Influence of hydrology and seasonality on DOC exports from three contrasting upland catchments

Variation in dissolved organic carbon (DOC) concentrations of surface waters is a consequence of process changes in the surrounding terrestrial environment, both within annual cycles and over the longer term. Long-term records (1987–2006) of DOC concentrations at six catchments (0.44–10.0 km²) across a climatic transect in Scotland were investigated for intra-annual relationships to evaluate potential long-term seasonal patterns. The intra-annual mode of DOC export contrasted markedly between catchments and appeared dependent on their hydrological characteristics. Catchments in wetter Central Scotland with high rainfall–runoff ratios, short transit times and well-connected responsive soils show a distinct annual periodicity in DOC concentrations throughout the long-term datasets. Increased DOC concentrations occurred between June and November with correspondingly lower DOC concentrations from December to May. This appears unrelated to discharge, and is dependent mainly on higher temperatures driving biological activity, increasing decomposition of available organic matter and solubility of DOC. The drier eastern catchments have lower rainfall–runoff ratios, longer transit times and annual drying–wetting regimes linked to changing connectivity of soils. These are characterised by seasonal DOC concentration–discharge relationships with an autumnal flush of DOC. Temperature influences the availability of organic matter for DOC transport producing a high DOC concentration–discharge relationship in summer/autumn and low DOC concentration–discharge relationship in winter/spring. These two distinct modes of seasonal DOC transport have important implications for understanding changes in DOC concentrations and export brought about by climate change (temperature and precipitation) and modelling of aquatic carbon losses from soil-types under different hydrological regimes.     

Changes in water colour between 1986 and 2006 in the headwaters of the River Nidd, Yorkshire, UK: a critique of methodological approaches and measurement of burning management

The effect of moorland management on the release of dissolved organic carbon (DOC) from blanket peat is currently a topic of great interest in the UK. A recent paper by Chapman et al. (Biogeochemistry, doi:10.1007/s10533-010-9474-x, 2010) reports on changes in humic colour/DOC concentrations in surface waters draining 15 upland peat catchments in the North Pennines (UK) over two decades, and examines the possible underlying drivers of those changes. Chapman et al. identify significant variation in water colour between adjacent catchments and over time. One potential driver of changes in DOC is managed moorland burning, and Chapman et al. state that their study provides evidence that burning has no effect on colour in upland catchment drainage waters. This observation counters a recent series of papers showing strong links between new moorland burn management on blanket peat soils and colour/DOC in catchment drainage waters. We have reviewed the methodological approach and results presented by Chapman et al. that relate to the assessment of managed burning, and show significant errors in the data used in their analysis. This has resulted in conclusions being drawn about the role of managed burning in DOC release that are not supported by evidence.     

Response of aquatic macrophytes in Northern Irish softwater lakes to forestry management; eutrophication and dissolved organic carbon

Softwater lakes provide a habitat for isoetid macrophytes, which are vulnerable to eutrophication and acidification. In Ireland many catchments of such lakes are currently planted with exotic conifers. Management of these plantations can lead to increases in lake water phosphorus (P), threatening the survival of softwater macrophytes. Regional increases of dissolved organic carbon (DOC) may also have a detrimental effect on aquatic plants. The persistence of the macrophyte flora in lakes with managed forested catchments in Northern Ireland was investigated by comparing the macrophyte community of 12 lakes surveyed in 2007 with a 1988-1990 survey. Contemporary data were compared with plant macrofossil records pre-dating 1900. Macrophyte abundance generally remained unchanged but Littorella uniflora (L.) Asch. and Fontinalis antipyretica Hedw. showed a significant decline since 1988-1990. Water colour, alkalinity, silica, total P, total soluble P and soluble reactive P increased; conductivity and chlorophyll a decreased in the lakes over time. These changes coincided with increased pH in precipitation and potentially elevated exports of DOC to water. Conifer plantation management appeared to have less impact on the macrophyte flora than expected from the elevated lake P concentrations. It appears that a large regional increase in DOC is also a threat to macrophyte abundance and diversity in these upland catchments and conservation efforts may be more successful in lakes with longer water residence times.     

Trends in Dissolved Organic Carbon in UK Rivers and Lakes

Several studies have highlighted an increase in DOC concentration in streams and lakes of UK upland catchments though the causal mechanisms controlling the increase have yet to be fully explained. This study, compiles a comprehensive data set of DOC concentration records for UK catchments to evaluate trends and test whether observed increases are ubiquitous over time and space. The study analysed monthly DOC time series from 198 sites, including 29 lakes, 8 water supply reservoirs and 161 rivers. The records vary in length from 8 to 42 years going back as far as 1961. Of the 198 sites, 153 (77%) show an upward trend in DOC concentration significant at the 95% level, the remaining 45 (23%) show no significant trend and no sites show a significant decrease in DOC concentration. The average annual increase in DOC concentration was 0.17 mg C/l/year. The dataset shows: (i) a spatial consistent upward trend in the DOC concentration independent of regional effects of rainfall, acid and nitrogen deposition, and local effects of land-use change; (ii) a temporally consistent increase in DOC concentration for period back as far as the 1960s; (iii) the increase in DOC concentration means an estimated DOC flux from the UK as 0.86 Mt C for the year 2002 and is increasing at 0.02 Mt C/year. Possible reasons for the increasing DOC concentration are discussed.     

The influence of catchment characteristics on the seasonality of carbon and nitrogen species concentrations in upland rivers of Northern Scotland

Data from 13 catchments with no arable land in Northern Scotland were used to develop empirical linear regression models of average monthly NO₃ ^? concentrations and average summer and winter concentrations for NH₄ ⁺, dissolved organic N (DON) and dissolved organic carbon (DOC) as a function of catchment characteristics. All catchments displayed a pronounced seasonal NO₃ ^? cycle. Variation in monthly mean NO₃ ^? concentration within and between catchments could be predicted from mean monthly air temperature using separate regression equations for temperatures < and ≥ 5 °C. Soil type, climate and land use influenced NH₄ ⁺ concentrations. In summer, concentrations of NH₄ ⁺ were largest in catchments with extensive areas of brown forest soils, which are less acidic and more base-rich than other upland soils. However, concentrations declined with increasing conifer cover and summer rainfall. In winter, however, % conifer cover had a positive effect, while higher temperature and higher humus iron podzol cover had negative influences. DON concentration decreased with increasing catchment elevation in both summer and winter. Surprisingly, concentrations of DON only displayed a positive relationship with percentage peat cover in the summer. The most important factor controlling DOC concentration was soil type, with a positive relationship being observed between DOC and peat and humus iron podzol coverage. Elevation was also important, but only in the winter when concentrations were negatively correlated with maximum catchment elevation. Overall, multivariate regression equations explained the spatial and seasonal variability in N species concentrations over a range of catchments within Northern Scotland.     

Ecosystem and Seasonal Control of Stream Dissolved Organic Carbon Along a Gradient of Land Use

We examined the influence of watershed land use and morphology on dissolved organic carbon (DOC) concentration in 32 south-central Ontario streams having varying agricultural land-use intensities in their catchments. For streams in this region, both univariate and multivariate regression models identify the proportion of the watershed with poorly drained soils (r ² up to 0.67) as a better predictor of stream DOC concentrations than any other landscape characteristic, including the proportion of the watershed as wetland. Agricultural land use did not strongly influence DOC concentrations in our study area; however, we do show that land-use changes could significantly alter the delivery of DOC to streams in the region. We also identify how landscape–DOC relationships change over a 2-year time period, as related to season, regional climatic conditions, soil moisture, and hydrology. Our results indicate that the relationships between landscape predictors and stream DOC concentrations are temporally dynamic. Strong temporal trends are shown seasonally and in association with climate, through its control of modelled soil moisture conditions. During periods of positive and negative deviation from normal soil moisture conditions, the relationships of DOC concentrations with landscape characteristics become less predictable. We show that these dominant patterns are likely a function of varying flow paths and that anthropogenic changes that affect soil moisture conditions or flow path will in turn strongly influence DOC dynamics. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

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.     

Controls on the release of dissolved organic carbon and nitrogen from a deciduous forest floor investigated by manipulations of aboveground litter inputs and water flux

Although dissolved organic matter (DOM) released from the forest floor plays a crucial role in transporting carbon and major nutrients through the soil profile, its formation and responses to changing litter inputs are only partially understood. To gain insights into the controlling mechanisms of DOM release from the forest floor, we investigated responses of the concentrations and fluxes of dissolved organic carbon (DOC) and nitrogen (DON) in forest floor leachates to manipulations of throughfall (TF) flow and aboveground litter inputs (litter removal, litter addition, and glucose addition) at a hardwood stand in Bavaria, Germany. Over the two-year study period, litter manipulations resulted in significant changes in C and N stocks of the uppermost organic horizon (Oi). DOC and DON losses via forest floor leaching represented 8 and 11% of annual litterfall C and N inputs at the control, respectively. The exclusion of aboveground litter inputs caused a slight decrease in DOC release from the Oi horizon but no change in the overall leaching losses of DOC and DON in forest floor leachates. In contrast, the addition of litter or glucose increased the release of DOC and DON either from the Oi or from the lower horizons (Oe + Oa). Net releases of DOC from the Oe + Oa horizons over the entire manipulation period were not related to changes in microbial activity (measured as rates of basal and substrate-induced respiration) but to the original forest floor depths prior to manipulation, pointing to the flux control by the size of source pools rather than a straightforward relationship between microbial activity and DOM production. In response to doubled TF fluxes, net increases in DOM fluxes occurred in the lower forest floor, indicating the presence of substantial pools of potentially soluble organic matter in the Oe + Oa horizons. In contrast to the general assumption of DOM as a leaching product from recent litter, our results suggest that DOM in forest floor leachates is derived from both newly added litter and older organic horizons through complex interactions between microbial production and consumption and hydrologic transport.     

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.     

Concentration and composition of dissolved organic carbon in streams in relation to catchment soil properties

Two adjacent catchments in the Otway Ranges of Victoria, Australia (Redwater and Clearwater) produce water with markedly different concentrations of dissolved organic carbon (DOC) during summer. Water from Redwater Creek had a DOC concentration of 32 mg L^–1, while water from Clearwater Creek had a DOC concentration of 3.8 mg L^–1. Examination of the catchments revealed that while climate, topography, vegetation and land use were similar, the soils were different. The objective of this study was to examine the relationship between the concentration and chemical composition of DOC in stream waters and the nature of soils in the two catchments. Soil mapping determined that clayey soils formed on Cretaceous sediments (Cretaceous soils) occurred throughout both catchments, but that Redwater Catchment also contained a large area (39%) of sandy soils formed on Tertiary sediments (Tertiary soils). The concentration of DOC in forest floor leachate was high in both the Tertiary and Cretaceous areas; however, the concentration of DOC in water draining areas dominated by Tertiary soils was greater than that in water draining areas dominated by Cretaceous soils. Laboratory experiments showed that the Cretaceous soils had higher adsorption capacities for forest floor leachate DOC than the Tertiary soils. The difference in DOC concentrations of the streams was therefore attributed to the difference in adsorption capacity of catchment soils for DOC. Adsorption capacities of the soils were found to be a function of their clay contents and specific surface areas.Solid-state³C nuclear magnetic resonance spectroscopy and pyrolysis-mass spectrometry were used to determine the chemical structure of DOC found in streams and forest floor leachate samples and that remaining in solution after interaction with soil. Chemistry of DOC in forest floor leachate was similar before and after interaction with soil, indicating no preferential adsorption of a particular type of carbon. Thus, differences between the chemical structure of stream DOC and forest floor leachate DOC could be attributed to microbial modifications during its movement through soils and into the streams, rather than losses by adsorption.     

Heterotrophic bacterial growth efficiency and community structure at different natural organic carbon concentrations

Batch cultures of aquatic bacteria and dissolved organic matter were used to examine the impact of carbon source concentration on bacterial growth, biomass, growth efficiency, and community composition. An aged concentrate of dissolved organic matter from a humic lake was diluted with organic compound-free artificial lake water to obtain concentrations of dissolved organic carbon (DOC) ranging from 0.04 to 2.53 mM. The bacterial biomass produced in the cultures increased linearly with the DOC concentration, indicating that bacterial biomass production was limited by the supply of carbon. The bacterial growth rate in the exponential growth phase exhibited a hyperbolic response to the DOC concentration, suggesting that the maximum growth rate was constrained by the substrate concentration at low DOC concentrations. Likewise, the bacterial growth efficiency calculated from the production of biomass and CO(2) increased asymptotically from 0.4 to 10.4% with increasing DOC concentration. The compositions of the microbial communities that emerged in the cultures were assessed by separation of PCR-amplified 16S rRNA fragments by denaturing gradient gel electrophoresis. Nonmetric multidimensional scaling of the gel profiles showed that there was a gradual change in the community composition along the DOC gradient; members of the beta subclass of the class Proteobacteria and members of the Cytophaga-Flavobacterium group were well represented at all concentrations, whereas members of the alpha subclass of the Proteobacteria were found exclusively at the lowest carbon concentration. The shift in community composition along the DOC gradient was similar to the patterns of growth efficiency and growth rate. The results suggest that the bacterial growth efficiencies, the rates of bacterial growth, and the compositions of bacterial communities are not constrained by substrate concentrations in most natural waters, with the possible exception of the most oligotrophic environments.     

Constant release of photosynthate from marine phytoplankton.

The release rate of dissolved organic carbon (DOC) by unialgal cultures and natural phytoplankton assemblages was constant over a wide range of dissolved inorganic carbon concentrations. DOC release was not proportional to the particulate organic carbon production rate. We postulate that intracellular DOC, fated for release, exists either as a separate pool from that leading to particulate organic carbon production or that there is active metabolic control on one portion of a common pool.     

Biogeochemical and hydrological controls on carbon export from a forested catchment in central Japan

Here we review research on the links between hydrological processes and the biogeochemical environment controlling the dynamics of dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) in temperate forested catchments. In addition, we present the results of original experiments. The spatial and temporal changes in DIC and DOC concentrations were investigated in tandem with observations of elementary belowground hydrological processes for a forested headwater catchment in central Japan. The soil CO₂ gas concentration, which is the source of DIC, increased with depth. The hydrological characteristics of groundwater also affected the spatial variation of partial pressure of dissolved CO₂ (pCO₂) in groundwater. The temporal variations in the soil CO₂ gas concentration and the pCO₂ values of groundwater suggested that the dynamics of DIC were strongly affected by biological activity. However, the geographical differences in DIC leaching were affected not only by the link between climatological conditions and biological activity, but also by other factors such as geomorphologic conditions. The DOC concentrations decreased with selective removal of hydrophobic acid during vertical infiltration. The major DOC-removal mechanisms were retention of metal-organic complexes to soil solids in the upper mineral soil layer and decomposition of DOC in the lower mineral soil layer. The responses of the DIC and DOC concentrations to changes in discharge during storm events were explained by the spatial variation in the DIC and DOC concentrations. Seasonal variation, which represents a long-term change, in stream water DOC concentrations was affected not only by the temporal variation in DOC concentrations in the topsoil, which may be affected by biological activity, but also by water movement, which transports DOC from the topsoil to stream water. These results indicate that both a biogeochemical approach and a method for evaluating the hydrological effects on carbon dynamics are critical for clarifying the carbon accumulation-and-release processes in forested ecosystems.     

Experimental whole‐lake increase of dissolved organic carbon concentration produces unexpected increase in crustacean zooplankton density

The observed pattern of lake browning, or increased terrestrial dissolved organic carbon (DOC) concentration, across the northern hemisphere has amplified the importance of understanding how consumer productivity varies with DOC concentration. Results from comparative studies suggest these increased DOC concentrations may reduce crustacean zooplankton productivity due to reductions in resource quality and volume of suitable habitat. Although these spatial comparisons provide an expectation for the response of zooplankton productivity as DOC concentration increases, we still have an incomplete understanding of how zooplankton respond to temporal increases in DOC concentration within a single system. As such, we used a whole‐lake manipulation, in which DOC concentration was increased from 8 to 11 mg L^?1 in one basin of a manipulated lake, to test the hypothesis that crustacean zooplankton production should subsequently decrease. In contrast to the spatially derived expectation of sharp DOC‐mediated decline, we observed a small increase in zooplankton densities in response to our experimental increase in DOC concentration of the treatment basin. This was due to significant increases in gross primary production and resource quality (lower seston carbon‐to‐phosphorus ratio; C:P). These results demonstrate that temporal changes in lake characteristics due to increased DOC may impact zooplankton in ways that differ from those observed in spatial surveys. We also identified significant interannual variability across our study region, which highlights potential difficulty in detecting temporal responses of organism abundances to gradual environmental change (e.g., browning).     

Bioavailability and composition of dissolved organic carbon and nitrogen in a near coastal catchment of south-western Australia

The bioavailability and composition of dissolved organic carbon (DOC) and nitrogen (DON) were examined in 10 major sub-catchments of the Swan-Canning estuary, which bisects the city of Perth, in south-western Australia. Catchments contain a mix of forest, agriculture, and urban-dominated land-use, with the degree of development increasing near the city center. We incubated water samples from the 10 sub-catchments for 14 days at 25°C, and measured changes in DOC and DON and dissolved inorganic nitrogen (DIN). A greater proportion of DON (4–44%) was decomposed compared to DOC (1–17%). Both agricultural and urban catchments had high proportions of bioavailable DOC and DON, but overall DOC and DON losses were greatest in urban catchments. Using resin isolation techniques, we found that DOC was concentrated in the hydrophobic (humic) fraction, whereas DON had both hydrophobic and hydrophilic (non-humic) fractions. Hydrophobic DOC content was positively related to DOC decomposition. In contrast, DON decomposition was highly correlated with hydrophilic DON content and inversely related to the hydrophilic DOC/DON ratio, indicating a labile fraction of DON from non-humic sources. Taken together, these relationships suggest that bioavailable DOC may be supplied in part from terrestrial plant material, but bioavailable DON is likely to be from highly labile sources, possibly autochthonous or anthropogenic. Overall, labile DON was greater than initial DIN concentration at seven of ten sites and was even dominant in highly developed catchments. This study highlights the importance of organic N in urbanizing coastal catchments that, in addition to DIN, may serve as a readily available source of N for in-stream and estuarine production.     

Hydrological and biological processes modulate carbon,nitrogen and phosphorus flux from the St. Lawrence River to its estuary (Quebec,Canada)

Changes in atmospheric deposition, stream water chemistry, and solute fluxes were assessed across 15 small forested catchments. Dramatic changes in atmospheric deposition have occurred over the last three decades, including a 70% reduction in sulphur (S) deposition. These changes in atmospheric inputs have been associated with expected changes in levels of acidity, sulphate and base cations in streams. Soil retention of S appeared to partially explain rates of chemical recovery. In addition to these changes in acid–base chemistry we also observed unexpected changes in nitrogen (N) biogeochemistry and nutrient stoichiometry of stream water, including decreased stream N concentrations. Among all catchments the average flux of dissolved inorganic nitrogen (DIN) was best predicted by average runoff, soil chemistry (forest floor C/N) and levels of acid deposition (both S and N). The rate of change in stream DIN flux, however, was much more closely correlated with reductions in rates of S deposition rather than those of DIN. Unlike DIN fluxes, the average concentrations as well as the rates of decline in streamwater nitrate (NO₃) concentration over time were tightly linked to stream dissolved organic carbon/dissolved organic nitrogen ratios DOC/DON and DON/TP rather than catchment characteristics. Declines in phosphorus adsorption with increasing soil pH appear to contribute to the relationship between C, N, and P in our study catchments. Our observations suggest that catchment P availability and its alteration due to environmental changes (e.g. acidification) might have profound effects on N cycling and catchment N retention that have been largely unrecognized.     

Hydrological controls on dissolved organic carbon during snowmelt in the Snake River near Montezuma,Colorado

A quantitative understanding of the factors controlling the variation of dissolved organic carbon (DOC) in headwater streams is of scientific concern for at least two reasons. First, quantifying the overall carbon budgets of lotic systems is needed for a fundamental understanding of these systems. Second, DOC interacts strongly with other dissolved substances (heavy metals in particular) and plays an important role in the transport of contaminants.In the Snake River near Montezuma, Colorado, measurements of DOC from 1980 to 1986 show rapid decreases in concentration from a peak very early in the snowmelt period. Peak DOC concentrations occur approximately one month prior to peak discharge in the stream. The decline in DOC with time is approximately exponential, suggesting that a simple flushing mechanism can explain the response. We examined hydrological mechanisms to explain the observed variability of DOC in the Snake River by simulating the hydrological response of the catchment using TOPMODEL and routing the predicted flows through a simple model that accounted for temporal changes in DOC. Conceptually the DOC model represents a terrestrial (soil) reservoir in which DOC builds up during low flow periods and is flushed out by infiltrating meltwaters. The model reproduces the main features of the observed variation in DOC in the Snake River and thus lays the foundation for quantitatively linking hydrological processes with carbon cycling through upland catchments. Model results imply that a significant fraction of the soils in the Snake River catchment contribute DOC to the stream during peak discharge. Our work represents one of the first attempts to quantitatively describe the hydrological controls on DOC dynamics in a headwater stream. These controls are studied through the model by imposing mass balance constraints on both the flux of water through the various DOC source areas and the amount of DOC that can accumulate in these areas.     

Influences of dissolved organic carbon on stream water chemistry in two forested catchments in central Sweden

Stream water chemistry in two headwater streams draining two small, adjacent catchments in Bispgården, central Sweden was studied during the 2003 and 2004 growing seasons. The two catchments closely resemble each other in regard to size, shape and drainage density, with the major difference found in the area of wetland lining the streambeds. The emphasis of the study was to investigate the stream water chemistry of these closely resembling catchments, regarding the quality and quantity of dissolved organic carbon (DOC) and its influence on the concentrations of di- and trivalent cations. The streams showed significant differences in the content and size distribution of DOC and in the distribution of cations between the different size fractions. For both streams the high flow events induced by precipitation influenced the chemistry of the streams through increase of organic matter and its associated cations. Fanbergsbäcken, with relatively low pH and high DOC concentration, had a greater amount of high molecular mass (HMM) DOC to which approximately 75% of Al and Fe and about 50% of Ca and Mg were associated. Gråbergsbäcken, with a higher pH and lower DOC level, had approximately 65% of Al, 40% of Fe and 30% of Ca and Mg associated to its HMM DOC fraction. Sixteen different low molecular mass organic acids were found in the stream water, of which oxalic and lactic acid were present in the highest concentrations.     

Site‐to‐site variability and temporal trends of DOC concentrations and fluxes in temperate forest soils

Here, we report site‐to‐site variability and 12–14 year trends of dissolved organic carbon (DOC) from organic layers and mineral soils of 22 forests in Bavaria, Germany. DOC concentrations in the organic layer were negatively correlated with mean annual precipitation and elevation whereas air temperature had a positive effect on DOC concentrations. DOC fluxes in subsoils increased by 3 kg ha^?1 yr^?1 per 100 mm precipitation or per 100 m elevation. The highest DOC concentrations were found under pine stands with mor humus. Average DOC concentrations in organic layer leachates followed the order: pine>oak>spruce>beech. However, the order was different for mean DOC fluxes (spruce>pine>oak>beech) because of varying precipitation regimes among the forest types. In 12 of 22 sites, DOC concentrations of organic layer leachates significantly increased by 0.5 to 3.1 mg C L^?1 yr^?1 during the sampling period. The increase in DOC concentration coincided with decreasing sulfate concentration, indicating that sulfate concentration is an important driver of DOC solubility in the organic layer of these forest sites. In contrast to the organic layer, DOC concentrations below 60 cm mineral soil depth decreased by <0.1–0.4 mg C L^?1 yr^?1 at eight sites. The negative DOC trends were attributed to (i) increasing adsorption of DOC by mineral surfaces resulting from desorption of sulfate and (ii) increasing decay of DOC resulting from decreasing stabilization of DOC by organo‐Al complexes. Trends of DOC fluxes from organic layers were consistent with those of DOC concentrations although trends were only significant at seven sites. DOC fluxes in the subsoil were with few exceptions small and trends were generally not significant. Our results suggest that enhanced mobilization of DOC in forest floors contributed to the increase of DOC in surface waters while mineral horizons did not contribute to increasing DOC export of forest soils.     

Fluorescence index as an indicator of dissolved organic carbon quality in hydrologic flowpaths of forested tropical watersheds

Over two hundred samples were collected in tropical headwater forested catchments in the lowland Amazon basin near Juruena, Mato Grosso Brazil. These were analyzed for fluorescence characteristics and DOC concentrations, and represented a range of terrestrial hydrologic flowpaths and first-order streams during baseflow and stormflow conditions. The fluorescence index (FI) of McKnight et al. (2001) was found to have a significant relationship with DOC concentrations for stream water at baseflow conditions, but FI values within individual terrestrial flowpaths and stormflow varied little for the range of DOC concentrations observed. FI values were seen to increase for increasing residence time of water within the terrestrial ecosystem, while DOC concentration decreased for increasing hydrologic residence time. The FI of terrestrial flow paths indicated that DOC became increasingly characterized by microbially derived carbon for flow paths with longer residence times, on the order through fall and overland flow < percolating soil water < groundwater. Base flow samples of stream water had a mean FI value of 1.78, compared with 1.51 and 1.44 for through fall and overland flow, respectively, and 1.65 for percolating soil water. The FI values for stream water at base flow were also seen to vary seasonally, and were inversely proportional to DOC concentrations over time.