODIS and SeaWiFS with mean absolute percent differences from field measurements of 9.3 ± 7.3% for DOC, 19 ± 14% for aCDOM(355), 15.5 ± 12% for aCDOM(443), and 8.6 ± 4.9% for the CDOM spectral slope. To our knowledge, the algorithms presented here represent the first validated algorithms for satellite retrieval of aCDOM, DOC, and CDOM spectral slope in the coastal ocean. Satellite imagery demonstrates the importance of riverine/estuarine discharge from Chesapeake Bay and Delaware Bay to the export of CDOM and DOC to the coastal ocean. Between spring and summer, photooxidation has a significant impact on CDOM distributions resulting in a pronounced decrease in aCDOM between the midshelf and continental slope region of the MAB. The satellite‐derived DOC products demonstrate the net ecosystem production of DOC of 12 to 34 μmol C L−1 between spring and summer. The aCDOM algorithms presented here are applicable to other coastal regions and can also be used to retrieve DOC using region‐specific aCDOM to DOC relationships.

Production of Dissolved Organic Carbon in Canadian Forest Soils

To identify the controls on dissolved organic carbon (DOC) production, we incubated soils from 18 sites, a mixture of 52 forest floor and peats and 41 upper mineral soil samples, at three temperatures (3, 10, and 22°C) for over a year and measured DOC concentration in the leachate and carbon dioxide (CO2) production from the samples. Concentrations of DOC in the leachate were in the range encountered in field soils ( 50 mg l−1). There was a decline in DOC production during the incubation, with initial rates averaging 0.03–0.06 mg DOC g−1 soil C day−1, falling to averages of 0.01 mg g−1 soil C day−1; the rate of decline was not strongly related to temperature. Cumulative DOC production rates over the 395 days ranged from less than 0.01 to 0.12 mg g−1 soil C day−1 (0.5–47.6 mg g−1 soil C), with an average of 0.021 mg g−1 soil C day−1 (8.2 mg g−1 soil C). DOC production rate was weakly related to temperature, equivalent to Q10 values of 0.9 to 1.2 for mineral samples and 1.2 to 1.9 for organic samples. Rates of DOC production in the organic samples were correlated with cellulose (positively) and lignin (negatively) proportion in the organic matter, whereas in the mineral samples C and nitrogen (N) provided positive correlations. The partitioning of C released into CO2–C and DOC showed a quotient (CO2–C:DOC) that varied widely among the samples, from 1 to 146. The regression coefficient of CO2–C:DOC production (log10 transformed) ranged from 0.3 to 0.7, all significantly less than 1. At high rates of DOC production, a smaller proportion of CO2 is produced. The CO2–C:DOC quotient was dependent on incubation temperature: in the organic soil samples, the CO2–C:DOC quotient rose from an average of 6 at 3 to 16 at 22°C and in the mineral samples the rise was from 7 to 27. The CO2–C:DOC quotient was related to soil pH in the organic samples and C and N forms in the mineral samples.

Performance of a membrane biofilm reactor for denitrification with methane

In this study, a membrane biofilm reactor was investigated for aerobic methane oxidation coupled indirectly to denitrification, a process potentially useful for denitrification of nitrate-contaminated waters and wastewaters using methane as external electron donor. Methane and oxygen were supplied from the interior of a silicone tube to a biofilm growing on its surface. We found that the membrane biofilm reactor was to some extent self-regulating in the supply of methane and oxygen. Although the intramembrane partial pressures of methane and oxygen were varied, the oxygen-to-methane ratio penetrating the membrane tended towards 1.68. Both nitrate removal rate and dissolved organic carbon (DOC) production rate appeared to be positively correlated with intramembrane methane pressure. Based on measured nitrate removal rates, DOC production rates, and nitrate removal efficiency, the possibility of using this method for treatment of a hypothetical wastewater was evaluated.

Effect of water table drawdown on peatland dissolved organic carbon export and dynamics

AbstractPeatlands play an important role in the global carbon cycle, and loss of dissolved organic carbon (DOC) has been shown to be important for peatland carbon budgets. The objective of this study was to determine how net production and export of DOC from a northern peatland may be affected by disturbance such as drainage and climate change.The study was conducted at a poor fen containing several pool–ridge complexes: (1) control site–no water table manipulation; (2) experimental site–monitored for one season in a natural state and then subjected to a water table drawdown for 3 years; (3) drained site–subjected to a water table drawdown 9 years prior to monitoring. The DOC concentration was measured in pore water along a microtopographic gradient at each site (hummock, lawn and hollow), in standing water in pools, and in discharge from the experimental and drained sites. The initial water table drawdown released ∼3 g of carbon per square metre in the form of DOC, providing a large pulse of DOC to downstream ecosystems. This value, however, represents only 1–9% of ecosystem respiration at this site. Seasonal losses of DOC following drainage were 8–11 g of carbon per square metre, representing ∼17% of the total carbon exchange at the experimental study site. Immediately following water table drawdown, DOC concentrations were elevated in pore water and open water pools. In subsequent seasons, DOC concentration in the pool declined, but remained higher than the control site even 11 years after water‐table drawdown. This suggests continued elevated net DOC production under lower water table conditions likely related to an increase in vegetation biomass and larger water table fluctuations at the experimental and drained sites. However, the increase in concentration was limited to initially wet microforms (lawns and hollows) reflecting differences in vegetation community changes, water table and soil subsidence along the microtopographic gradient. Copyright © 2008 John Wiley & Sons, Ltd and Her Majesty the Queen in right of Canada.

Examination of the potential relationship between droughts, sulphate and dissolved organic carbon at a wetland‐draining stream

AbstractRising dissolved organic carbon (DOC) concentrations observed at a number of sites in the northern hemisphere over recent decades are the subject of much debate, and recent reports suggest a link between DOC patterns in surface waters and changes in sulphate (SO4) related to droughts or deposition. In order to investigate the potential influence of changes in SO4 concentration on DOC patterns in south‐central Ontario, we used long‐term (1980–2001) stream monitoring data from a wetland‐dominated catchment (Plastic Lake‐1 subcatchment, PC1) that has been the focus of intensive investigations of both SO4 and DOC dynamics. Annual average volume‐weighted DOC concentration increased significantly between 1980 and 2001, whereas SO4 concentration declined, but the decrease was not significant due to large increases in SO4 that occurred during drought years. There was no relationship between SO4 and DOC in annual data series; however, seasonal analyses indicated significant negative correlations between SO4 and DOC concentrations in spring (March–April–May), summer (June–July–August) and fall (September–October–November). In spring, DOC concentration was negatively correlated with flow whereas SO4 concentrations increased with flow, and their opposing relationships with discharge explain the negative correlation between SO4 and DOC in this season. In summer and fall, low SO4 concentrations occur during periods of low flow as a result of microbial SO4 reduction, whereas correspondingly high DOC concentrations in the summer and fall can be attributed to optimal conditions (i.e. stagnant flow, warm temperatures) for DOC production in the wetland. Increases in SO4 (and acidity) following droughts were not associated with declines in DOC; instead the primary impact of droughts on DOC was to limit DOC export due to diminished stream flow. Rather than an acidification effect, we suggest that negative relationships between SO4 and DOC were either directly (spring) or indirectly (summer/fall) caused by underlying relationships with hydrology.

Determining net dissolved organic carbon production in the hydrographically complex western Arctic Ocean

Each spring and summer, the Chukchi Sea in the western Arctic Ocean experiences a short but intense phytoplankton bloom that is controlled by sea ice cover, nutrient availability, and river runoff. During the bloom, dissolved inorganic carbon (DIC) concentrations are drawn down in the mixed layer (shelf, ~Δ100 µmol kg−1; shelf‐break, ~Δ50 µmol kg−1; basin, <Δ5 µmol kg−1) and converted to particulate organic carbon (POC) and dissolved organic carbon (DOC). We use δ18O and salinity to estimate DOC inputs by river and sea ice melt, allowing the marine portion of the DOC pool to be compared between seasons and to facilitate estimates of net DOC production in the region. We estimate that ~10% of the DIC consumed during net community production is converted to DOC. The remaining carbon deficit (90% of DIC drawdown) is most likely exported from the mixed layer as POC. In 2002 a carbon (C) mass balance was developed for the Chukchi Sea that showed that rates of vertical export were highest over the Chukchi Shelf near Point Barrow (64 mmol C m− d−1), but more commonly ranged from 10 to 35 mmol C m− d−1 over the remainder of the shelf. At the shelf‐break and in the Canada Basin export production was <10 mmol C m− d−1. DOC plays a relatively small role as a carbon sink in the system, but quantifying its contributions provides more robust estimates of export production in the Arctic.

Rapid turnover of DOC in temperate forests accounts for increased CO2 production at elevated temperatures

The evidence for the contribution of soil warming to changes in atmospheric CO(2) concentrations and carbon stocks of temperate forest ecosystems is equivocal. Here, we use data from a beech/oak forest on concentrations and stable isotope ratios of dissolved organic carbon (DOC), phosphate buffer-extractable organic carbon, soil organic carbon (SOC), respiration and microbial gross assimilation of N to show that respired soil carbon originated from DOC. However, the respiration was not dependent on the DOC concentration but exceeded the daily DOC pool three to four times, suggesting that DOC was turned over several times per day. A mass flow model helped to calculate that a maximum of 40% of the daily DOC production was derived from SOC and to demonstrate that degradation of SOC is limiting respiration of DOC. The carbon flow model on SOC, DOC, microbial C mobilization/immobilization and respiration is linked by temperature-dependent microbial and enzyme activity to global warming effects of CO(2) emitted to the atmosphere.

Denitrification from drinking water using a membrane bioreactor: Chemical and biochemical feasibility

Interest is growing in developing membrane bioreactors (MBRs) to replace ion exchange for nitrate removal from drinking water. However, few published studies have successfully managed to retain exogenous or biologically derived carbon. This study determined an optimum C:N by substrate breakthrough rather than maximum nitrate removal. By dosing <C:N 1.52, ethanol (electron donor) could be retained and nitrate removal up to 92% could be observed. Carbon limited control led to nitrite formation <0.35 mg NO 2 −-N, which was oxidised to <0.02 mg NO 2 −-N L −1 by chlorination. Residual organics showed little competition with nitrite during short-term chlorination; thus chlorine demand could be predicted by the nitrite residual (5.0±0.1 mg Cl 2 mg NO 2 −-N −1). Biomass was completely retained and low residual dissolved organic carbon (DOC) (0.4±0.7 mg L −1) indicated high rejection of influent DOC and soluble microbial products (SMPs). Residual DOC exhibited low reactivity with long-term chlorination (trihalomethane formation potential (THMFP) 72.7±7.7 μg L −1); however, the impact of substrate breakthrough on THMFP was significant. Immersed heterotrophic MBR produced consistent high quality product water at steady state, comparing favourably to previously trialled heterotrophic fixed film membrane and packed bed processes.

Elevated CO2 Effects on Peatland Plant Community Carbon Dynamics and DOC Production

Northern peatlands are important stores of carbon and reservoirs of biodiversity that are vulnerable to global change. However, the carbon dynamics of individual peatland plant species is poorly understood, despite the potential for rising atmospheric CO2 to affect the vegetation’s contribution to overall ecosystem carbon function. Here, we examined the effects of 3 years exposure to elevated CO2 (eCO2) on (a) peatland plant community composition and biomass, and (b) plant carbon dynamics and the production of dissolved organic carbon (DOC) using a 13CO2 pulse–chase approach. Results showed that under eCO2, Sphagnum spp. cover declined by 39% (P < 0.05) and Juncus effusus L. cover increased by 40% (P < 0.001). There was a concurrent increase in above- and belowground plant biomass of 115% (P < 0.01) and 96% (P < 0.01), respectively. Vascular species assimilated and turned over more 13CO2-derived carbon than Sphagnum spp. (49% greater turnover of assimilated 13C in J. effusus and F. ovina L. leaf tissues compared with Sphagnum, P < 0.01). Elevated CO2 also produced a 66% rise in DOC concentrations (P < 0.001) and an order of magnitude more ‘new’ exudate 13DOC than control samples (24 h after 13CO2 pulse-labelling 2.5 ± 0.5 and 0.2 ± 0.1% in eCO2 and control leachate, respectively, P < 0.05). We attribute the observed increase in DOC concentrations under eCO2 to the switch from predominantly Sphagnum spp. to vascular species (namely J. effusus), leading to enhanced exudation and decomposition (litter and peat). The potential for reduced peatland carbon accretion, increased DOC exports and positive feedback to climate change are discussed.

The influence of organic acids in relation to acid deposition in controlling the acidity of soil and stream waters on a seasonal basis

Much uncertainty still exists regarding the relative importance of organic acids in relation to acid deposition in controlling the acidity of soil and surface waters. This paper contributes to this debate by presenting analysis of seasonal variations in atmospheric deposition, soil solution and stream water chemistry for two UK headwater catchments with contrasting soils. Acid neutralising capacity (ANC), dissolved organic carbon (DOC) concentrations and the Na:Cl ratio of soil and stream waters displayed strong seasonal patterns with little seasonal variation observed in soil water pH. These patterns, plus the strong relationships between ANC, Cl and DOC, suggest that cation exchange and seasonal changes in the production of DOC and seasalt deposition are driving a shift in the proportion of acidity attributable to strong acid anions, from atmospheric deposition, during winter to predominantly organic acids in summer.

Release of dissolved organic matter during oxic and anoxic decomposition of salt marsh cordgrass

Chromophoric dissolved organic matter (CDOM), as the light absorbing fraction of bulk dissolved organic matter (DOM), plays a number of important roles in the global and local biogeochemical cycling of dissolved organic carbon (DOC) and in controlling the optical properties of estuarine and coastal waters. Intertidal areas such as salt marshes can contribute significant amounts of the CDOM that is exported to the ocean, but the processes controlling this CDOM source are not well understood. In this study, we investigate the production of DOM and CDOM from the decomposition of two salt marsh cordgrasses, Spartina patens, a C 4 grass, and Typha latifolia, a C 3 grass, in well-controlled laboratory experiments. During the seven-week incubation period of the salt marsh grasses in oxic and anoxic seawater, changes in dissolved organic carbon (DOC) concentrations, dissolved nitrogen (DN) concentrations, stable carbon isotopic composition of DOC (DOC- δ 13C), and CDOM fluorescence demonstrate a significant contribution of DOC and CDOM to estuarine waters from salt marsh plants, such as Spartina and Typha species. In the natural environment, however, the release processes of CDOM from different cordgrass species could be controlled largely by the in situ oxic and anoxic conditions present during degradation which affects both the production and decomposition of DOC and CDOM, as well as the optical properties of CDOM in estuarine and coastal waters.

Flux of dissolved organic carbon from U.K. rivers

The rise in dissolved organic carbon (DOC) concentrations in the Northern Hemisphere in recent decades has been taken as indicative of increased turnover of terrestrial carbon and in particular in the vital reserve of peat. Hypotheses for this rise have been hotly debated in the literature. This study proposes that, although trends in concentration have been the main focus of attention, it is trends in DOC flux that need to be examined to understand changes in terrestrial carbon storage. This study uses data from 208 rivers over the period 1975–2003 to calculate the trend of DOC flux from Great Britain. The DOC flux peaks at 1.68 Mt C/yr, or an export of 6.6 tonnes C/km2/yr. The trend in flux is inconsistent with the pattern of atmospheric deposition, but regression analysis shows that the major control is hydrological throughput followed by an underlying upward, linear temporal trend. There is a small but significant effect ascribable to drought and storage effects. This suggests that there is additional biogeochemical production as a result of drought but that the overwhelming control in DOC flux is controlled by hydrological throughput overprinting an underlying increase in trend in DOC production. The underlying trend is consistent with increases in air temperature and or atmospheric CO2.

Modeling the mechanisms that control in‐stream dissolved organic carbon dynamics in upland and forested catchments

We present a new, process‐based model of soil and stream water dissolved organic carbon (DOC): the Integrated Catchments Model for Carbon (INCA‐C). INCA‐C is the first model of DOC cycling to explicitly include effects of different land cover types, hydrological flow paths, in‐soil carbon biogeochemistry, and surface water processes on in‐stream DOC concentrations. It can be calibrated using only routinely available monitoring data. INCA‐C simulates daily DOC concentrations over a period of years to decades. Sources, sinks, and transformation of solid and dissolved organic carbon in peat and forest soils, wetlands, and streams as well as organic carbon mineralization in stream waters are modeled. INCA‐C is designed to be applied to natural and seminatural forested and peat‐dominated catchments in boreal and temperate regions. Simulations at two forested catchments showed that seasonal and interannual patterns of DOC concentration could be modeled using climate‐related parameters alone. A sensitivity analysis showed that model predictions were dependent on the mass of organic carbon in the soil and that in‐soil process rates were dependent on soil moisture status. Sensitive rate coefficients in the model included those for organic carbon sorption and desorption and DOC mineralization in the soil. The model was also sensitive to the amount of litter fall. Our results show the importance of climate variability in controlling surface water DOC concentrations and suggest the need for further research on the mechanisms controlling production and consumption of DOC in soils.

Production of dissolved organic carbon by sloppy feeding in the copepods Acartia tonsa, Centropages typicus, and Temora longicornis

With 14C‐labeled phytoplankton as tracer, dissolved organic carbon (DOC) produced by sloppy feeding was measured for three species of marine copepods (Acartia tonsa, Centropages typicus, and Temora longicornis) grazing on the differently sized phytoplankton (Rhodomonas salina, Heterocapsa rotundata, and Ditylum brightwelli). DOC production per prey cleared was not dependent on prey concentration, whereas the relative size of the prey was important. When the prey was more than 39 times smaller than the copepod (measured as equivalent spherical diameter) no DOC production by sloppy feeding was measured. When the prey was larger, the fraction of carbon cleared that was lost as DOC by sloppy feeding was a function of the copepod‐prey size ratio. The DOC production by the three species could be predicted from a common regression.

Distribution characteristics of dissolved organic carbon in annular wetland soil-water solutions through soil profiles in the Sanjiang Plain, Northeast China

Overwhelming evidence reveals that concentrations of dissolved organic carbon (DOC) have increased in streams which brings negative environmental impacts. DOC in stream flow is mainly originated from soil-water solutions of watershed. Wetlands prove to be the most sensitive areas as an important DOC reserve between terrestrial and fluvial biogeosystems. This reported study was focused on the distribution characteristics and the controlling factors of DOC in soil-water solutions of annular wetland, i.e., a dishing wetland and a forest wetland together, in the Sanjiang Plain, Northeast China. The results indicate that DOC concentrations in soil-water solutions decreased and then increased with increasing soil depth in the annular wetland. In the upper soil layers of 0–10 cm and 10–20 cm, DOC concentrations in soil-water solutions linearly increased from edge to center of the annular wetland (R2 = 0.3122 and R2 = 0.443). The distribution variations were intimately linked to DOC production and utilization and DOC transport processes in annular wetland soil-water solutions. The concentrations of total organic carbon (TOC), total carbon (TC) and Fe(II), DOC mobility and continuous vertical and lateral flow affected the distribution variations of DOC in soil-water solutions. The correlation coefficients between DOC concentrations and TOC, TC and Fe(II) were 0.974, 0.813 and 0.753 respectively. These distribution characteristics suggested a systematic response of the distribution variations of DOC in annular wetland soil-water solutions to the geometry of closed depressions on a scale of small catchments. However, the DOC in soil pore water of the annular wetland may be the potential source of DOC to stream flow on watershed scale. These observations also implied the fragmentation of wetland landscape could bring the spatial-temporal variations of DOC distribution and exports, which would bring negative environmental impacts in watersheds of the Sanjiang Plain.

Relative importance of inefficient feeding and consumer excretion to organic carbon flux from Daphnia

Summary1. Sloppy and inefficient feeding by zooplankton is generally thought to make a major contribution to the regeneration of the dissolved organic carbon (DOC) pool in aquatic environments. In this study, we tested experimentally the regeneration of DOC by a freshwater zooplankter feeding on two species of phytoplankton at different food concentrations and C : P ratios. We separated the DOC production because of inefficient feeding (pre‐ingestive regeneration) and zooplankton excretion and faeces release (postdigestive regeneration).2. Within a brief incubation period (10 min), DOC production in the presence of Daphnia was not significantly different from that in the control treatment without grazers. During a longer incubation period (4 h), the amounts of radiocarbon retained in the algal cells per se were constant or were not different from those in the control treatments. These experimental results strongly suggest that inefficient feeding did not contribute significantly to DOC production in the grazer–prey system.3. During the 4‐h incubation, calculations of the DOC per ingestion rate (i.e. DOC produced by Daphnia alone) showed that food concentration and algal species did not affect the relative DOC production, but there was considerable difference at different algal C : P ratios and grazer densities. We found that direct excretion of DOC by Daphnia occurred rapidly following food digestion and accounted for &gt;65% of the total DOC production. Maximum DOC leakage from Daphnia faeces contributed less to DOC production than the grazer excretion, except under P‐limited conditions.4. This study highlights the dominant role of postingestive process, especially the direct excretion by zooplankton, in DOC production in a grazer–prey system.

Response of Oxidative Enzyme Activities to Nitrogen Deposition Affects Soil Concentrations of Dissolved Organic Carbon

Recent evidence suggests that atmospheric nitrate (NO 3 − ) deposition can alter soil carbon (C) storage by directly affecting the activity of lignin-degrading soil fungi. In a laboratory experiment, we studied the direct influence of increasing soil NO 3 − concentration on microbial C cycling in three different ecosystems: black oak–white oak (BOWO), sugar maple–red oak (SMRO), and sugar maple–basswood (SMBW). These ecosystems span a broad range of litter biochemistry and recalcitrance; the BOWO ecosystem contains the highest litter lignin content, SMRO had intermediate lignin content, and SMBW leaf litter has the lowest lignin content. We hypothesized that increasing soil solution NO 3 − would reduce lignolytic activity in the BOWO ecosystem, due to a high abundance of white-rot fungi and lignin-rich leaf litter. Due to the low lignin content of litter in the SMBW, we further reasoned that the NO 3 − repression of lignolytic activity would be less dramatic due to a lower relative abundance of white-rot basidiomycetes; the response in the SMRO ecosystem should be intermediate. We increased soil solution NO 3 − concentrations in a 73-day laboratory incubation and measured microbial respiration and soil solution dissolved organic carbon (DOC) and phenolics concentrations. At the end of the incubation, we measured the activity of β-glucosidase, N-acetyl-glucosaminidase, phenol oxidase, and peroxidase, which are extracellular enzymes involved with cellulose and lignin degradation. We quantified the fungal biomass, and we also used fungal ribosomal intergenic spacer analysis (RISA) to gain insight into fungal community composition. In the BOWO ecosystem, increasing NO 3 − significantly decreased oxidative enzyme activities (−30% to −54%) and increased DOC (+32% upper limit) and phenolic (+77% upper limit) concentrations. In the SMRO ecosystem, we observed a significant decrease in phenol oxidase activity (−73% lower limit) and an increase in soluble phenolic concentrations (+57% upper limit) in response to increasing NO 3 − in soil solution, but there was no significant change in DOC concentration. In contrast to these patterns, increasing soil solution NO 3 − in the SMBW soil resulted in significantly greater phenol oxidase activity (+700% upper limit) and a trend toward lower DOC production (−52% lower limit). Nitrate concentration had no effect on microbial respiration or β-glucosidase or N-acetyl-glucosaminidase activities. Fungal abundance and basidiomycete diversity tended to be highest in the BOWO soil and lowest in the SMBW, but neither displayed a consistent response to NO 3 − additions. Taken together, our results demonstrate that oxidative enzyme production by microbial communities responds directly to NO 3 − deposition, controlling extracellular enzyme activity and DOC flux. The regulation of oxidative enzymes by different microbial communities in response to NO 3 − deposition highlights the fact that the composition and function of soil microbial communities directly control ecosystem-level responses to environmental change.

Connecting organic carbon in stream water and soils in a peatland catchment

We investigated the spatial and temporal connectivity between concentrations of organic carbon in stream water and the soil carbon pool in a small (1.3 km2) upland catchment in NE Scotland by comparing downstream changes in dissolved organic carbon (DOC) with spatial changes in the soil carbon pool. Stream water DOC concentrations increased downstream to reach a maximum in the center of the catchment and then decreased in the lower part of the catchment, a pattern that was linked to spatial changes in the soil carbon pool. Subcatchment analysis of the soil carbon pool and DOC export showed that the strong relationship between percent of peat coverage and stream water DOC in the upper part of the catchment did not hold farther downstream, where freely draining mineral soils become spatially more important. Temporal variations in stream water and soil water DOC concentrations in the central part of the catchment were studied using samples collected from two depths (0.20–0.45 m and 0.65–0.95 m) in the two most important soil types there (histosols and fluvisols). Stream water DOC concentrations were only related to soil water DOC in shallow peat. DOC concentration in shallow peat and stream water were both positively correlated with temperature. Multivariate regression analysis showed that 55% of the variation in stream water DOC concentrations could be explained by a model including stream temperature, discharge, and soil solution DOC concentrations in the shallow histosols. We conclude that the linkage between stream water DOC concentrations and the soil C pool in the upper 1.5 km of the stream reach are likely to be driven by temperature‐related DOC production in near‐surface histosols. Farther downstream, this relationship between organic carbon in the soil and stream becomes weaker as other processes (e.g., lower inputs from minerals soils and allochthonous within‐stream processing of DOC) become more important.

Drain blocking: An effective treatment for reducing dissolved organic carbon loss and water discolouration in a drained peatland

Peatlands are an important terrestrial carbon store. However, heightened levels of degradation in response to environmental change have resulted in an increased loss of dissolved organic carbon (DOC) and an associated rise in the level of discolouration in catchment waters. A significant threat to peatland sustainability has been the installation of artificial drainage ditches. However, recent restoration schemes have pursued drain blocking as a possible strategy for reducing degradation, fluvial carbon loss and water discolouration. This paper investigates the effect of open cut drainage and the impact of drain blocking on DOC and colour dynamics in blanket peat soil–water solutions. Three treatments (intact peat, drained peat and drain-blocked peat) were monitored in an upland blanket peat catchment in the UK. DOC and colour values were significantly higher on the drained slopes compared with those of the intact peat, which in turn had greater DOC and colour values than the drain-blocked slopes. Consequently, drain blocking is shown to be a highly successful technique in reducing both the DOC concentration and level of discolouration in soil waters, even to values lower than those observed for the intact site, which suggests a process of store exhaustion and flushing may operate. The colour per carbon unit (C/C) ratio was significantly higher at the drain-blocked site than either the intact or the drained treatments, while the E4/E6 ratio (fulvic acid/humic acid) was significantly lower at the blocked site compared to the two other treatments. The high C/C and low E4/E6 ratios indicate that drain blocking also modifies the composition of DOC, such that darker-coloured humic substances become more dominant compared to the intact site. This implies disturbance to DOC production and/or transportation processes operating within the peat.

Copper bioavailability in the rhizosphere of maize ( Zea mays L.) grown in two Italian soils

In this study, potentially bioavailable copper was estimated in two soils (a fungicide polluted and a natural soil) using a passive sampling technique, DGT. As plants can alter copper mobility and bioavailability in the soil, the rhizosphere properties of Zea mays L. were investigated using rhizoboxes. Compared to the total concentration, the soluble and the potentially bioavailable copper concentration in the bulk soils were generally low (less than 0.20% and 0.06% respectively), with a sixfold increase in the rhizosphere of the polluted soil. Our results suggest that maize cultivation in a polluted vineyard soil could increase the potentially available fraction of copper. DGTs showed a good sensitivity to soil properties and to root-induced changes in the rhizosphere, but the potentially bioavailable copper could not be related to the copper concentration in the above ground parts of maize. The results suggest that DGT may be used to predict some effects of the cultivation of polluted soils, for example, metal mobility and increased availability, but they cannot mimic the uptake of a tolerant plant. For both soils, dissolved organic carbon (DOC) concentrations were threefold higher in the rhizosphere than in the bulk soil, whilst bioaccumulation in leaves and roots was not significant. DOC production, usually effective in ion mobilization and assimilation, may help also in the reduction of Cu uptake at toxic concentrations. The sequestration of available Cu in soil and soil solution by DOC seems to contribute to maize tolerance.