Characterisation and reactivity continuum of dissolved organic matter in forested headwater catchments of Andean Patagonia

相似文献   

Regulation of Bacterial Growth Rates by Dissolved Organic Carbon and Temperature in the Equatorial Pacific Ocean

Abstract The effect of dissolved organic matter (DOM) and temperature on bacterial production was examined in the equatorial Pacific Ocean. Addition of glucose, glucose plus ammonium, or free amino acids stimulated bacterial production ([³H]thymidine incorporation), whereas changes in bacterial abundance were either negligible or much less than changes in bacterial production. The average bacterial growth rate also greatly increased following DOM additions, whereas in contrast, addition of ammonium alone never affected production, bacterial abundance, or growth rates. Since the large glucose effect was not observed in previous studies of cold oceanic waters, several experiments were conducted to examine DOM-temperature interactions. These experiments suggest that bacteria respond more quickly and to a greater extent to DOM additions at higher temperatures, which may explain apparently conflicting results from previous studies. We also examined how temperate affects the kinetic parameters of sugar uptake. Maximum uptake rates (V_max) of glucose and mannose increased with temperature (Q₁₀= 2.4), although the half-saturation constant (K_m) was unaffected; K_m+ S was roughly equal to glucose concentrations (S) measured by a high pressure liquid chromographic technique. Bacterial production and growth rates appear to be limited by DOM in the equatorial Pacific, and thus bacterial production follows primary production over large spatial and temporal scales in this oceanic regime, as has been observed in other aquatic systems. Although temperature may not limit bacterial growth rates in the equatorial Pacific and similar warm waters, it could still affect how bacteria respond to changes in DOM supply and help set steady-state DOM concentrations. Received: 26 July 1995; Revised: 19 January 1996     

Effects of High Dissolved Inorganic and Organic Carbon Availability on the Physiology of the Hard Coral Acropora millepora from the Great Barrier Reef

Coral reefs are facing major global and local threats due to climate change-induced increases in dissolved inorganic carbon (DIC) and because of land-derived increases in organic and inorganic nutrients. Recent research revealed that high availability of labile dissolved organic carbon (DOC) negatively affects scleractinian corals. Studies on the interplay of these factors, however, are lacking, but urgently needed to understand coral reef functioning under present and near future conditions. This experimental study investigated the individual and combined effects of ambient and high DIC (pCO₂ 403 μatm/ pH_Total 8.2 and 996 μatm/pH_Total 7.8) and DOC (added as Glucose 0 and 294 μmol L^-1, background DOC concentration of 83 μmol L^-1) availability on the physiology (net and gross photosynthesis, respiration, dark and light calcification, and growth) of the scleractinian coral Acropora millepora (Ehrenberg, 1834) from the Great Barrier Reef over a 16 day interval. High DIC availability did not affect photosynthesis, respiration and light calcification, but significantly reduced dark calcification and growth by 50 and 23%, respectively. High DOC availability reduced net and gross photosynthesis by 51% and 39%, respectively, but did not affect respiration. DOC addition did not influence calcification, but significantly increased growth by 42%. Combination of high DIC and high DOC availability did not affect photosynthesis, light calcification, respiration or growth, but significantly decreased dark calcification when compared to both controls and DIC treatments. On the ecosystem level, high DIC concentrations may lead to reduced accretion and growth of reefs dominated by Acropora that under elevated DOC concentrations will likely exhibit reduced primary production rates, ultimately leading to loss of hard substrate and reef erosion. It is therefore important to consider the potential impacts of elevated DOC and DIC simultaneously to assess real world scenarios, as multiple rather than single factors influence key physiological processes in coral reefs.     

Chemistry and Dynamics of Dissolved Organic Matter in a Temperate Coniferous Forest on Andic Soils: Effects of Litter Quality

Dissolved organic matter (DOM) plays an important role in transporting carbon and nitrogen from forest floor to mineral soils in temperate forest ecosystems. Thus, the retention of DOM via sorption or microbial assimilation is one of the critical steps for soil organic matter formation in mineral soils. The chemical properties of DOM are assumed to control these processes, yet we lack fundamental information that links litter quality, DOM chemistry, and DOM retention. Here, we studied whether differences in litter quality affect solution chemistry and whether changes in litter inputs affect DOM quality and removal in the field. The effects of litter quality on solution chemistry were evaluated using chemical fractionation methods for laboratory extracts and for soil water collected from a temperate coniferous forest where litter inputs had been altered. In a laboratory extraction, litter type (needle, wood, root) and the degree of decomposition strongly influenced solution chemistry. Root litter produced more than 10 times more water-extractable dissolved organic N (DON) than any other litter type, suggesting that root litter may be most responsible for DON production in this forest ecosystem. The chemical composition of the O-horizon leachate was similar under all field treatments (doubled needle, doubled wood, and normal litter inputs). O-horizon leachate most resembled laboratory extracts of well-decomposed litter (that is, a high proportion of hydrophobic acids), in spite of the significant amount of litter C added to the forest floor and a tendency toward higher mean DOM under doubled-Litter treatments. A lag in DOM production from added litter or microbial modification might have obscured chemical differences in DOM under the different treatments. Net DOM removal in this forest soil was strong; DOM concentration in the water deep in the mineral soil was always low regardless of concentrations in water that entered the mineral soil and of litter input manipulation. High net removal of DOM from O-horizon leachate, in spite of extremely low initial hydrophilic neutral content (labile DOM), coupled with the lack of influence by season or soil depth, suggests that DOM retention in the soil was mostly by abiotic sorption.     

Carbonate recrystallization in root-free soil and rhizosphere of Triticum aestivum and Lolium perenne estimated by 14C labeling

The photo- and bio-degradation of dissolved organic matter (DOM) in water from the Broad River were investigated in laboratory experiments using a solar simulator to control the intensity and exposure of samples to irradiation. The water samples included a natural assemblage of microorganisms, and the daily exposure of samples to irradiation was varied to distinguish the relative contributions of photochemical and biological degradation. Concentrations of dissolved organic carbon (DOC) and specific components of DOM, including chromophoric DOM (CDOM), lignin phenols and amino acids, were monitored to investigate the reactivity and predominant pathway of degradation of these DOM components. Biodegradation was primarily responsible for the overall remineralization of DOC and losses of the amino acid component of DOM, whereas photodegradation was primarily responsible for losses of the chromophoric and lignin phenol components of DOM. The rates of photodegradation of lignin phenols were strongly influenced by the presence of methoxy groups on the aryl ring. Syringyl (S) phenols have two methoxy substitutions, vanillyl (V) phenols have one methoxy substitution, and p-hydroxy (P) phenols are not substituted with methoxy groups. Photochemical decay constants were highest for S phenols, lowest for P phenols and followed a consistent pattern (S > V > P) in the experiments. The carbon-normalized yields of amino acids and lignin phenols were found to be useful molecular indicators of the highly reactive (i.e. labile) components of biodegradable and photodegradable DOM, respectively.     

Hydroxyl Radical Production from Irradiated Arctic Dissolved Organic Matter

The hydroxyl radical (OH·) plays an important role in the environmental chemistry and biogeochemistry of surface waters. OH· acts as a strong oxidant within the irradiated water column, and affects the bioavailability, cycling, and mineralization of dissolved organic matter (DOM), the speciation and redox state of important trace metals e.g., iron and copper, and the fate of persistent organic pollutants (POPs). The generation of this species from irradiated DOM may be especially important in Arctic surface waters during the boreal summer, which contains high levels of DOM and experiences continual solar irradiance. Here, we investigate the OH· produced from laser irradiated Arctic DOM isolated from Toolik Lake, AK (68°38′ N, 149°43′ W). We measured the wavelength dependence of OH· production for aqueous solutions of DOM and report that the greatest OH· production occurs at wavelengths less than 360 nm. OH· production rates ranged from 1.7 (±0.1)×10⁻⁷ M h⁻¹ to 6.4 (±0.2)×10⁻⁷ M h⁻¹, with the rate depending on both irradiation wavelength and to a lesser degree the method used to isolate the DOM matrix. These findings lead to a better understanding of the potentially important photo-oxidation processes that may impact DOM cycling in the Arctic.     

Composition,Dynamics, and Fate of Leached Dissolved Organic Matter in Terrestrial Ecosystems: Results from a Decomposition Experiment

Fluxes of dissolved organic matter (DOM) are an important vector for the movement of carbon (C) and nutrients both within and between ecosystems. However, although DOM fluxes from throughfall and through litterfall can be large, little is known about the fate of DOM leached from plant canopies, or from the litter layer into the soil horizon. In this study, our objectives were to determine the importance of plant-litter leachate as a vehicle for DOM movement, and to track DOM decomposition [including dissolve organic carbon (DOC) and dissolved organic nitrogen (DON) fractions], as well as DOM chemical and isotopic dynamics, during a long-term laboratory incubation experiment using fresh leaves and litter from several ecosystem types. The water-extractable fraction of organic C was high for all five plant species, as was the biodegradable fraction; in most cases, more than 70% of the initial DOM was decomposed in the first 10 days of the experiment. The chemical composition of the DOM changed as decomposition proceeded, with humic (hydrophobic) fractions becoming relatively more abundant than nonhumic (hydrophilic) fractions over time. However, in spite of proportional changes in humic and nonhumic fractions over time, our data suggest that both fractions are readily decomposed in the absence of physicochemical reactions with soil surfaces. Our data also showed no changes in the ¹³C signature of DOM during decomposition, suggesting that isotopic fractionation during DOM uptake is not a significant process. These results suggest that soil microorganisms preferentially decompose more labile organic molecules in the DOM pool, which also tend to be isotopically heavier than more recalcitrant DOM fractions. We believe that the interaction between DOM decomposition dynamics and soil sorption processes contribute to the ¹³C enrichment of soil organic matter commonly observed with depth in soil profiles. published online 2004     

Influence of Litter Diversity on Dissolved Organic Matter Release and Soil Carbon Formation in a Mixed Beech Forest

We investigated the effect of leaf litter on below ground carbon export and soil carbon formation in order to understand how litter diversity affects carbon cycling in forest ecosystems. ¹³C labeled and unlabeled leaf litter of beech (Fagus sylvatica) and ash (Fraxinus excelsior), characterized by low and high decomposability, were used in a litter exchange experiment in the Hainich National Park (Thuringia, Germany). Litter was added in pure and mixed treatments with either beech or ash labeled with ¹³C. We collected soil water in 5 cm mineral soil depth below each treatment biweekly and determined dissolved organic carbon (DOC), δ¹³C values and anion contents. In addition, we measured carbon concentrations and δ¹³C values in the organic and mineral soil (collected in 1 cm increments) up to 5 cm soil depth at the end of the experiment. Litter-derived C contributes less than 1% to dissolved organic matter (DOM) collected in 5 cm mineral soil depth. Better decomposable ash litter released significantly more (0.50±0.17%) litter carbon than beech litter (0.17±0.07%). All soil layers held in total around 30% of litter-derived carbon, indicating the large retention potential of litter-derived C in the top soil. Interestingly, in mixed (ash and beech litter) treatments we did not find a higher contribution of better decomposable ash-derived carbon in DOM, O horizon or mineral soil. This suggest that the known selective decomposition of better decomposable litter by soil fauna has no or only minor effects on the release and formation of litter-derived DOM and soil organic matter. Overall our experiment showed that 1) litter-derived carbon is of low importance for dissolved organic carbon release and 2) litter of higher decomposability is faster decomposed, but litter diversity does not influence the carbon flow.     

An accounting of C‐based trace gas release during abiotic plant litter degradation

Recent studies showed that photochemical breakdown (photodegradation) of plant material accounts for a substantial portion of litter decomposition and subsequent trace gas release in ecosystems under high radiative load and low precipitation. In the absence of solar radiation, thermal degradation may also cause trace gas release at temperatures below the ignition point. These observations suggest that the abiotic processes of photodegradation and thermal degradation of plant litter may be important in understanding global trace gas budgets. In a laboratory incubation study, we performed a simultaneous carbon (C) accounting of CO₂, CO, and CH₄ produced as a byproduct of photodegradation and thermal degradation of six different plant litter types that varied in chemical composition. The patterns of trace gas release during photodegradation and thermal degradation differed considerably across the six plant materials, suggesting that chemical composition of litter may influence the rates of abiotic degradation. There was a strong positive correlation between the rates of trace gas release during photodegradation and temperature. A significant portion of trace gases were produced during low temperature (< 100 °C) thermal degradation of litter in the absence of solar radiation, which was also positively correlated to temperature. In addition, both thermal degradation and photodegradation occurred in the absence of O₂. This indicates that the mechanism formerly accepted as photo‐oxidation may only be one of several photodegradation processes. We speculate that the direct breakdown of chemical groups such as carboxyl, carbonyl, and methoxyl groups may result in CO₂, CO, and CH₄ release. We suggest that the combined processes of thermal and photodegradation of litter may be a previously under accounted source of C‐based trace gases from terrestrial systems.     

Use of ESI-FTICR-MS to Characterize Dissolved Organic Matter in Headwater Streams Draining Forest-Dominated and Pasture-Dominated Watersheds

Electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICR-MS) has proven to be a powerful technique revealing complexity and diversity of natural DOM molecules, but its application to DOM analysis in grazing-impacted agricultural systems remains scarce. In the present study, we presented a case study of using ESI-FTICR-MS in analyzing DOM from four headwater streams draining forest- or pasture-dominated watersheds in Virginia, USA. In all samples, most formulas were CHO compounds (71.8–87.9%), with other molecular series (CHOS, CHON, CHONS, and CHOP (N, S)) accounting for only minor fractions. All samples were dominated by molecules falling in the lignin-like region (H/C = 0.7–1.5, O/C = 0.1–0.67), suggesting the predominance of allochthonous, terrestrial plant-derived DOM. Relative to the two pasture streams, DOM formulas in the two forest streams were more similar, based on Jaccard similarity coefficients and nonmetric multidimensional scaling calculated from Bray-Curtis distance. Formulas from the pasture streams were characterized by lower proportions of aromatic formulas and lower unsaturation, suggesting that the allochthonous versus autochthonous contributions of organic matter to streams were modified by pasture land use. The number of condensed aromatic structures (CAS) was higher for the forest streams, which is possibly due to the controlled burning in the forest-dominated watersheds and suggests that black carbon was mobilized from soils to streams. During 15-day biodegradation experiments, DOM from the two pasture streams was altered to a greater extent than DOM from the forest streams, with formulas with H/C and O/C ranges similar to protein (H/C = 1.5–2.2, O/C = 0.3–0.67), lipid (H/C = 1.5–2.0, O/C = 0–0.3), and unsaturated hydrocarbon (H/C = 0.7–1.5, O/C = 0–0.1) being the most bioreactive groups. Aromatic compound formulas including CAS were preferentially removed during combined light+bacterial incubations, supporting the contention that black carbon is labile to light alterations. Collectively, our data demonstrate that headwater DOM composition contains integrative information on watershed sources and processes, and the application of ESI-FTICR-MS technique offers additional insights into compound composition and reactivity unrevealed by fluorescence and stable carbon isotopic measurements.     

Effects of Dissolved Organic Matter Photoproducts and Mineral Nutrient Supply on Bacterial Growth in Mediterranean Inland Waters

Sunlight reacts with dissolved organic matter (DOM) modifying its availability as bacterial substrate. We assessed the impact of DOM photoproducts and mineral nutrient supply on bacterial growth in seven inland waters from the South of Spain, where DOM is characterized by low chromophoric content and long residence time. Factorial experiments were performed with presence vs absence of DOM photoproducts and mineral nutrient supply. In six of the seven experiments, we found a significant and negative effect of DOM photoproducts on bacterial growth and a significant and positive effect of mineral nutrient supply. The interaction of these two factors leaded to a compensation of negative effects of photoproducts by availability of mineral nutrients. Dissolved organic matter diagenetic status and the ionic environment where organic carbon is dissolved can be influencing bacterial DOM processing.     

Effects of Watershed History on Dissolved Organic Matter Characteristics in Headwater Streams

Dissolved organic matter (DOM) is recognized as a major component in the global carbon cycle and is an important driver in aquatic ecosystem function. Climate, land use, and forest cover changes all impact stream DOM and alter biogeochemical cycles in terrestrial environments. We determined the temporal variation in DOM quantity and quality in headwater streams at a reference watershed (REF), a watershed clear-cut 30 years ago (CC), and a watershed converted to a white pine plantation 50 years ago (WP) at the US Forest Service, Coweeta Hydrologic Laboratory, in the Nantahala Mountains of western North Carolina, USA. Average stream dissolved organic carbon (DOC) concentrations in CC or WP were 60 and 80% of those in REF, respectively. Stream DOM composition showed that the difference was mainly due to changes in humic-like components in chromophoric DOM. In addition, excitation–emission matrix fluorescence data with parallel factor analysis indicate that although the concentration of protein-like components did not differ significantly among watersheds, their relative abundance showed an enrichment in CC and WP compared to REF. The ratio of humic acid-type to fulvic acid-type components was highest and lowest at REF and WP, respectively. Our data suggest that forest ecosystem disturbance history affects the DOM quantity and quality in headwater streams over decades as a result of changes in watershed soil organic matter characteristics due to differences in organic matter inputs.     

Dissolved Organic Matter Concentration and Quality Influences upon Structure and Function of Freshwater Microbial Communities

Past studies have suggested that the concentration and quality of dissolved organic matter (DOM) may influence microbial community structure. In this study, we cross-inoculated the bacterial communities from two streams and a dystrophic lake that varied in DOM concentration and chemistry, to yield nine fully crossed treatments. We measured dissolved organic carbon (DOC) concentration and heterotrophic microbial community productivity throughout a 72-h incubation period, characterized DOM quality by molecular weight, and determined microbial community structure at the initial and final time points. Our results indicate that all bacterial inoculate sources had similar effects upon DOC concentration and DOM quality, regardless of the DOM source. These effects included an overall decrease in DOM M _W and an initial period of DOC concentration variability between 0-24h. In contrast, microbial communities and their metabolic rates converged to profiles that reflected the DOM source upon which they were growing, regardless of the initial bacterial inoculation. The one exception was that the bacterial community from the low-concentration and low-molecular-weight DOM source exhibited a greater denaturing gradient gel electrophoresis (DGGE) band richness when grown in its own DOM source than when grown in the highest concentration and molecular weight DOM source. This treatment also exhibited a higher rate of productivity. In general, our data suggest that microbial communities are selected by the DOM sources to which they are exposed. A microbial community will utilize the low-molecular-weight (or labile) DOM sources as well as parts of the high-molecular-weight (refractory) DOM, until a community develops that can efficiently metabolize the more abundant high-molecular-weight source. This experiment examines some of the complex interactions between microbial community selection and the combined factors of DOM quality and concentration. Our data suggest that the roles of aerobic aquatic heterotrophic bacteria in carbon cycling, as well as the importance of high-molecular-weight DOM as a carbon source, may be more complex than is conventionally recognized.     

Ecosystem Modulation of Dissolved Carbon Age in a Temperate Marsh-Dominated Estuary

We measured the concentrations and isotopic values (¹⁴C and ¹³C) of dissolved inorganic, dissolved organic, and particulate organic carbon (DIC, DOC, and POC, respectively) in the Parker River watershed and estuary in Massachusetts, USA, to determine the age of carbon (C) entering the estuary and how estuarine processing affects the quantity and apparent age of C transported to the Gulf of Maine. The watershed measurements indicated the transport of ¹⁴C-enriched modern DIC and DOC and variably aged POC from the watershed to the estuary. The transport of organic matter from the watershed was dominated by DOC transport, with POC making up less than 10% of the total. Surveys within the watershed aimed at determining which land-use type dominated the DOC export indicated that wetlands, although they made up only around 20% of the land use, could be responsible for approximately 75% of the DOC export. We therefore conclude that the wetland land uses of the Parker River watershed are exporting mainly ¹⁴C-enriched modern DOC. DIC isotopes indicate that the source of DIC in the Parker River watershed is dominated by the weathering of noncarbonate parent material by ¹⁴C-enriched carbon dioxide (CO₂) originating from the respiration of young organic matter in soils. Transects in the estuary displayed net additions of all C species. For DOC and DIC, the export of this internally added DOC and DIC was approximately equal to the amount being exported from the watershed, showing the importance of focusing on estuaries when estimating the export of C to the coastal ocean. With respect to DIC, the total input is even larger when the atmospheric exchange of excess pCO₂ is calculated. The ¹⁴C-DOC and ¹⁴C-DIC transects indicate that the internally added DOC and DIC is ¹⁴C-enriched modern material. The source of this material is the fringing marshes and estuarine phytoplankton, with the relative importance of these two sources changing over time. Taken together, the bulk C and ¹⁴C measurements show that the estuary is adding significant quantities of young DOC despite the presence of vast quantities of old marsh peat flanking the entire estuary. Furthermore, the DIC data indicate that ¹⁴C-enriched modern material is what is fueling the majority of heterotrophic respiration within the system.     

Dissolved Organic Carbon in Headwater Streams and Riparian Soil Organic Carbon along an Altitudinal Gradient in the Wuyi Mountains,China

Stream water dissolved organic carbon (DOC) correlates positively with soil organic carbon (SOC) in many biomes. Does this relationship hold in a small geographic region when variations of temperature, precipitation and vegetation are driven by a significant altitudinal gradient? We examined the spatial connectivity between concentrations of DOC in headwater stream and contents of riparian SOC and water-soluble soil organic carbon (WSOC), riparian soil C:N ratio, and temperature in four vegetation types along an altitudinal gradient in the Wuyi Mountains, China. Our analyses showed that annual mean concentrations of headwater stream DOC were lower in alpine meadow (AM) than in subtropical evergreen broadleaf forest (EBF), coniferous forest (CF), and subalpine dwarf forest (SDF). Headwater stream DOC concentrations were negatively correlated with riparian SOC as well as WSOC contents, and were unrelated to riparian soil C:N ratio. Our findings suggest that DOC concentrations in headwater streams are affected by different factors at regional and local scales. The dilution effect of higher precipitation and adsorption of soil DOC to higher soil clay plus silt content at higher elevation may play an important role in causing lower DOC concentrations in AM stream of the Wuyi Mountains. Our results suggest that upscaling and downscaling of the drivers of DOC export from forested watersheds when exploring the response of carbon flux to climatic change or other drivers must done with caution.     

Impact of Dissolved Organic Matter on the Desorption and Mineralization Rates of Naphthalene

The persistence of organic contaminants in plant-soil systems will be largely determined by the basic processes controlling bioavailability: desorption and biodegradation. Both processes can be affected by the presence of and variations in dissolved organic matter (DOM). To evaluate potential effects of DOM in surface soil remediation systems, the kinetics of naphthalene desorption from soil and mineralization in soil solution in the presence of DOM were studied in completely mixed batch reactors (CMBRs). Three different DOM solutions were studied. These were obtained from water extraction of two different soils (muck-highly organic and alfalfa field-agricultural) and a third prepared by reconstituting a fulvic acid reference standard purchased commercially. Neither the desorption rate nor equilibrium partitioning of naphthalene was affected by the presence of varying DOM solutions. The effect on mineralization was evaluated based on an evaluation of a first-order rate constant produced by nonlinear regression of data fitted to a ¹⁴CO₂ production model. For the muck DOM solution, alfalfa DOM solution, and fulvic acid DOM solution, first-order rate constants were 0.0474, 0.0657, 0.0422 min_?1, respectively. Using uncertainty analysis, these differences were found not to be significant at a 90% confidence level. Although not apparent in this data, the impact of DOM on the mechanisms of desorption and biodegradation in a plant -soil environment may be important for other contaminant/organism/soil combinations. With the increased interest in phytoremediation and land farming as means of treating organic contaminants, further investigation of such interactions may be warranted.     

Uncoupling of Bacterial and Terrigenous Dissolved Organic Matter Dynamics in Decomposition Experiments

The biodegradability of terrigenous dissolved organic matter (tDOM) exported to the sea has a major impact on the global carbon cycle, but our understanding of tDOM bioavailability is fragmentary. In this study, the effects of preparative tDOM isolation on microbial decomposition were investigated in incubation experiments consisting of mesocosms containing mesohaline water from the Baltic Sea. Dissolved organic carbon (DOC) consumption, molecular DOM composition, bacterial activities, and shifts in bacterial community structure were compared between mesocosms supplemented with riverine tDOM, either as filtered, particle-free river water or as a concentrate obtained by lyophilization/tangential ultrafiltration, and those containing only Baltic Sea water or river water. As shown using ultra-high-resolution mass spectrometry (15 Tesla Fourier-transform ion cyclotron resonance mass spectrometry, FT-ICR-MS) covering approximately 4600 different DOM compounds, the three DOM preparation protocols resulted in distinct patterns of molecular DOM composition. However, despite DOC losses of 4–16% and considerable bacterial production, there was no significant change in DOM composition during the 28-day experiment. Moreover, tDOM addition affected neither DOC degradation nor bacterial dynamics significantly, regardless of the tDOM preparation. This result suggested that the introduced tDOM was largely not bioavailable, at least on the temporal scale of our experiment, and that the observed bacterial activity and DOC decomposition mainly reflected the degradation of unknown, labile, colloidal and low-molecular weight DOM, both of which escape the analytical window of FT-ICR-MS. In contrast to the different tDOM preparations, the initial bacterial inoculum and batch culture conditions determined bacterial community succession and superseded the effects of tDOM addition. The uncoupling of tDOM and bacterial dynamics suggests that mesohaline bacterial communities cannot efficiently utilize tDOM and that in subarctic estuaries other factors are responsible for the removal of imported tDOM.     

Effect of Native Vegetation Loss on Stream Ecosystem Processes: Dissolved Organic Matter Composition and Export in Agricultural Landscapes

Stream and river ecosystems are dependent on energetic inputs from their watersheds and thus shifts in land use from forest cover to agriculture will affect stream community composition and function. The disruption of forest-aquatic linkages alters the organic matter resources in agricultural streams. Dissolved organic matter (DOM) is the dominant form of organic matter in aquatic ecosystems, and a microbial energy source that is important for stream respiration. The concentrations and characteristics of DOM are regulated by both terrestrial (for example, terrestrial organic matter supply) and in-stream processes (for example, microbial respiration and periphyton production) that are influenced by land management. The effects of watershed land use and topographic, soil and climatic variables on DOM quantity (dissolved organic carbon concentration and load), source (terrestrial or in-stream) and quality (composition and lability) were measured in 14 streams across an agricultural land-use gradient. DOC concentration was positively correlated with watershed pasture cover and negatively correlated with watershed relief. No watershed variables were important correlates of DOC load. Stream DOM was primarily of terrestrial origin, but DOM in agricultural streams had a greater proportion of sources from in-stream sources. This may be due to reduced connection with riparian vegetation and increased in-stream primary production. We suggest that maintaining watershed tree cover greater than 52% and ensuring less than 10% of the length of riparian corridor is cleared for pasture could minimize changes to DOM composition. This is important to avoid flow-on effects for stream ecosystem processes that are mediated by DOM. Long-term DOM monitoring will be valuable for assessing the functional impacts of land-use change.     

Environmental Dissolved Organic Matter Governs Biofilm Formation and Subsequent Linuron Degradation Activity of a Linuron-Degrading Bacterial Consortium

It was examined whether biofilm growth on dissolved organic matter (DOM) of a three-species consortium whose members synergistically degrade the phenylurea herbicide linuron affected the consortium''s integrity and subsequent linuron-degrading functionality. Citrate as a model DOM and three environmental DOM (eDOM) formulations of different quality were used. Biofilms developed with all DOM formulations, and the three species were retained in the biofilm. However, biofilm biomass, species composition, architecture, and colocalization of member strains depended on DOM and its biodegradability. To assess the linuron-degrading functionality, biofilms were subsequently irrigated with linuron at 10 mg liter⁻¹ or 100 μg liter⁻¹. Instant linuron degradation, the time needed to attain maximal linuron degradation, and hence the total amount of linuron removed depended on both the DOM used for growth and the linuron concentration. At 10 mg liter⁻¹, the final linuron degradation efficiency was as high as previously observed without DOM except for biofilms fed with humic acids which did not degrade linuron. At 100 μg liter⁻¹ linuron, DOM-grown biofilms degraded linuron less efficiently than biofilms receiving 10 mg liter⁻¹ linuron. The amount of linuron removed was more correlated with biofilm species composition than with biomass or structure. Based on visual observations, colocalization of consortium members in biofilms after the DOM feed appears essential for instant linuron-degrading activity and might explain the differences in overall linuron degradation. The data show that DOM quality determines biofilm structure and composition of the pesticide-degrading consortium in periods with DOM as the main carbon source and can affect subsequent pesticide-degrading activity, especially at micropollutant concentrations.     

Porewater Stoichiometry of Terminal Metabolic Products, Sulfate, and Dissolved Organic Carbon and Nitrogen in Estuarine Intertidal Creek-bank Sediments

Porewater equilibration samplers were used to obtain porewater inventories of inorganic nutrients (NH₄⁺, NO_x, PO₄³⁻), dissolved organic carbon (DOC) and nitrogen (DON), sulfate (SO₄²⁻), dissolved inorganic carbon (DIC), hydrogen sulfide (H₂S), chloride (Cl⁻), methane (CH₄) and reduced iron (Fe²⁺) in intertidal creek-bank sediments at eight sites in three estuarine systems over a range of salinities and seasons. Sulfate reduction (SR) rates and sediment particulate organic carbon (POC) and nitrogen (PON) were also determined at several of the sites. Four sites in the Okatee River estuary in South Carolina, two sites on Sapelo Island, Georgia and one site in White Oak Creek, Georgia appeared to be relatively pristine. The eighth site in Umbrella Creek, Georgia was directly adjacent to a small residential development employing septic systems to handle household waste. The large data set (>700 porewater profiles) offers an opportunity to assess system-scale patterns of porewater biogeochemical dynamics with an emphasis on DOC and DON distributions. SO₄²⁻ depletion (SO₄²⁻)_Dep was used as a proxy for SR, and (SO₄²⁻)_Dep patterns agreed with measured (³⁵S) patterns of SR. There were significant system-scale correlations between the inorganic products of terminal metabolism (DIC, NH₄⁺ and PO₄³⁻) and (SO₄²⁻)_Dep, and SR appeared to be the dominant terminal carbon oxidation pathway in these sediments. Porewater inventories of DIC and (SO₄²⁻)_Dep indicate a 2:1 stoichiometry across sites, and the C:N ratio of the organic matter undergoing mineralization was between 7.5 and 10. The data suggest that septic-derived dissolved organic matter with a C:N ratio below 6 fueled microbial metabolism and SR at a site with development in the upland. Seasonality was observed in the porewater inventories, but temperature alone did not adequately describe the patterns of (SO₄²⁻)_Dep, terminal metabolic products (DIC, NH₄⁺, PO₄³⁻), DOC and DON, and SR observed in this study. It appears that production and consumption of labile DOC are tightly coupled in these sediments, and that bulk DOC is likely a recalcitrant pool. Preferential hydrolysis of PON relative to POC when overall organic matter mineralization rates were high appears to drive the observed patterns in POC:PON, DOC:DON and DIC:DIN ratios. These data, along with the weak seasonal patterns of SR and organic and inorganic porewater inventories, suggest that the rate of hydrolysis limits organic matter mineralization in these intertidal creek-bank sediments.