Showing papers on "Organic matter published in 2005"

Extensive sorption of organic compounds to black carbon, coal, and kerogen in sediments and soils: mechanisms and consequences for distribution, bioaccumulation, and biodegradation

Abstract: Evidence is accumulating that sorption of organic chemicals to soils and sediments can be described by “dual-mode sorption”: absorption in amorphous organic matter (AOM) and adsorption to carbonaceous materials such as black carbon (BC), coal, and kerogen, collectively termed “carbonaceous geosorbents” (CG). Median BC contents as a fraction of total organic carbon are 9% for sediments (number of sediments, n ≈ 300) and 4% for soils (n = 90). Adsorption of organic compounds to CG is nonlinear and generally exceeds absorption in AOM by a factor of 10−100. Sorption to CG is particularly extensive for organic compounds that can attain a more planar molecular configuration. The CG adsorption domain probably consists of surface sites and nanopores. In this review it is shown that nonlinear sorption to CG can completely dominate total sorption at low aqueous concentrations (<10-6 of maximum solid solubility). Therefore, the presence of CG can explain (i) sorption to soils and sediments being up to 2 orders of m...

Controls on nitrogen cycling in terrestrial ecosystems: a synthetic analysis of literature data

Abstract: Isotope pool dilution studies are increasingly reported in the soils and ecology literature as a means of measuring gross rates of nitrogen (N) mineralization, nitrification, and inorganic N assimilation in soils. We assembled data on soil characteristics and gross rates from 100 studies conducted in forest, shrubland, grassland, and agricultural systems to answer the following questions: What factors appear to be the major drivers for production and consumption of inorganic N as measured by isotope dilution studies? Do rates or the relationships between drivers and rates differ among ecosystem types? Across a wide range of ecosystems, gross N mineralization is positively correlated with microbial biomass and soil C and N concentrations, while soil C:N ratio exerts a negative effect on N mineralization only after adjusting for differences in soil C. Nitrification is a log-linear function of N mineralization, increasing rapidly at low mineralization rates but changing only slightly at high mineralization rates. In contrast, NH 4 1 assimilation by soil microbes increases nearly linearly over the full range of mineralization rates. As a result, nitrification is proportionately more important as a fate for NH4 1 at low mineralization rates than at high mineralization rates. Gross nitrification rates show no relationship to soil pH, with some of the fastest nitrification rates occurring below pH 5 in soils with high N mineralization rates. Differences in soil organic matter (SOM) composition and concentration among ecosystem types in- fluence the production and fate of mineralized N. Soil organic matter from grasslands appears to be inherently more productive of ammonium than SOM from wooded sites, and SOM from deciduous forests is more so than SOM in coniferous forests, but differences appear to result primarily from differing C:N ratios of organic matter. Because of the central importance of SOM characteristics and concentrations in regulating rates, soil organic matter depletion in agricultural systems appears to be an important determinant of gross process rates and the proportion of NH4 1 that is nitrified. Addition of 15 N appears to stimulate NH4 1 consumption more than NO3 2 consumption processes; however, the magnitude of the stim- ulation may provide useful information regarding the factors limiting microbial N trans- formations.

Labile Organic Matter Fractions as Central Components of the Quality of Agricultural Soils: An Overview

Abstract: Total soil organic matter content is a key attribute of soil quality since it has far-reaching effects on soil physical, chemical, and biological properties. However, changes in contents of organic carbon (C) and total nitrogen (N) occur only slowly and do not provide an adequate indication of important short-term changes in soil organic matter quality that may be occurring. Labile organic matter pools can be considered as fine indicators of soil quality that influence soil function in specific ways and that are much more sensitive to changes in soil management practice. Particulate organic matter consists of partially decomposed plant litter, and it acts as a substrate and center for soil microbial activity, a short-term reservoir of nutrients, a food source for soil fauna and loci for formation of water stable macroaggregates. Dissolved (soluble) organic matter consists of organic compounds present in soil solution. This pool acts as a substrate for microbial activity, a primary source of mineralizable N, sulfur (S), and phosphorus (P), and its leaching greatly influences the nutrient and organic matter content and pH of groundwater. Various extractable organic matter fractions have also been suggested to be important, including hot water-extractable and dilute acid-extractable carbohydrates, which are involved in stabilization of soil aggregates, and permanganate-oxidizable C. Measurement of potentially mineralizable C and N represents a bioassay of labile organic matter using the indigenous microbial community to release labile organic fractions of C and N. Mineralizable N is also an important indicator of the capacity of the soil to supply N for crops. It is concluded that individual labile organic matter fractions are sensitive to changes in soil management and have specific effects on soil function. Together they reflect the diverse but central effects that organic matter has on soil properties and processes. (c) 2005 Elsevier Inc.

Similar response of labile and resistant soil organic matter pools to changes in temperature

Abstract: Our understanding of the relationship between the decomposition of soil organic matter (SOM) and soil temperature affects our predictions of the impact of climate change on soil-stored carbon. One current opinion is that the decomposition of soil labile carbon is sensitive to temperature variation whereas resistant components are insensitive. The resistant carbon or organic matter in mineral soil is then assumed to be unresponsive to global warming. But the global pattern and magnitude of the predicted future soil carbon stock will mainly rely on the temperature sensitivity of these resistant carbon pools. To investigate this sensitivity, we have incubated soils under changing temperature. Here we report that SOM decomposition or soil basal respiration rate was significantly affected by changes in SOM components associated with soil depth, sampling method and incubation time. We find, however, that the temperature sensitivity for SOM decomposition was not affected, suggesting that the temperature sensitivity for resistant organic matter pools does not differ significantly from that of labile pools, and that both types of SOM will therefore respond similarly to global warming.

Competitive sorption reactions between phosphorus and organic matter in soil: a review

Abstract: The incorporation of organic matter ( OM) in soils that are able to rapidly sorb applied phosphorus ( P) fertiliser reportedly increases P availability to plants. This effect has commonly been ascribed to competition between the decomposition products of OM and P for soil sorption sites resulting in increased soil solution P concentrations. The evidence for competitive inhibition of P sorption by dissolved organic carbon compounds, derived from the breakdown of OM, includes studies on the competition between P and (i) low molecular weight organic acids (LOAs), (ii) humic and fulvic acids, and (iii) OM leachates in soils with a high P sorption capacity. These studies, however, have often used LOAs at 1 - 100 mM, concentrations much higher than those in soils ( generally < 0.05 mM). The transience of LOAs in biologically active soils further suggests that neither their concentration nor their persistence would have a practical benefit in increasing P phytoavailability. Higher molecular weight compounds such as humic and fulvic acids also competitively inhibit P sorption; however, little consideration has been given to the potential of these compounds to increase the amount of P sorbed through metal - chelate linkages. We suggest that the magnitude of the inhibition of P sorption by the decomposition products of OM leachate is negligible at rates equivalent to those of OM applied in the field. Incubation of OM in soil has also commonly been reported as reducing P sorption in soil. However, we consider that the reported decreases in P sorption ( as measured by P in the soil solution) are not related to competition from the decomposition products of OM breakdown, but are the result of P release from the OM that was not accounted for when calculating the reduction in P sorption.

Tracing the production and degradation of autochthonous fractions of dissolved organic matter by fluorescence analysis

Abstract: We present the results of a mesocosm experiment investigating the production and utilization of autochthonous dissolved organic matter (DOM) by the plankton community under different inorganic nutrient regimes. Fluorescence spectroscopy combined with parallel factor analysis was applied to study the dynamics of autochthonous DOM. Seven independent fluorescent fractions were identified, differing in their spectral characteristics, production rates, and sensitivity to photochemical and microbial degradation processes. Five different humic fractions, a marine protein, and a peptide fluorescence were found. The five humic fractions were produced microbially, with the greatest production occurring under combined Si- and P-limiting conditions. The two proteinaceous fractions were produced during exponential growth of phytoplankton, irrespective of biomass composition. Photodegradation was an important sink for the microbially derived humic material, and the marine protein material was susceptible to both photoand microbial degradation. The amount of carbon bound in dissolved organic matter (DOM) in the world’s oceans is similar to that bound as atmospheric carbon dioxide (Siegenthaler and Sarmiento 1993). As a result of increasing interest in the global carbon cycle, research of DOM has intensified over the last 30 yr. Although the supply of terrestrially derived DOM to the world’s oceans is considerable and it often dominates coastal seas, it is thought to represent only about 2‐3% of the total oceanic DOM pool (Opsahl and Benner 1997). The majority of marine DOM is therefore autochthonous (i.e., produced in the marine environment by phytoplankton). The major sources of autochthonous DOM include extracellular release by phytoplankton, release by grazers, and viral lysis of plankton (Nagata 2000). A large fraction of the DOM produced by these processes is consumed and respired rapidly by microbial activity; as a result, it can be difficult to measure. A smaller, more refractory fraction accumulates in seawater and is degraded over longer timescales (Nagata 2000). This fraction consists of both large, complex molecules (humic material) formed by condensation reactions and lower molecular weight organic matter from bacterial cell structures (Harvey et al. 1983; Nagata 2000). DOM consists of a complex mixture of compounds as a result of its variety of sources and continual reworking by photochemical and microbial degradation processes (Scully 1

The Depth Distribution of Soil Organic Carbon in Relation to Land Use and Management and the Potential of Carbon Sequestration in Subsoil Horizons

Abstract: Routine soil surveys for estimating the soil organic carbon (SOC) pool account for a soil depth of about 1 m Deeper soil horizons, however, may have a high capacity to sequester significant amounts of SOC as the turnover time and chemical recalcitrance of soil organic matter (SOM) increases with depth The subsoil carbon (C) sequestration may be achieved by higher inputs of fairly stable organic matter to deeper soil horizons This can be achieved directly by selecting plants/cultivars with deeper and thicker root systems that are high in chemical recalcitrant compounds like suberin Furthermore, recalcitrant compounds could be a target for plant breeding/biotechnology to promote C sequestration A high surface input of organic matter favors the production of dissolved organic carbon that can be transported to deeper soil horizons and thus contribute to the subsoil C storage By promoting the activity of the soil fauna, organic matter can be transferred to deeper soil layers and stabilized (eg, in earthworm casts) Manipulating the subsoil microorganisms may result in higher amounts of fairly stable aliphatic compounds The subsoil below 1‐m depth may have the potential to sequester between 760 and 1520 Pg C These estimates are, however, highly uncertain and more studies on C storage in subsoil horizons and the assessment of the chemical nature of subsoil organic C are needed

Review: organic matter removal from soils using hydrogen peroxide, sodium hypochlorite, and disodium peroxodisulfate

Abstract: We compare the performance of three most accepted reagents for organic matter removal: hydrogen peroxide (H 2 O 2 ), sodium hypochlorite (NaOCI) and disodium peroxodisulfate (Na 2 S 2 O 8 ). Removal of organic matter from soil is mostly incomplete with the efficiency of removal depending on reaction conditions and sample properties. Generally, NaOCI and Na 2 S 2 O 8 are more effective in organic C removal than H 2 O 2 . Alkaline conditions and additives favoring dispersion and/or desorption of organic matter, such as sodium pyrophosphate, seem to be crucial for C removal. Pyrophosphate and additives for pH control (bicarbonate) may irreversibly adsorb to mineral surfaces. In soils with a large proportion of organic matter bound to the mineral matrix, for example subsoils, or rich in clay-sized minerals (Fe oxides, poorly crystalline Fe and Al phases, expandable phyllosilicates), C removal can be little irrespective of the reagents used. Residual organic C seems to seems to represent largely refractory organic matter, and comprises mainly pyrogenic materials and aliphatic compounds. If protected by close association with minerals, other organic constituents such as low-molecular weight carboxylic acids, lignin-derived and N-containing compounds may escape chemical destruction. For determination of mineral phase properties, treatment with H 2 O 2 should be avoided since it may promote organic-assisted dissolution of poorly crystalline minerals at low pH, disintegration of expandable clay minerals, and transformation of vermiculite into mica-like products due to NH + 4 fixation. Sodium hypochlorite and Na 2 S 2 O 8 are less harmful for minerals than H 2 O 2 . While the NaOCI procedure (pH 9.5) may dissolve Al hydroxides, alkaline conditions favor the precipitation of metals released upon destruction of organic matter. Prolonged heating to >40°C during any treatment may transform poorly crystalline minerals into more crystalline ones. Sodium hypochlorite can be used at 25°C, thus preventing heat-induced mineral alteration.

Post-photosynthetic fractionation of stable carbon isotopes between plant organs--a widespread phenomenon.

Abstract: Discrimination against 13C during photosynthesis is a well-characterised phenomenon. It leaves behind distinct signatures in organic matter of plants and in the atmosphere. The former is depleted in 13C, the latter is enriched during periods of preponderant photosynthetic activity of terrestrial ecosystems. The intra-annual cycle and latitudinal gradient in atmospheric 13C resulting from photosynthetic and respiratory activities of terrestrial plants have been exploited for the reconstruction of sources and sinks through deconvolution by inverse modelling. Here, we compile evidence for widespread post-photosynthetic fractionation that further modifies the isotopic signatures of individual plant organs and consequently leads to consistent differences in delta13C between plant organs. Leaves were on average 0.96 per thousand and 1.91 per thousand more depleted than roots and woody stems, respectively. This phenomenon is relevant if the isotopic signature of CO2-exchange fluxes at the ecosystem level is used for the reconstruction of individual sources and sinks. It may also modify the parameterization of inverse modelling approaches if it leads to different isotopic signatures of organic matter with different residence times within the ecosystems and to a respiratory contribution to the average difference between the isotopic composition of plant organic matter and the atmosphere. We discuss the main hypotheses that can explain the observed inter-organ differences in delta13C.

Ecological stoichiometry in freshwater benthic systems: recent progress and perspectives

Abstract: SUMMARY 1. Ecological stoichiometry deals with the mass balance of multiple key elements [e.g. carbon (C), nitrogen (N), phosphorus (P)] in ecological systems. This conceptual framework, largely developed in the pelagic zone of lakes, has been successfully applied to topics ranging from population dynamics to biogeochemical cycling. More recently, an explicit stoichiometric approach has also been used in many other environments, including freshwater benthic ecosystems. 2. Description of elemental patterns among benthic resources and consumers provides a useful starting point for understanding causes of variation and stoichiometric imbalance in feeding interactions. Although there is considerable overlap among categories, terrestrially-derived resources, such as wood, leaf litter and green leaves have substantially higher C : nutrient ratios than other resources of both terrestrial and aquatic origin, such as periphyton and fine particulate organic matter. The elemental composition of these resources for benthic consumers is modulated by a range of factors and processes, including nutrient availability and ratios, particle size and microbial colonisation. 3. Among consumers in benthic systems, bacteria are the most nutrient-rich, followed (in descending order) by fishes, invertebrate predators, invertebrate primary consumers, and fungi. Differences in consumer C : nutrient ratios appear to be related to broad-scale phylogenetic differences which determine body size, growth rate and resource allocation to structural body constituents (e.g. P-rich bone). 4. Benthic consumers can influence the stoichiometry of dissolved nutrients and basal resources in multiple ways. Direct consumption alters the stoichiometry of food resources by increasing nutrient availability (e.g. reduced boundary layer thickness on substrata) or through removal of nutrient-rich patches (e.g. selective feeding on fungal patches within leaf litter). In addition, consumers alter the stoichiometry of resources and dissolved nutrient pools through the return of egested or excreted nutrients. In some cases, consumer excretion supplies a large proportion of the nutrients required by algae and heterotrophic microbes and alters elemental ratios of dissolved nutrient pools. 5. Organic matter decomposition in benthic systems is accompanied by significant changes in the elemental composition of organic matter. Microbial colonisation of leaf litter influences C : nutrient ratios, and patterns of microbial succession (e.g. fungi followed by bacteria) may be under some degree of stoichiometric control. Large elemental imbalances

Stabilization of dissolved organic matter by sorption to the mineral soil

Abstract: The main process by which dissolved organic matter (DOM) is retained in forest soils is likely to be sorption in the mineral horizons that adds to stabilized organic matter (OM) pools. The objectives of this study were to determine the extent of degradation of sorbed OM and to investigate changes in its composition during degradation. DOM of different origins was sorbed to a subsoil and incubated for 1 year. We quantified mineralized C by frequent CO2 measurements in the headspace of the incubation vessels and calculated mean residence times by a double exponential model. Mineralization of C of the corresponding DOM in solution was used as a control to estimate the extent of DOM stabilization by sorption. Changes in the composition of sorbed OM during the incubation were studied by spectroscopic (UV, fluorescence) and isotope (13C, 14C) measurements after hot-water extraction of OM. The fraction of sorbed organic C mineralized during the incubation was only one-third to one-sixth of that mineralized in solution. The mean residence time of the most stable OM sample was estimated to increase from 28 years in solution to 91 years after sorption. For highly degradable DOM samples, the portion of stable C calculated by a double exponential model nearly doubled upon sorption. With less degradable DOM the stability increased by only 20% after sorption. Therefore, the increase in stability due to sorption is large for labile DOM high in carbohydrates and relatively small for stable DOM high in aromatic and complex molecules. Nevertheless, in terms of stability the rank order of OM types after sorption was the same as in solution. Furthermore, the extent of sorption of recalcitrant compounds was much larger than sorption of labile compounds. Thus, sorptive stabilization of this stable DOM sample was four times larger than for the labile ones. We conclude that stabilization of OM by sorption depends on the intrinsic stability of organic compounds sorbed. We propose that the main stabilization processes are selective sorption of intrinsically stable compounds and strong chemical bonds to the mineral soil and/or a physical inaccessibility of OM to microorganisms. The UV, fluorescence and 13C measurements indicated that aromatic and complex compounds, probably derived from lignin, were preferentially stabilized by sorption of DOM. The 13C and 14C data showed that degradation of the indigenous OM in the mineral soil decreased after sorption of DOM. We estimated DOM sorption stabilizes about 24 Mg C ha−1 highlighting the importance of sorption for accumulation and preservation of OM in soil.

Poorly crystalline mineral phases protect organic matter in acid subsoil horizons

Abstract: Summary Soil minerals are known to influence the biological stability of soil organic matter (SOM). Our study aimed to relate properties of the mineral matrix to its ability to protect organic C against decomposition in acid soils. We used the amount of hydroxyl ions released after exposure to NaF solution to establish a reactivity gradient spanning 12 subsoil horizons collected from 10 different locations. The subsoil horizons represent six soil orders and diverse geological parent materials. Phyllosilicates were characterized by X-ray diffraction and pedogenic oxides by selective dissolution procedures. The organic carbon (C) remaining after chemical removal of an oxidizable fraction of SOM with NaOCl solution was taken to represent a stable organic carbon pool. Stable organic carbon was confirmed as older than bulk organic carbon by a smaller radiocarbon (14C) content after oxidation in all 12 soils. The amount of stable organic C did not depend on clay content or the content of dithionite–citrate-extractable Fe. The combination of oxalate-extractable Fe and Al explained the greatest amount of variation in stable organic C (R2 = 0.78). Our results suggest that in acid soils, organic matter is preferentially protected by interaction with poorly crystalline minerals represented by the oxalate-soluble Fe and Al fraction. This evidence suggests that ligand exchange between mineral surface hydroxyl groups and negatively charged organic functional groups is a quantitatively important mechanism in the stabilization of SOM in acid soils. The results imply a finite stabilization capacity of soil minerals for organic matter, limited by the area density of reactive surface sites.

Burial of terrestrial organic matter in marine sediments: A re-assessment

Abstract: [1] Calculations based on recent observations indicate that approximately one third of the organic matter presently being buried in marine sediments may be of terrestrial origin, with the majority of this terrestrial organic matter (TOM) burial occurring in muddy, deltaic sediments. These calculations further suggest that the remineralization of terrestrial organic matter in the oceans is also much less efficient than that of marine organic matter. These two underappreciated observations have important implications in terms of our understanding of the controls on the global carbon cycle. From a paleoceanographic perspective, the results presented here also suggest that changes in TOM burial on glacial-interglacial timescales have the potential to impact the global carbon cycle (i.e., atmospheric CO2 levels).

Short-term and residual availability of nitrogen after long-term application of organic fertilizers on arable land

Abstract: Knowledge on short-term and long-term availability of nitrogen (N) after application of organic fertilizers (e.g., farmyard manure, slurry, sewage sludge, composts) provides an important basis to optimize fertilizer use with benefits for the farmer and the environment. Nitrogen from many organic fertilizers often shows little effect on crop growth in the year of application, because of the slow-release characteristics of organically bound N. Furthermore, N immobilization after application can occur, leading to an enrichment of the soil N pool. However, this process finally increases the long-term efficiency of organic fertilizers. Short-term N release from organic fertilizers, measured as mineral-fertilizer equivalents (MFE), varies greatly from 0% (some composts) to nearly 100% (urine). The most important indicators to be used for predicting the short-term availability of N are total and NH + 4 -N contents, C : N ratio (especially of the decomposable organic fraction), and stability of the organic substances. Processing steps before organic fertilizers are applied in the field particularly can influence N availability. Composting reduces mineral-N content and increases the stability of the organic matter, whereas anaerobic fermentation increases NH + 4 -N content as well as the stability of organic matter, but decreases the C : N ratio remarkably, resulting in a product with a high content of directly available N. Nevertheless, long-term effects of organic fertilizers rather slowly releasing N have to be considered to enable optimization of fertilizer use. After long-term application of organic fertilizers, the overall N-use efficiency is adequate to a MFE in the range of 40%-70%.

Organic carbon content of sediments as an indicator of stress in the marine benthos

Abstract: While organic matter in sediments is an important source of food for benthic fauna, an overabundance can cause reductions in species richness, abundance, and biomass due to oxygen depletion and buildup of toxic by-products (ammonia and sulphide) associated with the breakdown of these materials. Moreover, increasing organic content of sediment is often accompanied by other chemical stressors co-varying with sediment particle size. In the present study, synoptic data on the structure of macroinfaunal communities and total organic carbon (TOC) content of sediment were obtained from 951 stations representing 7 coastal regions of the world: the northern Black Sea (Crimean and Caucasian coasts); eastern Mediterranean Sea (Greece); North Sea (Ekofisk oil field); Firth of Clyde and Liverpool Bay, UK; Seto Inland Sea, Japan; Boston Harbor and Massachusetts Bay, USA and estuaries of the southeastern USA. Macroinfaunal and TOC data were examined to look for patterns of association consistent with conceptual model predictions and to identify TOC critical points corresponding to major shifts in the benthic data. Species richness, Hurlbert's E (Sn), was selected as the primary response parameter. Results suggested that risks of reduced species richness from organic loading and other associated stressors in sediments should be relatively low at TOC concentrations less than about 10 mg g -1 , high at concentrations greater than about 35 mg g -1 , and intermediate at concentrations in between. Predictive ability across these ranges was high based on results of re-sampling simulation. While not a measure of causality, it is anticipated that these TOC critical points may be used as a general screening-level indicator for evaluating the likelihood of reduced sediment quality and associated bioeffects over broad coastal areas receiving organic wastes and other pollutants from human activities.

Stable isotope food web studies: a case for standardized sample treatment

Abstract: The enrichment of the stable isotopes 13C and 15N across trophic levels is a commonly used tool in studies on organic matter flow and food webs. An accepted standard for pre-analysis sample preparation, however, is still missing. Thus, potential methodological bias in single studies may hamper comparability and scalability of data from different sources. Sample CaCO3 content introduces a positive bias in ?13C measurements and a negative bias in ?15N measurements. The acidification of samples to remove inorganic carbonate significantly reduces both ?13C and ?15N. As a standard procedure we recommend (i) to acidify samples with as little hydrochloric acid (HCl) as possible using the drop-by-drop technique and (ii) to restrain from rinsing after HCl application.

Ecological risk assessment of polycyclic aromatic hydrocarbons in sediments: Identifying sources and ecological hazard

Abstract: Polycyclic aromatic hydrocarbons (PAHs) are nearly ubiquitous contaminants of freshwater and marine sediments. Sediment PAHs are derived from combustion of organic matter, fossil fuels, and biosynthesis by microbes. Pyrogenic PAHs, particularly those associated with combustion particles (soot), have a low accessibility and bioavailability in sediments. Polycyclic aromatic hydrocarbons associated with petroleum, creosote, or coal tar in sediments may have a moderate accessibility/bioavailability, particularly if the PAHs are part of a nonaqueous phase liquid (NAPL) phase that is in contact with sediment pore water. We present a method for estimating the hazard of complex PAH assemblage in sediments to benthic organisms. Concentrations of all PAHs in sediment pore water are estimated by an equilibrium partitioning model relative to concentrations in bulk sediment. Predicted log Koc values can be used for predicting sediment/water partitioning of petrogenic PAH, but empirically derived log Kd values are needed to predict partitioning of pyrogenic PAH. A hazard quotient (HQ) for each PAH is calculated as the ratio of the estimated concentration in pore water to the chronic toxicity of the PAH determined by a log Kow/toxicity model. Hazard quotients for all PAH in a sample are summed to produce a hazard index (HI), which is a measure of the worst-case estimated hazard of the sediment PAH to benthic organisms. The results of this study show that the integration of HI results with PAH source data provides insights into the causes of sediment toxicity that are useful in an ecological risk assessment.

Nitrogen mineralization from organic residues: research opportunities.

Abstract: Research on nitrogen (N) mineralization from organic residues is important to understand N cycling in soils. Here we review research on factors controlling net N mineralization as well as research on laboratory and field modeling efforts, with the objective of highlighting areas with opportunities for additional research. Among the factors controlling net N mineralization are organic composition of the residue, soil temperature and water content, drying and rewetting events, and soil characteristics. Because C to N ratio of the residue cannot explain all the variability observed in N mineralization among residues, considerable effort has been dedicated to the identification of specific compounds that play critical roles in N mineralization. Spectroscopic techniques are promising tools to further identify these compounds. Many studies have evaluated the effect of temperature and soil water content on N mineralization, but most have concentrated on mineralization from soil organic matter, not from organic residues. Additional work should be conducted with different organic residues, paying particular attention to the interaction between soil temperature and water content. One- and two-pool exponential models have been used to model N mineralization under laboratory conditions, but some drawbacks make it difficult to identify definite pools of mineralizable N. Fixing rate constants has been used as a way to eliminate some of these drawbacks when modeling N mineralization from soil organic matter, and may be useful for modeling N mineralization from organic residues. Additional work with more complex simulation models is needed to simulate both gross N mineralization and immobilization to better estimate net N mineralized from organic residues.

Composition of Organic Matter Fractions for Explaining Wettability of Three Forest Soils

Abstract: Soil organic matter (SOM) as a solid or as a film at mineral surfaces affects wetting properties in unsaturated soil Soil organic matter mostly consists of a heterogeneous mixture of components with hydrophilic and hydrophobic functional groups This paper analyzes relations of SOM to soil wettability by considering functional group compositions of different soluble fractions Forest soil samples from two loamy sand Cambisol profiles (locations Chorin and Steigerwald) and from a Podzol (Waldstein) were used to obtain water [SOM(W)] and sodium pyrophosphate [SOM(PY)] soluble SOM fractions The hydrophobic (A) and hydrophilic (B) functional groups of bulk soil SOM and of the soluble fractions were evaluated using transmission Fourier-transform infrared (FT-IR) spectroscopy Advancing liquid-solid contact angles (CA) were determined by using the capillary rise method For soil organic carbon (SOC) contents 10 g kg -1 Although hydrophilic groups in FT-IR spectra of SOM(W), SOM(PY), and bulk soil dominated (ie, A/B ratios between 008 and 05), soil wettability was reduced (ie, CA between 88 and 52°) Soil specific relations between CA and A/B ratios could be obtained after introducing relatively soil type independent factors, G As exponential functions of the SOC/clay relation, the G-factors imitate the effectiveness of functional groups with respect to wettability The results suggest that in addition to SOC content, the SOM composition may improve explanations of soil wettability if the spatial orientation of SOM functional groups at the SOM-mineral surface in the presence of sorption sites and polyvalent cations is considered

Polyparameter linear free energy relationships for estimating the equilibrium partition of organic compounds between water and the natural organic matter in soils and sediments.

Abstract: Values of the organic-carbon-based partition coefficient (Koc) have often been estimated using one-parameter linear free energy relationships (op-LFERs), which include both correlations between log Koc and log Kow, where Kow is the octanol−water partition coefficient, and op-LFERs that are based on first-order molecular connectivity indices. For chemicals with tendencies toward strong hydrogen-bonding or other specific interactions with the organic phase, however, these methods are not sufficiently accurate. Polyparameter LFERs (pp-LFERs) address these shortcomings by explicitly considering contributions toward free energy change from multiple kinds of molecular interactions with both water and bulk organic phases. This paper reviews pp-LFER theory and presents the development of a new pp-LFER for organic chemical partitioning with soil/sediment organic matter (SOM) using a data set of 356 carefully selected experimental values of log Koc for 75 chemicals, including apolar, monopolar, and bipolar compound...

Biodegradation of hexachlorocyclohexane (HCH) by microorganisms.

Abstract: The organochlorine pesticide Lindane is the gamma-isomer of hexachlorocyclohexane (HCH). Technical grade Lindane contains a mixture of HCH isomers which include not only gamma-HCH, but also large amounts of predominantly alpha-, beta- and delta-HCH. The physical properties and persistence of each isomer differ because of the different chlorine atom orientations on each molecule (axial or equatorial). However, all four isomers are considered toxic and recalcitrant worldwide pollutants. Biodegradation of HCH has been studied in soil, slurry and culture media but very little information exists on in situ bioremediation of the different isomers including Lindane itself, at full scale. Several soil microorganisms capable of degrading, and utilizing HCH as a carbon source, have been reported. In selected bacterial strains, the genes encoding the enzymes involved in the initial degradation of Lindane have been cloned, sequenced, expressed and the gene products characterized. HCH is biodegradable under both oxic and anoxic conditions, although mineralization is generally observed only in oxic systems. As is found for most organic compounds, HCH degradation in soil occurs at moderate temperatures and at near neutral pH. HCH biodegradation in soil has been reported at both low and high (saturated) moisture contents. Soil texture and organic matter appear to influence degradation presumably by sorption mechanisms and impact on moisture retention, bacterial growth and pH. Most studies report on the biodegradation of relatively low (< 500 mg/kg) concentrations of HCH in soil. Information on the effects of inorganic nutrients, organic carbon sources or other soil amendments is scattered and inconclusive. More in-depth assessments of amendment effects and evaluation of bioremediation protocols, on a large scale, using soil with high HCH concentrations, are needed.