Showing papers on "Organic matter published in 1990"

Evidence from chronosequence studies for a low carbon-storage potential of soils

Abstract: OVER most of the Earth's land surface, the amount of carbon stored in soil organic matter exceeds by a factor of two or three the amount stored in living vegetation. This pool of soil carbon is large (1.5 × 1018 g)1,2 and plays a dynamic part in the geochemical carbon cycle. Prentice and Fung3 have suggested that terrestrial vegetation and soils would act as a large sink for atmospheric carbon dioxide if its concentration were twice the present level. Here I use data from chronosequence studies to show that the production of refractory humus substances in soils sequesters only ∼0.4 × 1015 g C yr−1 from the atmosphere, accounting for just 0.7% of terrestrial net primary production. Moreover, agricultural practices tend, on balance, to cause a release of soil carbon to the atmosphere4,5. Thus if the terrestrial biosphere is indeed to act as a carbon sink under future elevated levels of carbon dioxide, this would be more likely to be the result of changes in the distribution and biomass of terrestrial vegetation than of changes in the accumulation of soil organic matter.

Characterization of dissolved organic matter in the Black Sea by fluorescence spectroscopy

Abstract: THE natural fluorescence properties of sea water provide a means of elucidating the complex chemical composition and diverse sources of dissolved organic matter (DOM) in sea water1–6. The positions of excitation and emission maxima for a wide range of natural water samples show remarkable similarity7. High-sensitivity fluorescence spectroscopic studies8 have shown recently that emission maxima for marine and coastal waters differ by 20 nm when the excitation wavelength is 313 nm. Here we present evidence from three-dimensional excitation emission matrix (EEM) spectroscopy that at least three fluorophores are present in waters of the Black Sea. Distinct changes in the relative abundance of these fluorophores are observed as a function of depth. We suggest that three-dimensional fluorescence spectroscopy can be used to distinguish between different types and sources of DOM in natural waters. These findings may have important applications in the field of remote sensing of phytoplankton pigments. For example, a better understanding of the sources of DOM components will help in correcting9,10 remotely sensed data for the presence of gelbstoff (yellow-coloured DOM11, which plays an important part in radiation absorption by surface waters).

Interpreting Rock-Eval pyrolysis data using graphs of pyrolizable hydrocarbons vs. total organic carbon

Abstract: Kerogen, the major organic component of sedimentary rocks, is commonly analyzed by Rock-Eval pyrolysis. Plotting the data on a graph of S[2] vs. total organic carbon (TOC) and determining the regression equation is the best method for determining the true average hydrogen index and measuring the adsorption of hydrocarbon by the rock matrix. Such a plot also indicates the type of kerogen present and avoids the problem of increasing hydrogen index with total organic carbon content. With the S[2] vs. TOC diagram, the organic component of different suites of samples may be compared and their petroleum-generation potentials established. As an example, the diagrams are used to evaluate the sedimentary environments and petroleum potential of the Paleogene evaporitic sediments of the Mulhouse basin, Alsace, France.

Effect of Organic Matter Application on Methane Emission from Some Japanese Paddy Fields

Abstract: Emission rates of CH4 from four Japanese paddy fields were measured throughout the cultivation period in 1988 by using the closed chamber method. Large seasonal variations of the CH4 flux were observed. The emission was closely related to the decrease of the redox potential (Eh) in paddy soils. Drainage and supplementary application of mineral fertilizer substantially reduced the CH4 emission. Emission rates of CH4 differed markedly with the soil types. The highest rate was observed in a paddy field consisting of Peat soil (44.8 g-CH4/m2 during a cultivation period), followed by Gley soil (8.0-27.0). The emission rates in the Andosols were significantly lower (0.6-12.6). Application of rice straw at a rate of 6-9 t/ha to the paddy fields increased the CH4 emission rates 1.8- to 3.5-fold. Application of compost slightly increased the CH4 emission. Annual emission rates of CH4 from individual plots were positively correlated with the contents of readily mineralizable carbon (RMC) in paddy soils col...

Evidence from carbon isotope measurements for diverse origins of sedimentary hydrocarbons.

Abstract: The organic matter found in sedimentary rocks must derive from many sources; not only from ancient primary producers but also from consumers and secondary producers. In all of these organisms, isotope effects can affect the abundance and distribution of 13C in metabolites. Here, by using an improved form of a previously described technique in which the effluent of a gas chromatograph is continuously analysed isotopically, we report evidence of the diverse origins of sedimentary organic matter. The record of 13C abundances in sedimentary carbonate and total organic carbon can be interpreted in terms of variations in the global carbon cycle. Our results demonstrate, however, that isotope variations within sedimentary organic mixtures substantially exceed those observed between samples of total organic carbon. Resolution of isotope variations at the molecular level offers a new and convenient means of refining views both of localized palaeoenvironments and of control mechanisms within the global carbon cycle.

Predicting long-term patterns of mass loss, nitrogen dynamics, and soil organic matter formation from initial fine litter chemistry in temperate forest ecosystems

Abstract: Long-term decomposition data are presented for several types of foliar and fine root litter in different stands in Wisconsin and Massachusetts, USA Changes in mass remaining as well as nitrogen

Influence of organic matter from soils and sediments from various origins on the sorption of some chlorinated aliphatic hydrocarbons: implications on KOC correlations.

Abstract: Sorption of nonionic compounds is strongly dependent on the content as well as the nature of the organic matter in soils and sediments. The composition and the structure of organic matter varies due to its origin and geological history and strongly influences the sorption affinity for nonionic organic compounds. Organic matter in unweathered shales and high-grade coals shows enhanced sorption (> 1 order of magnitude) compared to organic matter in recent soils or geologically young material and low-grade coals. The results obtained indicate a decrease in sorption with increasing proportions of oxygen-containing functional groups in natural organic substances. A first approximation to estimate sorption coefficients for various organic matter is provided by an empirical correlation between the hydrogen/oxygen (H/O) atomic ratio as an index of the oxidation of the organic matter and the organic carbon normalized sorption coefficients (K{sub OC}). This approximation also permits adjustment of K{sub OC} values derived from K{sub OW} data.

Formation of carbonyl compounds from UV‐induced photodegradation of humic substances in natural waters: Fate of riverine carbon in the sea

Abstract: Low-molecular-weight (LMW) carbonyl compounds, e.g. formaldehyde, acetaldehyde, and the a-keto acid glyoxylate, were produced in a wide variety of natural waters upon irradiation with sunlight. Production rates were linearly related (r2 > 0.98) to initial absorbance at 300 nm and initial fluorescence (360/460 nm) in all waters tested. Photochemical production was also linearly related to loss (photobleaching) of absorbance and fluorescence during irradiation, irrespective of irradiation time and prior photobleaching history of the sample. These results were attributed to absorption of light by humic substances in the waters, as determined by experiments where purified humic and fulvic extracts were added to open-ocean water. The wavelengths in the solar spectrum responsible for photoproduction of LMW carbonyl compounds and bleaching of dissolved organic matter light absorbance are in the UV-B region (280-320 nm). These results, plus results from time-course irradiations, suggest that photoproduction of carbonyl compounds from humic substances is closely related to photobleaching of absorbance. Based on our photoproduction rates of LMW carbonyl compounds, we estimate that the half-life of humic-rich riverine dissolved organic C in the oceanic mixed layer is 5-l 5 yr.

The role of organic matter in soil compactibility: a review of some practical aspects.

Abstract: The effect of variation in the amount, composition, form, origin and distribution of organic matter on the reaction of soil to compactive loading under both laboratory and field conditions is reviewed. For experimental purposes, soils having different organic matter contents may be obtained using samples from a number of sites or from a single site at which different organic matter contents have developed as a result of differences in crop management practices, or by the incorporation of organic material in laboratory samples or in field soils. Compactibility tests made with uniaxial compression and impact loading equipment may not give rise to similar conclusions concerning the role of organic matter. Living roots, and to a lesser extent dead roots, provide a filamentous network which, like geotextiles, resists compactive loads. Fungal hyphae have a similar action, particularly within aggregates. Highly humified material increases the stability and strength of aggregates, and hence decreases compactibility. Soil management practices influence the form, amount and distribution of organic matter in soils. The incorporation of straw or other bulky organic materials results in local concentrations of undecomposed material. Undecomposed organic litter may accumulate at the soil surface or within the top few centimetres after zero tillage. Under forest crops, a heavy cover of woody material acts as a protective mat during the passage of loaded wheels. Increases in organic matter may reduce compactibility by increasing resistance to deformation and/or by increasing elasticity (rebound effects). Compactibility is sensitive to even quite small changes in the amount of organic matter. These observations have important implications in the improvement of soil management to avoid over-compaction problems in crop production.

Hydroxyl radical photoproduction in the sea and its potential impact on marine processes.

Abstract: Photochemical production rates and steady-state concentrations of hydroxyl radicals (.OH) were measured in sunlight-irradiated seawater. Values ranged from 110 nanomolar per hour and 12 x 10(-18) molar in coastal surface water to 10 nanomolar per hour and 1.1 x 10(-18) molar in open ocean surface water. The wavelengths responsible for this production are in the ultraviolet B region (280 to 320 nanometers) of the solar spectrum. Dissolved organic matter (DOM) appears to be the main source for .OH over most of the oceans, but in upwelling areas nitrite and nitrate photolysis may also be important. DOM in the deep sea is degraded more readily by .OH (and its daughter radicals), by a factor of 6 to 15, than is DOM in open-ocean surface water. This finding may in part bear on major discrepancies among current methods for measuring dissolved organic carbon in seawater.

Stability of soil aggregates in relation to organic constituents and soil water content

Abstract: SUMMARY The effects of soil organic matter content, soil water content and duration of wet-sieving on aggregate stability of soils with contrasting cropping histories were investigated. Long-term pasture samples had a greater aggregate stability than long-term arable samples. However, air-drying aggregates before wet-sieving increased the aggregate stability of long-term pasture samples, but decreased that of long-term arable samples. With increasing duration of wet-sieving, the proportion of water-stable aggregates declined until a near-constant value was reached for each sample. Thus, within a sample there are aggregates possessing a wide range of stabilities; with increasing time under arable cropping there is an increase in the proportion of unstable aggregates present, and the measured aggregate stability, therefore, declines. Unstable aggregates (defined as those dispersed after wet-sieving for 1 min) generally had lower organic matter content than stable ones (those still intact after sieving for 15 min). The aggregate stability of a regrassed site (13 years of arable plus 2 years of pasture) was markedly higher than that of a corresponding site from 15 years of arable cropping. Nonetheless, levels of organic matter (organic C, total N and hydrolysable carbohydrate) were almost identical at the two sites. However, aggregates from the regrassed site did have a higher biomass C and water-extractable carbohydrate content than those from the 15-year arable site. For a group of soils with varying cropping histories, aggregate stability was significantly more closely correlated with hot water-extractable carbohydrate content than with organic C or hydrolysable carbohydrate content. It is suggested that the hot water-extractable carbohydrate fraction may represent a pool of carbohydrate involved in the formation of stable aggregates.

Biogeochemistry of carbon in the Amazon River

Abstract: Depth-integrated, discharge-weighted water samples were collected over 1,800km ofthe Amazon River on eight cruises at different stages of the hydrograph, 1982-1984. Fine (FPOC, 163 pm) and coarse (CPOC, > 63 rm) particulate organic carbon as weight percentage of suspended sediment varied between 0.9-1.5% for FPOC and 0.5-3.49/o for CPOC. Concentrations of FPOC ranged from 5 mg liter-’ upriver to 2 mg liter-’ downriver in the mainstem and from 6 mg liter-’ inthe Rio Madeira to i 1 in the Rio Negro. CPOC had similar distribution patterns. but with concentrations 15.-30% those of FPOC. Dissolved organic carbon (DOC) averaged 4-6 mg liter-r in the mainstem and up to 12 ml; liter -I in the Rio Negro. Upriver dissolved inorganic carbon (DIC) concentrations of about 1,200 @LM were diluted by tributaries and floodplain drainage to 600 PM at the most downriver site. Evasion ofCO,, invasion of O,, and in situ oxidation were of comparable magnitude, 3-8 pmol m-2 s-r. The average export of total organic carbon (TOC) was 36.1 Tg yr-i (8.5 g m-2 yr-I), of which 62% was DOC, 34% was FPOC, and 4% was CPOC. TOC inputs were insufficient to support in situ oxidation by a factor of at least two. A relatively small, rapidly cycling pool of labile organic matter may coexist with a much larger pool of more refractory material.

Effect of tillage on soil organic carbon mineralization estimated from 13C abundance in maize fields

Abstract: SUMMARY Three methods of cultivation, conventional tillage (CT), superficial tillage (ST) and no-tillage (NT), were applied for 17 years to continuous maize. Their effect on soil organic carbon content was investigated through measurements of carbon and 13C/12C ratios, using the natural difference in 13C content between C3 plants and maize, which is a C4 plant. Because the soil had carried C3 plants before the experiment started, the organic carbon remaining from that time (C3,-carbon), was distinguished from the carbon derived from maize. Comparison between continuous wheat and maize plots showed that organic matter from both maize and wheat decomposed without significant 13C enrichment, whereas older C3-carbon was enriched by 1.5% compared to that of fresh wheat material. From the initial 3.6 kg C m−2 in the topsoil (0–30 cm), 0.95 were mineralized in the CT treatment, but only 0.45 in NT. The mineralization was the same in the tilled layer of ST as in CT. The CT treatment accumulated 1.1 kg C m−2 of maize-derived carbon and the NT treatment 0.8. The mineralization of initial C3-carbon was the same at all depths between 0 and 30 cm in the NT treatment; 75% of the carbon derived from maize was found in the 0–5 cm layer.

Influence of organic matter on phosphate adsorption by aluminium and iron oxides in sandy soils

Abstract: SUMMARY The phosphate adsorption capacity (Pmax) of samples from various horizons of five Danish podzolized soils were investigated before and after organic matter removal. Removal of organic matter had no direct influence on Pmax suggesting that organic matter did not compete with phosphate for adsorption sites. In the soils investigated aluminium and iron oxides were the main phosphate adsorbents. Thus, more than 96% of the variation in Pmax could be accounted for by poorly crystalline aluminium and iron oxides (extractable by oxalate) and by well-crystallized iron oxides (taken as the difference between dithionite-citrate-bicarbonate-extractable iron and oxalate-extractable iron). Organic matter affected phosphate adsorption indirectly by inhibiting aluminium oxide crystallization. The resulting poorly crystalline oxides had high Pmax. In contrast, the influence of organic matter on the crystallinity of the iron oxides, and therefore on their capacity to adsorb phosphate, seemed limited.

Microbial Ectoenzymes in Aquatic Environments

Abstract: During the past decade an increasing number of ecological studies have considered the complexity of aquatic environments. One major outcome of these studies has been an accelerated interest in the role of microheterotrophs and the mode by which organic matter is made available to them. The heterotrophic microorganisms are the key level at which the metabolism of the whole ecosystem is affected, i.e., nutrient cycling, organic matter transformation and mineralization, and energy flow. The measurement of microbial activity in natural waters is very important for understanding the dynamic aspects of the functioning of the whole ecosystem.

Estimate of organic matter turnover rate in a savanna soil by 13C natural abundance measurements.

Abstract: The medium-term dynamics of organic matter in a sandy savanna soil has been investigated at Lamto (Ivory Coast) through changes in the 13C/12C ratio induced by vegetation changes from the C4 to the C3 photosynthetic pathway. After 25 yr of protection from fire, a soil previously covered by C4 grassland vegetation had been progressively colonized by C3 woody plants. Although the total C content did not show significant changes. 52–70% of the original C4 carbon was turned over when vegetation cover was changed. Turnover of coarse organic debris (> 250 μm) was much greater (97%) than the mineralization (50%) of clay-associated fractions (< 20 μm).

Wetland soil formation in the rapidly subsiding Mississippi River Deltaic Plain: mineral and organic matter relationships.

Abstract: The elevation of submerging coastal marshes is maintained by vertical accretion of mineral and organic matter. Submergence rates currently exceed 1·0 cm year −1 in the Mississippi Deltaic Plain and are expected to increase. Mineral matter-organic matter relationships were examined in surface profiles of Mississippi Deltaic Plain soil from both Active Delta Zone marsh (which receives freshwater and mineral sediment from the Atchafalaya or Mississippi Rivers) and Inactive Delta Zone marsh (which relies on rainfall for freshwater and on reworked sediments for mineral matter) to gain insights into marsh soil structure and formation. Mineral and organic matter accounted for 4–14% of soil volume. The remainder was pore space and was occupied by water and entrapped gases. Organic matter occupied more volume than mineral matter in all but saline marsh soil. The regular influx of mineral matter to active fresh marsh resulted in active fresh marsh soil containing twice as much mineral and organic matter as inactive fresh marsh soil. Within the Inactive Delta Zone, the volume of mineral and organic matter increased from fresh (inland) to saline (seaward) marshes. Saline marsh soil required 1·7 times as much mineral matter as brackish marsh soil to vertically accrete at similar rates, possibly as a result of soil bulk density requirements of the dominant saline marsh plant, Spartina alterniflora . Vertical accretion rates were highest in the Active Delta Zone, probably as a result of increased mineral matter availability and delivery. Current, best estimates of the combination of mineral and organic matter required (g m −2 year −1 ) to maintain marsh surface-water level relationship are fresh marsh: organic matter = 1700 + 269 x , mineral matter = 424 x ; brackish marsh: organic matter = 553 + 583 x , mineral matter = 1052 x ; saline marsh: organic matter = 923 + 601 x , mineral matter = 1798 x , where x = the rate of submergence (cm year −1 ).

Modelling organic matter dynamics in different soils.

Abstract: A mathematical model was developed to describe carbon (C) and nitrogen (N) cycling in different soil types, e.g. clay and sandy soils. Transformation rates were described by first-order kinetics. Soil organic matter is divided into four fractions (including microbial biomass pool) and three fractions of residues. The fraction of active soil organic matter was assumed to be affected by the extent of physical protection within the soil, as was the soil microbial biomass. The extent of protection influenced the steady state level of the model, and, hence, the mineralization rates. The mineralization rate in fine-textured soils is lower than in coarse-textured soils; in fine-textured soils a larger proportion of the soil organic matter may be physically protected. The availability of organic materials as a substrate for microorganisms is not only determined by their chemical composition, but also by their spatial distribution in the soil. (Abstract retrieved from CAB Abstracts by CABI’s permission)

Intensification of recycling of organic matter at the sea floor near ocean margins

Abstract: QUANTIFICATION of the flux of organic carbon from the ocean surface to the deep ocean and sea floor is crucial to the prediction of future levels of atmospheric carbon dioxide and the interpretation of organic carbon variations in marine sediments. We report here the results of a study of benthic exchange and metabolism based on in situ benthic flux-chamber, in situ oxygen microelectrode and shipboard pore-water measurements performed off the central California continental shelf. At the base of the continental slope, the rates of benthic carbon mineralization exceed 0.8 mol C m−2 yr−1. Seaward of the slope and rise, however, rates decrease to <0.05 mol C m−2yr−1 in the North Pacific central gyre. These results indicate that, across the northeast Pacific, about half of the input of organic carbon to the sea floor occurs within 500 km of the continental slope. Measured rates of benthic carbon oxidation near the continental margin exceed the fluxes of organic carbon determined from previous sediment-trap studies by a factor of about three. In this region, therefore, either traps significantly underestimate the mean flux of vertically sinking particulate organic carbon (perhaps by under-sampling important episodic events) or more than half of the organic-carbon input is due to processes that by-pass traps, such as near-bottom lateral or active biological transport.

Fire-induced transformation of soil organic matter from an oak forest: an experimental approach to the effects of fire on humic substances

Abstract: This study is about the effects of forest fire on the organic matter from a Dystric Xerochrept under Quercus rotundifolia. In laboratory experiments carried out to simulate the effects of fire on isolated humic fractions, several physicochemical characteristics were described in the heated samples,

Geochemistry of sulfur in fossil fuels

Abstract: Geochemistry of Sulfur in Petroleum Systems Geochemistry of Sulfur in Coal Environmental Aspects of the Combustion of Sulfur-Bearing Fuels Polysulfide Reactions in the Formation of Organosulfur and Other Organic Compounds in the Geosphere Isolation of Sulfur Compounds from Petroleum Microbial Metabolism of Organosulfur Compounds in Petroleum Geochemistry of Organic and Inorganic Sulfur in Ancient and Modern Lacustrine Environments: Case Studies of Freshwater and Saline Lakes Characterization of Organic Matter in Sulfur-Rich Lacustrine Sediments of Miocene Age Incorporation of Sulfur into Recent Organic Matter in a Carbonate Environment Sulfur and Pyrite in Precursors for Coal and Associated Rocks: A Reconnaissance Study of Three Modern Sites Formation of Iron Sulfides in Modern Salt Marsh Sediments Sulfur K-Edge X-ray Absorption Spectroscopy of Petroleum Asphaltenes and Model Compounds Direct Determination of Total Organic Sulfur in Coal Elemental Sulfur in Bituminous Coals Multiple-Heteroatom-Containing Sulfur Compounds in a High Sulfur Coal Organosulfur Consituents in Rasa Coal Distribution of Organic-Sulfur-Containing Structures in High-Organic-Content Sulfur Coals Characterization of Organic Sulfur Compounds in Coals and Single-Coal Macerals Spatial Variation of Organic Sulfur in Coal Characterization of Organosulfur Compounds in Oklahoma Coals by Pyrolysis-Gas Chromatography Coal Desulfurization by Programmed-Temperature Pyrolysis and Oxidation Nature and Geochemistry of Sulfur-Containing Compounds in Alberta Petroleums Identification of Alkylthiophenes Occurring in the Geosphere by Synthesis of Authentic Standards Organic Sulfur Compounds and Other Biomarkers as Indicators of Palaeosalinity Alkylthiophenes as Sensitive Indicators of Palaeoenvironmental Changes: A Study of a Cretaceous Oil Shale from Jordan Characterization of Organically Bound Sulfur in High-Molecular-Weight, Sedimentary Organic Matter Analysis of Maturity-Related Changes in the Organic Sulfur Composition of Kerogens by Flash Pyrolysis-Gas Chromatography Isotopic Study of Coal-Associated Hydrogen Sulfide Pyrolysis of High-Sulfur Monterey Kerogens: Stable Isotopes of Sulfur, Carbon, and HydrogenWSulfur Isotope Data Analysis of Crude Oils from the Bolivar Coastal Fields Distribution of Organic Sulfur Compounds in Mesozoic and Cenozoic Sediments Carbon Isotope Fractionation during Oxidation of Light Hydrocarbon Gases: Relevance to Thermochemical Sulfate Reduction in Gas Reservoirs

Organic matter accumulation and nitrogen mineralization during secondary succession in heathland ecosystems.

Abstract: SUMMARY (1) Five series of plots in heathlands were selected in which the above-ground biomass and the litter (L) and humus (FH) layer had been removed between one and fifty years ago. Secondary succession in these heathlands was studied by comparing the amounts of soil organic matter, above-gound biomass, below-ground biomass and the annual nitrogen mineralization in plots of different ages. (2) The amounts of organic matter in the L and FH layers increased with age while the communities were dominated by Calluna vulgaris or Erica tetralix. Where dwarf shrubs were replaced by Molinia caerulea the amount of soil organic matter did not further increase. (3) Both above-ground and below-ground biomass in the communities dominated by dwarf shrubs increased with age to about 1300 and 800 g m-2, respectively, but were much lower in the Molinia-dominated plots (c. 400 and 500 g m-2). (4) The annual nitrogen mineralization remained at a low level, or even decreased, during the first ten years after turf removal. Thereafter the mineralization rate increased with an increasing amount of organic matter in the L and FH layers. Multiple regression analysis revealed that the amount of organic matter in the L and FH layers and the biomass percentage of Molinia together explained 84% of the observed variance in the annual nitrogen mineralization. It is proposed that Molinia has a positive effect on the mineralization rate. (5) In communities dominated by Calluna or Erica all the nitrogen that entered the ecosystem appeared to accumulate in the plant biomass and the soil organic matter, whereas losses of nitrogen from the ecosystem appeared to be negligible. In Moliniadominated communities part of the nitrogen that entered the system was lost to deeper soil layers, to the atmosphere or to herbivores.

Experimental evidence quantifying the role of benthic invertebrates in organic matter dynamics of headwater streams

Abstract: SUMMARY. 1. The insecticide methoxychlor was applied seasonally to one of three small headwater streams in the southern Appalachian Mountains in North Carolina, U.S.A. The initial application caused massive invertebrate drift (>1,000,000 organisms/week) and resulted in a community with few shredders and reduced abundances of most insect taxa. 2. Bacterial densities and microbial respiration rates were not affected by treatment. 3. Disruption of the invertebrate community resulted in significant reductions in leaf litter processing rates (50–74% reduction depending on leaf species) and in the amount of leaf litter processed annually (reduction of 25–28%). 4. Reductions in leaf litter processing rates resulted in significant reductions in fine particulate organic matter (FPOM) export. Declines in both concentration and total export were detectable within 1 week of treatment. Annual FPOM export was reduced to 33% of pretreatment levels. Alteration to the invertebrate community had a much greater effect on FPOM export than a severe (50–200 year) drought. 6. Course particulate organic matter (CPOM) export was not significantly influenced by treatment but was influenced by hydrologic differences among years.

Survival of the soil microbial biomass at elevated temperatures

Abstract: A grassland soil was incubated for 240 days at either 15. 25 or 35°C. Changes in soil microbial biomass were monitored throughout the incubations in three ways: from soil ATP content, by the fumigation-extraction method and by a new version of the fumigation-extraction method in which carbohydrate released by fumigation is measured, rather than total C. All three methods of measuring biomass concur in showing that the biomass drifted downwards very slowly at 15° (with a mean decline of 0.11% per day) and 25°C (0.21% per day), whereas it fell precipitously during the first 50 days of incubation at 35°C (1.72% per day over the 0–50 day period). Both CO 2 output and biomass C remained remarkably constant over the 150–240 day period in all the incubations. The specific death rates of the biomass over this period were 0.0072, 0.016 and 0.25 day −1 at 15. 25 and 35°C respectively. The corresponding biomass turnover-times were 139, 62 and 4 days. Raising the incubation temperature had two effects: the rate of release of substrate increased but the specific death rate increased even more. The activation energy of death was 130kJ mol −1 , similar to values for thermal death in bacteria, suggesting that thermal denaturation was the process responsible for the enhanced death rate at 35°C. Although some of the CO 2 liberated during these prolonged incubations must have come from the initial microbial biomass, through cryptic growth and consumption of intracellular C reserves, most can only have come from soil organic matter. Some chemical or biochemical process (or processes) must operate on soil organic mailer to release a steady flow of substrate to the waiting biomass. The measurement of carhohydrate released by fumigation may provide a convenient way of measuring soil microbial biomass and deserves further testing.

Molecular size distributions of dissolved organic matter

Abstract: Discrete molecular size distributions of dissolved organic matter (DOM) in natural waters and wastewaters are usually determined using an array of ultrafiltration membranes in stirred cells However, many researchers neglect membrane rejection, resulting in an underestimation of DOM in low molecular weight size classes In this paper, we develop a simple model, based on a permeation coefficient, that can be used to improve the accuracy of size distributions Since permeation coefficients are a function of membrane material and water composition, these coefficients must be determined for every water sample The utility of the model is demonstrated by comparing unadjusted and adjusted size distributions of dissolved organic carbon (DOC) in ground waters and municipal wastewaters For example, when membrane rejection is neglected, 41% of the DOC in Biscayne Aquifer ground water has an apparent molecular weight greater than 5,000 amu Size distributions determined using the permeating coefficient model, however, indicate only 16% of the DOC is greater than 5,000 amu

Use of organic carbon and loss-on-ignition to estimate soil organic matter in different soil types and horizons

Abstract: Loss-on-ignition (LOI) and the organic C content have been used to estimate soil organic matter. Organic matter is often estimated from organic C by applying a factor of 1.724. Several authors have examined the relationship between LOI, used as an estimate of organic matter, and C by simple linear regressions. In the present study, this approach was examined in relation to two sets of data. LOI overestimates organic matter in soils with significant proportions of clay minerals because of bound water, and correcting for bound water gives some LOI: C ratios of less than 1. It is concluded that differences in the nature of the organic matter in different soils and horizons make the simple regression approach unsuitable. More attention needs to be paid to studies of the nature of the organic matter.