Dissolved Organic C Research Articles

Effects of Landscape Age on Soil Organic Matter Processing in Northern Alaska

Much of the previous research on arctic soil biogeochemistry has focused largely on the effects of climate, organisms, and relief rather than on geologic factors. However, a few studies of limited scope suggest that biogeochemical cycling in tundra soils may be significantly affected by variation in glacial history. To determine if variation in landscape age-related differences in geochemistry influence rates of organic matter decomposition in tundra soils, we conducted soil analyses and a 5-mo soil incubation study across a chronosequence of landscape ages in northern Alaska. Our results indicate that there were landscape-age related controls on microbial activity, with higher microbial biomass, rates of respiration, and production of dissolved organic C (DOC) and N (DON) in soils from the older landscape ages than in soils from younger, less weathered landscapes. This biological boundary corresponded to a chemical boundary, as older soils had lower pH (4.5–5 vs. 6.5) and exchangeable cation concentrations than the youngest soils. There were no differences among landscape ages in total C stocks in organic soils; however, total N stocks in the organic soil decreased with increasing landscape age. Differences among landscape ages in microbial activity due to geochemical variation may have important consequences for tundra C budgets because differences in soil pH and cation content exist throughout the circumpolar region.

Metal binding to dissolved organic matter and adsorption to ferrihydrite in shallow peat groundwaters: Application to diamond exploration in the James Bay Lowlands, Canada

The speciation and solubility of kimberlite pathfinder metals (Ni, Nd, Ba and K) in shallow peat groundwaters is investigated over the Yankee, Zulu and Golf kimberlites in the Attawapiskat region, James Bay Lowlands, Canada. The purpose of this study is to examine the relationship between dissolved organic matter (DOM) complexation with kimberlite pathfinder metals and determine the spatial distribution of those metals in shallow peat groundwaters along sampling transects over subcropping kimberlites. Nickel, Nd, Ba and K complexation with DOM and the adsorption of these metals onto ferrihydrite were calculated using Visual MINTEQ 3.0 and the NICA-Donnan database. Calculations predict almost 100% of soluble Nd, Ni and Ba form complexes with DOM at sampling sites with little to no contribution from upwelling groundwater (i.e., dissolved organic C (DOC) concentrations = 40–132 mg/L, pH = 3.9–5.5, and log ionic strength 6� 3). In only the most ombrotrophic peat groundwater conditions does a majority fraction of K bind to DOM. By contrast, under conditions with large contributions from upwelling groundwaters (i.e., DOC concentrations 640 mg/L, pH = 5.5–6.5, and log ionic strength = � 3t o� 2), as little as 10% of Nd and Ni, and 0% K and Ba are predicted to complex with DOM. The modeling calculations suggest the dominant control on metal–DOM complexation, particularly with respect to Ni and Nd, is competitive effects for DOM binding sites due to elevated ionic strength where there is evidence of strong groundwater upwelling. Visual MINTEQ modeling of metal adsorption on ferrihydrite surfaces predicts that under strong upwelling conditions, Ni and Nd are scavenged from solution due to increased ferrihydrite precipitation and decreased fractions of metals complexed with DOM. Analytical geochemical data are consistent with model predictions of metal adsorption on ferrihydrite. Total dissolved Ni and Nd concentrations at sites of strong upwelling are up to five times lower than waters with little to no upwelling and log ferrihydrite saturation indices (logSI ferr) indicate precipitation (values up to 5) at sites of strong groundwater upwelling. Where the majority of Ni and Nd complex with DOM and ferrihydrite is highly under saturated (logSIferr = � 18 to � 5), the concentrations of total Ni and Nd are elevated compared to other sites along sampling transects. Metal complexation with DOM effectively inhibits metal scavenging from solution via adsorption and/or from forming secondary mineral precipitates. Also, because alkaline earth metals do not compete strongly with Ni and Nd for adsorption sites on ferrihydrite surfaces, but do compete strongly for insoluble organic sites, Ni and Nd are more likely to adsorb onto ferrihydrite.

Organic carbon amendments for passive in situ treatment of mine drainage: Field experiments

Abstract A field-scale experiment was conducted to evaluate various organic C sources as amendments for passive treatment of tailings pore water. Varied mixtures of peat, spent-brewing grain (SBG) and municipal biosolids (MB) were assessed for the potential to promote dissimilatory sulfate reduction (DSR) and metal-sulfide precipitation. Five amended cells and one control were constructed in the vadose zone of a sulfide- and carbonate-rich tailings deposit, and the geochemistry, microbiology and mineralogy were monitored for 4 a. Increases in pore-water concentrations of dissolved organic C (DOC) and decreases in aqueous SO 4 concentrations of >2500 mg L −1 were observed in cells amended with peat + SBG and peat + SBG + MB. Removal of SO 4 was accompanied by shifts in δ 34 S-SO 4 values of >+30‰, undersaturation of pore water with respect to gypsum [CaSO 4 ·2H 2 O], and increased populations of SO 4 -reducing bacteria (SRB). Decreases in aqueous concentrations of Zn, Mn, Ni, Sb and Tl were observed for these cells relative to the control. Organic C introduction also supported growth of Fe-reducing bacteria (IRB) and increases in Fe and As concentrations. Enhanced Fe and As mobility occurred in all cells; however, maximum concentrations were observed in cells amended with MB. Subsequent decreases in Fe and As concentrations were attributed to DSR and metal-sulfide precipitation. The common presence of secondary Zn-S and Fe-S phases was observed by field emission-scanning electron microscopy (FE-SEM) and energy dispersive X-ray (EDS) spectroscopy. Selective extractions indicated that large decreases in water-soluble SO 4 occurred in cells that supported DSR. Furthermore, amendments that supported DSR generally were characterized by slight decreases in solid-phase concentrations of extractable metal(loid)s. Amendment of tailings with organic C amendments that supported ongoing DOC production and DSR was essential for sustained treatment.

Chemical properties of anaerobic digestates affecting C and N dynamics in amended soils

The optimisation of digestate recycling as fertilisers, based on both environmental and agricultural criteria, requires an evaluation of the effects on C and N dynamics in soil. In the present paper, six digestates from several anaerobic co-digestion experiments, using pig or cattle slurry as the main substrate, were evaluated in short-term incubations in soil.Digestate properties such as dissolved organic-C (DOC), biochemical oxygen demand (BOD) and digestate organic-C mineralised in the soil during the first 7 days represented properly the digestate biodegradability. These, together with their ratios with respect to the total nitrogen (TN) concentration in the digestate, were reliable parameters with respect to defining the C and N dynamics in the soil and hence the N-fertiliser potential of the digested materials. Therefore, highly biodegradable digested materials, represented in the present study by digestates from cattle slurry–glycerine mixtures were not suitable for agricultural use as they caused a high CO2–C production and led to N-immobilisation and/or denitrification after their application to soil. Contrastingly, for less biodegradable digested materials (BOD5d<6.0gO2L−1 fresh weight, DOC<5.5gL−1 fresh weight and DOC/TN<1.5), less CO2–C was evolved and ammonium was rapidly nitrified in soil—being an available N source for crops.

Natural Attenuation of Zn, Cu, Pb and Cd in Three Biosolids-Amended Soils of Contrasting pH Measured Using Rhizon Pore Water Samplers

The effects of application of biosolids, at four rates, to an alkaline (pH 8.4), neutral (pH 7.0) and acidic (pH 4.0) soil on concentrations of Cu, Zn, Pb, Cd and dissolved organic C in soil solution were measured over a 170-day period in a laboratory incubation study using Rhizon pore water samplers. Applications of biosolids decreased solution pH in the alkaline soil, increased it in the acidic soil and had little effect in the neutral soil. In general, increasing application rates of biosolids progressively increased EC and concentrations of dissolved organic C (DOC), Cu, Zn, and to a lesser extent Cd and Pb, in soil solution. Concentrations of DOC and concentrations of solution Cu, Zn, and to a lesser extent solution Cd and Pb, decreased over the incubation period. In all three soils, concentrations of solution Cu and Zn were closely positively correlated with DOC concentrations and similar positive but weaker correlations were found for solution Cd and Pb. For the alkaline and neutral soils, concentrations of solution Cu, Zn, Cd and Pb were generally negatively correlated with solution pH but for the acidic soil, positive correlations for Cu and Zn were recorded. The percentage reduction in solution Cu and Zn, between 0 and 170 days incubation, increased with increasing rates of biosolids in the acid soil (where biosolids applications increased pH) but the reverse was the case for the alkaline soil (where pH fell following biosolids applications). Greatest percentage reduction in soluble Cu and Zn occurred in the neutral soil which had the greatest BET surface area, clay and organic matter contents and therefore the greatest capacity to adsorb heavy metal cations. It was concluded that solution pH, dissolved organic C and the intrinsic capacity of the soil to remove metals from solution, were the main factors interacting to regulate heavy metal cation solubility in the biosolids-amended soils.

Mitigation of methane and nitrous oxide emissions from flood-irrigated rice by no incorporation of winter crop residues into the soil

Winter cover crops are sources of C and N in flooded rice production systems, but very little is known about the effect of crop residue management and quality on soil methane (CH4) and nitrous oxide (N2O) emissions. This study was conducted in pots in a greenhouse to evaluate the influence of crop residue management (incorporated into the soil or left on the soil surface) and the type of cover-crop residues (ryegrass and serradella) on CH4 and N2O emissions from a flooded Albaqualf soil cultivated with rice (Oryza sativa L.). The closed chamber technique was used for air sampling and the CH4 and N2O concentrations were analyzed by gas chromatography. Soil solution was sampled at two soil depths (2 and 20 cm), simultaneously to air sampling, and the contents of dissolved organic C (DOC), NO3-, NH4+, Mn2+, and Fe2+ were analyzed. Methane and N2O emissions from the soil where crop residues had been left on the surface were lower than from soil with incorporated residues. The type of crop residue had no effect on the CH4 emissions, while higher N2O emissions were observed from serradella (leguminous) than from ryegrass, but only when the residues were left on the soil surface. The more intense soil reduction verified in the deeper soil layer (20 cm), as evidenced by higher contents of reduced metal species (Mn2+ and Fe2+), and the close relationship between CH4 emission and the DOC contents in the deeper layer indicated that the sub-surface layer was the main CH4 source of the flooded soil with incorporated crop residues. The adoption of management strategies in which crop residues are left on the soil surface is crucial to minimize soil CH4 and N2O emissions from irrigated rice fields. In these production systems, CH4 accounts for more than 90 % of the partial global warming potential (CH4+N2O) and, thus, should be the main focus of research.

Carbon and nitrogen biogeochemistry of a Prairie Pothole wetland, Stutsman County, North Dakota, USA

The concentration and form of dissolved organic C (DOC) and N species (NH4+ and NO3-) were investigated as part of a larger hydrogeochemical study of the Cottonwood Lake Study Area within the Prairie Potholes region. Groundwater, pore water and surface wetland water data were used to help characterize the relationships between surface and groundwater with respect to nutrient dynamics. Photosynthesis and subsequent decomposition of vegetation in these hydrologically dynamic wetlands generates a large amount of dissolved C and N, although the subsurface till, derived in part from organic matter rich Pierre Shale, is a likely secondary source of nutrients in deeper groundwater. While surface water DOC concentrations ranged from 2.2 to 4.6 mM, groundwater values were 0.15 mM to 3.7 mM. Greater specific UV absorbance (SUVA254) in the wetland water column and in soil pore waters relative to groundwater indicate more reactive DOC in the surface to near-surface waters. Circumneutral wetlands had greater SUVA254, possibly because of variations in vegetation communities. The dominant inorganic nitrogen species was NH4+ in both wetland water and most ground water samples. The exceptions were 3 wells with NO3- ranging from 38 to 115 μM. Shallow groundwater wells (Well 28 and Well 13S) with greater connection to wetland surface water had greater NH4+ concentrations (1.1 mM and 120 μM) than other well samples (3–90 μM). Pore water nutrient chemistry was more similar to surface water than ground water. Nitrogen results suggest reducing conditions in both groundwater and surface water, possibly due to the microbial uptake of O2 by decaying vegetation in the wetland water column, labile organic C available in shallow groundwater, or the oxidation of pyrite associated with the subsurface.

Biogeochemical processes in a clay formation in situ experiment: Part C – Organic contamination and leaching data

Data interpretation of the Porewater Chemistry (PC) experiment at the Mont Terri Rock Laboratory has led to unexpected observations of anaerobic microbial processes which caused important geochemical perturbations of the Opalinus Clay water in the borehole. The increases of acetate to 146 mg C/L, of DIC to 109 mg C/L and of CH4 to 0.5 mg C/L were unexpected and could not be explained without the presence of a C source in the system. The organic C fuelling the observed microbial activity was until then unknown. Leaching tests were performed on several polymers used for the fabrication of the PC equipment to identify the source of organic matter (OM). Polyethylene (PE) appears to be very inert and does not release detectable concentrations of dissolved organic C (DOC) (<1 ppb) into the water. Polyurethane (PU) leaches out a dozen different organic compounds accounting for only 13 μg DOC/g PU. Under the conditions of the leaching tests, 1 g of polyamide (PA, Nylon) also releases ∼512 μg of the plasticizer N-Butyl-Benzene-Sulfonamide (NBBS). Soaking tests with polyethylene samples immersed in acetone under conditions similar to those used to remove grease spots on the porous PE filter prior to installation showed that acetone could have been trapped in the PE filter, corresponding to an initial concentration of 1.5 g acetone/L of water. However, the accumulated amount of organic C taken into account from all these components was insufficient to satisfactorily explain the observed microbially mediated reducing perturbation. Finally, large amounts of dissolved organic C were found to be released in the system by the jelly polymer filling the reference compartment of the pH and Eh electrodes permanently installed over 5 years in flow-through cells on the water circulation loop of the PC experiment. Glycerol was further identified by chromatographic analysis as the main organic compound released by the electrodes. From the analysis results, as well as from the geochemical calculations, the most likely primary organic C source fuelling the microbial perturbation was glycerol released from the polymeric gel filling the reference electrodes (1.6 g glycerol/electrode). Other sources, such as acetone, may also have contributed to microbial processes, but only to a minor extent.

Gaseous and fluvial carbon export from an Amazon forest watershed

The transfer of carbon (C) from Amazon forests to aquatic ecosystems as CO2 supersaturated in groundwater that outgases to the atmosphere after it reaches small streams has been postulated to be an important component of terrestrial ecosystem C budgets. We measured C losses as soil respiration and methane (CH4) flux, direct CO2 and CH4 fluxes from the stream surface and fluvial export of dissolved inorganic C (DIC), dissolved organic C (DOC), and particulate C over an annual hydrologic cycle from a 1,319-ha forested Amazon perennial first-order headwater watershed at Tanguro Ranch in the southern Amazon state of Mato Grosso. Stream pCO2 concentrations ranged from 6,491 to 14,976 μatm and directly-measured stream CO2 outgassing flux was 5,994 ± 677 g C m−2 y−1 of stream surface. Stream pCH4 concentrations ranged from 291 to 438 μatm and measured stream CH4 outgassing flux was 987 ± 221 g C m−2 y−1. Despite high flux rates from the stream surface, the small area of stream itself (970 m2, or 0.007% of watershed area) led to small directly-measured annual fluxes of CO2 (0.44 ± 0.05 g C m2 y−1) and CH4 (0.07 ± 0.02 g C m2 y−1) per unit watershed land area. Measured fluvial export of DIC (0.78 ± 0.04 g C m−2 y−1), DOC (0.16 ± 0.03 g C m−2 y−1) and coarse plus fine particulate C (0.001 ± 0.001 g C m−2 y−1) per unit watershed land area were also small. However, stream discharge accounted for only 12% of the modeled annual watershed water output because deep groundwater flows dominated total runoff from the watershed. When C in this bypassing groundwater was included, total watershed export was 10.83 g C m−2 y−1 as CO2 outgassing, 11.29 g C m−2 y−1 as fluvial DIC and 0.64 g C m−2 y−1 as fluvial DOC. Outgassing fluxes were somewhat lower than the 40–50 g C m−2 y−1 reported from other Amazon watersheds and may result in part from lower annual rainfall at Tanguro. Total stream-associated gaseous C losses were two orders of magnitude less than soil respiration (696 ± 147 g C m−2 y−1), but total losses of C transported by water comprised up to about 20% of the ± 150 g C m−2 (±1.5 Mg C ha−1) that is exchanged annually across Amazon tropical forest canopies.

Soil Carbon Change through 2 m during Forest Succession Alongside a 30-Year Agroecosystem Experiment

Abstract Forest succession (FS) and no-till (NT) agriculture are generally assumed to have a beneficial effect on surficial soil organic C (SOC) stocks compared with conventional tillage (CT) management; however, land use effects to depths &amp;gt;30 cm remain uncertain. In this research we compared SOC contents and composition to 2 m under CT, NT, and FS at the 30-year Horseshoe Bend agroecosystem experiment in Athens, Georgia, USA. Soils from 0 to 2 m were fractionated into particulate organic C (POC) (53-2000 μm) and fine C (&amp;lt;53 μm) fractions, and bulk soil δ13C signatures were determined. Soils from 0 to 28 cm were dry- and wet-sieved to estimate aggregate stability. Soil solutions were also collected at 0, 15, and 100 cm for dissolved organic C (DOC) analysis. Full-profile (0-2 m) SOC storage is 52 Mg ha−1 in CT, 60 Mg ha−1 in NT, and 62 Mg ha−1 in FS. Significant differences are limited to 0-5 cm and are linked to enhanced aggregate stability under NT and FS. Increases in subsoil POC under FS and changes in soil δ13C and C/N ratio indicate that substantial subsoil C cycling has occurred. DOC fluxes at 0 cm were significantly greater under NT (200 kg ha−1 year−1) and FS (210 kg ha−1 year−1) than under CT (80 kg ha−1 year−1). DOC fluxes at 15 cm are estimated to be 20 kg ha−1 year−1 under CT and NT and 40 kg ha−1 year−1 under FS. At 100 cm, DOC fluxes are 2 kg ha−1 year−1, regardless of land use. An increase in FS POC of 2 Mg ha−1 from 15 to 100 cm outweighs cumulative differences in DOC input to this layer, implicating deep forest rooting and bioturbation as active mechanisms in subsoil C change. Whereas differences in SOC content were concentrated near the surface, dynamic changes in C cycling extend well below the plow layer.

Three-source-partitioning of microbial biomass and of CO2 efflux from soil to evaluate mechanisms of priming effects

Abstract We propose and successfully applied a new approach for 3-source-partitioning based on a combination of 14 C labeling with 13 C natural abundance. By adding 14 C-labeled glucose to soil after C 3 – C 4 vegetation change, we partitioned three C sources in three compartments, namely CO 2 , microbial biomass and dissolved organic C (DOC). This enabled us to estimate mechanisms and sources of priming effects (PE). Glucose application at low and high rate (GL: 100 and GH: 1000 μg C g −1 , respectively) caused positive PE both short-term (during 1–3 days) and long-term (3–55 days). Despite a 10-fold difference in the amount of substrate added, the PE observed was larger by a factor of only 1.6 at the high versus low rate of glucose. The real and apparent priming effects were distinguished by partitioning of microbial C for glucose-C and SOM-derived C. As the amount of primed CO 2 respired during short-term PE was 40% lower than microbial C, and the contribution of soil C in microbial biomass did not increase, we concluded that such short-term PE was apparent and was mainly caused by accelerated microbial turnover (at GL) and by pool substitution (at GH). Both the amount of primed CO 2 –C, which was 1.3–2.1 times larger than microbial C, and the increased contribution of soil C in microbial biomass allowed us to consider the long-term PE as being real. The sole source of real PE (GL treatment) was the “recent” soil organic matter, which is younger than 12-year-old C. The real PE-induced by a glucose amount exceeding microbial biomass (GH) was due to the almost equal contribution of ‘recent’ ( 12 years) C. Thus, the decomposition of old recalcitrant SOM was induced only by an amount of primer exceeding microbial C. We conclude that combining 14 C labeling with 13 C natural abundance helped disentangle three C sources in CO 2 , microbial biomass and DOC and evaluate mechanisms and sources of PE.

Variability in dissolved organic matter fluorescence and reduced sulfur concentration in coastal marine and estuarine environments

Abstract Fluorescence characterization of dissolved organic matter (DOM) and measurements of Cr-reducible sulfide (CRS) are presented for 72 coastal marine and estuarine water samples obtained from the USA and Canada. Each sample is identified according to source: terrigenous, autochthonous, wastewater or mixed. Fluorescence data are resolved into contributions from humic, fulvic, tyrosine and tryptophan-like fluorophores. Humic and fulvic-like fluorophores correlate well with dissolved organic C (DOC) ( r 2 = 0.73 and 0.71, respectively) but tyrosine and tryptophan-like fluorophores show no correlation with DOC. Quality factors are identified by normalization of fluorescence contributions to DOC. Humic and fulvic components show no statistical differences between sources but the amino acid-like fluorescence quality factors show significant variations between source, with highest values for autochthonous sources (0.07 ± 0.01 arbitrary fluorescence units per mg of C) versus low values (0.015 ± 0.005) for terrigenous source waters. CRS concentrations are highly variable from 0.07 ± 0.01 to 7703 ± 98 nM and do no correlate with DOC except when terrigenous source waters ( n = 13) are separated out from the total sample set ( r 2 = 0.55). There is an open question in the literature; does DOC source matter in terms of protective effects towards metal toxicity? Here is shown that DOC molecular-level quality does vary and that this variation is mostly in terms of the contributions of amino acids to total fluorescence.

Individual and combined effects of disturbance and N addition on understorey vegetation in a subarctic mountain birch forest

Questions: What are the effects of repeated disturbance and N-fertilization on plant community structure in a mountain birch forest? What is the role of enhanced nutrient availability in recovery of understorey vegetation after repeated disturbance? How are responses of soil micro-organisms to disturbance and N-fertilization reflected in nutrient allocation patterns and recovery of understorey vegetation after disturbance? Location: Subarctic mountain birch forest, Finland. Methods: We conducted a fully factorial experiment with annual treatments of disturbance (two levels) and N-fertilization (four levels) during 1998–2002. We monitored treatment effects on above-ground plant biomass, plant community structure and plant and soil nutrient concentrations. Results: Both disturbance and N-fertilization increased the relative biomass of graminoids. The increase of relative biomass of graminoids in the disturbance treatment was over twice that of the highest N-fertilization level, and N-fertilization further increased their relative biomass after disturbance. As repeated disturbance broke the dominance of evergreen dwarf shrubs, it resulted in a situation where deciduous species, graminoids and herbs dominated the plant community. Although relative biomass of deciduous dwarf shrubs declined with N-fertilization, it did not cause a shift in plant community structure, as evergreen dwarf shrubs remained dominant. Both disturbance and N-fertilization increased the N concentration in vascular plants, whereas microbial biomass N and C were not affected by the treatments. Concentrations of NH4+, dissolved organic N (DON) and dissolved organic C (DOC) increased in the soil after N-fertilization, whereas concentrations of NH4+ and DON decreased after disturbance. Conclusions: Disturbances caused by e.g. humans or herbivores contribute more to changes in the understorey vegetation structure than increased levels of N in subarctic vegetation. Fertilization accelerated the recovery potential after repeated disturbance in graminoids. Microbial activities did not limit plant growth.

Does anoxia affect mercury cycling at the sediment–water interface in the Gulf of Trieste (northern Adriatic Sea)? Incubation experiments using benthic flux chambers

Abstract Coastal areas in the northernmost part of the Adriatic Sea (Gulf of Trieste and the adjacent Grado Lagoon) are characterized by high levels of Hg, both in sediments and in the water column, mainly originating from the suspended material inflowing through the Isonzo/Soca River system, draining the Idrija (NW Slovenia) mining district, into the Gulf of Trieste. Hypoxic and anoxic conditions at the sediment–water interface (SWI) are frequently observed in the Gulf of Trieste and in the lagoon, due to strong late summer water stratification and high organic matter input. Mercury mobility at the SWI was investigated at three sampling points located in the Gulf of Trieste (AA1, CZ) and in the Grado Lagoon (BAR). Experiments were conducted under laboratory conditions at in situ temperature, using a dark flux chamber simulating an oxic–anoxic transition. Temporal variations of dissolved Hg and methylmercury (MeHg) as well as O2, NH 4 + , NO 3 - + NO 2 - , PO 4 3 - , H2S, dissolved Fe and Mn, dissolved inorganic C (DIC) and dissolved organic C (DOC) were monitored simultaneously. Benthic Hg fluxes were higher under anoxic conditions than in the oxic phase of the experiment. Methyl Hg release was less noticeable (low or absent) in the oxic phase, probably due to similar methylation and demethylation rates, but high in the anoxic phase of the experiment. The MeHg flux was linked to SO4 reduction and dissolution of Fe (and Mn) oxyhydroxides, and formation of sulphides. Re-oxygenation was studied at sampling point CZ, where concentrations of MeHg and Hg dropped rapidly probably due to re-adsorption onto Fe (Mn) oxyhydroxides and enhanced demethylation. Sediments, especially during anoxic events, should be, hence, considered as a primary source of MeHg for the water column in the northern Adriatic coastal areas.

Soil microbial biomass and activity response to repeated drying–rewetting cycles along a soil fertility gradient modified by long-term fertilization management practices

The effects of repeated drying–rewetting (DRW) cycles on the microbial biomass and activity in soils taken from long-term field experiment plots with different fertilization (FERT) management practice histories were studied. We investigated the hypothesis that soil response to DRW cycles differs with soil fertility gradient modified by FERT management practices. The soils were incubated for 51 days, after exposure to either nine or three DRW cycles, or remaining at constant moisture content (CMC) at field capacity. We found that both DRW and FERT significantly affected soil properties including NH 4–N, NO 3–N, dissolved organic C (DOC), microbial biomass C (Cmic), basal soil respiration rate (BSR), urease activity (URE) and dehydrogenase activity (DHD). Except for NH 4–N and BSR, variation in the properties was largely explained by FERT, followed by DRW, and then their interaction. Irrespective of the soils' FERT treatment, repeated DRW cycles significantly raised the DOC and Cmic levels compared with CMC, and the DRW cycles also resulted in a significant decline in BSR and URE and increase in DHD, probably because the organisms were better-adapted to the drying and rewetting stresses. The variations in soil biological properties caused by DRW cycles showed a significantly negative relationship with the soil organic C content measured prior to the start of the DRW experiments, suggesting that soils with higher fertility are better able to maintain their original biological functions (i.e., have a higher functional stability) in response to DRW cycles.

Body size and population dynamics of enchytraeids with different disturbance histories and nutrient dynamics

The population dynamics of the enchytraeid Cognettia sphagnetorum originating from an unmanaged forest (FP), a clear-cut area (CCP) or a plot treated with birch ash (APP) and the effects of population origin on labile C and N dynamics were investigated. Twenty individuals of C. sphagnetorum were introduced in microcosms containing humus from the unmanaged forest devoid of enchytraeids and amended with sucrose, and incubated for 14 weeks. Triplicate microcosms from FP, CCP and APP treatments were destructively sampled every second week and enchytraeid population density, individual length, nematode abundance and trophic structure, humus properties and dissolved organic C (DOC) and N (DON), and NH4–N in soil were determined. The enchytraeid body size was initially smaller in CCP and APP than in FP. The enchytraeid propagation rate was lower and individual size less variable in APP than in FP or CCP, and although enchytraeid size increased in all treatments, exponential population models indicated that APP was less stable. Nematode community was dominated by bacterial-feeders especially in the microcosms with APP. N mineralization rate was lower and DOC decomposition rate greater in APP systems. The results show that C. sphagnetorum is more sensitive to wood ash than clear-cutting, and its altered body size distribution has the potential to affect the dynamics of soluble nutrients.

The origins and behaviour of carbon in a major semi-arid river, the Murray River, Australia, as constrained by carbon isotopes and hydrochemistry

Abstract δ13C values of dissolved inorganic C (DIC), dissolved organic C (DOC), and particulate organic C (POC) together with δ18O and δ2H values of water, δ34S values of dissolved SO4, and major ion concentrations were measured in the Murray River and its tributaries between November 2005 and April 2007 to constrain the origins and behaviour of riverine C. δ13CDIC values in the Murray River vary between −9.5 and −4.7‰ with a range of

Adsorption of Antibiotics by Montmorillonite and Kaolinite

Agricultural antibiotics are commonly present in animal manures that are applied to soil. The objective of this study was to examine the influence of pH, ionic strength, background electrolyte cation type, and dissolved organic C (DOC) concentration on the adsorption of chlortetracycline (CTC), tylosin (TYL), and sulfamethazine (SMT) to montmorillonite and kaolinite. Surface complexation and isotherm models were used to analyze antibiotic adsorption. Adsorption of CTC and TYL decreased with increasing pH and ionic strength. Adsorption also decreased when Ca2+ was substituted for Na+ as the background electrolyte and when DOC was added. The CTC and TYL isotherms were Langmuirian. We concluded that ion exchange was the predominant process responsible for CTC and TYL retention. The adsorption of SMT generally decreased with increasing pH in response to the deprotonation of the phenolic amine group and the formation of the nonionic SMT molecule. The retention of SMT was generally unaffected by ionic strength and background electrolyte type, and the isotherms were either linear (montmorillonite) or S type; however, adsorption increased on the addition of DOC and the isotherm was Langmuirian. We postulated that ion exchange was the predominant retention mechanism for SMT in pH &lt; 4 systems but that weak nonionic interactions predominated in the pH 5 to 6 range. Antibiotic adsorption as a function of pH was satisfactorily characterized by surface complexation models that considered outer sphere complexation (cation exchange), although these models were not unique (models that considered inner sphere complexation also satisfactorily described adsorption). Similarly, the adsorption isotherms were generally well characterized by the Dubinin–Radushkevich adsorption isotherm model.

Initial soil responses to experimental warming in two contrasting forest ecosystems, Eastern Tibetan Plateau, China: Nutrient availabilities, microbial properties and enzyme activities

In order to understand the effects of projected global warming on soils in different land-use types, we compared the impacts of warming on soils in two contrasting forest ecosystems (a dragon spruce plantation and a natural forest) using the open-top chamber (OTC) method in the Eastern Tibetan Plateau of China. The OTC on average enhanced daily mean soil temperatures by 0.61 °C (plantation) and by 0.55 °C (natural forest) throughout the growing season, respectively. Conversely, soil volumetric moisture declined by 4.10% in the plantation and by 2.55% in the natural forest, respectively. Warming did not affect dissolved organic C (DOC) and N (DON) in the plantation but significantly increased them in the natural forest. Elevated temperature significantly increased net N mineralization rates and extractable inorganic N pools in both sites. Warming had no effects on microbial biomass C (MBC) and N (MBN) and their ratios (MBC/MBN) in the plantation and significantly increased MBC and MBN only late in the growing season in the natural forest. Warming did not affect basal respiration in the plantation but significantly increased it in the natural forest. No clear change was observed in metabolic quotient between warming regimes for both forest types. Experimental warming tended to increase invertase and urease in both forest soils. Measured pools related to N turnover generally showed significant interactions in warming, forest type and sampling date. Taken together, our results indicate that responses of soils to experimental warming depend strongly on forest managements and seasons.

The importance of the fallow period for N2O and CH4 fluxes and nitrate leaching in a Mediterranean irrigated agroecosystem

The aim of this study was to evaluate the pattern of nitrous oxide (N2O) and methane (CH4) fluxes, and leaching losses of nitrate (NO3−) and dissolved organic C (DOC), during a fallow–onion crop–fallow cycle in a Mediterranean area. The importance of the fallow (intercrop) period and the type of fertilizer were also evaluated. Goat and chicken manure (M) from an organic farm, digested pig slurry (DPS) and urea (U) were applied at a rate of 110 kg N ha−1 and compared with a zero N treatment (Control). The crop period contributed more than each fallow period to the total N2O emission (ranging from 70 to 85% of the total emission, depending on the treatment). The variability of rainfall during fallow periods affected N2O emissions, with the highest fluxes observed in the second fallow, which was the wetter. Negative net fluxes of N2O (0 to −0.4 mg N2O‐N m−2 day−1) were mainly observed during the irrigation period and in fallow periods. The type of fertilizer had no effect on N2O fluxes, but influenced the CH4 oxidation. The largest CH4 emission was from the manure treatment (2.4 mg CH4‐C m−2 day−1) during the irrigation period. The lowest NO3− but highest DOC leaching rates were measured during the second fallow period from the manure treated plots (0.2 kg NO3−‐N ha−1 and 3.9 kg C ha−1), which also had the highest drainage. The use of OM, therefore, seems to be a suitable method to reduce the environmental impacts associated with N leaching as well as increase the potential to denitrify NO3− in groundwater.