Extractable Dissolved Organic Carbon Research Articles

Assessment of Aqueous Extraction Methods on Extractable Organic Matter and Hydrophobic/Hydrophilic Fractions of Virgin Forest Soils

The assessment of water-extractable organic matter using an autoclave can provide useful information on physical, chemical, and biological changes within the soil. The present study used virgin forest soils from Chini Forest Reserve, Langkawi Island, and Kenyir Forest Reserve (Malaysia), extracted using different extraction methods. The dissolved organic carbon (DOC), total dissolved nitrogen (TDN), total dissolved phosphorus (TDP), and ammonium-nitrate content were higher in the autoclave treatments, up to 3.0, 1.3, 1.2, and 1.4 times more than by natural extraction (extracted for 24 h at room temperature). Overall, the highest extractable DOC, TDN, TDP, ammonium and nitrate could be seen under autoclaved conditions 121 °C 2×, up to 146.74 mg C/L, 8.97 mg N/L, 0.23 mg P/L, 5.43 mg N mg/L and 3.47 N mg/L, respectively. The soil extracts became slightly acidic with a higher temperature and longer duration. Similar trends were observed in the humic and nonhumic substances, where different types of soil extract treatments influenced the concentrations of the fractions. Different soil extraction methods can provide further details, thus widening the application of soil extracts, especially in microbes.

Effects of composited organic mobilizing agents and their application periods on cadmium absorption of Sorghum bicolor L. in a Cd-contaminated soil

Organic mobilizing agents have been advocated for phytoremediation of heavy metals contaminated soils, while the effects of application period of such agents remain unclear. A pot experiment was conducted, with two composited organic agents (oxalic acid or citric acid + dissolved organic fertilizer (OA + DOF and CA + DOF)) and four application periods (seeding, jointing, flag leaf and heading stages) of sorghum (Sorghum bicolor L.), to investigate their impacts on Cd bioavailability in soil. Results indicated that application of the two composited agents increased soil dissolved organic carbon (DOC) and DTPA extractable Cd by 7.31–49.13%, Cd contents in roots and shoots by 21.49–72.10%, bioaccumulation factor (BCF) and translocation factor (TF) of shoots by 4.44–71.99%, while reduced soil pH by 0.25–0.53 units, respectively. Most of these indices increased with the application periods, and largely peaked with their application during the flag leaf to heading stages. Meanwhile, the maximum sorghum biomass (132.84 g pot−1) and Cd bioaccumulation quantity (BCQ, 0.71 mg pot−1) in shoots were obtained for the CA + DOF applied at the heading. The DTPA extractable Cd was closely related to soil pH and DOC. Similar close relationships were observed between the Cd contents in shoots and soil DTPA extractable Cd, pH and DOC. The BCQ of Cd was positively related to the shoots biomass rather than their Cd contents. Therefore, the sorghum combined with the CA + DOF may be advocated as an alternative phytoremediation mode in Cd-contaminated soils, and the mobilizing agent should be primarily applied at the heading stage.

The role of the understory in litter DOC and nutrient leaching in boreal forests

Dissolved organic carbon (DOC) derived from plant litter plays an important role in the ecosystem carbon balance and soil biogeochemistry. However, in boreal coniferous forests no integrated understanding exists of how understory vegetation contributes to litter leaching of DOC, nitrogen (N) and phosphorus (P) with different bioavailability at the forest stand level. We characterized water extractable leachates from fresh and decayed litter of dominant canopy and understory sources in a boreal coniferous forest, in order to explore the contribution of understory vegetation as a source of both total and bioavailable forms of DOC, N and P. Recently produced litter from deciduous species (including Vaccinium myrtillus) yielded the highest amounts of DOC. However, this leaching potential decreased exponentially with mass loss through litter decay. The DOC lability generally showed little interspecific variation, although wood derived DOC was more recalcitrant. Lability decreased progressively with litter aging. Water extractable nutrients increased proportionally with DOC, and roughly a quarter (N) or half (P) had directly bioavailable inorganic forms. Scaled to annual litterfall at the forest stand, understory vegetation contributed ~ 80% of the water extractable DOC and nutrients from fresh litter, with > 60% coming from Vaccinium myrtillus alone. However, as litter decomposes, the data suggest a lower leaching potential is maintained with a larger contribution from needle, wood and moss litter. Our study shows that understory vegetation, especially V. myrtillus, is a key driver of litter DOC and nutrient leaching in boreal coniferous forests.

Soil Aggregates as a Source of Dissolved Organic Carbon to Streams: An Experimental Study on the Effect of Solution Chemistry on Water Extractable Carbon

Over the past two decades, headwater streams of the northern hemisphere have shown increased amounts of dissolved organic carbon (DOC), coinciding with decreased acid deposition. The exact nature of the mechanistic link between precipitation composition and stream water DOC is still widely debated in the literature. We hypothesize that soil aggregates are the main source of stream water DOC and that DOC release is greater in organic rich, riparian soils versus hillslope soils. To test these hypotheses, we collected soils from two main landscape positions (hillslope and riparian zones) from the acid-impacted Sleepers River Research Watershed in northeastern Vermont. We performed aqueous soil extracts with solutions of different ionic strength (IS) and composition to simulate changes in soil solution. We monitored dynamic changes in soil particle size, aggregate architecture and composition, leachate DOC concentrations, dissolved organic matter (DOM) characteristics with fluorescence spectroscopy and trends in bioavailability. In low IS solutions, extractable DOC concentrations were significantly higher, particle size (by laser diffraction) was significantly smaller and organic material was separated from mineral particles in scanning electron microscope observations. Furthermore, higher DOC concentrations were found in Na+ compared to Ca2+ solutions of the same IS. These effects are attributed to aggregate dispersion due to expanding diffuse double layers in decreased IS solutions and to decreased bridging by divalent cations. Landscape position impacted quality but not quantity of released DOC. Overall, these results indicate that soil aggregates might be one important link between Critical Zone inputs (i.e. precipitation) and exports in streams.

Vulnerability of wetland soil carbon stocks to climate warming in the perhumid coastal temperate rainforest

The perhumid coastal temperate rainforest (PCTR) of southeast Alaska has some of the densest soil organic carbon (SOC) stocks in the world (>300 Mg C ha−1) but the fate of this SOC with continued warming remains largely unknown. We quantified dissolved organic carbon (DOC) and carbon dioxide (CO2) yields from four different wetland types (rich fen, poor fen, forested wetland and cedar wetland) using controlled laboratory incubations of surface (10 cm) and subsurface (25 cm) soils incubated at 8 and 15 °C for 37 weeks. Furthermore, we used fluorescence characterization of DOC and laboratory bioassays to assess how climate-induced soil warming may impact the quality and bioavailability of DOC delivered to fluvial systems. Soil temperature was the strongest control on SOC turnover, with wetland type and soil depth less important in controlling CO2 flux and extractable DOC. The high temperature incubation increased average CO2 yield by ~40 and ~25% for DOC suggesting PCTR soils contain a sizeable pool of readily biodegradable SOC that can be mineralized to DOC and CO2 with future climate warming. Fluxes of CO2 were positively correlated to both extractable DOC and percent bioavailable DOC during the last few months of the incubation suggesting mineralization of SOC to DOC is a strong control of soil respiration rates. Whether the net result is increased export of either carbon form will depend on the balance between the land to water transport of DOC and the ability of soil microbial communities to mineralize DOC to CO2.

Dissolved Black Carbon in the Headwaters-to-Ocean Continuum of Paraíba Do Sul River, Brazil

Rivers annually carry 25-28 Tg carbon in the form of pyrogenic dissolved organic matter (dissolved black carbon, DBC) into the ocean, which is equivalent to about 10% of the entire riverine land-ocean flux of dissolved organic carbon (DOC). The objective of this study was to identify the main processes behind the release and turnover of DBC on a riverine catchment scale. As a model system, we chose the headwater-to-ocean continuum of Paraiba do Sul River (Brazil), the only river system with long-term DBC flux data available. The catchment was originally covered by Atlantic rain forest (mainly C3 plants) which was almost completely destroyed over the past centuries by slash-and-burn. As a result, large amounts of wood-derived charcoal reside in the soils. Today, fire-managed pasture and sugar cane (both dominated by C4 plants) cover most of the catchment area. Water samples were collected along the river, at the main tributaries, and also along the salinity gradient in the estuary and up to 35 km offshore during three different seasons. DBC was determined on a molecular level as benzenepolycarboxylic acids (BPCAs). Stable carbon isotopes (δ13C) were determined in solid phase extractable DOC (SPE-DOC) to distinguish C4 and C3 sources. Our results clearly show a relationship between hydrology and DBC concentrations in the river, with highest DBC concentrations and fluxes in the wet season (flux of 770 moles .sec 1 in 2013 and 59 moles .sec 1 in 2014) and lowest in the dry season (flux of 27 moles .sec 1). This relationship indicates that DBC is mainly mobilized from the upper soil horizons during heavy rainfalls. The relationship between DBC concentrations and δ13C-SPE-DOC indicated that most of DBC in the river system originated from C3 plants, i.e. from the historic burning event of the Atlantic rain forest. A conservative mixing model could largely reproduce the observed DBC fluxes within the catchment and the land to ocean continuum. Comparably slight deviations from conservative mixing were accompanied by changes in the molecular composition of DBC (i.e. the ratio of benzenepenta- to benzenehexacarboxylic acid) that are indicative for photodegradation of DBC.

Adsorption behavior and mechanism of pentachlorophenol on reed biochars: pH effect, pyrolysis temperature, hydrochloric acid treatment and isotherms

This study aims to investigate the impacts of pH, inorganic fractions and dissolved organic carbon (DOC) on pentachlorophenol (PCP) adsorption on reed biochars prepared by pyrolysis temperatures ranging from 300°C to 600°C, as well as illustrating the adsorption mechanisms. The adsorption capacities for PCP on acid-washed reed biochars were higher than that of original reed biochars. The dedication of neutral PCP molecules (PCP0) and ionized PCP species (PCP−) to the overall adsorption at pH 3.0–12.0 was calculated. The relative contribution of PCP0 was above 53.4% at pH 3.0–5.0, and PCP− was dominant for adsorption at pH 6.0–12.0 with the contributions of 43.2%-100%. The inorganic fractions in the biochars present the negligible or negative effect on PCP adsorption due to its high hydrophilic. The water extractable DOC decreased the adsorption capacity of PCP on reed biochars. The isotherms of PCP adsorption on reed biochars were well described by Freundlich model, and the adsorption mechanisms were mainly interpreted as π–π and hydrophobic interactions. These observations are helpful for exploiting reed biochars as engineered absorbents to remove PCP from aqueous solution.

Effects of Organic Matter Concentration in Soil on Phytoavailability of Cadmium in Medicinal Plants

The safety of plant species used as a source for herbal medicines and dietary supplements has recently been questioned due to poisonings associated with the presence of cadmium (Cd) in these plants. These plants can derive Cd from their presence in the soil. Organic matter (OM) concentrations in soils could affect the availability of Cd for plants. To determine the effect of OM concentration in soil on the concentration of plant available Cd and uptake of this toxic element by medicinal plants, soil and plant samples were collected from 102 fields supporting for 5 species of medicinal plants in 6 province of South Korea. Concentrations of OM and dissolved organic carbon (DOC) in soils affected the phytoavailability of Cd. One M NH₄OAc extractable Cd concentration in soil increased with increasing OM concentrations. There were significantly positive relationships between 1 M NH₄OAc extractable Cd concentration and OM concentration in soil and between 1 M NH₄OAc extractable Cd concentration and DOC concentration. Likewise, OM and DOC concentrations significantly affected Cd concentration in medicinal plant soils. Cadmium concentration in medicinal plants increased with increasing OM concentration in soil [Cd concentration (mg kg -1) = 0.179 + 1.424 × 10 -3 OM concentrations, R 2 = 0.042 * ] and with DOC concentration [Cd concentration (mg kg -1 ) = 0.150 + 5.870 × 10 -4 DOC concentrations, R 2 = 0.124 *** ]. These results might result from Cd-DOC complex which is easily absorbed Cd form by plant root. Dissolved organic carbon concentration had more positive relationship with Cd concentration in medicinal plants and 1 M NH₄OAc extractable Cd concentration in soils than OM. Cadmium concentration in all 5 species of medicinal plant (Atractylodes macrocephala Koidzumi, Astragalus membranaceus, Codonopsis lanceolata, Platycodon grandiflorum, and Rehmannia glutinosa) significantly increased with increasing DOC concentration in soil. From the above results, formation of Cd-DOC complex caused by OM application might be mainly attributed to increase in Cd concentration in medicinal plants.

Efficient removal of recalcitrant deep-ocean dissolved organic matter during hydrothermal circulation

Oceanic dissolved organic carbon (DOC) is an important carbon pool, similar in magnitude to atmospheric CO2, but the fate of its oldest forms is not well understood. Hot hydrothermal circulation may facilitate the degradation of otherwise un-reactive dissolved organic matter, playing an important role in the long-term global carbon cycle. The oldest, most recalcitrant forms of DOC, which make up most of oceanic DOC, can be recovered by solid-phase extraction. Here we present measurements of solid-phase extractable DOC from samples collected between 2009 and 2013 at seven vent sites in the Atlantic, Pacific and Southern oceans, along with magnesium concentrations, a conservative tracer of water circulation through hydrothermal systems. We find that magnesium and solid-phase extractable DOC concentrations are correlated, suggesting that solid-phase extractable DOC is almost entirely lost from solution through mineralization or deposition during circulation through hydrothermal vents with fluid temperatures of 212-401°C. In laboratory experiments, where we heated samples to 380°C for four days, we found a similar removal efficiency. We conclude that thermal degradation alone can account for the loss of solid-phase extractable DOC in natural hydrothermal systems, and that its maximum lifetime is constrained by the timescale of hydrothermal cycling, at about 40 million years

Interactive effects of biochar ageing in soils related to feedstock, pyrolysis temperature, and historic charcoal production

Biochar is suggested for soil amelioration and carbon sequestration, based on its assumed role as the key factor for the long-term fertility of Terra preta soils. Several studies have shown that certain biochar properties can undergo changes through ageing processes, especially regarding charge characteristics. However, only a few studies determined the changes of different biochars under the same incubation conditions and in different soils. The objective of this study was to characterize the changes of pine chip (PC)- and corn digestate (CD)-derived biochars pyrolyzed at 400 or 600°C during 100days of laboratory incubation in a historical kiln soil and an adjacent control soil. Separation between soil and biochar was ensured by using mesh bags. Especially, changes in charge characteristics depended on initial biochar properties affected by feedstock and pyrolysis temperature and on soil properties affected by historic charcoal production. While the cation exchange capacity (CEC) markedly increased for both CD biochars during incubation, PC biochars showed no or only slight increases in CEC. Corresponding to the changes in CEC, ageing of biochars also increased the amount of acid functional groups with increases being in average about 2-fold higher in CD biochars than in PC biochars. Further and in contrast to other studies, the surface areas of biochars increased during ageing.Changes in CEC and surface acidity of CD biochar were more pronounced after incubation in the control soil, while surface area increase was higher in the kiln soil. Since the two acidic forest soils used in this study did not greatly differ in physical or chemical properties, the main process for inducing these differences in the buried biochar most likely is related to the differences in dissolved organic carbon (DOC). Although the kiln soil contained about 50% more soil organic carbon due to the presence of charcoal particles, extractable DOC was lower and less aromatic than in the adjacent control soil, likely due to strong sorption of dissolved organic matter (DOM) onto charcoal particles. We suggest that higher sorption of DOM onto the surface of biochar in the control soil provided additional acid functional groups and thus increased the surface charge to a greater extent than in the DOC poorer kiln soil. Hence, biochars incubated in the kiln soil showed less changes in CEC and surface acidity. Higher availability of DOM in the control soil could also stimulate microbial activity to a larger extent, resulting in higher oxidation rates of biochars incubated in the control soil.

Extractable DOC and DON from a dry-land long-term rotation and cropping system in Texas, USA

Soil solution chemistry may be responsible for the solubility and subsequent losses of DOC and DON from terrestrial ecosystems. The objective of this research was to determine whether there was a significant difference in recoverable soil dissolved organic carbon (DOC), organic nitrogen (DON), and dissolved inorganic nitrogen (DIN) extracted with cold and hot water, 10mM CaCl2, 2M KCl and 0.5M K2SO4 in four long-term agricultural tillage and cropping management systems and whether the type of extractant, tillage or cropping regime was responsible for decoupling the relationship between DOC and DON. The recovery of DOC and DON from soils was highly dependent upon the method of extraction. 10mM CaCl2 recovered the least DOC, which ranged from 49 to 139μgg−1, while 0.5M K2SO4 recovered the most DOC and ranged from 176 to 290μgg−1. Hot water extracted significantly greater DON than any of the other extractants among all cropping and tillage systems. The relationship between DON and DOC was strong and significant in hot and cold water extracts (R2=0.94 and 0.55 respectively), but their relationship was decoupled with chemical extractants and a continuous cropping.

Dissolved organic matter in relation to nutrients (N and P) and heavy metals in surface runoff water as affected by temporal variation and land uses – A case study from Indian River Area, south Florida, USA

Transport of dissolved organic matter in runoff water plays a critical role in the increased inputs of nutrients (N and P) and metals (Fe, Cu and Zn) in water, and the deposition of muck sediments in the Saint Lucie Estuary and Indian River Lagoon, Florida. The objectives of this study were to investigate the variation of dissolved organic carbon (DOC) and dissolved nitrogen (DN) in surface runoff water with varying land uses, and to explore their possible relations with the leaching losses of nutrients N and P, and metals Fe, Cu, and Zn from different land uses. For this, surface runoff water from two major agricultural production systems (citrus grove and vegetable farm) in south Florida was collected weekly using pre-installed autosamplers during Jan. 2008–Mar. 2010. Meanwhile, runoff water from twelve field sites with varying land uses (such as ranch, golf course, and forest) was collected monthly using a grab sampler from Aug. 2008 to April, 2010, and soils were collected at the depth of 0–15cm and 15–30cm twice a year from autosampler-installed locations and on Nov. 25th, 2008 from grab sampling locations as well. In the field, water samples were measured for electrical conductivity, dissolved oxygen, and temperature; in the laboratory, water samples were analyzed for pH, total solids, turbidity, DOC, DN, total P (TP), and metals Cu, Fe and Zn with or without passing a 0.45μm filter membrane. After passing a 2mm sieve, fresh soil samples were analyzed for moisture, pH, electrical conductivity, 0.5M K2SO4 solution extractable DOC/DN, and Mehlich 3 extractable P, Fe, Cu, and Zn. Spatial and temporal variations in the concentrations of DOC and DN in runoff water were observed, with the greatest average concentrations of DOC and DN from ranch (∼28 and ∼2.3mgL−1) and the lowest from golf course (∼16mgL−1) for DOC and from forest and nursery for DN (∼1.5–1.6mgL−1), respectively. Temporal fluctuations of DOC and DN in runoff water from varying land uses were in good accord with rainfall intensity and frequency of rainfall. Temporal variations of TP and metals Fe, Cu, and Zn were influenced by climatic condition as well. Citrus grove exported greater amounts of DOC and DN in runoff water (27.6 and 3.51mgL−1 of median value) than vegetable farms (13.8 and 2.26mgL−1 of median value). Annual cumulative loads of DOC and total N in 2008, 2009 or 2010 were >20kgha−1, greater than those of TP (<13.5kgha−1) and metals Fe, Cu, and Zn (<1.5kgha−1). Ranch soil had the highest extractable DOC concentration (>100mgkg−1), accounting for 1% of its total organic carbon. Soil extractable DN had significant correlations with extractable P and Cu/Zn in soils. In conclusion, soil total organic carbon pool and its availability had an important effect on DOC and DN in runoff water. Land use type and rainfall event were the two influential factors affecting concentrations of DOC and DN, and the export of nutrients N and P, and metals in runoff water.

Characterization of extracellular polymeric substances (EPS) from periphyton using liquid chromatography-organic carbon detection–organic nitrogen detection (LC-OCD-OND)

A protocol was developed to extract, fractionate, and quantitatively analyze periphyton extracellular polymeric substances (EPS), which obtains both information on the molecular weight (M r) distribution and protein and polysaccharide content. The EPS were extracted from freshwater periphyton between July and December 2011. Organic carbon (OC) compounds from different EPS extracts were analyzed using liquid chromatography-organic carbon detection–organic nitrogen detection (LC-OCD-OND), and total protein and polysaccharide content were quantified. Four distinct OC fractions, on the basis of M r, were identified in all extracts, corresponding to high M r biopolymers (≥80–4 kDa), degradation products of humic substances (M r not available), low M r acids (10–0.7 kDa), and small amphiphilic/neutral compounds (3–0.5 kDa). Low C/N ratios (4.3 ± 0.8) were calculated for the biopolymer fractions, which represented 16–38 % of the measured dissolved organic carbon (DOC), indicating a significant presence of high M r proteins in the EPS. Protein and polysaccharide represented the two major components of EPS and, when combined, accounted for the measured DOC in extracts. Differences in specific OC fractions of EPS extracts over the course of the study could be quantified using this method. This study suggests that LC-OCD-OND is a new valuable tool in EPS characterization of periphyton.Electronic supplementary materialThe online version of this article (doi:10.1007/s11356-012-1228-y) contains supplementary material, which is available to authorized users.

Biodegradation of dissolved organic carbon in soil extracts and leachates from a temperate forest stand and its relationship to ultraviolet absorbance

The amount and biodegradability of dissolved organic carbon (DOC) in forest floors can contribute to carbon sequestration in soils and the release of CO2-C from soil to the atmosphere. There is only limited knowledge about the biodegradation of DOC in soil extracts and leachates due to the limitations inherent in degradation experiments. Differences in the biodegradation of DOC were studied in forest soil extracts using cold and hot water and 4 mmol/L CaCl2 solution and in soil leachates sampled under different conditions over a wide range of DOC concentrations. From these results, we developed a simple and rapid method for determining the biodegradable organic C in forest floors. The hot water extracts and CaCl2 extracts after CH3Cl fumigation contained higher concentrations of biodegradable organic C than the cold water extracts and CaCl2 extracts before fumigation, with rapid DOC degradation occurring 24–48 h after incubation with an inoculum, followed by slow DOC degradation till 120–168 h into the incubation. During a 7-d incubation with an inoculum, the variation in DOC degradation in the different soil extracts was consistent with the change in special UV absorbance at 254 nm. Relatively higher levels of biodegradable organic C were detected in soil leachates from the forest canopy than in forest gaps between April and October 2008 (P <0.05). Relatively lower concentrations of DOC and biodegradable organic C were observed in soil leachates from N-fertilized plots during the growing season compared with the control, with the exception of the plot treated with KNO3 at a rate of 45 kg N ha−1 a−1. Around 77.4% to 96.3% of the variability in the biodegradable organic C concentrations in the forest floors could be accounted for by the initial DOC concentration and UV absorbance at 254 nm. Compared with the conventional inoculum incubation method, the method of analyzing UV absorbance at 254 nm is less time consuming and requires a much smaller sample volume. The results suggest that the regression models obtained using the initial DOC concentration and UV absorbance can provide a rapid, simple and reliable method for determining the biodegradable organic C content, especially in field studies involving relatively large numbers of samples.

Properties of successive horizons in a thick forest floor ( mor) reflect a sequence of soil acidification

Successive horizons of thick forest floors may be considered as a litterfall decomposition sequence, but the evolution of physico-chemical properties along such a sequence is poorly known. Here, we have characterized the physico-chemical properties within a thick forest floor and evaluated their potential impacts on the release of Dissolved Organic Carbon (DOC). Seven sub-horizons of a mor forest floor were sampled in a 200 year-old beech stand on Podzol. We performed total analysis, measured the concentrations of exchangeable cations and determined the concentration of ions and DOC in water extracts. From Ol to Oh, sharp gradients were observed: (1) total concentrations of Ca, Mg and Mn decreased, while total concentrations of Si, Al, K and Na increased, (2) exchangeable Ca, Mg, K and Na concentrations decreased while exchangeable H and Al concentrations increased and (3) leachate pH value dropped from 6.3 to 4.2. These gradients can be compared to the evolution of physico-chemical properties of a soil undergoing acidification. However, mineral incorporation into forest floor likely prevented a decline in total K and Na concentrations. Such a vertical heterogeneity of physico-chemical properties within the forest floor can affect DOC release: low pH value and high Mn concentrations in Of could promote DOC production by lignin degradation, while high Al concentrations in Oh could reduce DOC solubility.

Effects of elevated nitrogen deposition on soil microbial biomass carbon in major subtropical forests of southern China

The effects of elevated nitrogen deposition on soil microbial biomass carbon (C) and extractable dissolved organic carbon (DOC) in three types of forest of southern China were studied in November, 2004 and June, 2006. Plots were established in a pine forest (PF), a mixed pine and broad-leaved forest (MF) and monsoon evergreen broad-leaved forest (MEBF) in the Dinghushan Nature Reserve. Nitrogen treatments included a control (no N addition), low N (50 kg N/(hm2·a)), medium N (100 kg N/(hm2·a)) and high N (150 kg N/(hm2·a)). Microbial biomass C and extractable DOC were determined using a chloroform fumigation-extraction method. Results indicate that microbial biomass C and extractable DOC were higher in June, 2006 than in November, 2004 and higher in the MEBF than in the PF or the MF. The response of soil microbial biomass C and extractable DOC to nitrogen deposition varied depending on the forest type and the level of nitrogen treatment. In the PF or MF forests, no significantly different effects of nitrogen addition were found on soil microbial biomass C and extractable DOC. In the MEBF, however, the soil microbial biomass C generally decreased with increased nitrogen levels and high nitrogen addition significantly reduced soil microbial biomass C. The response of soil extractable DOC to added nitrogen in the MEBF shows the opposite trend to soil microbial biomass C. These results suggest that nitrogen deposition may increase the accumulation of soil organic carbon in the MEBF in the study region.

Soil organic matter and litter chemistry response to experimental N deposition in northern temperate deciduous forest ecosystems

AbstractThe effects of atmospheric nitrogen (N) deposition on organic matter decomposition vary with the biochemical characteristics of plant litter. At the ecosystem‐scale, net effects are difficult to predict because various soil organic matter (SOM) fractions may respond differentially. We investigated the relationship between SOM chemistry and microbial activity in three northern deciduous forest ecosystems that have been subjected to experimental N addition for 2 years. Extractable dissolved organic carbon (DOC), DOC aromaticity, C : N ratio, and functional group distribution, measured by Fourier transform infrared spectra (FTIR), were analyzed for litter and SOM. The largest biochemical changes were found in the sugar maple–basswood (SMBW) and black oak–white oak (BOWO) ecosystems. SMBW litter from the N addition treatment had less aromaticity, higher C : N ratios, and lower saturated carbon, lower carbonyl carbon, and higher carboxylates than controls; BOWO litter showed opposite trends, except for carbonyl and carboxylate contents. Litter from the sugar maple–red oak (SMRO) ecosystem had a lower C : N ratio, but no change in DOC aromaticity. For SOM, the C : N ratio increased with N addition in SMBW and SMRO ecosystems, but decreased in BOWO; N addition did not affect the aromaticity of DOC extracted from mineral soil. All ecosystems showed increases in extractable DOC from both litter and soil in response to N treatment. The biochemical changes are consistent with the divergent microbial responses observed in these systems. Extracellular oxidative enzyme activity has declined in the BOWO and SMRO ecosystems while activity in the SMBW ecosystem, particularly in the litter horizon, has increased. In all systems, enzyme activities associated with the hydrolysis and oxidation of polysaccharides have increased. At the ecosystem scale, the biochemical characteristics of the dominant litter appear to modulate the effects of N deposition on organic matter dynamics.

Seasonal fluctuation in microbial biomass and activity along a natural nitrogen gradient in a drained peatland

The effects of peat total N on the dissolved N and C concentrations and microbial biomass and activity and their range of seasonal fluctuation were studied in a drained peatland forest in Finland. Seasonal fluctuations in the concentrations of extractable dissolved organic (DON) and inorganic nitrogen (DIN) compounds and extractable dissolved organic carbon (DOC), microbial C and N, ergosterol, net and gross N mineralisation rates were investigated during two growing seasons along a natural peat N gradient in a drained peatland. Significant seasonal fluctuations in NH 4 + and DOC concentrations, microbial C and N, but not in ergosterol or microbial C-to-N ratios in the peat, were observed during the 1999 and 2000 growing seasons. The peat total N concentration affected extractable DON and DOC, but not DIN concentrations in the peat. A negative correlation was found between total N concentration in peat and microbial N and C, and a positive correlation between total N and ergosterol, in peat with N concentrations of up to 2%. Gross mineralisation rates did not show any correlation, whereas net mineralisation rates showed a significant positive correlation with the total N concentration of the peat in both 1999 and 2000.

Extramatrical ectomycorrhizal mycelium contributes one-third of microbial biomass and produces, together with associated roots, half the dissolved organic carbon in a forest soil.

• A large-scale tree-girdling experiment enabled estimates in the field of the contribution of extramatrical mycelium of ectomycorrhizal (ECM) fungi to soil microbial biomass and by ECM roots and fungi to production of dissolved organic carbon (DOC). • Tree-girdling was made early (EG) or late (LG) during the summer to terminate the flow of photosynthate to roots and ECM fungi. Determination of microbial C (Cmicr ) and microbial N in root-free organic soil was performed by using the fumigation-extraction technique; extractable DOC was determined on unfumigated soil. • Soil Cmicr was 41% lower on LG than on control plots 1month after LG, whereas at the same time (that is, 3months after EG), the Cmicr was 23% lower on EG than on control plots. Extractable DOC was 45% lower on girdled plots than control plots. • Our results, which are of particular interest as they were obtained directly in the field, clearly demonstrate the important contribution by extramatrical ECM mycelium to soil microbial biomass and by ECM roots to the production of DOC, a carbon source for other microbes.

Dynamics of extractable organic carbon in Spodosol forest floors

Transport of dissolved organic carbon (DOC) from the forest floor to the mineral soil in Spodosols affects many biogeochemical processes. The nature of the C reservoir in organic soil immediately before leaching, however, is not clear. Extractable organic C may partition between dissolved and undissolved phases, or reside in entrained water prior to leaching. We used laboratory incubations and extractions to test which model more accurately describes the reservoir from which DOC is removed. We also measured the rate at which C enters the extractable reservoir, how that rate changes with time, and the partitioning relationship between this reservoir and bulk solution. In addition, we tested whether the composition of extractable DOC changes with time. Extractable organic C was stored in an adsorbed phase. DOC in a solution of hydrophobic acids isolated from soil solution reduced dissolution of additional DOC by 41–44%. Furthermore, in each extraction, approximately 1 mg DOC remained in the unextracted phase for every 16–29 mg DOC 1 −1 in the aqueous phase. This partitioning relationship was not consistent with storage of DOC in entrained water alone. Soil produced 48 μg g −1 h −1 of DOC when extracted repeatedly at 1-h intervals, but only 1 μg g −1 h −1 when extracted at 1-week intervals. Extractable organic C production is fast (230 μg g −1 d −1) within 1 day of the last extraction, and then declines to a constant rate (15 μg g −1 d −1). Hydrophobic acids accounted for 75% of the extracted DOC after a 1-week incubation, but contributed only 50% of the increase in the extractable pool in subsequent weeks. Laboratory incubations and extractions of Oa material provided useful, consistent data for studying the dynamics of DOC removal from soil.