Availability Of Organic Substrates Research Articles

Stable carbon isotope fractionation by sulfate-reducing bacteria.

Biogeochemical transformations occurring in the anoxic zones of stratified sedimentary microbial communities can profoundly influence the isotopic and organic signatures preserved in the fossil record. Accordingly, we have determined carbon isotope discrimination that is associated with both heterotrophic and lithotrophic growth of pure cultures of sulfate-reducing bacteria (SRB). For heterotrophic-growth experiments, substrate consumption was monitored to completion. Sealed vessels containing SRB cultures were harvested at different time intervals, and delta(13)C values were determined for gaseous CO(2), organic substrates, and products such as biomass. For three of the four SRB, carbon isotope effects between the substrates, acetate or lactate and CO(2), and the cell biomass were small, ranging from 0 to 2 per thousand. However, for Desulfotomaculum acetoxidans, the carbon incorporated into biomass was isotopically heavier than the available substrates by 8 to 9 per thousand. SRB grown lithoautotrophically consumed less than 3% of the available CO(2) and exhibited substantial discrimination (calculated as isotope fractionation factors [alpha]), as follows: for Desulfobacterium autotrophicum, alpha values ranged from 1.0100 to 1.0123; for Desulfobacter hydrogenophilus, the alpha value was 0.0138, and for Desulfotomaculum acetoxidans, the alpha value was 1.0310. Mixotrophic growth of Desulfovibrio desulfuricans on acetate and CO(2) resulted in biomass with a delta(13)C composition intermediate to that of the substrates. The extent of fractionation depended on which enzymatic pathways were used, the direction in which the pathways operated, and the growth rate, but fractionation was not dependent on the growth phase. To the extent that environmental conditions affect the availability of organic substrates (e.g., acetate) and reducing power (e.g., H(2)), ecological forces can also influence carbon isotope discrimination by SRB.

Structural and functional analysis of a microbial mat ecosystem from a unique permanent hypersaline inland lake: ‘La Salada de Chiprana’ (NE Spain)

The benthic microbial mat community of the only permanent hypersaline natural inland lake of Western Europe, ‘La Salada de Chiprana’, northeastern Spain, was structurally and functionally analyzed. The ionic composition of the lake water is characterized by high concentrations of magnesium and sulfate, which were respectively 0.35 and 0.5 M at the time of sampling while the total salinity was 78 g l −1. Community composition was analyzed by microscopy, high-performance liquid chromatography (HPLC) pigment analyses and by studying culturable bacteria from different functional groups. Therefore, denaturing gradient gel electrophoresis (DGGE) was applied on most probable number (MPN) dilution cultures. Microscopy revealed that a thin layer of Chloroflexus-like bacteria overlaid various cyanobacteria-dominated layers each characterized by different morphotypes. DGGE analysis of MPN dilution cultures from distinct mat layers showed that various phylotypes of anoxygenic phototrophic, aerobic heterotrophic, colorless sulfur-, and sulfate-reducing bacteria were present. The mats were furthermore functionally studied and attention was focussed on the relationship between oxygenic primary production and the flow of carbon through the microbial community. Microsensor techniques, porewater and sediment photopigment analysis were applied in order to estimate oxygenic photosynthetic rates, daily dynamics of (in)organic carbon porewater concentration and migration behavior of phototrophs. Chiprana microbial mats produced dissolved organic carbon (DOC) both during the day and night. It was estimated that 14% of the mats gross photosynthetic production and 49% of the mats net photosynthetic production diffused out of the mat in the form of low molecular mass fatty acids, although these compounds made up only 2% of the total DOC pool. The high flux of dissolved fatty acids from the microbial mat to the water column may explain why in this system Chloroflexus-like bacteria proliferate on top of the cyanobacterial layers since these photoheterotrophic bacteria grow preferably on organic phototrophic exudates. Furthermore it may also explain why high numbers of viable sulfate-reducing bacteria were found in the fully oxygenated sediment surface layers. These organisms apparently do not have to compete with aerobic heterotrophic community members due to the ample availability of organic substrates. Moreover, the high production of DOC strongly indicates that the mat community was nutrient limited in its growth. Photopigment analysis revealed furthermore that chlorophyll a (Chl a) and three of its allomeres had a complementary depth distribution what suggests that the Chl a allomeres are functional adaptations to differences in light quality and/or quantity and may be species specific.

Ectoenzymatic ratios in relation to particulate organic matter distribution (Ross Sea, Antarctica).

The results of a study on ectoenzymatic activity (the enzyme activity bound to particles larger than 0.2 micro m) and its relation to organic particle concentration are reported here. The sampling was carried out during the 1994 Antarctic spring, at a fixed station (Station 11) in the polynya of the Ross Sea, an area characterized by quick changes in sea ice cover. The sampling was repeated 4 times over a 20-day time period. The particulate organic matter distribution followed the physical structure of the water column, which depends on ice dynamics and is mainly determined by salinity. In the mixed-water surface layer (0-50 m) the concentrations were higher (on average 65.6 micro gC/L) than in the deeper water layer (50 m-bottom) (on average 19.1 micro gC/L). This distribution and quality, expressed by the protein:carbohydrate ratio, linked the particulate organic matter to the phytoplanktonic bloom which was in progress in the area. We determined the kinetic parameters of the glycolytic and proteolytic ectoenzymes and also the total activity for the proteolytic enzyme, in order to evaluate the contribution of the particle-bound activity. We observed higher values in the surface layer than in the deeper layer. b-Glucosidase activity ranged between 0.03 and 0.92 nmol L(-1) h(-1); b-N-acetylglucosaminidase activity was in the range of 0.04-0.58 nmol (L-1) (h-1). The total proteolytic activity (leucine aminopeptidase) ranged between 0.85 and 33.71 nmol L(-1) (h-1). The ectoproteolytic activity was about 35-60% of the total. The Km values were slightly higher for the proteolytic activity (on average 0.43 micro M for ectoproteolytic activity and 0.58 micro M for total proteolytic activity) than for the b-glucosidase (on average 0.36 micro M) and b-N-acetylglucosaminidase (on average 0.17 micro M), showing no remarkable variations in the water column. The ectoenzymatic ratios and their relationship with particulate organic substrates confirm the close link between organic substrate availability and degradation system response. The significant and positive correlations are not specific and suggest a prompt and efficient systemic response to the input of trophic resources. Nevertheless, changes in ectoenzyme activity and synthesis may act as adaptive responses to changing features of the ecosystem. In particular, variations in the proteolysis:glycolysis ratio depend on the functional features of the ecological system. In our study area this ratio is higher (about 10 or more) during production (particularly autotrophic) and lower (about 5 or less) during degradation/consumption events. The analysis of previous data, collected over a larger area characterized by different environmental conditions due to the changes of the pack ice cover, during the same cruise, confirms the existence of a significant relationship. Furthermore, the analysis of enzyme-uptake systems, expressed as Vmax:Km ratio, suggests that glycolytic ectoenzymes, although poorly expressed, may encourage microconsumers to grow rapidly on a wide range of organic substrates, including the refractory ones such as cellulose and chitin. However, low ectoenzyme potential exploitation rates of available organic substrates (on average about 5% for glycolytic and 12% for proteolytic ectoenzymes) would suggest that, during spring, zooplankton grazing or vertical and lateral transport are likely to play an important role in the removal of organic materials from the system.

Nitrogen processing gradients in subsurface-flow treatment wetlands—influence of wastewater characteristics

Nitrification, an aerobic microbial process, is generally considered to be the rate-limiting step for N removal in subsurface-flow (SSF) constructed wetlands treating organic wastewaters. SSF wetland nitrogen (N) processing gradients were investigated using cascade mesocosms comprised of five planted ( Schoenoplectus tabernaemontani), gravel-filled tanks operated in series, in order to determine the effects of organic substrate availability (measured as chemical oxygen demand, COD) and partial pre-nitrification on rates of N removal. Duplicate sets of cascades supplied with 23 mm d −1 of four different organic wastewaters provided COD:N ratios of ∼2–15 (<1–30 in individual tanks) and a range of N species balances (34–88% ammoniacal-N, NH 4-N; <1–40% oxidised N, NO x -N). Mass balances for organic N (Org-N), NH 4-N and NO x -N, and COD removal were calculated for each tank of the cascades. Concurrent measurements were made of plant growth and N uptake, sediment N accumulation and selected biogeochemical indicators (redox potential, pH, and CO 2 and CH 4 emissions). Using a simplified model of sequential N transformations and sinks, average net rates of N mineralisation ranged from 0.22 to 0.53 g m −2 d −1, nitrification from 0.56 to 2.15 g m −2 d −1, denitrification from 0.47 to 1.99 g m −2 d −1 (60–84% of measured N removal) and plant assimilation from 0.28 to 0.47 g m −2 d −1 in the cascade tanks. Nitrification and denitrification occurred concurrently with COD removal, even in the upstream stages of cascades receiving the higher-strength wastewaters. Surprisingly, neither net areal nitrification rates, nor first order nitrification rate constants ( k A) were correlated with COD removal rates (as a measure of heterotrophic oxygen demand). Nitrification rates were correlated with average NH 4-N concentrations in the cascade tanks, and were closely paralleled by net denitrification rates. Although k A for N mineralisation, nitrification and total N removal were highest for the partially pre-nitrified wastewater tested, considerably higher areal mass removals were achieved in the cascades receiving more concentrated ammonium-rich wastewaters. The oxygen demand required to support full nitrification of ammonia and mineralised Org-N in the cascades (without accounting for competitive heterotrophic oxygen demand) was in the upper range of that expected to be able to be supplied through surficial and plant-mediated oxygen transfer. Implications for understanding the nature of coupled nitrification–denitrification and COD removal in SSF treatment wetlands are discussed.

Inhibition of methanogens increases photo-dependent nitrogenase activities in anoxic paddy soil amended with rice straw.

The interaction between phototrophic dinitrogen fixers and methanogens was examined in soil slurries amended with rice straw using 2-bromoethanesulfonic acid (BES), a specific methanogenic inhibitor. Slurries incubated in light increased phototrophic nitrogenase activity (acetylene reducing activity), and showed growth of phototrophic purple bacteria and reduction of CH(4) emission, indicating outcompetition of purple bacteria with methanogens in photic zones. Adding BES effectively inhibited methane production and markedly increased phototrophic acetylene reducing activity accompanied with acetate accumulation, but did not affect populations of purple bacteria in the slurries. More acetate accumulated in the inhibited slurries incubated in dark. We suggest that increased availability of organic substrates for purple bacteria after stopping methanogenic consumption by BES caused the increased phototrophic acetylene reducing activity. These results indicate that, after purple bacteria grow enough, performance of their N(2) fixation may be limited by substrate availability, which methanogenesis may profoundly influence.

Inhibition of methanogens increases photo-dependent nitrogenase activities in anoxic paddy soil amended with rice straw

The interaction between phototrophic dinitrogen fixers and methanogens was examined in soil slurries amended with rice straw using 2-bromoethanesulfonic acid (BES), a specific methanogenic inhibitor. Slurries incubated in light increased phototrophic nitrogenase activity (acetylene reducing activity), and showed growth of phototrophic purple bacteria and reduction of CH 4 emission, indicating outcompetition of purple bacteria with methanogens in photic zones. Adding BES effectively inhibited methane production and markedly increased phototrophic acetylene reducing activity accompanied with acetate accumulation, but did not affect populations of purple bacteria in the slurries. More acetate accumulated in the inhibited slurries incubated in dark. We suggest that increased availability of organic substrates for purple bacteria after stopping methanogenic consumption by BES caused the increased phototrophic acetylene reducing activity. These results indicate that, after purple bacteria grow enough, performance of their N 2 fixation may be limited by substrate availability, which methanogenesis may profoundly influence.

Wetland rice soils as sources and sinks of methane: a review and prospects for research

Rice paddies are an important human-made ecosystem for the global CH4 budget. CH4, which is produced in the predominantly anaerobic bulk soil layers, is oxidized significantly before it reaches the atmosphere. Roots of rice, in addition to supporting the consumption of CH4, contribute to the total CH4 production in the soil. The various controls of CH4 emission from this ecosystem depend on the structure of plant and microbial communities and their interactions. Availability of organic substrates, electron acceptors and other soil- and plant-related factors influence the activities of microbial communities. Agronomic practices including fertilization and application of pesticides have effects on CH4 emission. Recent studies using molecular retrieval approaches with small subunit rRNA-encoding gene (rDNA) sequences and functional genes, showed the richness of diversity of the microbial community in rice paddy soils, which includes members of the Archaea and methanotrophs. There is need for further research to know the consequences, at the ecosystem level, of changes in microbial diversity and microbial communities in paddy soils. This will aid in understanding the mechanisms involved in the mitigating effects of certain agricultural practices.

Potential nitrogen immobilization in grassland soils across a soil organic matter gradient

Nitrogen additions to grasslands have increased historically and are likely to continue increasing given the current and projected land use patterns, urbanization and fossil fuel use. Nitrogen retention in both grassland and forest soils is often limited by organic substrate availability, but few studies have explicitly tested the relationship between soil carbon content and nitrogen retention. We initiated a laboratory study to directly assess the influence of soil organic matter content on potential nitrogen immobilization and turnover for soils collected from across a temperature gradient in the Great Plains region of the U.S. We measured soil organic carbon, total nitrogen and carbon–nitrogen ratios and estimated carbon mineralization and net nitrogen mineralization over 5- and 30-day laboratory incubations. We used the 15 N pool dilution assay to estimate gross nitrogen immobilization and nitrogen turnover for 5 day laboratory incubations. Soil organic carbon concentration and soil carbon–nitrogen ratios were negatively correlated with mean annual temperature in a linear regression model that accounted for 46–56% of the variability, respectively. Regional patterns in soil organic carbon content and small scale variability in substrate availability imposed by discontinuous plant cover together strongly influenced potential nitrogen immobilization. Potential carbon mineralization and nitrogen immobilization increased with increasing soil organic matter content. Soil organic carbon content accounted for 58% of the variation in potential rates of N immobilization. A strong correlation between nitrogen immobilization and carbon mineralization further suggests that rapid stabilization of nitrogen is facilitated by an active microbial community and the availability of a readily mineralizable organic substrate.

Effects of temperature and dissolved oxygen on hydrolysis of sewer solids

Effects of temperature and dissolved oxygen (DO) on kinetics of microbial transformation processes of suspended sewer sediment particles, suspended wastewater particles and wastewater were investigated. Microbial activity was estimated by the oxygen uptake rate. The effects were studied under conditions where biomass had not acclimated to changed temperature or DO conditions. Hydrolysis processes limited the availability of organic substrates for the aerobic microbial growth. The effects of temperature were interpreted applying an Arrhenius relationship. The average Arrhenius constants found for sewer sediment particles and wastewater particles differed significantly from the average Arrhenius constant found for wastewater. The effects of dissolved oxygen concentration were interpreted applying a saturation type relationship. No differences between sewer sediment particles, wastewater particles and wastewater were found for the oxygen saturation coefficients.

Relationship between ectoenzymatic activity and availability of organic substrates (Ross Sea, Antarctica): an experimental approach

Organic matter consumption and decomposition were studied in four experimental systems, having collected different organic substrates in the Ross Sea in December 1994. For the experimental approach selected, processes normally acting on a mixed pool of substances could be separated and the main features of each phenomenon could be focused on. Through the strict relationship between each experimental system and natural conditions shown by organic matter assessment, ectoenzymatic activity trends and their relation with Antarctic water substrates could be described. Through ice melting the water column becomes rich in large pools of substrates, as well as enzyme-producing micro-organisms, capable of quick development. The quantitative predominance of leucine-aminopeptidase throughout the year is well known, but its relative importance seems to decrease when, owing to production events, the environment is enriched with autotrophic- and heterotrophic-derived substances, leading to glycolytic enzymes expression. Thus, ectoenzymatic activity is supposed to be one of the factors responsible for organic matter variations, showing quantitative and qualitative changes depending on substrate availability.

Mechanisms of N 2 O and NO production in the soil profile of a drained and forested peatland, as studied with acetylene, nitrapyrin and dimethyl ether

Acetylene, dimethyl ether (DME) and 2-chloro-6-trichloromethyl pyridine (nitrapyrin) were used as inhibitors to study the contributions of nitrification and denitrification to the production of N2O and nitric oxide (NO) in samples taken from the soil profile of a peatland drained for forestry. Acetylene and DME inhibited 60–100% of the nitrification activity in field-moist samples from the 0–5 cm and 5–10 cm peat layers, whereas nitrapyrin had no inhibitory effect. In the 0–5 cm peat layer the N2O production could be reduced by up to 90% with inhibitors of nitrification, but in the 5–10 cm peat layer this proportion was 20–30%. All the inhibitors removed 96–100% of the nitrification potential in peat-water slurries from the 0–5 cm peat layer, but the 5–10 cm layer had a much lower nitrification activity, and here the efficiency of the inhibitors was more variable. Litter was the main net source of NO in the peat profile. NO3– production was lower in the litter layer than in the peat, whereas N2O production was much higher in the litter than in the peat. Denitrification was the most probable source of N2O and NO in the litter, which had a high availability of organic substrates.

Heterotrophic nitrogen fixation (acetylene reduction) during leaf-litter decomposition of two mangrove species from South Florida, USA

Heterotrophic nitrogen-fixation (acetylene reduction) was measured during decomposition (under dark conditions) of Rhizophora mangle L. and Avicennia germinans (L.) Stearn leaf litter. Nitrogen-fixation rates in leaf litter increased following 24 d incubation, then decreased after ≃44 d for both species. Maximum rates of 66.2 and 64.6 nmol C2H4 g−1 dry wt h−1 were reached by R. mangle and A. germinans leaf litter, respectively. Higher fixation rates of leaf litter were associated with an increase in water content and sediment particles on leaf surfaces of both species. Rates of nitrogen fixation by diazotrophs attached to sediment particles were not significantly different from zero. With additions of d-glucose, ethylene production rates increased by factors of 625-, 34- and 7-fold for sediment, R. mangle and A.␣germinans leaf litter, respectively, compared to rates prior to enrichment. These organically enhanced rates of nitrogen fixation on leaves could be accounted for by increased activity associated with attached sediment particles and not the leaf material. Total phenolics [reported as tannic acid equivalent (TAE) units] decreased nitrogen-fixation rates when added to d-glucose-enriched sediment at >20 mg TAE l−1. Phenolic compounds could explain the initial lag in rates of nitrogen fixation during leaf-litter decomposition of R. mangle (initial content of 110.8 mg TAE g−1 dry wt), but not of A. germinans (initial content of 23.4 mg TAE g−1 dry wt). The higher phenolic content and reportedly lower carbon substrate of R. mangle did not result in species-specific differences in either the magnitude or temporal pattern of nitrogen fixation compared to A. germinans leaf litter. We conclude that the availability of organic substrates leached from the leaf litter along with colonization by the heterotrophic diazotrophs (as indicated by sediment accumulation) controls nitrogen-fixation rates in a similar manner in the leaf litter of both species.

Growth promotion ability of zoysiagrass rhizosphere fungi in consecutive plantings of wheat and soybean

Several isolates ofPhoma sp., certain nonsporulating fungi, as well asPenicillium andTrichoderma, all isolated from zoysiagrass rhizosphere, promoted growth of wheat and soybean under greenhouse conditions. However, the ability of these rhizosphere fungi to enhance plant growth varied with the crop tested. For example, most of the fungi effectively promoted the growth of wheat, whereas only a few fungi were effective on soybean. In consecutive plantings of wheat and soybean grown in soil previously infested with these zoysiagrass rhizosphere fungi, the growth promotion ability of the fungi was lowered. However, addition of fresh potting medium appeared to restore their growth-promotive effects. It appears that the activation of plant growth-promoting fungi in soil might depend on the availability of organic substrates to colonize, as evidenced by the promotion of plant growth.

A study of the competing ion effects in the biosorption of metals

Bacterial dissimilatory reduction of iron and sulphur in extremely acidic environments is described. Evidence for reduction at two disused mine sites is presented, within stratified 'acid streamers' growths and in sediments from an acid mine drainage stream. A high proportion (approx. 40%) of mesophilic heterotrophic acidophiles were found to be capable of reducing ferric iron (soluble and insoluble forms) under microaerophilic and anoxic conditions. Mixed cultures of Thiobacillus ferrooxidans and Acidiphilium-like isolate SJH displayed cycling of iron in shake flask and fermenter cultures. Oxido-reduction of iron in mixed cultures was determined by oxygen concentration and availability of organic substrates. Some moderately thermophilic iron-oxidis- ing bacteria were also shown to be capable of reducing ferric iron under conditions of limiting oxygen when grown in glycerol/yeast extract or elemental sulphur media. Cycling of iron was observed in pure cultures of these acidophiles. Sulphate-reducing bacteria isolated from acid streamers could be grown in acidified glycerol/yeast extract media (as low as pH 2.9), but not in media used conventionally for their laboratory culture. An endospore-forming, non-motile rod resembling Desulfotomaculum has been isolated. This bacterium has a wide pH spectrum, and appears to be acid-tolerant rather than acidophilic.

Biogeochemical cycling of iron and sulphur in leaching environments

Bacterial dissimilatory reduction of iron and sulphur in extremely acidic environments is described. Evidence for reduction at two disused mine sites is presented, within stratified 'acid streamers' growths and in sediments from an acid mine drainage stream. A high proportion (approx. 40%) of mesophilic heterotrophic acidophiles were found to be capable of reducing ferric iron (soluble and insoluble forms) under microaerophilic and anoxic conditions. Mixed cultures of Thiobacillus ferrooxidans and Acidiphilium-like isolate SJH displayed cycling of iron in shake flask and fermenter cultures. Oxido-reduction of iron in mixed cultures was determined by oxygen concentration and availability of organic substrates. Some moderately thermophilic iron-oxidis- ing bacteria were also shown to be capable of reducing ferric iron under conditions of limiting oxygen when grown in glycerol/yeast extract or elemental sulphur media. Cycling of iron was observed in pure cultures of these acidophiles. Sulphate-reducing bacteria isolated from acid streamers could be grown in acidified glycerol/yeast extract media (as low as pH 2.9), but not in media used conventionally for their laboratory culture. An endospore-forming, non-motile rod resembling Desulfotomaculum has been isolated. This bacterium has a wide pH spectrum, and appears to be acid-tolerant rather than acidophilic.

Methods for physical separation and characterization of soil organic matter fractions

Turnover of soil organic matter (SOM) is coupled to the cycling of nutrients in soil through the activity of soil microorganisms. Biological availability of organic substrate in soil is related to the chemical quality of the organic material and to its degree of physical protection. SOM fractions can provide information on the turnover of organic matter (OM), provided the fractions can be related to functional or structural components in soil. Ultrasonication is commonly used to disrupt the soil structure prior to physical fractionation according to particle size, but may cause redistribution of OM among size fractions. The presence of mineral particles in size fractions can complicate estimations of OM turnover time within the fractions. Densiometric separation allows one to physically separate OM found within a specific size class from the heavier-density mineral particles. Nutrient contents and mineralization potential were determined for discrete size/density OM fractions isolated from within the macroaggregate structure of cultivated grassland soils. Eighteen percent of the total soil C and 25% of the total soil N in no-till soil was associated with fine-silt size particles having a density of 2.07-2.21 g/cm 3 isolated from inside macroaggregates (enriched labile fraction or ELF). The amount of C and N sequestered in the ELF fraction decreased as the intensity of tillage increased. The specific rate of mineralization (μg net mineral N/μg total N in the fraction) for macroaggregate-derived ELF was not different for the three tillage treatments but was greater than for intact macroaggregates. The methods described here have improved our ability to quantitatively estimate SOM fractions, which in turn has increased our understanding of SOM dynamics in cultivated grassland systems.

Soil structure: carbon and nitrogen metabolism

Physical factors confer biological stability on organic matter in soils through the constraints they place on the opportunities for reactions between substrate and enzyme and/or decomposer organism; and, in the context of C and nutrient turnover through decomposer biomass, on the limitations they impose on the activities of predators on prey. The concept of protection afforded to substrates because of their location in micropores of sizes which deny access to decomposer cells, or to microflora in pores which deny access to microfaunal predators, has heightened interest in soil microstructure as a determinant of the availability of organic substrates for decomposition and of the rates of survival of decomposer cells. Widely different methodologies have been used in studies which have attested to the importance of physical protection as mechanisms for stabilization of organic matter in soils. In this review we appraise some of the established methodologies and offer suggestions for extending their usefulness; and we briefly consider some new approaches where methodologies have not as yet been fully developed and whose potential for successful application to soils is unknown. Studies considered include those which seek to characterize enzymes, substrates and organic residues, and their location in situ, in soil aggregates. Characterization of enzymes in situ derives from the use of electron microscopy in conjunction with cytochemical techniques. Characterization of substrates and residues in situ derives from the application of techniques of electron microscopy (SEM, TEM), and spectroscopy (NMR, IR), to soil aggregates or to fractions thereof. The successful development of methodologies combining electron microscopy and autoradiography could demonstrate directly within soil aggregates, sites of activity of specific microorganisms or enzymes in response to selected conditions of substrate addition. Other studies seek to define more closely the relationships between decomposer activities and processes of C and N turnover and soil properties, especially those properties pertaining to soil microstructure. Water retention characteristics of a soil have been utilized to introduce substrates to different pore size locations within aggregates, in order to determine the effects of substrate location on C turnover. Physical fractionation techniques have been utilized to determine the location within aggregates of decomposer organisms, in order to assess the stability of fraction C and N in microbial biomass and organic residues resulting from the metabolism of introduced substrates.

Factors affecting the oxidation of elemental sulfur in soils

The use of elemental sulfur (S0) to alleviate widespread S deficiencies in agricultural soils is limited by the unpredictability of its oxidation to plant available sulfate. Here we review the biological, fertilizer and soil-related factors that control S0 oxidation. Sulfur oxidation in soil is mediated primarily by microorganisms, and thus it is the size, composition and activity of the microbial community which dictate oxidation rates. Because S0 oxidation is a biological process, it is strongly influenced by factors directly affecting microbial activity including soil temperature, water potential, and aeration. In many soils these factors represent the primary constraints to S0 oxidation. Oxidation is also influenced by the effective surface area of the S exposed to microbial activity. Thus oxidation is favored by reducing the particle size and abundant populations of heterotrophic bacteria and fungi capable of oxidizing S0, thus the availability of organic substrates from residue additions or root exudates may also affect S oxidation. Previous application of S0 may increase oxidation rates in many soils, presumably by stimulating S0 oxidizing populations. The large number of factors that govern S0 oxidation account for the variability in oxidation rates among soils, climatic regions, and agronomic practices. Many of these factors are subject to agronomic control, however, and it should be possible to devise S fertilizer strategies that exploit the slow release characteristics of S0 to meet crop demands efficiently in a variety of conditions.

Invertebrates and nutrients in a Mediterranean vineyard mulched with subterranean clover (Trifolium subterraneum L.)

In the 25-year-old vineyard of a farm near the Maremma National Park (Central Italy), replicate plots were established with a mulch of Trifolium subterraneum L. or cultivation and two levels of fertilizer. The mai objective of this research was to analyse responses by soil invertebrates and nutrients after introducing a herbaceous component into the system; the yield was also considered. The greater availability of organic substrate and the reduced cultivation as a result of green mulching increased the soil mesofauna biomass, especially detritivores. With time, a significant difference became evident between the populations of invertebrates present in the mulched plots and those in the cultivated plots. Over 2 years, most nutrients (Ca, K, P, and N) were significantly increased in the mulched plots compared to the cultivated plots. Grape yields were consistently higher in the bare plots. No significant differences were found in fertilizer effects.

Ethylene Accumulation in Soil in Response to Organic Amendments

AbstractOne of the major factors affecting ethylene (C2H4) accumulation in soil is the availability of organic substrates. Various organic compounds were screened for their stimulatory effect on C2H4 accumulation in soil. Since L‐methionine (L‐met), a sole precursor of C2H4 synthesis in plants, is also an excellent precursor for microbial biosynthesis of C2H4, all the compounds tested were added on a C‐equivalent basis with respect to 1 g L‐met kg−1 soil. Incubation was carried out under ambient conditions (24 ± 3 °C) for 14 d and the soil was maintained at field capacity (−33 kPa). Among the 63 compounds tested, the majority of amino acids, organic acids, carbohydrates, proteins, alcohols, and met analogs promoted C2H4 generation to a greater degree than did the unamended soil. Use of 1‐aminocyclopropane‐1‐carboxylic acid (acc) enhanced C2H4 synthesis much more than any other treatment (2400‐fold over the unamended nonsterile control). Of the five vitamins tested, inositol was the most effective in promoting C2H4 production. L‐Ethionine (L‐eth) was an excellent precursor among the five met analogs. Nonbiological production of C2H4 was observed only upon treatment with L‐met, L‐cysteine, acc, ethanol, and L‐eth, which were 7.9, 19.0, 3.0, and 4.2% and 2.1‐fold, respectively, of the total C2H4 detected under nonsterile amended conditions. Since the majority of these compounds are excreted by plant roots, the rhizosphere may be an active site for C2H4 generation.