Breakdown Of Organic Matter Research Articles

Interactive effects of elevated CO2, N deposition and climate change on extracellular enzyme activity and soil density fractionation in a California annual grassland

AbstractElevated CO2, N deposition and climate change can alter ecosystem‐level nutrient cycling both directly and indirectly. We explored the interactive effects of these environmental changes on extracellular enzyme activity and organic matter fractionation in soils of a California annual grassland. The activities of hydrolases (polysaccharide‐degrading enzymes and phosphatase) increased significantly in response to nitrate addition, which coincided with an increase in soluble C concentrations under ambient CO2. Water addition and elevated CO2 had negative but nonadditive effects on the activities of these enzymes. In contrast, water addition resulted in an increase in the activities of lignin‐degrading enzymes (phenol oxidase and peroxidase), and a decrease in the free light fraction (FLF) of soil organic matter. Independent of treatment effects, lignin content in the FLF was negatively correlated with the quantity of FLF across all samples. Lignin concentrations were lower in the aggregate‐occluded light fraction (OLF) than the FLF, and there was no correlation between percent lignin and OLF quantity, which was consistent with the protection of soil organic matter in aggregates. Elevated CO2 decreased the quantity of OLF and increased the OLF lignin concentration, however, which is consistent with increased degradation resulting from increased turnover of soil aggregates. Overall, these results suggest that the effects of N addition on hydrolase activity are offset by the interactive effects of water addition and elevated CO2, whereas water and elevated CO2 may cause an increase in the breakdown of soil organic matter as a result of their effects on lignin‐degrading enzymes and soil aggregation, respectively.

Stochastic Modeling of Chemical Oxygen Demand Transformations in Gravity Sewers

Wastewater quality characteristics in terms of biomass, its substrates, and the corresponding kinetic and stoichiometric parameters were determined based on 109 wastewater samples originating from five different campaigns in four different sewer networks. Quality parameters were determined by model calibration of measured wastewater oxygen uptake rates applying a model that describes the aerobic breakdown of wastewater organic matter. Thereafter, the distributions of the parameters were analyzed. Two of the five datasets were obtained at the upstream end of a five-km-long, intercepting gravity sewer. For each of these upstream wastewater samples, downstream samples were collected with a delay corresponding to the residence time. The upstream distributions of the wastewater composition were used as boundary conditions for a Monte Carlo simulation. The calculated downstream distributions were compared to the measured downstream distributions and good agreement was observed.

Stochastic Modeling of Chemical Oxygen Demand Transformations in Gravity Sewers

Wastewater quality characteristics in terms of biomass, its substrates, and the corresponding kinetic and stoichiometric parameters were determined based on 109 wastewater samples originating from five different campaigns in four different sewer networks. Quality parameters were determined by model calibration of measured wastewater oxygen uptake rates applying a model that describes the aerobic breakdown of wastewater organic matter. Thereafter, the distributions of the parameters were analyzed. Two of the five datasets were obtained at the upstream end of a five‐km‐long, intercepting gravity sewer. For each of these upstream wastewater samples, downstream samples were collected with a delay corresponding to the residence time. The upstream distributions of the wastewater composition were used as boundary conditions for a Monte Carlo simulation. The calculated downstream distributions were compared to the measured downstream distributions and good agreement was observed.

Metabolic interactions in methanogenic and sulfate-reducing bioreactors

In environments where the amount of electron acceptors is insufficient for complete breakdown of organic matter, methane is formed as the major reduced end product. In such methanogenic environments organic acids are degraded by syntrophic consortia of acetogenic bacteria and methanogenic archaea. Hydrogen consumption by methanogens is essential for acetogenic bacteria to convert organic acids to acetate and hydrogen. Several syntrophic cocultures growing on propionate and butyrate have been described. These syntrophic fatty acid-degrading consortia are affected by the presence of sulfate. When sulfate is present sulfate-reducing bacteria compete with methanogenic archaea for hydrogen and acetate, and with acetogenic bacteria for propionate and butyrate. Sulfate-reducing bacteria easily outcompete methanogens for hydrogen, but the presence of acetate as carbon source may influence the outcome of the competition. By contrast, acetoclastic methanogens can compete reasonably well with acetate-degrading sulfate reducers. Sulfate-reducing bacteria grow much faster on propionate and butyrate than syntrophic consortia.

Free radicals, antioxidants, and soil organic matter recalcitrance

SummaryHighly reactive, and potentially damaging, free radicals are readily generated in our oxygen‐rich environment, and are ubiquitous in biological systems. However, plants and animals have evolved protection against them with a range of antioxidant molecules, such as vitamins C and E, many of which are phenolic compounds. These stop the destructive chain reaction of free radical formation by being transformed into unreactive, stable free radicals. The biodegradation of food involves oxidation by free radicals, and is retarded by antioxidants. Similarly, the biodegradation of plant residues in soils involves free radicals; so the questions arise: (i) do soils have antioxidants, and (ii) what function might they have? The evidence suggests that they probably do have antioxidants. First, plant and animal remains added to soils will contain antioxidants. These are likely to persist for a time, particularly tannins, which are polyphenolic compounds with known antioxidant properties and which are relatively resistant to degradation. Second, studies using electron spin resonance spectroscopy have shown that humic materials contain stable semiquinone free radicals, and that their concentration increases as humification progresses. These semiquinone species are most likely to be derived from the reaction of phenolic compounds with reactive radicals. If this is the case, the phenolics are acting as antioxidants, because they are scavenging the reactive free radicals and terminating the oxidative chain reaction responsible for soil organic matter (SOM) degradation. Thus the soil's antioxidant capacity could control the rate of breakdown of organic matter in the more labile pools and could provide a chemical mechanism for the recalcitrance of SOM. Current available evidence for the nature of the recalcitrant pool in SOM is discussed in the light of this hypothesis, and the experimental approaches necessary for testing it are outlined.

The effects of The Quesungual Agroforestry System of western Honduras on soil macrofauna

Agricultural practices that promote increased diversity and abundance of soil macrofauna may improve soil quality and productivity, due to the influence of soil macrofauna on organic matter breakdown, nutrient cycling and soil structure. Southern Lempira department in Honduras is an environment where farmers could benefit greatly from such an increase in soil quality. The landscape is hilly, with steep slopes and shallow soils that are susceptible to erosion. The need for shorter fallow periods, a decrease in the use of slash-and-burn agriculture, and promotion by extension agents has resulted in the large-scale adoption of an indigenous agroforestry system known as the ‘Quesungual System’, based on slash-andmulch of vegetation, rotation of maize, sorghum and beans and inclusion of naturally regenerated trees and shrubs within cropped fields. This paper presents research on the effects of four common land use types (secondary forest, recently cleared agroforestry fields, mature agroforestry fields and silvopastoral fields) in the Quesungual area on soil macrofauna abundance and community composition. Sampling methods recommended by the Tropical Soil Biology and Fertility Institute were used to collect soil macrofauna in three representative fields of each type of land use, giving 12 study sites in total. Soil macrofauna communities were highly variable, both within representative fields, between representative

Effects of stream restoration on ecosystem functioning: detritus retentiveness and decomposition

Summary Increasing degradation of ecological conditions in streams because of human activities has prompted widespread restoration attempts; however, the ecological consequences of restoration remain poorly understood. We explored the effects of restoration through placement of boulders into the channel in the Ume River catchment in northern Sweden, where tributary streams were extensively channelized to facilitate the transport of timber in the 19th and early 20th centuries. Retentiveness and breakdown of coarse particulate organic matter (CPOM), two key ecological functions in low‐order streams most likely to be affected by channelization, were compared between restored, channelized and unimpacted reference stream sites. Artificial leaves were used to assess short‐term CPOM retentiveness, while CPOM breakdown was estimated as the mass loss of alder (Alnus spp.) leaf packs placed in coarse mesh litter bags. Also, the taxonomic richness, abundance, biomass and evenness of the leaf‐eating invertebrates (shredders) on the retrieved leaf material were quantified. Detailed field measurements were carried out to identify geomorphological and hydraulic controls of CPOM retentiveness and breakdown at the study sites. CPOM retentiveness reflected most strongly the density of boulders and submerged woody debris at the study sites. Restored sites were on average twice as retentive as channelized sites and significantly more retentive than reference sites when discharge was controlled. Current velocity at bank‐full flow was the single most important predictor of CPOM mass loss, implying that mechanical fragmentation was substantial during high flows; other apparent controls of CPOM breakdown included water temperature and shredder abundance. CPOM mass loss was similar between restored and reference sites. However, breakdown was slightly faster at most channelized sites, consistent with higher hydraulic stress during high flow conditions. The shredder assemblages that colonized the litter bags were similar in richness, abundance, biomass and evenness between treatments. Synthesis and applications. In channelized forest streams, low retentiveness and fast mechanical fragmentation during high flows contribute to the rapid depletion of benthic CPOM following leaf abscission in autumn, thereby weakening the heterotrophic energy pathways that probably support much of the biological production in these systems. Our results illustrate that restoration by replacement of boulders can successfully reverse these impacts of channelization and thus contribute to the efficient ecological functioning of impacted streams.

Nitrogen and carbon partitioning in diagenetic and hydrothermal minerals from Paleozoic Black Shales, (Selwyn Basin, Yukon Territories, Canada)

Selected mineralized black shales of Devonian age from the Selwyn Basin, Northwest Territories (Canada) were analyzed by Nuclear Reaction Analyses (NRA) and electron microprobe for nitrogen and carbon in silicates, sulfides, phosphates and organic matter in order to give new insights on nitrogen and carbon fractionation processes during diagenesis and hydrothermal infiltration. Hydrothermal feldspars show tri-modal composition: albite, high nitrogen-bearing K-feldspar (∼56 mol% buddingtonite (NH 4AlSi 3O 8·1/2H 2O, hydrous ammonium-feldspar, ∼51 mol% orthoclase) and hyalophane (∼32 mol% celsian). Barium-rich feldspars (hyalophane) contain lowest nitrogen contents. Potassium and nitrogen are positively correlated, while nitrogen and barium are negatively correlated due to the replacement of monovalent NH 4 + by divalent Ba 2+. The Ba-rich K-feldspar rim shows penetrative textures towards an internal K–N-rich core that is interpreted as diffusive overgrowth. These feldspars are interpreted to be deposited from hot hydrothermal Ba-bearing fluids. The second important nitrogen carrier is organic matter (from 0.6 to 0.66 wt.%). Hydrothermal quartz (N=527 ppm), diagenetic biogenic F-rich apatite (conodonts: N=468 ppm,), biogenic Fe–Ni sulfides (N=380–620 ppm) and abiogenic Ni–Fe sulfides (N>440 ppm) contain homogeneously distributed nitrogen with amounts 10-fold lower than those measured in organic matter. A two-step nitrogen-release model is suggested to explain the nitrogen-partitioning in these minerals. Primary organic matter breakdown is considered to liberate nitrogen, phosphate and sulfur to pore fluids and the water column, providing nutrients for vent fauna growth. Sulfurization, due to microbial sulfate reduction, and silicification of the vent fauna releases nitrogen in a second step. Minor nitrogen was trapped as organic molecules in conodonts, while the majority was transported by hydrothermal fluids and was incorporated as ammonium in feldspars substituting for potassium.

Modeling of Clay Liner Desiccation

The potential for the desiccation of clay liner component of composite liners due to temperature field generated by breakdown of organic matter in municipal solid waste landfills is examined using a model proposed by Zhou and Rowe. In these analyses, a set of fully coupled governing equations expressed in terms of displacement, capillary pressure, air pressure, and temperature increase are used, and numerical results are solved by using finite element method with a mass-conservative numerical scheme. The model results are shown to be in encouraging agreement with experimental data for a problem involving heating of a landfill liner. The fully coupled transient fields ~temperature, horizontal stress change, suction head, and volumetric water content! are then examined for two types of composite liner system, one involving a geomembrane over a compacted clay liner ~CCL! and the other involving a geomembrane over a geosynthetic clay liner ~GCL!. It is shown that there can be significant water loss and horizontal stress change in both the CCL and GCL liner even with a temperature increase as small as 20°C. The time to reach steady state decreases as boundary temperature increases. Under a 30°C temperature increase, it takes 5 years to reach the steady state water content with a GCL liner but 50 years with a CCL liner. The effects of various parameters, such as hydraulic conductivity and thickness of the liner, on the performance of the liner are discussed.

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

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.

INFLUENCE OF AERATION RATE AND LIQUID TEMPERATURE ON AMMONIA EMISSION RATE AND MANURE DEGRADATION IN BATCH AEROBIC TREATMENT

The influence of aeration rates (26, 63, and 100 .g O2 g VS-1 min-1) at different temperatures (5C, 15C, and25C) on ammonia emission and breakdown of organic materials was studied using 15.3 L batch reactors for 21 days. Theremoval of total Kjeldahl nitrogen (TKN) ranged from 9.7% to 37.7% during the experiment, with higher values correspondingto higher manure temperatures and aeration rates. Ammonia loss ranged from 0.7% to 25.8% of initial TKN and increasedwith increase in manure temperature and aeration rate. In a similar pattern, the ratio of volatilized ammonia gas to removedTKN varied from 6.5 to 68.5. In addition, high BOD5 removal was also observed when both the temperature and the aerationrate were high, with the exception for airflow rates between 26 and 63 .g O2 g VS-1 min-1 for the manure at temperaturesbetween 5C and 15C. The combinations of high aeration rate (100 .g O2 g VS-1 min-1) with low temperature (5C) or lowaeration rate (26 .g O2 g VS-1 min-1) with high temperature (15C to 25C) were found to be desirable to reduce ammoniaemission while still achieving satisfactory BOD5 removal at the same time.

Carbon and Noble Gas Isotopes in the Tengchong Volcanic Geothermal Area, Yunnan, Southwestern China

Abstract Carbon and noble gas isotope analyses are reported for bubbling gas samples from the Tengchong volcanic geothermal area near the Indo‐Eurasian suture zone. All samples contain a resolvable component of mantle‐derived 3He. Occurrence of mantle‐derived 3He coincides with surface volcanism. However, 3He occurs over a larger geographic area than do surface volcanics. δ13C values for CO2 and CH4 vary from −33.4 ‰ to 1.6 ‰ and from −52.8 ‰ to −2.8 ‰, respectively. He and C isotope systematics indicate that CO2 and CH4 in the CO2‐rich gases originated predominantly from magmatic component mixed with crustal CO2 produced from carbonate. However, breakdown of organic matter and near‐surface processes accounts for the CH4 and CO2 in N2‐rich gases. 3He/4He ratio distribution pattern suggests that mantle‐derived He and heat sources of high‐temperature system in central Tengchong originate from a hidden magma reservoir at subsurface. CO2‐rich gases with the highest 3He/4He ratio (5.2 Ra) may be representative of the Tengchong magmatic component. Compared with MORB, this relative low 3He/4He ratio could be fully attributed to either deep crustal contamination, or radioactive aging, or past contamination of the local mantle by U‐ and Th‐rich subducted crustal material. However, a combination of low 3He/4He, high radiogenic 4He/40Ar ratio and identical CO2/3He and δ13CO2 relative to MORB may suggest addition of prior subductedd crsustal material (ca 1%–2%) to the MORB reservoir around 1.3 Ga ago, which is essentially compatible with the LIL‐elements, and Sr‐Nd‐Pb isotopes of volcanic rocks.

Bound residues: environmental solution or future problem?

The paper examines the issue of bound residues from the viewpoint of the risk assessment procedures employed for environmental protection. It considers, on one hand, the evidence that such residues are so tightly bound to soil organic matter as to be essentially unavailable; and on the other, the perspective that we should not be loading up the environment with compounds whose future behaviour and release we cannot predict. Existing knowledge of the mechanisms by which residues bind to soil organic matter suggests that release will be closely dependent on soil organic matter breakdown. Simple models of organic matter turnover suggest that the release following single applications of individual compounds will be very slow; but the significance of releases following repeated application over many years of a number of compounds needs to be investigated further. Applying environmental risk assessment techniques is complicated by the difficulty in identifying parent molecules and potential metabolites in the bound residue fraction. The paper concludes that for single additions of individual compounds, bound residues probably do represent an environmental solution. But the long-term significance of bound residues formed from multiple additions of a number of compounds is less clear. The paper recommends that future work should emphasise the biological significance of such residues and their release.

Impacts of chronic nitrogen additions vary seasonally and by microbial functional group in tundra soils

Previous studies have shown that fertilization with nitrogen depresses overall microbial bio- mass and activity in soil. In the present study we broaden our understanding of this phenomenon by studying the seasonality of responses of specific microbial functional groups to chronic nitrogen addi- tions in alpine tundra soils. We measured soil enzyme activities, mineralization kinetics for 8 substrates, biomass of 8 microbial functional groups, and changes in N and carbon pools in the soil. Our approach allowed us to compare the ability of the soil microbial biomass to utilize various substrates in addition to allowing us to estimate changes in biomass of microbial functional groups that are involved in carbon and nitrogen cycling. Overall microbial activity and biomass was reduced in fertilized plots, whereas pools of N in the soil and microbial biomass N were higher in fertilized plots. The negative effects of N were most prominent in the summer. Biomass of the dominant microbial functional groups recovered in fertilized soils during the winter and nitrogen storage in microbial biomass was higher in fertilized soils in the autumn and winter than in the summer. Microbial immobilization of N may therefore be a sig- nificant sink for added N during autumn and winter months when plants are not active. One large mi- crobial group that did not recover in the winter in fertilized soils was phenol mineralizers, possibly in- dicating selection against microbes with enzyme systems for the breakdown of phenolic compounds and complex soil organic matter. Overall, this work is a step towards understanding how chronic N addi- tions affect the structure and biogeochemical functioning of soil microbial communities.

Ring-testing and Field-validation of a Terrestrial Model Ecosystem(TME) – An Instrument for Testing Potentially Harmful Substances: Effects of Carbendazim on Organic Matter Breakdown and Soil Fauna Feeding Activity

Organic matter (OM) decomposition and soil fauna feeding activity were integrated as functional endpoints into ecotoxicological tests with intact-soil-core Terrestrial Model Ecosystems (TMEs). Cellulose filter paper served as standardized OM and was either inserted into the top soil or placed on the soil surface for a period of up to 16 weeks. Faunal feeding activity was assessed by the bait-lamina method. The fungicide carbendazim, applied at six dosages ranging from 0.36 kg/ha to 87.5 kg a.i./ha, served as a model chemical. To validate the results from the TME test, a field study was run in parallel. In TMEs the cellulose paper inserted into the soil was decomposed faster than under field conditions. The carbendazim-induced effects on OM decomposition in TMEs and in the field were comparable and followed a clear dose-response relationship. The calculated EC50 values after 8 weeks of incubation were 9.5, 7.1 and 2.1 kg carbendazim/ha for grassland TMEs, grassland field and arable TMEs, respectively. The feeding activity of the soil fauna showed a large variability. The EC50 values for the effect of carbendazim on bait-lamina consumption ranged between 2.0 and 56 kg a.i./ha. Effects on decomposition were correlated with effects on enchytraeids and earthworms but not with effects on bait-lamina consumption.

Bacterial metabolism and growth efficiency in lakes: The importance of phosphorus availability

We investigated bacterial responses to variations in dissolved organic carbon (DOC) and nutrient availability by a comparative analysis of bacterial metabolism in lakes ranging from oligotrophic to eutrophic. Bacterial growth, respiration, and growth efficiency were quantified in lake water dilution cultures performed in 20 lakes located in eastern Quebec, Canada, which varied with respect to both DOC and nutrient concentrations. Intrinsic growth rates of the bacteria ranged from 0.1 to 1.4 d−, bacterial cell‐specific respiration rates ranged from 0.4 to 7.2 fg C cell−1 h−1, and growth efficiencies ranged from 6.7% to 51.6%. These variations were unrelated to bulk DOC concentrations. Instead, growth rate and efficiency were positively related to total phosphorus concentrations. Specific respiration rate, on the other hand, decreased with increasing phosphorus concentrations, and the magnitude of respiration, on a per‐cell basis, strongly influenced observed growth efficiencies. In a series of substrate enrichment experiments, additions of glucose alone failed to stimulate a response in growth rate, mean cell biovolume, or the potential biomass yield in dilution cultures, but all responded positively to phosphorus additions. Our results show that bacterial metabolism and the fate of DOC input to lake microbial communities are strongly dependent on phosphorus availability, rather than total carbon availability. Extreme oligotrophy appears to place high respiratory demands on the bacterioplankton, resulting in very low bacterial growth efficiencies and consequently greater DOC flow to CO2 than to biomass available for transfer to higher trophic levels.

Lake metabolism: Relationships with dissolved organic carbon and phosphorus

Recent literature has suggested that for many lakes and rivers, the respiratory breakdown of organic matter (R) exceeds production of organic matter by photosynthesis (gross primary production [GPP]) within the water body. This metabolic balance (GPP > R; “heterotrophy”) implies that allochthonous organic matter supports a portion of the aquatic ecosystem's respiration. Evidence that many lakes are heterotrophic comes from diverse approaches, and debate remains over the circumstances in which heterotrophy exists. The methods used to estimate GPP and R and the limited extent of lake types studied, especially with respect to dissolved organic carbon (DOC) and total phosphorus (TP) concentrations, are two reasons for differing conclusions. We deployed O2 and CO2 sondes to measure diel gas dynamics in the surface waters of 25 lakes. From these data, we calculated GPP, R, and net ecosystem production (NEP = GPP − R). Over the broad range in TP and DOC among the lakes, diel CO2 and O2 changed on a near 1 : 1 molar ratio. Metabolism estimates from the two gases were comparable, except at high pH. Most lakes in our data set had negative NEP, but GPP and R appeared to be controlled by different factors. TP correlated strongly with GPP, whereas DOC correlated with R. At low DOC concentrations, GPP and R were nearly equal, but, at higher DOC, GPP and R uncoupled and lakes had negative NEP. Strong correlations between lake metabolism and landscape related variables suggest that allochthonous carbon influences lake metabolism.

Heavy Metals in Soil, Plants and Groundwater Following High-Rate Sewage Sludge Application to Land

The concentrations, solubility and mobility of Cr, Cu, Ni, Pb and Zn were measured over a four year period in soil from a site that had received over 1000 t ha-1 wet, undigested, sewage sludge (on average, 15% dry solids). The pH of this light-textured sandy soil was markedly reduced after sludge application (to ≤4 in some samples), presumably as a result of breakdown of the unstable organic matter, nitrification of the NH4 +-N and sulphide oxidation. As a consequence, soil solution concentrations of Cu, Ni, and especially, Zn were initially elevated, and this was reflected in high plant uptake of Zn and elevated levels of all three metals in some groundwater wells. An extensive liming programme resulted in soil pH values generally between 5 and 6, more normal for this soil, in the following years. Soil solution metal concentrations were substantially lower, e.g., Zn from a high of 27 mg kg-1 in 1995 to 0.04 mg kg-1 in the equivalent sample in 1999. Herbage Zn concentrations declined accordingly and overall there was a strong relationship between plant metal uptake and soil solution concentration of this element (R2 = 0.84), although not for any of the other metals. Our results suggested that, for this soil, pH was by far the greatest determinant of metal solubility and that the metal source, whether sewage sludge or geochemical, had little influence. Results from extractants that solubilise other metal phases, i.e., NaNO3, EDTA and HNO3, are also presented and discussed.

Corn and weed residue decomposition in northeast Ohio organic and conventional dairy farms

Increasingly, farmers claim that management practices can significantly influence soil quality. For instance, it is common practice for dairy farms in their rotation cycle to harvest grain and then leave stubble and weeds over winter on the soil surface before ploughing in spring ahead of planting crops. Corn stubble and weeds protect soil in winter, decompose through the seasons, and release nutrients that are utilized by crops and microorganisms. Also, plant residues may harbor increased soil decomposing organisms that feed up on them. Both conventional and organic farmers tend to follow these practices with the major difference that in the conventional systems, inorganic fertilizers and pesticides are applied. Organic matter breakdown in soils under different management (organic versus conventional as in this case) may be different because some inorganic chemicals are known to affect soil decomposers and also these might be less abundant in soils with less food resources, since the organic farmers rely totally on soil fertility derived from organic source. This hypothesis was tested in an on-farm decomposition study conducted in an organic and a conventional farm that grew corn ( Zea mays L.) for dairy cattle feed, in Wayne County, OH. Mesh bags containing crop and weed residues were laid on the soil surface at the onset of winter and sampled thereafter to determine decomposition. Mass loss was significantly different among substrates, with crop residues decomposing faster than weeds. Mass and nitrogen (N) loss varied among substrates and variations were greater in summer. Between farms mass and N loss were significantly higher in the organic farm in summer. Earthworm population density was significantly higher in the organic farm. In summary, corn residues decomposed faster than weed residues, and differences in decomposition among substrates increased in summer. Decomposition was faster in summer at both farms, however, it was higher at the organic one, where more soil decomposers were found. The higher metabolic activity in the soil of the organic farm in warmer weather suggests a more active soil biota, which may be crucial for farms whose main or sole source of nutrients derives from organic matter decomposition.

Assessing the effects of environmental pollutants on soil organisms, communities, processes and ecosystems

A wide range of pollutants reach the soils of natural and managed ecosystems in concentrations that can affect their function. These chemicals, which include pesticides, heavy metals, acid deposition and a range of industrial chemicals, can reach soils in many different ways and by various routes. The ecological impacts of these chemicals on agricultural systems can involve effects at the: (i) organism population level, in terms of individual life histories (birth rate, numbers, growth, mortality); (ii) at the community level in terms of effects on plant/plant, plant/microbial, or plant/faunal interactions, species diversity and on soil food webs; (iii) at the ecosystem level, effects on primary and secondary productivity, organic matter breakdown and nutrient cycling; (iv) at the landscape level, changes in spatial heterogeneity of plants and soil organisms, material transfer of soil and nutrients, and hydrologic transfers of nutrients. Currently available methods of assessing the effect of pollutants include single species laboratory tests, a few multi-species assays, and integrated soil microcosms and terrestrial model ecosystems. The latter two methods produce data on the effects of pollutants on populations, communities and ecosystems as well as the fate of pollutants.