Average Dissolved Organic Carbon Concentrations Research Articles

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.

Effect of moisture and temperature variation on DOC release from a peatland: Conflicting results from laboratory, field and historical data analysis

Peatlands are large repositories of atmospheric carbon and concern has been raised over the stability of this carbon store because increasing dissolved organic carbon (DOC) concentrations have been observed in peatland drainage waters. A number of potential causes have been proposed in the literature, and conflicting results among studies conducted at different spatial and temporal scales suggest that the methodological approach may be an important confounding factor. The objective of this study was to determine the influence of moisture and temperature on DOC release from a south-central Ontario peatland during the fall (a major export period) following three commonly used approaches: laboratory microcosms, an intensive field study and analysis of long-term data (1980–2008). The effect of variations in temperature and moisture differed among microcosm, field study and analysis of the long-term record. Water content was important at the microcosm scale as DOC concentration and aromaticity increased with peat water-saturation. Drought caused a decrease in DOC concentration and pH, and an increase in sulphate and base cation concentrations. In contrast, the field study indicated that DOC concentration was strongly associated with temperature, and weakly correlated (negatively) with stream discharge. Average fall DOC concentration (but not export) increased over the 29 year record, and was correlated with fall discharge and precipitation (negative) and summer precipitation and fall stream pH (positive). As no common strong predictor of fall DOC was found at three scales of investigation at a single, well-studied site, it may be unreasonable to expect to identify a universal driver behind the widespread increase in DOC concentration.

Effects of grazing and experimental warming on DOC concentrations in the soil solution on the Qinghai-Tibet plateau

Little information is available about the effects of global warming and land management on dissolved organic carbon (DOC) concentration in soil solution in the field. Here, for the first time, we used a free-air temperature enhancement (FATE) system in a controlled warming-grazing experiment in 2006 and 2007 to test the hypothesis that grazing modifies the response of soil solution DOC concentration to experimental warming. Warming with no-grazing (WNG) significantly increased the average soil solution DOC concentration to 40 cm soil depth by 14.1 and 17.2% compared with no-warming with no-grazing (NWNG) in 2006 and 2007 respectively based on 1.3–1.4 °C soil temperature increase. However, the lack of significant differences among warming with grazing (WG), no-warming with grazing (NWG) and NWNG indicate that moderate grazing modified the effect of warming on DOC concentration in the soil solution. The effect of grazing on DOC concentration in the soil solution varied with sampling date and soil depth. Generally, the direct contribution of soil temperature and soil moisture to variation of DOC concentration in the soil solution was small. Positive correlations were observed between soil solution DOC concentration in the surface soil and standing death quality and belowground biomass. The Lignin:N ratio in the standing death and belowground biomass at 10 cm soil depth explained 60% of the variation of mean DOC concentration at 10 cm soil depth. Soil moisture and belowground biomass explained 79% of the variation of the mean soil solution DOC concentration to 40 cm soil depth in 2007.

Disinfection Byproduct Formation Potentials of Wetlands, Agricultural Drains, and Rivers and the Effect of Biodegradation on Trihalomethane Precursors

Trihalomethane (THM) precursors are a significant problem in the San Joaquin River (SJR) watershed, an important source of drinking water for >20 million people. Trihalomethane precursors diminish drinking water quality and are formed during natural decomposition of organic matter in aquatic systems. This study sought to identify sources of chlorine-reactive dissolved organic carbon (DOC) in the SJR watershed and to determine if wetlands were more important sources of THM precursors than nearby rivers and agricultural drains. The effects of biodegradation on DOC quality and quantity were investigated and analyzed across drainage type. Results show wetland drainage contained comparable bromide concentrations and organic carbon aromaticity but contained more than two times the average DOC concentrations found in agricultural drains and rivers. Wetland DOC did not have an increased propensity to form THMs when compared with the other drainage types, despite significantly higher wetland formation potentials. The higher formation potentials measured in wetland drainages were attributed to higher DOC concentrations; the positive correlation found between DOC and trihalomethane formation potentials (THMFPs) and the significantly higher wetland DOC concentrations suggest that increased wetland restoration could result in increased THMFPs in the SJR watershed. Wetland THM precursors were more resistant to biodegradation than THM precursors from agricultural and river samples. Results suggest that THM precursors in the SJR were from algae and were biodegraded.

Dissolved Organic Carbon Concentrations in Throughfall and Soil Waters at Level II Monitoring Plots in Norway: Short- and Long-Term Variations

We investigated concentrations of dissolved organic carbon (DOC) in throughfall and soil solutions at 5, 15 and 40-cm depth in 16 Norway spruce and two Scots pine plots throughout Norway between 1996 and 2006. Average DOC concentrations ranged from 2.3 to 23.1 mg/l and from 1.1 to 53.5 mg/l in throughfall water and soil solutions, respectively. Concentrations of DOC in throughfall and soil waters varied seasonally at most plots with peaks in the growing season. By contrast to recently reported positive long-term trends in DOC concentrations in surface waters between 1986 and 2003, soil water data from 1996 to 2006 showed largely negative trends in DOC concentrations and no significant trends in throughfall. However, regression analysis for individual sites, particularly at 5- and 15-cm soil depths, showed that DOC concentrations in soil water were significantly and negatively related to non-marine sulphate (SO4) and chloride (Cl−). The lack of a long-term increase in DOC in soil water in the period May 1996–December 2006 may be due to the relatively small changes in the deposition of SO4 and Cl− in this period.

The role of hydrologic regimes on dissolved organic carbon composition in an agricultural watershed

Willow Slough, a seasonally irrigated agricultural watershed in the Sacramento River valley, California, was sampled weekly in 2006 in order to investigate seasonal concentrations and compositions of dissolved organic carbon (DOC). Average DOC concentrations nearly doubled from winter baseflow (2.75 mg L −1) to summer irrigation (5.14 mg L −1), while a concomitant increase in carbon-normalized vanillyl phenols (0.11 mg 100 mg OC −1 increasing to 0.31 mg 100 mg OC −1, on average) indicates that this additional carbon is likely vascular plant-derived. A strong linear relationship between lignin concentration and total suspended sediments ( r 2 = 0.79) demonstrates that agricultural management practices that mobilize sediments will likely have a direct and significant impact on DOC composition. The original source of vascular plant-derived DOC to Willow Slough appears to be the same throughout the year as evidenced by similar syringyl to vanillyl and cinnamyl to vanillyl ratios. However, differing diagenetic pathways during winter baseflow as compared to the rest of the year are evident in acid to aldehyde ratios of both vanillyl and syringyl phenols. The chromophoric dissolved organic matter (CDOM) absorption coefficient at 350 nm showed a strong correlation with lignin concentration ( r 2 = 0.83). Other CDOM measurements related to aromaticity and molecular weight also showed correlations with carbon-normalized yields (e.g. specific UV absorbance at 254 nm ( r 2 = 0.57) and spectral slope ( r 2 = 0.54)). Our overall findings suggest that irrigated agricultural watersheds like Willow Slough can potentially have a significant impact on mainstem DOC concentration and composition when scaled to the entire watershed of the main tributary.

Influence of seasonal changes in runoff and extreme events on dissolved organic carbon trends in wetland- and upland-draining streams

Dissolved organic carbon (DOC) fluxes at eight headwater basins in south-central Ontario were strongly related to seasonal streamflow, and extreme events contributed to both interannual and intercatchment variability. Six catchments with high stream DOC and greater peatland coverage exhibited a different seasonal pattern of DOC concentration compared with two catchments with low DOC and less wetland influence. In wetland-dominated catchments, DOC concentrations decreased during fall wet-up and spring melt, and because of the dominance of the spring melt period in annual budgets, variations in spring flow explained 39%–48% of the intervariability in DOC concentration. Significant increases in average DOC concentration between 1980 and 2001 at all six wetland-dominated catchments were driven by relatively high DOC concentrations in the latter years of record, consistent with low spring flow in these years, and were not translated into greater DOC export to downstream lakes. Localized rainstorms in summer and fall resulted in differences in DOC export among adjacent catchments, and a single fall storm in September 1998 was only detected at one of six catchments draining into Harp Lake but accounted for one-quarter of the annual tributary DOC load to the lake.

Release of nitrogen and trace metal species from field stacked biosolids

Concerns over elevated nitrate (NO3-) levels found in groundwater near former biosolid stockpiling locations resulted in the Maine Department of Environmental Protection (MDEP) imposing stricter regulations governing the stockpiling of biosolids in October 2002. The goals of this study were to measure the amount and speciation of nitrogen (N) and trace metals leaving stockpiled biosolids and travelling through the soil column. The biosolids were placed on plastic-lined cells to collect all leachate. Ammonium (NH4+), ranging from 2000 to 4900 mg L(-1), was the dominant N species (90% of total N) in the leachate from the Class B lime-stabilized biosolids in the lined cell experiment. Nitrate (NO3-) and nitrite (NO2-) concentrations were negligible, remaining below 0.25 and 0.1 mg L(-1), respectively. Dissolved organic carbon (DOC) concentrations as high as 8900 mg L(-1) and chemical oxygen demand (COD) as high as 37 000 mg L(-1) were measured in the leachate leaving the lined cell. Fifteen zero-tension pan lysimeters (ZTP-lysimeter) were installed in a 90 m2 plot at depth intervals of 30, 60, and 100 cm. Leachate passing through the soil column underlying the biosolids stockpile was collected in the ZTP-lysimeters. The average ZTP-lysimeter NH4+ concentrations ranged from 1400 mg L(-1) at 60 cm depth to 145 mg L(-1) at 90 cm depth. The average ZTP-lysimeter DOC concentrations ranged from 2000 mg L(-1) at 60 cm to 525 mg L(-1) at 90 cm. Trace metal determinations of the leachate collected from the lined cell and ZTP-lysimeters showed arsenic loading rates exceeded the state limits of 0.5 kg ha(-1) year(-1) by an order of magnitude. Arsenic concentrations were in excess of several thousand milligrams per litre in the lined-cell leachate and several hundred milligrams per litre in the ZTP-lysimeters as deep as 90 cm under the biosolid stockpile. Phosphorus, iron and manganese in excess of several thousand milligrams per litre were observed in both the lined-cell leachate and ZTP-lysimeters. Significant concentrations of other trace metals were found at depth in the zero-tension ZTP-lysimeter plot. Trace metals were largely mobilized by the DOC from the biosolids and due to the presence of anaerobic environment, especially in the underlying soil.

Cadmium Release in Soil Solutions at Low Moisture Content

Speciation of cadmium (Cd) was studied in four spiked agricultural soils at moisture content corresponding to 1.2 times field moisture capacity (FMC) and in the range from 1.2 FMC to soil–water 1∶10. Cadmium desorption isotherms were nonlinear in all soils, resulting in the decrease in Cd partition coefficient with loading. The Windermere Humic Aqueous Model (WHAM VI) was applied to predict Cd concentration in the solutions, and predicted values were compared with the measured ones. Based on total Cd content in soils, with concentrations of dissolved organic carbon (DOC), calcium (Ca), magnesium (Mg), and sodium (Na) and soil solution pH as the input variables, WHAM VI predicted Cd concentration in soil solutions with the root‐mean‐square error (RMSE) of log[Cd] RMSElog[Cd]=0.54 (n=37). Using total Cd content in soils, average concentrations of Ca and DOC in soil solutions, and soil pH instead of soil solution pH enabled prediction of Cd concentration in soil solution with RMSElog[Cd]=0.56. Calculation of Cd concentration as a function of moisture content resulted in RMSElog[Cd]=0.25 (n=20).

Dissolved Organic Carbon Fluxes under Bare Soil

The flux of dissolved organic carbon (DOC) in soil facilitates transport of nutrients and contaminants in soil. There is little information on DOC fluxes and the relationship between DOC concentration and water flux in agricultural soils. The DOC fluxes and concentrations were measured during 2.5 yr using 30 automatic equilibrium tension plate lysimeters (AETPLs) at 0.4 m and 30 AETPLs at 1.20-m depth in a bare luvisol, previously used as an arable soil. Average annual DOC fluxes of the 30 AETPLS were 4.9 g C m(-2) y(-1) at 0.4 m and 2.4 g C m(-2) y(-1) at 1.2 m depth. The average leachate DOC concentrations were 17 mg C L(-1) (0.4 m) and 9 mg C L(-1) (1.2 m). The DOC concentrations were unrelated to soil moisture content or average temperature and rarely dropped below 9 mg C L(-1) (0.4 m) and 5 mg C L(-1) (1.2 m). The variability in cumulative DOC fluxes among the plates was positively related to leachate volume and not to average DOC concentrations at both depths. This suggests that water fluxes are the main determinants of spatial variability in DOC fluxes. However, the largest DOC concentrations were inversely proportional to the mean water velocity between succeeding sampling periods, suggesting that the maximal net DOC mobilization rate in the topsoil is limited. Elevated DOC concentrations, up to 90 mg C L(-1), were only observed at low water velocities, reducing the risks of DOC-facilitated transport of contaminants to groundwater. The study emphasizes that water flux and velocity are important parameters for DOC fluxes and concentrations.

Export of organic carbon in run‐off from an Amazonian rainforest blackwater catchment

AbstractRainfall, run‐off and dissolved and particulate organic carbon concentrations were measured to investigate the hydrological export of organic carbon out of the blackwater Igarapé Asu rainforest catchment over a two‐year period. Annual rainfall was above average (2442 mm) at 2976 mm in 2002 and below average at 2054 mm in 2003. Surface run‐off dominated the flow out of the catchment, with groundwater outflow being negligible. Streamflow totals amounted to 1362 mm in 2002 and 780 mm in 2003. Dissolved organic carbon (DOC) concentrations in rainfall were similar to those measured in rainfall elsewhere in the Amazon Basin at 1·2 mg l−1, leading to atmospheric DOC deposition estimates of 3·5 g m−2 in 2002 and 2·4 g m−2 in 2003. Daily average DOC concentrations in run‐off ranged from 8 mg l−1 under low flow conditions to 27 mg l−1 during large quickflow events. Suspended sediment (10–2000‐µm size fraction) consisted for 28% of carbon and had a median concentration of 4·1 mg l−1. Daily run‐off varied between 1·2–2·5 mm day−1 during dry periods with corresponding organic carbon exports of 0·009–0·031 g m−2 day−1. Exports associated with large storms were much higher, reaching a daily maximum of 1·02 g m−2 day−1 for a discharge event of 38·4 mm. Export of carbon during the wet seasons amounted to 70% of the total. Annual exports in river water were different between the years because of differences in run‐off, varying between 26·2 g C m−2 in 2002 and 11·7 g C m−2 in 2003. Organic carbon exports were dominated by DOC, with exports in sediment constituting 6–8% of the total. Net carbon export, corrected for rainfall inputs, amounted to 22·7 and 9·3 g m−2 in 2002 and 2003, respectively. The present study indicates that accounting for exports of organic carbon in stream run‐off (averaging 19·0 g m−2 year−1 over the 2 years) would reduce the rainforest sink strength, estimated at 300–400 g m−2 year−1 from eddy covariance measurements, by 5–6% for this blackwater catchment. Copyright © 2006 John Wiley &amp; Sons, Ltd.

Dissolved organic carbon in ridge-axis and ridge-flank hydrothermal systems

The circulation of hydrothermal fluid through the upper oceanic crustal reservoir has a large impact on the chemistry of seawater, yet the impact on dissolved organic carbon (DOC) in the ocean has received almost no attention. To determine whether hydrothermal circulation is a source or a sink for DOC in the oceans, we measured DOC concentrations in hydrothermal fluids from several environments. Hydrothermal fluids were collected from high-temperature vents and diffuse, low-temperature vents on the basalt-hosted Juan de Fuca Ridge axis and also from low-temperature vents on the sedimented eastern flanks. High-temperature fluids from Main Endeavour Field (MEF) and Axial Volcano (AV) contain very low DOC concentrations (average = 15 and 17 μM, respectively) compared to background seawater (36 μM). At MEF and AV, average DOC concentrations in diffuse fluids (47 and 48 μM, respectively) were elevated over background seawater, and high DOC is correlated with high microbial cell counts in diffuse fluids. Fluids from off-axis hydrothermal systems located on 3.5-Ma-old crust at Baby Bare Seamount and Ocean Drilling Program (ODP) Hole 1026B had average DOC concentrations of 11 and 13 μM, respectively, and lowered DOC was correlated with low cell counts. The relative importance of heterotrophic uptake, abiotic sorption to mineral surfaces, thermal decomposition, and microbial production in fixing the DOC concentration in vent fluids remains uncertain. We calculated the potential effect of hydrothermal circulation on the deep-sea DOC cycle using our concentration data and published water flux estimates. Maximum calculated fluxes of DOC are minor compared to most oceanic DOC source and sink terms.

Concentration and Fluxes of Dissolved Organic Carbon (DOC) in Three Norway Spruce Stands along a Climatic Gradient in Sweden

Leaching of dissolved organic carbon (DOC) from the forest floor and transport in soil solution into the mineral soil are important for carbon cycling in boreal forest ecosystems. We examined DOC concentrations in bulk deposition, throughfall and in soil solutions collected under the O and B horizons in three Norway spruce stands along a climatic gradient in Sweden. Mean annual temperature for the three sites was 5.5, 3.4 and 1.2 °C. At each site we also examined the effect of soil moisture on DOC dynamics along a moisture gradient (dry, mesic and moist plots). To obtain information about the fate of DOC leached from the O horizon into the mineral soil, 14C measurements were made on bulk organic matter and DOC. The concentration and fluxes of DOC in O horizon leachates were highest at the southern site and lowest at the northern. Average DOC concentrations at the southern, central and northern sites were 49, 39 and 30 mg l−1, respectively. We suggest that DOC leaching rates from O horizons were related to the net primary production of the ecosystem. Soil temperature probably governed the within-year variation in DOC concentration in O horizon leachates, but the peak in DOC was delayed relative to that of temperature, probably due to sorption processes. Neither soil moisture regime (dry, mesic or moist plots) nor seasonal variation in soil moisture seemed to be of any significance for the concentration of DOC leached from the O horizon. The 14C measurements showed that DOC in soil solution collected below the B horizon was derived mainly from the B horizon itself, rather than from the O horizon, indicating a substantial exchange (sorption–desorption reactions) between incoming DOC and soil organic carbon in the mineral soil.

A preliminary study on the distribution characteristics of nutrients (N, P, Si, C) in the Wujiang River Basin

The distribution of nutrients (N, P, Si, C) in the Wujiang River surface water was studied during the high-flow and low-flow periods in 2002. The results showed that nitrate nitrogen (NO3-N) is the main form of dissolved inorganic nitrogen (DIN) in the Wujiang River Basin. It accounts for about 90% of DIN. The average NO3-N concentrations in the mainstream are 147.5 µM in the high-flow period and 158.0 µM in the low-flow period, respectively. The average concentrations of total phosphorus (TP) are 6.43 µM in the high-flow period and 4.18 µM in the low-flow period, respectively. Of the various forms of phosphorus, particulate phosphorus (PP) has the highest percentage (62.9%) of TP in the high-flow period. In the low-flow period, however, phosphate is the main form of phosphorus, which accounts for 49% of TP. With the Wujiangdu Reservoir as the boundary, the concentrations of DIN and phosphorus in the upper reaches are different from those in the lower reaches of the Wujiang River. As a whole, the concentrations of DIN and phosphorus are both higher in the low-flow period than in the high-flow period. The spatial and temporal variations of DIN and phosphorus concentrations suggested that DIN and phosphorus come from agricultural and domestic wastewaters and groundwaters and that the Wujiangdu Reservoir has an important impact on the concentrations and distribution of DIN and phosphorus in the Wujiang River. The distribution patterns of dissolved silica (DSi) and dissolved organic carbon (DOC) are similar. Both of them maintain no change in the whole course of the river and their concentrations (with the exception of the reservoir itself) are higher in the high-flow period than in the low-flow period. The average DSi and DOC concentrations in the mainstream are 85.4, 84.6 µM in the high-flow period and 60.8, 53.9 µM in the low-flow period, respectively. The concentrations of nutrients in most of the major tributaries are lower than in the mainstream. This suggested that the contributions of most tributaries are relatively small but importance should be attached to the influence of some individual tributaries such as the Qingshuijiang River and the Weng’ an River on the mainstream.

Winter-Time Climatic Control on Dissolved Organic Carbon Export and Surface Water Chemistry in an Adirondack Forested Watershed

Although most of forested watersheds in temperate and boreal regions are snow-covered for a substantial portion of the year, responses of biogeochemical processes under the snow pack to climatic fluctuations are poorly understood. We investigated responses of dissolved organic carbon (DOC) and surface water chemistry in stream and lake discharge waters draining the Arbutus Lake Watershed in the Adirondacks of New York State to climatic fluctuations during the snow-covered months from December through April. Interannual variability in stream discharge corresponded to changes in air temperature and snow pack depth across the winter months. Concentrations of DOC in stream water draining a subcatchment showed immediate positive responses to rising temperatures and subsequent increases in runoff during most snowmelt events. Increases in DOC concentrations usually coincided with decreases in pH and increases in total aluminum (Al) concentrations, while the correlations between concentrations of DOC and SO4(2-) or base cations were negative. Although changes in air temperature, snow pack depth, and runoff were all significantly correlated with stream water concentrations of major solutes, stepwise linear regression found that runoff was the best predictor of solute concentrations. Results of stepwise linear regression with long-term monthly monitoring data collected at the lake outlet showed weaker but still consistent climatic effects on interannual variations in concentrations of DOC and other solutes. Over the 17 winter periods from December 1983 through April 2000, changes in seasonal average concentrations of DOC, H+, and Al in lake discharge generally corresponded to interannual variations in temperature, precipitation, and runoff, while SO4(2-) and base cations displayed an opposite trend. The results suggest that snowmelt-mediated DOC responses to temperature fluctuations during the winter months might offset increases in the surface water pH caused by decreasing acidic deposition and pose a potential hazard of Al toxicity in surface waters.

Effects of altitude on aphid‐mediated processes in the canopy of Norway spruce

Abstract 1 The abundance of aphids and their honeydew are important in shaping the ecology of food web interactions and nutrient cycling in forests of Norway spruce. Here, the effects of the different environmental conditions at two study sites located at different altitudes (500 m, 765 m a.s.l.), in the Fichtelgebirge, north‐eastern Bavaria, Germany, on the abundance of Cinara pilicornis and their influence on epiphytic microorganisms on shoots of Picea abies were compared. Subsequent changes in throughfall fluxes were measured over a period of 12 weeks beneath infested and reference trees. In a laboratory experiment, the effects of ultraviolet (UV) radiation on microbial mortality and C and N concentrations in leachates were determined.2 The warmer and drier conditions at the low altitude site favoured an early onset to aphid multiplication in spring compared with the high altitude site, where aphid numbers peaked 3 weeks later.3 The presence of honeydew was associated with a significant increase in the total number of cultured epiphytic filamentous fungi, yeasts and bacteria in 12 of the 18 sample units, indicating better culturability or growth, whereas altitude had no significant effect on cultured cell numbers. By contrast to the reference trees, the high dissolved organic carbon (DOC) and hexose‐C fluxes beneath infested trees at the peak in aphid abundance, in June and July, resulted in a concomitant decrease in the fluxes of total inorganic nitrogen beneath infested trees (low altitude: −19.7%; high altitude: −52.3%). Fluxes of organic nitrogen were significantly higher beneath infested trees at the time of infestation. Similarly, potassium fluxes in throughfall increased 1.6–2.0‐fold in response to aphid infestation.4 The exposure of infested and uninfested shoots of Norway spruce to UV‐A and UV‐B radiation only weakly affected epiphytic microbial mortality and did not affect the concentrations of the different nitrogen compound in leachates. However, bacteria, tended to be more active in the leachates collected from infested shoots, which resulted in the higher concentrations of aminosugar‐N. The aphids had a more pronounced effect on the concentrations of DOC in leachates, with average DOC concentrations being 4.2‐fold higher than in leachates from uninfested shoots.5 It is suggested that, even at very low densities, aphids exert a strong influence via honeydew on the performance of microorganisms, and nutrient and energy flow, in spruce forests.

Effects of asynchronous fluctuations in DOC supply and bacterial growth on biodegradation efficiency

We present a model of the biodegradation system in which dissolved organic carbon (DOC) is supplied by a continuous flow of wastewater, aerobically decomposed by heterotrophic bacteria, and continuously discharged downstream. In this model, the rate at which DOC is supplied to the system ( u) and the maximum growth rate of bacteria ( k) fluctuate periodically. We demonstrate analytically how temporal patterns (amplitude, period, and phase difference) in these two fluctuations influence the average DOC concentration and bacterial biomass. These patterns affect: (i) the temporal variability of DOC concentration and (ii) correlations among the fluctuating DOC concentration, the maximum bacterial growth rate, and bacterial biomass. The temporal variability of DOC tends to increase as the relative amplitude of u to k and the fluctuation period increase, leading to a decrease in the average bacterial biomass and biodegradation efficiency. On the other hand, a higher positive (or negative) correlation among fluctuations leads to higher (lower) bacterial biomass and biodegradation efficiency. The sign of the correlations depends on the phase difference between u and k, and tends to be negative at longer periods. The temporal variability of DOC and the effects of correlations among fluctuations determine whether the average DOC concentration is higher or lower than the equilibrium DOC concentration when u and k are temporally constant. Furthermore, the patterns of the fluctuations determine whether the bacterial community is maintained or washed out. In addition, we evaluate numerically the magnitude of the effect of fluctuations. In particular, the dependence on the phase difference can generate a greater than two-fold difference in the average DOC concentration. These results imply that the temporal control of the DOC supply rate, synchronized with the fluctuations of the bacterial community, can improve the efficiency of biodegradation and reduce the DOC concentration in the outflow from a system, even if the total load of DOC delivered to the system is not reduced.

Dissolved organic carbon distributions in the Bransfield and Gerlache Straits, Antarctica

Abstract During FRUELA’95 cruise, seawater samples were collected at the Bransfield and Gerlache Straits for the analysis of dissolved organic carbon (DOC) profiles throughout the water column. An excess of DOC probably derived from phytogenic material was observed in the upper mixed layer (UML; average: +22±13 μmol C l−1), compared to the constant concentration of refractory DOC below 400 m (44±4 μmol C l−1). The average excess DOC concentration was higher than the particulate organic carbon concentration indicating the major contribution of DOC to carbon export in this area. However, large spatial variability of DOC in the upper mixed layer (52–102 μmol C l−1) was observed: excess DOC contributed from 15% to 57% to the actual DOC concentration. Maximum average DOC concentrations in the UML were recorded in the Gerlache Strait (71 μmol C l−1) and in the Gerlache–Bransfield confluence (80 μmol C l−1), whereas minimum values were recorded in the Bransfield Strait (61 μmol C l−1). Several shelf and slope stations showed a slight increase of DOC (5–10 μmol C l−1) in the deep layer which might be related to organic matter release from the underlying sediments. Considering the net DOC release from phytoplankton, the low bacterial biomass and the reduced vertical DOC export, the DOC excess could build up in about 6 days for most of the sampling stations. The probable fate of the DOC excess is the eastwards horizontal transport by the Bransfield Current out of the study area.

Rainwater dissolved organic carbon: Concentrations and global flux

Dissolved organic carbon (DOC) is a major component of both marine (23 μM) and continental (161 μM) rain, present in concentrations greater than nitric and sulfuric acids combined. Rain is a significant source of DOC to surface seawater (90 × 1012 g C yr−1), equivalent to the magnitude of river input of DOC to the open ocean and half the magnitude of carbon buried in marine sediments per year on a global scale. Current models of global carbon cycling focus primarily on inorganic forms of carbon and are unable to account for approximately 20% of the global carbon dioxide, suggesting a significant missing carbon sink. Quantification of the average DOC concentration in marine rain allows calculation of the global rainwater flux of DOC of 430 ± 150 × 1012 g C yr−1. When inorganic carbon is included, this rainwater carbon flux becomes 510 ± 170 × 1012 g C yr−1, which, although not the same carbon, is equivalent in magnitude to over one third of the missing carbon sink.

Dissolved Organic Carbon in a Large Macrotidal Estuary (the Humber, UK): Behaviour During Estuarine Mixing

Dissolved organic carbon (DOC) was measured during four cruises to the Yorkshire Ouse–Humber Estuary, covering contrasting seasons. Despite the high turbidity and the complete flushing times in the `Humber proper' (downstream from Trent Falls), DOC was not sensitive to bacterial oxidation and/or chemical precipitation at any time. More than 96% of the variability in the DOC profiles are simply explained by conservative mixing of the two end-members. The average DOC concentration at zero salinity (C*), was ∼20% higher than levels measured in the landward end of the Yorkshire Ouse (C 0). C* showed an inverse relationship with river discharge, illustrating a significant contribution from sources other than natural processes in the catchment area. The difference between C* and C 0 was due to a conspicuous DOC peak observed year-round about the turbidity maximum zone of the tidal Ouse (upstream from Trent Falls). Both anthropogenic waste discharges and biogeochemical production within the turbidity maximum are compatible with the observed inverse relationship with river flows.