Distribution of dissolved organic carbon and nitrogen in a coral reef

Dissolved organic matter (DOM) concentrations in a fringing coral reef were measured for both carbon and nitrogen with the analytical technique of high-temperature catalytic oxidation. Because of high precision of the analytical system, not only the concentrations of dissolved organic carbon and nitrogen (DOC and DON, respectively) but the C:N ratio was also determined from the distribution of DOC and DON concentrations. The observed concentrations of DOC and DON ranged 57–76 and 3.8–5.6 μmol l⁻¹, respectively. The C:N ratios of the DOM that was produced on the reef flat were very similar between seagrass- and coral-dominated areas; the C:N ratio was 10 on average. The C:N ratio of DOM was significantly higher than that of particulate organic matter (POM) that was produced on the reef flat. Production rates of DOC were measured on the reef flat during stagnant periods and accounted for 3–7% of the net primary production, depending on the sampling site. The production rate of DON was estimated to be 10–30% of the net uptake of dissolved inorganic N in the reef community. Considering that the DOM and POM concentrations were not correlated with each other, a major source of the reef-derived DOM may be the benthic community and not POM such as phytoplankton. It was concluded that a widely distributed benthic community in the coral reef released C-rich DOM to the overlying seawater, conserving N in the community.     

Colloidal and dissolved organic matter in lake water: Carbohydrate and amino acid composition,and ability to support bacterial growth

Bacterial utilization of dissolved organic matter (DOM) was studied in water from a humic and a clearwater oligotrophic lake. Indigenous bacteria were inoculated into either 0.2 m natural filtered lake water, or lake water enriched fivefold with colloidal DOM >100 kD but below 0.2 m. Consumption of DOM was followed from changes in concentrations of total dissolved organic carbon (DOC), dissolved combined and free carbohydrates and amino acids (DCCHO and DFCHO, and DCAA and DFAA, respectively) and by uptake of monosaccharide and amino acid radioisotopes. DCCHO and DCAA made up 8% (humic lake) to 33–44% (clear-water lake) of the natural DOC pools, while DFCHO and DFAA contributed at most 1.7% to the DOC pools. Addition of >100 kD DOM increased the DOC concentrations by 50% (clearwater lake) to 92% (humic lake), but it only resulted in a higher bacterial production (by 63%) in the humic lake. During the incubations 13 to 37% of the DOC was assimilated by the bacteria, at estimated growth efficiencies of 4–8%. Despite the measured reduction of DOC, statistically significant changes of specific organic compounds, especially of DCCHO and DCAA, generally did not occur. Probably the presence of high molecular weight DOC interfered with the applied analytical procedures. Addition of radiotracers indicated, however, that DFAA sustained 17–58% and 29–100% of the bacterial carbon and nitrogen requirements, respectively, and that glucose met 1–3% of the bacterial carbon requirements. Thus, our experiments indicate that radiotracers, rather than measurements of concentration changes, should be used in studies of bacterial utilization of DOC in freshwaters with a high content of humic or high molecular weight organic matter.     

Dissolved organic matter in Arctic multi-year sea ice during winter: major components and relationship to ice characteristics

Ice cores were collected between 10.03.93 and 15.03.93 along a 200 m profile on a large ice floe in Fram Strait. The ice was typical of Arctic multi-year ice, having a mean thickness along the profile of 2.56 ±0.53 m. It consisted mostly of columnar ice (83%) grown through congelation of seawater at the ice bottom, and the salinity profiles were characterized by a linear increase from 0 psu at the top to values ranging between 3 and 5 psu at depth. Distributions of dissolved organic carbon (DOC) and nitrogen (DON) and major nutrients were compared with ice texture, salinity and chlorophyll a. DOC, DON, dissolved inorganic nitrogen (DIN), NH₄ ⁺ and NO₂ ^– were present in concentrations in excess of that predicted by dilution curves derived from Arctic surface water values. Only NO₃ ^– was depleted, although not exhausted. High DOC and DON values in conjunction with high NH₄ ⁺ levels indicated that a significant proportion of the dissolved organic matter (DOM) was a result of decomposition/grazing of ice algae and/or detritus. The combination of high NH₄ ⁺ and NO₂ ^– points to regeneration of nitrogen compounds. There was no significant correlation between DOC and Chl a in contrast to DON, which had a positively significant correlation with both salinity and Chl a, and the distribution of DOM in the cores might best be described as a combination of both physical and biological processes. There was no correlation between DOC and DON suggesting an uncoupling of DOC and DON dynamics in multi year ice.     

Depleted dissolved organic carbon and distinct bacterial communities in the water column of a rapid-flushing coral reef ecosystem

Coral reefs are highly productive ecosystems bathed in unproductive, low-nutrient oceanic waters, where microbially dominated food webs are supported largely by bacterioplankton recycling of dissolved compounds. Despite evidence that benthic reef organisms efficiently scavenge particulate organic matter and inorganic nutrients from advected oceanic waters, our understanding of the role of bacterioplankton and dissolved organic matter (DOM) in the interaction between reefs and the surrounding ocean remains limited. In this study, we present the results of a 4-year study conducted in a well-characterized coral reef ecosystem (Paopao Bay, Moorea, French Polynesia) where changes in bacterioplankton abundance and dissolved organic carbon (DOC) concentrations were quantified and bacterial community structure variation was examined along spatial gradients of the reef:ocean interface. Our results illustrate that the reef is consistently depleted in concentrations of both DOC and bacterioplankton relative to offshore waters (averaging 79 μmol l⁻¹ DOC and 5.5 × 10⁸ cells l⁻¹ offshore and 68 μmol l⁻¹ DOC and 3.1 × 10⁸ cells l⁻¹ over the reef, respectively) across a 4-year time period. In addition, using a suite of culture-independent measures of bacterial community structure, we found consistent differentiation of reef bacterioplankton communities from those offshore or in a nearby embayment across all taxonomic levels. Reef habitats were enriched in Gamma-, Delta-, and Betaproteobacteria, Bacteriodetes, Actinobacteria and Firmicutes. Specific bacterial phylotypes, including members of the SAR11, SAR116, Flavobacteria, and Synechococcus clades, exhibited clear gradients in relative abundance among nearshore habitats. Our observations indicate that this reef system removes oceanic DOC and exerts selective pressures on bacterioplankton community structure on timescales approximating reef water residence times, observations which are notable both because fringing reefs do not exhibit long residence times (unlike those characteristic of atoll lagoons) and because oceanic DOC is generally recalcitrant to degradation by ambient microbial assemblages. Our findings thus have interesting implications for the role of oceanic DOM and bacterioplankton in the ecology and metabolism of reef ecosystems.     

Bacterial Contribution to Dissolved Organic Matter in Eutrophic Lake Kasumigaura,Japan

Incubation experiments using filtered waters from Lake Kasumigaura were conducted to examine bacterial contribution to a dissolved organic carbon (DOC) pool. Bacterial abundance, bacterial production, concentrations of DOC, total dissolved amino acids (TDAA), and total dissolved neutral sugars (TDNS) were monitored during the experiments. Bacterial production during the first few days was very high (20 to 35 μg C liter⁻¹ day⁻¹), accounting for 40 to 70% of primary production. The total bacterial production accounted for 34 to 55% of the DOC loss during the experiment, indicating high bacterial activities in Lake Kasumigaura. The DOC degradation was only 12 to 15%, whereas the degradation of TDAA and TDNS ranged from 30 to 50%, suggesting the preferential usage of TDAA and TDNS. The contribution of bacterially derived carbon to a DOC pool in Lake Kasumigaura was estimated using d-amino acids as bacterial biomarkers and accounted for 30 to 50% of the lake DOC. These values were much higher than those estimated for the open ocean (20 to 30%). The ratio of bacterially derived carbon to bulk carbon increased slightly with time, suggesting that the bacterially derived carbon is more resistant to microbial degradation than bulk carbon. This is the first study to estimate the bacterial contribution to a DOC pool in freshwater environments. These results indicate that bacteria play even more important roles in carbon cycles in freshwater environments than in open oceans and also suggests that recent increases in recalcitrant DOC in various lakes could be attributed to bacterially derived carbon. The potential differences in bacterial contributions to dissolved organic matter (DOM) between freshwater and marine environments are discussed.     

A stormflow/baseflow comparison of dissolved organic matter concentrations and bioavailability in an Appalachian stream

Patterns of dissolved organic carbon (DOC) and nitrogen (DON) delivery were compared between times of stormflow and baseflow in Paine Run, an Appalachian stream draining a 12.4 km² forested catchment in the Shenandoah National Park (SNP), Virginia. The potential in-stream ecological impact of altered concentrations and/or chemical composition of DOM during storms also was examined, using standardized bacterial bioassays. DOC and DON concentrations in Paine Run were consistently low during baseflow and did not show a seasonal pattern. During storms however, mean DOC and DON concentrations approximately doubled, with maximum concentrations occurring on the rising limb of storm hydrographs. The rapid response of DOM concentration to changes in flow suggests a near-stream or in-stream source of DOM during storms. Stormflow (4% of the time, 36% of the annual discharge) contributed >50% of DOC, DON and NO₃ ^– flux in Paine Run during 1997. In laboratory bacterial bioassays, growth rate constants were higher on Paine Run stormflow water than on baseflow water, but the fraction of total DOM which was bioavailable was not significantly different. The fraction of the total stream DOC pool taken up by water column bacteria was estimated to increase from 0.03 ± 0.02% h^–1 during baseflow, to 0.15 ± 0.04% h^–1 during storms. This uptake rate would have a minimal effect on bulk DOM concentrations in Paine Run, but storms may still have considerable impact on the bacterial stream communities by mobilizing them into the water column and by supplying a pulse of DOM.     

Dissolved organic matter under native Cerrado and Pinus caribaea plantations in the Brazilian savanna

The transformation of native Cerrado into Pinus caribaea Morelet plantations changes the DOM dynamics including changed rates of mineralisation, denitrification, and C export to the groundwater. To examine the differences in quantity, temporal dynamics, and quality of DOM between Cerrado and Pine plantations we collected rainfall, throughfall, stemflow, litter leachate (under pine only) and soil solution at 15, 80, and 200 cm depth in weekly intervals during the rainy seasons 1997/98 and 1998/99. We determined total dissolved organic carbon (DOC) concentrations and assessed DOM quality by separating hydrophilic and hydrophobic fractions and by NMR analysis of organic layer extracts. The rainfall had a mean DOC concentration of 2.6 mg L^–1. The mean concentrations of DOC in the throughfall of the pine plantations (5.0–10.5 mg L^–1) were significantly above those of Cerrado (2.6–4.9 mg L^–1). During the first part of the rainy seasons (October–December), the concentrations of DOC in the soil solution (15–200 cm depth) under Cerrado and pine did not differ significantly. During the second part of both rainy seasons (January–April), the concentrations of DOC in the soil solution under Cerrado (4.4–5.1 mg L^–1) exceeded those under PI (1.4–2.7 mg L^–1). Possible explanations of the latter include higher DOM input into the Cerrado soil and a stronger retention and/or faster mineralisation of the pine DOM than of the Cerrado DOM in the mineral soil. As the structural composition of DOM extracted from the organic layer under Cerrado and pine did not differ significantly, faster mineralisation was the most likely explanation for partly lower DOC concentrations in the soil solution under pine than under Cerrado. This assumption was supported by increasing contributions of hydrophobic DOM to total DOM with increasing depth under pine while, under Cerrado, the DOM composition did not change with depth. The reason for DOM mineralisation under pine was probably the higher N availability because total N concentrations were 11–23 times higher under pine than under Cerrado.     

High molecular weight dissolved organic matter enrichment selects for methylotrophs in dilution to extinction cultures

The role of bacterioplankton in the cycling of marine dissolved organic matter (DOM) is central to the carbon and energy balance in the ocean, yet there are few model organisms available to investigate the genes, metabolic pathways, and biochemical mechanisms involved in the degradation of this globally important carbon pool. To obtain microbial isolates capable of degrading semi-labile DOM for growth, we conducted dilution to extinction cultivation experiments using seawater enriched with high molecular weight (HMW) DOM. In total, 93 isolates were obtained. Amendments using HMW DOM to increase the dissolved organic carbon concentration 4x (280 μM) or 10x (700 μM) the ocean surface water concentrations yielded positive growth in 4–6% of replicate dilutions, whereas <1% scored positive for growth in non-DOM-amended controls. The majority (71%) of isolates displayed a distinct increase in cell yields when grown in increasing concentrations of HMW DOM. Whole-genome sequencing was used to screen the culture collection for purity and to determine the phylogenetic identity of the isolates. Eleven percent of the isolates belonged to the gammaproteobacteria including Alteromonadales (the SAR92 clade) and Vibrio. Surprisingly, 85% of isolates belonged to the methylotrophic OM43 clade of betaproteobacteria, bacteria thought to metabolically specialize in degrading C1 compounds. Growth of these isolates on methanol confirmed their methylotrophic phenotype. Our results indicate that dilution to extinction cultivation enriched with natural sources of organic substrates has a potential to reveal the previously unsuspected relationships between naturally occurring organic nutrients and the microorganisms that consume them.     

Dissolved Organic Matter concentration and composition in the forests and streams of Olympic National Park, WA

Dissolved organic carbon (DOC) concentration and dissolved organic matter (DOM) character were investigated in soil water (15 and 40 cm) and streams at eleven sites in Olympic National Park. In addition, the effect of added nitrogen on soil water DOM concentration and composition was tested. Forested plots covering a gradient of precipitation, climate, slope, and aspect in Olympic National Park were fertilized with the addition of 20, 10 and zero (control) kg urea-N ha^–1 y^–1. Seven sites had the two different fertilizer treatments and control plots, while the additional four sites had no fertilizer treatments. Soil water DOC concentrations ranged from 0.5 mg C/L to 54.1 mg C/L, with an average value of 14.1 mg C/L. Streams had low DOC concentrations ranging from 0.2 mg C/L to 4.4 mg C/L, with an average value of 1.2 mg C/L. DOM composition was examined with regard to molar ratios, H:C, O:C and N:C, index of unsaturation, average carbon oxidation state, and specific absorbance. Fertilizer had no consistent effect on either DOM concentration or composition across the study sites. Soil depth influenced both DOM concentration and composition. Shallow soil water DOM had greater concentrations, higher specific absorbance, a higher degree of unsaturation, and had lower molar ratios compared to deep soil water samples. Overall, changes in DOM stoichiometry and specific absorbance as a function of soil depth were consistent despite the diversity of the forested study sites sampled.     

氮沉降对杉木人工林土壤可溶性有机质数量和结构的影响

氮沉降作为现在乃至未来气候变化的趋势之一,其可能深刻影响土壤可溶性有机质的数量和结构。选取我国中亚热带杉木人工林不同深度土壤（0-10 cm和10-20 cm）进行氮沉降试验,利用光谱技术研究氮沉降对土壤可溶性有机质数量和结构的影响。试验设对照（CT,0 kg hm^-2 a^-1）、高氮（HN：80 kg hm^-2 a^-1）、低氮（LN：40 kg hm^-2 a^-1）3种处理。结果表明：（1）在0-10 cm和10-20 cm土层,HN、LN处理的土壤可溶性有机碳和可溶性有机氮含量显著高于CT。（2）在0-10 cm和10-20 cm土层,1月时HN、LN处理的芳香性指数和腐殖化程度都显著高于CT,而4月时HN、LN处理的芳香性指数和腐殖化程度都显著低于CT。除了氮含量的直接影响外,RDA （冗余分析）表明,两土层中土壤含水量、pH和土壤有机碳是驱动氮沉降对土壤DOM数量和结构的重要环境因子。因此,氮沉降对土壤DOM的影响是复杂的,未来尤其应该注重氮沉降对生态系统影响的季节模式。     

Stream dissolved organic matter bioavailability and composition in watersheds underlain with discontinuous permafrost

We examined the impact of permafrost on dissolved organic matter (DOM) composition in Caribou-Poker Creeks Research Watershed (CPCRW), a watershed underlain with discontinuous permafrost, in interior Alaska. We analyzed long term data from watersheds underlain with varying degrees of permafrost, sampled springs and thermokarsts, used fluorescence spectroscopy, and measured the bioavailabity of dissolved organic carbon (DOC). Permafrost driven patterns in hydrology and vegetation influenced DOM patterns in streams, with the stream draining the high permafrost watershed having higher DOC and dissolved organic nitrogen (DON) concentrations, higher DOC:DON and greater specific ultraviolet absorbance (SUVA) than the streams draining the low and medium permafrost watersheds. Streams, springs and thermokarsts exhibited a wide range of DOC and DON concentrations (1.5–37.5 mgC/L and 0.14–1.26 mgN/L, respectively), DOC:DON (7.1–42.8) and SUVA (1.5–4.7 L mgC⁻¹ m⁻¹). All sites had a high proportion of humic components, a low proportion of protein components, and a low fluorescence index value (1.3–1.4), generally consistent with terrestrially derived DOM. Principal component analysis revealed distinct groups in our fluorescence data determined by diagenetic processing and DOM source. The proportion of bioavailable DOC ranged from 2 to 35%, with the proportion of tyrosine- and tryptophan-like fluorophores in the DOM being a major predictor of DOC loss (p < 0.05, R ² = 0.99). Our results indicate that the degradation of permafrost in CPCRW will result in a decrease in DOC and DON concentrations, a decline in DOC:DON, and a reduction in SUVA, possibly accompanied by a change in the proportion of bioavailable DOC.     

Cultivation and Growth Characteristics of a Diverse Group of Oligotrophic Marine Gammaproteobacteria

Forty-four novel strains of Gammaproteobacteria were cultivated from coastal and pelagic regions of the Pacific Ocean using high-throughput culturing methods that rely on dilution to extinction in very low nutrient media. Phylogenetic analysis showed that the isolates fell into five rRNA clades, all of which contained rRNA gene sequences reported previously from seawater environmental gene clone libraries (SAR92, OM60, OM182, BD1-7, and KI89A). Bootstrap analyses of phylogenetic reliability did not support collapsing these five clades into a single clade, and they were therefore named the oligotrophic marine Gammaproteobacteria (OMG) group. Twelve cultures chosen to represent the five clades were successively purified in liquid culture, and their growth characteristics were determined at different temperatures and dissolved organic carbon concentrations. The isolates in the OMG group were physiologically diverse heterotrophs, and their physiological properties generally followed their phylogenetic relationships. None of the isolates in the OMG group formed colonies on low- or high-nutrient agar upon their first isolation from seawater, while 7 of 12 isolates that were propagated for laboratory testing eventually produced colonies on 1/10 R2A agar. The isolates grew relatively slowly in natural seawater media (1.23 to 2.63 day⁻¹), and none of them grew in high-nutrient media (>351 mg of C liter⁻¹). The isolates were psychro- to mesophilic and obligately oligotrophic; many of them were of ultramicrobial size (<0.1 μm³). This cultivation study revealed that sporadically detected Gammaproteobacteria gene clones from seawater are part of a phylogenetically diverse constellation of organisms mainly composed of oligotrophic and ultramicrobial lineages that are culturable under specific cultivation conditions.     

Net release of dissolved organic matter by the scleractinian coral Acropora pulchra

The net production of dissolved organic matter (DOM) and dissolved combined and free amino acids (DCAA and DFAA, respectively) by the hermatypic coral Acropora pulchra was measured in the submerged condition, and the production rates were normalized to the coral surface area, tissue biomass, and net photosynthetic rates by zooxanthellae. When normalized to the unit surface area, the production rates of dissolved organic carbon and nitrogen (DOC and DON, respectively) were 37 and 4.4 nmol cm^− 2 h^− 1, respectively. Comparing with the photosynthetic rate by zooxanthellae, which was measured by ¹³C-tracer accumulation in the soft tissue of the coral colony, the release rate of DOC corresponded to 5.4% of the daily net photosynthetic production. The tissue biomass of the coral colony was 178 µmol C cm^− 2 and 23 µmol N cm^− 2, indicating that the release of DOC and DON accounted for 0.021% h^− 1 and 0.019% h^− 1 of the tissue C and N, respectively. The C:N ratios of the released DOM (average 8.4) were not significantly different from those of the soft tissue of the coral colonies (average 7.7). While DFAA did almost not accumulate in the incubated seawater, DCAA was considerably released by the coral colonies at the rate of 2.1 nmol cm^− 2 h^− 1 on average. Calculating C and N contents of the hydrolyzable DCAA, it was revealed that about 20% and 50%–60% of the released bulk DOC and DON, respectively, were composed of DCAA.     

Fluorescence characteristics and biodegradability of dissolved organic matter in forest and wetland soils from coastal temperate watersheds in southeast Alaska

Understanding how the concentration and chemical quality of dissolved organic matter (DOM) varies in soils is critical because DOM influences an array of biological, chemical, and physical processes. We used PARAFAC modeling of excitation–emission fluorescence spectroscopy, specific UV absorbance (SUVA₂₅₄) and biodegradable dissolved organic carbon (BDOC) incubations to investigate the chemical quality of DOM in soil water collected from 25 cm piezometers in four different wetland and forest soils: bog, forested wetland, fen and upland forest. There were significant differences in soil solution concentrations of dissolved organic C, N, and P, DOC:DON ratios, SUVA₂₅₄ and BDOC among the four soil types. Throughout the sampling period, average DOC concentrations in the four soil types ranged from 9–32 mg C l⁻¹ and between 23–42% of the DOC was biodegradable. Seasonal patterns in dissolved nutrient concentrations and BDOC were observed in the three wetland types suggesting strong biotic controls over DOM concentrations in wetland soils. PARAFAC modeling of excitation–emission fluorescence spectroscopy showed that protein-like fluorescence was positively correlated (r ² = 0.82; P < 0.001) with BDOC for all soil types taken together. This finding indicates that PARAFAC modeling may substantially improve the ability to predict BDOC in natural environments. Coincident measurements of DOM concentrations, BDOC and PARAFAC modeling confirmed that the four soil types contain DOM with distinct chemical properties and have unique fluorescent fingerprints. DOM inputs to streams from the four soil types therefore have the potential to alter stream biogeochemical processes differently by influencing temporal patterns in stream heterotrophic productivity.     

Effects of Dissolved and Complexed Copper on Heterotrophic Bacterial Production in San Diego Bay

Bacterial abundance and production, free (uncomplexed) copper ion concentration, total dissolved copper concentration, dissolved organic carbon (DOC), total suspended solids (TSS), and chlorophyll a were measured over the course of 1 year in a series of 27 sample “Boxes” established within San Diego Bay. Water was collected through a trace metal-clean system so that each Box’s sample was a composite of all the surface water in that Box. Bacterial production, chlorophyll a, TSS, DOC, and dissolved copper all generally increased from Box 1 at the mouth of the Bay to Box 27 in the South or back Bay. Free copper ion concentration generally decreased from Box 1 to Box 27 presumably due to increasing complexation capacity within natural waters. Based on correlations between TSS, chlorophyll a, bacterial production or DOC and the ratio of dissolved to free Cu ion, both DOC and particulate (bacteria and algae) fractions were potentially responsible for copper complexation, each at different times of the year. CuCl₂ was added to bacterial production assays from 0 to 10 μg L⁻¹ to assess acute copper toxicity to the natural microbial assemblage. Interestingly, copper toxicity appeared to increase with decreases in free copper from the mouth of the Bay to the back Bay. This contrasts the free-ion activity model in which higher complexation capacity should afford greater copper protection. When cell-specific growth rates were calculated, faster growing bacteria (i.e. toward the back Bay) appeared to be more susceptible to free copper toxicity. The protecting effect of natural dissolved organic material (DOM) concentrated by tangential flow ultrafiltration (>1 kDa), illite and kaolinite minerals, and glutathione (a metal chelator excreted by algae under copper stress) was assessed in bacterial production assays. Only DOM concentrate offered any significant protection to bacterial production under increased copper concentrations. Although the potential copper protecting agents were allowed to interact with added copper before natural bacteria were added to production assays, there may be a temporal dose–response relationship that accounts for higher toxicity in short production assays. Regardless, it appears that effective natural complexation of copper in the back portions of San Diego Bay limits exposure of native bacterial assemblages to free copper ion, resulting in higher bacterial production.     

Detrital Controls on Soil Solution N and Dissolved Organic Matter in Soils: A Field Experiment

We established a long-term field study in an old growth coniferous forest at the H.J. Andrews Experimental Forest, OR, USA, to address how detrital quality and quantity control soil organic matter accumulation and stabilization. The Detritus Input and Removal Treatments (DIRT) plots consist of treatments that double leaf litter, double woody debris inputs, exclude litter inputs, or remove root inputs via trenching. We measured changes in soil solution chemistry with depth, and conducted long-term incubations of bulk soils from different treatments in order to elucidate effects of detrital inputs on the relative amounts and lability of different soil C pools. In the field, the addition of woody debris increased dissolved organic carbon (DOC) concentrations in O-horizon leachate and at 30 cm, but not at 100 cm, compared to control plots, suggesting increased rates of DOC retention with added woody debris. DOC concentrations decreased through the soil profile in all plots to a greater degree than did dissolved organic nitrogen (DON), most likely due to preferential sorption of high C:N hydrophobic dissolved organic matter (DOM) in upper horizons; percent hydrophobic DOM decreased significantly with depth, and hydrophilic DOM had a much lower and less variable C:N ratio. Although laboratory extracts of different litter types showed differences in DOM chemistry, percent hydrophobic DOM did not differ among soil solutions from different detrital treatments in the field, suggesting that microbial processing of DOM leachate in the field consumed easily degradable components, thus equalizing leachate chemistry among treatments. Total dissolved N leaching from plots with intact roots was very low (0.17 g m⁻² year⁻¹), slightly less than measured deposition to this very unpolluted forest (~s 0.2 g m⁻² year⁻¹). Total dissolved N losses showed significant increases in the two treatments without roots whereas concentrations of DOC decreased. In these plots, N losses were less than half of estimated plant uptake, suggesting that other mechanisms, such as increased microbial immobilization of N, accounted for retention of N in deep soils. In long-term laboratory incubations, soils from plots that had both above- and below-ground litter inputs excluded for 5 years showed a trend towards lower DOC loss rates, but not lower respiration rates. Soils from plots with added wood had similar respiration and DOC loss rates as control soils, suggesting that the additional DOC sorption observed in the field in these soils was stabilized in the soil and not readily lost upon incubation.     

Hydrologic Drivers and Seasonality of Dissolved Organic Carbon Concentration, Nitrogen Content, Bioavailability, and Export in a Forested New England Stream

We present the results of a full year of high-resolution monitoring of hydrologic event-driven export of stream dissolved organic matter (DOM) from the forested Bigelow Brook watershed in Harvard Forest, Massachusetts, USA. A combination of in situ fluorescent dissolved organic matter (FDOM) measurement, grab samples, and bioassays was utilized. FDOM was identified as a strong indicator of concentration for dissolved organic carbon (DOC, r ² = 0.96), dissolved organic nitrogen (DON, r ² = 0.81), and bioavailable DOC (BDOC, r ² = 0.81). Relationships between FDOM and concentration were utilized to improve characterization of patterns of hydrological event-driven export and the quantification of annual export. This characterization was possible because DOM composition remained relatively consistent seasonally; however, a subtle shift to increased fluorescence per unit absorbance was observed for summer and fall seasons and percent BDOC did increase slightly with increasing concentrations. The majority of export occurred during pulsed hydrological events, so the greatest impact of bioavailable exports may be on downstream aquatic ecosystems. Export from individual events was highly seasonal in nature with the highest flow weighted mean concentrations (DOC_FW) being observed in late summer and fall months, but the highest total export being observed for larger winter storms. Seasonal trends in DOC export coincide with weather driven changes in surface and subsurface flow paths, potential for depletion and rebuilding of a flushable soil organic matter pool, and the availability of terrestrial carbon sources such as leaf litter. Our approach and findings demonstrate the utility of high frequency FDOM measurement to improve estimates of intra-annual temporal trends of DOM export.     

Distinct Optical Chemistry of Dissolved Organic Matter in Urban Pond Ecosystems

Urbanization has the potential to dramatically alter the biogeochemistry of receiving freshwater ecosystems. We examined the optical chemistry of dissolved organic matter (DOM) in forty-five urban ponds across southern Ontario, Canada to examine whether optical characteristics in these relatively new ecosystems are distinct from other freshwater systems. Dissolved organic carbon (DOC) concentrations ranged from 2 to 16 mg C L^-1 across the ponds with an average value of 5.3 mg C L^-1. Excitation-emission matrix (EEM) spectroscopy and parallel factor analysis (PARAFAC) modelling showed urban pond DOM to be characterized by microbial-like and, less importantly, by terrestrial derived humic-like components. The relatively transparent, non-humic DOM in urban ponds was more similar to that found in open water, lake ecosystems than to rivers or wetlands. After irradiation equivalent to 1.7 days of natural solar radiation, DOC concentrations, on average, decreased by 38% and UV absorbance decreased by 25%. Irradiation decreased the relative abundances of terrestrial humic-like components and increased protein-like aspects of the DOM pool. These findings suggest that high internal production and/or prolonged exposure to sunlight exerts a distinct and significant influence on the chemistry of urban pond DOM, which likely reduces its chemical similarity with upstream sources. These properties of urban pond DOM may alter its biogeochemical role in these relatively novel aquatic ecosystems.     

Sorption of dissolved organic matter by mineral soils of the Siberian forest tundra

Because of low net production in arctic and subarctic surface water, dissolved organic matter (DOM) discharged from terrestrial settings plays an important role for carbon and nitrogen dynamics in arctic aquatic systems. Sorption, typically controlling the export of DOM from soil, may be influenced by the permafrost regime. To confirm the potential sorptive control on the release of DOM from permafrost soils in central northern Siberia, we examined the sorption of DOM by mineral soils of Gelisols and Inceptisols with varying depth of the active layer. Water‐soluble organic matter in the O horizons of the Gelisols was less (338 and 407 mg C kg^?1) and comprised more dissolved organic carbon (DOC) in the hydrophobic fraction (HoDOC) (63% and 70%) than in the O horizons of the Inceptisols (686 and 706 mg C kg^?1, 45% and 48% HoDOC). All A and B horizons from Gelisols sorbed DOC strongly, with a preference for HoDOC. Almost all horizons of the Inceptisols showed a weaker sorption of DOC than those of the Gelisols. The C horizons of the Inceptisols, having a weak overall DOC sorption, sorbed C in the hydrophilic fraction (HiDOC) stronger than HoDOC. The reason for the poor overall sorption and also the preferential sorption of HiDOC is likely the high pH (pH>7.0) of the C horizons and the smaller concentrations of iron oxides. For all soils, the sorption of HoDOC related positively to oxalate‐ and dithionite–citrate‐extractable iron. The A horizons released large amounts of DOC with 46–80% of HiDOC. The released DOC was significantly (r=0.78, P<0.05) correlated with the contents of soil organic carbon. From these results, we assume that large concentrations of DOM comprising large shares of HiDOC can pass mineral soils where the active layer is thin (i.e. in Gelisols), and enter streams. Soils with deep active layer (i.e. Inceptisols), may release little DOM because of more frequent infiltration of DOM into their thick mineral horizons despite their smaller contents of reactive, poorly crystalline minerals. The results obtained for the Inceptisols are in agreement with the situation observed for streams connecting to Yenisei at lower latitudes than 65°50′ with continuous to discontinuous permafrost. The smaller sorption of DOM by the Gelisols is in agreement with the larger DOM concentrations in more northern catchments. However, the Gelisols preferentially retained the HoDOC which dominates the DOC in streams towards north. This discrepancy can be explained by additional seepage water from the organic horizons that is discharged into streams without intensive contact with the mineral soil.