Showing papers in "Soil Biology & Biochemistry in 1999"

Microbiological precipitation of CaCO3

Abstract: The process of microbial mineral plugging in porous media is common in nature. We examined physical and biochemical properties of CaCO3 precipitation induced by Bacillus pasteurii, an alkalophilic soil microorganism. X-ray diffraction analysis quantified the composition of the mineral deposited in sand and identified the CaCO3 crystal as calcite. Examination by scanning electron microscopy identified bacteria in the middle of calcite crystals, which acted as nucleation sites. The rate of microbiological CaCO3 precipitation correlated with cell growth and was significantly faster than that of chemical precipitation. Biochemical properties of urease (urea amidohydrolase, E.C. 3.5.1.5) from B. pasteurii that was indirectly involved in CaCO3 precipitation were examined to understand the kinetics of the microbiological process. Urease from B. pasteurii exhibited a relatively low affinity for urea at pH 7.0 with a Km of 41.6 mM and Vmax of 3.55 mM min−1 mg−1 protein and increased affinity at pH 7.7 with a Km of 26.2 mM and Vmax of 1.72 mM min−1 mg−1 protein. Results of kinetic studies indicate that urease activity and its affinity to urea are significantly high at the pH where calcite precipitation is favorable. Our findings further suggest a potential use of the microbial calcite precipitation process in remediation of the surface and subsurface of porous media.

Field management effects on soil enzyme activities

Abstract: There is growing recognition for the need to develop sensitive indicators of soil quality that reflect the effects of land management on soil and assist land managers in promoting long-term sustainability of terrestrial ecosystems. Eleven soil enzymes assays were investigated relative to soil management and soil quality at two study sites. Soils were sampled from the Vegetable Crop Rotation Plots (VRP) (established in 1989 in humid western Oregon) which compared continuous fescue (Festuca arundinacea) and four winter cover crop treatments in annual rotation with a summer vegetable crop. The second site was the Residue Utilization Plots (RUP) (initiated in 1931 in semi-arid Eastern Oregon) which is under a winter wheat–summer fallow and compared inorganic N, green manure and beef manure treatments. Soil also was sampled at the research center from a nearby grass pasture that is on the same soil type. The enzymes were α- and β-glucosidase, α- and β-galactosidase, amidase, arylsulfatase, deaminase, fluorescein diacetate hydrolysis, invertase, cellulase and urease. At both sites there was a significant treatment effect for each enzyme tested (P<0.05). Enzyme activities (except α- and β-glucosidase and α- and β-galactosidase) were generally higher in continuous grass fields than in cultivated fields. In cultivated systems, activity was higher where cover crops or organic residues were added as compared to treatments without organic amendments. It was found that use of air-dried soil samples provided the same ranking of treatments by a number of enzyme assays and would facilitate adoption of these assays for practical or commercial applications. Deaminase was not a good indicator of soil quality, while β-glucosidase was suggested as an assay that reflects soil management effects and has microbial ecological significance because of its role in the C cycle.

Bacterial and fungal abundance and biomass in conventional and no-tillage agroecosystems along two climatic gradients

Abstract: Microbial community composition may be an important determinant of soil organic matter (SOM) decomposition rates and nutrient turnover and availability in agricultural soils. Soil samples were collected from six long-term tillage comparison experiments located along two climatic gradients to examine the effects of no-tillage (NT) and conventional tillage (CT) management on bacterial and fungal abundance and biomass and to examine potential controls on the relative abundances of bacteria and fungi in these two systems. Samples were divided into 0–5 and 5–20 cm depth increments and analyzed for bacterial and fungal abundance and biomass, total C and N, particulate organic matter C and N (POM-C and N), soil water content, texture, pH, and water-stable aggregate distributions. Soil moisture, which varied by tillage treatment and geographically with climate, ranged from 0.05 to 0.35 g g−1 dry soil in the surface 0–5 cm and 0.15 to 0.28 g g−1 dry soil at 5–20 cm. Measured organic matter C and N fractions and mean weight diameter (MWD) of water-stable aggregates were significantly higher in NT relative to CT at three of the six sites. Fungal hyphal length ranged from 19 to 292 m g−1 soil and was 1.9 to 2.5 times higher in NT compared to CT surface soil across all sites. Few significant tillage treatment differences in soil physical and chemical properties or in fungal abundance and biomass were observed at 5–20 cm. Bacterial abundance and biomass were not consistently influenced by tillage treatment or site location at either depth. The proportion of the total biomass composed of fungi ranged from 10 to 60% and was significantly higher in NT compared to CT surface soil at five of six sites. Proportional fungal biomass was not strongly related to soil texture, pH, aggregation, or organic C and N fractions, but was positively related to soil moisture (r=0.67; P<0.001). The relationship between soil moisture and the degree of fungal dominance was due to the positive response of fungal biomass and the lack of response of bacterial biomass to increasing soil moisture across the range of measured soil water contents. Tillage treatment effects on fungal biomass and proportional fungal abundance were not significant when the data were analyzed by analysis of covariance with soil moisture as the covariate. These results suggest that observed tillage treatment and climate gradient effects on fungi are related to differences in soil moisture. Further research is needed, however, to determine how tillage-induced changes in the soil environment shape microbial community composition in agroecosystems.

Soil organic matter quality interpreted thermodynamically

Abstract: Soil organic matter quality in the sense of how easily carbon in the soil organic matter can be mineralised is a major determinant of soil carbon storage and rate of mineralisation of nutrients. Its origin has so far remained elusive and a number of indices, such as C-to-N-ratio, lignin concentration and other combinations of chemical constituents have been used as substitutes for quality. We suggest here that quality is the number of enzymatic steps required to release as carbon dioxide a carbon atom from an organic compound . The larger the number of steps the lower is the quality of the carbon atom. Such a measure connects quality to thermodynamics. It also explains the rapid decrease in decomposition rate with decreasing quality suggested in the q-theory of organic matter dynamics and shows that the decomposition rate of low quality substrates has a stronger temperature dependence than that of high quality substrates.

Moisture effects on microbial activity and community structure in decomposing birch litter in the Alaskan taiga

Abstract: We carried out a field experiment to evaluate the effect of moisture regime on microbial biomass and activity in birch litter in the Alaskan taiga. Litter bags were placed in one of three treatments: continuously moist (0.5 cm water d−1), cycling (0.5 cm water weekly), and `natural', which experienced two natural dry–wet cycles of 2 weeks dry followed by rain. The experiment lasted for 1 month. Each week we collected litter bags and analyzed microbial respiration and biomass C and N. In the last two cycles we analyzed bacterial substrate use on Biolog GN plates. There were strong overall correlations between biomass, respiration and litter moisture content. However, the different treatments had significantly different rates of respiration, biomass and respiratory quotient (qCO2) that could not be explained by moisture content directly. The natural treatment had lower respiration rates and biomass than the wet or cycling samples, indicating that the 2-week droughts in the natural treatment reduced microbial populations and activity to a greater degree than did shorter droughts. Episodic drying and rewetting considerably decreased the number of Biolog substrates used. This experiment showed that the size and functioning of the litter microbial community was strongly affected by its stress history.

Seasonal changes in soil microbial communities along a fertility gradient of temperate grasslands

Abstract: This study aimed to: (1) determine whether soil microbial communities along a gradient from intensive (fertilized) to low-input (unfertilized) grassland management, shift in their composition as shown by an increase in the abundance of fungi relative to bacteria and (2) whether these shifts in soil microbial communities vary depending on season. At all sample dates soil microbial biomass-C and -N, and the total abundance of phospholipid fatty acids (PLFA) were highest in unfertilized, undrained treatments and lowest in fertilized and drained grassland. Similarly, microbial activity, measured as CO2-C respiration, was found to be at its lowest in the most intensively managed grassland. Measures of microbial biomass showed a high degree of seasonality, having summer maxima and winter minima. In contrast, PLFA measures had spring maxima and autumn minima. Seasonal and management differences were also observed within the microbial community. PLFA profiles revealed that most individual fatty acids were highest in the unfertilized treatments, and lowest in fertilized grassland. The fungal-to-bacterial biomass ratio was also highest in the unfertilized and lowest in the fertilized soils, suggesting that higher microbial biomass in former were more due to the growth of fungi than bacteria. As with total PLFA, the abundance of individual fatty acids showed a spring maximum and an autumn minimum. Seasonal differences in PLFA patterns were shown to be related to soil mineral-N and soil moisture contents. Factors controlling shifts in microbial community structure between sample dates and sites are discussed in relation to other studies. A critical assessment of the different measures of microbial biomass is also given. Overall, the findings of this study support the thesis that fungi play a more significant role in soil biological processes of low-input, unfertilized grasslands, than in intensively managed systems.

Soil, pH and N availability effects on net nitrification in the forest floors of a range of boreal forest stands

Abstract: Net nitrification in forest soils is potentially problematic as it can promote nitrogen and cation losses as well as soil acidification and favor competing vegetation in regenerating forests. Rates of net nitrification have been associated with vegetation and soil types. However, this phenomenon is difficult to predict. Our objectives were to describe the relationships between stand types and net nitrification in the forest floor of five stand types (Paper birch: Betula papyrifera , trembling aspen: Populus tremuloides , white spruce: Picea glauca , jack pine: Pinus banksiana , white cedar: Thuja occidentalis ) typical of the boreal mixedwood of northwestern Quebec and to experimentally verify the effect of pH and ammonium availability as well as the occurrence of persistent allelopathic inhibitors on net nitrification rates. All samples came from well-drained lacustrine clay soils and all forest stands originated from wildfires. The net accumulation of nitrate and ammonium was measured during the course of 6-week laboratory incubations. Nitrate accumulation was highest under aspen and birch, low under white spruce and cedar and not detectable under jack pine. The forest floor of jack pine had the lowest pH and exchangeable bases and the widest C:N ratio. Ammonium addition did not promote a significant increase in net nitrification. Increase of forest floor pH had a positive effect on net nitrification while acidification depressed it. The absence of net nitrate production in jack pine forest floor appeared to be related to the absence of nitrifiers as it was stimulated by the addition of a small amount of nitrifying forest floor. Our results suggest that jack pine and aspen, which occupy the same habitat, may have the capacity to change soil conditions in order to favor or depress nitrification. Observed rates of nitrification under these species is coherent with their preference for a particular form of mineral N. Although pH appeared as an important regulator of net nitrification, the effects of other factors are yet to be established.

Response of soil microbial biomass, urease and xylanase within particle size fractions to long-term soil management

Abstract: We investigated whether long-term management of a Haplic Phaeocem (FAO) influences organic C, microbial biomass and enzyme activities in the bulk soil and in particle size fractions. The following treatments were used: (1) no fertilization (control), (2) NPK, (3) 20t FYM ha−1 (farmyard manure), (4) 20t FYM ha−1+NPK, (5) 30t FYM ha−1 and (6) 30t FYM ha−1+NPK. The fertilizers were applied to plots of a crop rotation (sugar beet, potatoes, winter wheat, spring barley, alfalfa); inorganic fertilizers were applied every year, FYM was applied every second year. Particle size fractions were obtained by low-energy sonication (0.25 kJ g−1) and a combination of wet sieving and centrifugation, releasing the size fractions 2000–250, 250–63, 63–2, 2–0.1 and <0.1 μm. FYM increased microbial biomass, N-mineralization, urease, arginine deaminase and alkaline phosphatase activity of the bulk soil, whereas the effect of additional NPK fertilization depended on the enzyme assayed. Xylanase activity of the bulk soil was mainly influenced by the quality and quantity of the residues and by the amount of below-ground plant biomass. C and N contents (related to fraction dry weight) increased with diminishing particle size. The mean recoveries of C and N contents after the fractionation procedure were 96.8 and 97.1%, respectively. Organic amendments (20 vs 30 t FYM ha−1) induced an equal increase of organic C and total N in all particle size fractions (2000–250, 250–63, 63–2, 2–0.1 and <0.1 μm). Unfertilized soils were characterized by a higher C-to-N ratio of the particles (2000–250 and 250–63 μm) than the organically and inorganically fertilized soils. Microbial ninhydrin-reactive N was recovered to 73.6% after the fractionation procedure. Highest concentrations of ninhydrin-reactive N were found in the clay (2–0.1 μm) and silt fraction (63–2 μm), lowest in the coarse sand fraction (2000–250 μm). The increased ninhydrin-reactive N in the bulk soil after long-term amendment with FYM is mainly attributed to an increase of ninhydrin-reactive N in the clay and silt fractions. Therefore, long-term organic amendments increased the capacity of the small-sized fractions to protect soil microorganisms. Urease activity was mainly located in the 63–2 and 2–0.1 μm fractions, whereas the coarse and fine sand particles accumulated disproportionately higher amounts of xylanase. The predominance of xylanase and urease in different particle size fractions depends apparently not only on the location of soil microorganisms and their substrates but also on the mechanisms of enzymes to adsorb and bind onto mineral and organic particles.

Cytokinin production by Paenibacillus polymyxa

Abstract: The production of hormones has been suggested to be one of the mechanisms by which plant growth-promoting rhizobacteria (PGPR) stimulate plant growth. To evaluate whether the free-living soil bacterium, Paenibacillus polymyxa, releases the hormone group cytokinins and, if so, their identity, the content of cytokinins in the growth media, before and after cultivation of this bacterium, was determined by immunoaffinity chromatography (IAC). This method allows the isolation of almost all known cytokinins and their metabolites. Separation and characterization were done by high performance liquid chromatography (HPLC) with on-line ultraviolet (UV) detection, and final identification was by gas chromatography-mass spectrometry. Iso-pentenyladenine (iP) was identified in the two defined media used for the cultivation of P. polymyxa, but not earlier than at its late stationary growth. A third medium, supplemented with yeast extract, contained iso-pentenyladenine riboside (iPR) and some additional cytokinin-like substances before inoculation. When the same medium was sampled after the cultivation of P. polymyxa up to its logarithmic growth phase, the cytokinin concentration had decreased. After prolonged cultivation of P. polymyxa, small amounts of iP appeared in all three media, and iPR had disappeared from the yeast-containing medium, which indicates that the bacterium can metabolize cytokinins.

Phosphate solubilisation in solution culture by the soil fungus Penicillium radicum

Abstract: Phosphorus is a plant nutrient which is rapidly made immobile and less available for plant use after addition to the soil as a soluble fertiliser. Phosphate-solubilising microorganisms may be able to improve the P nutrition of plants and thus stimulate plant growth. Penicillium radicum, a phosphate-solubilising fungus isolated from the rhizosphere of wheat roots, has shown promise in plant growth promotion. Its ability to solubilise inorganic phosphate was studied in vitro. The fungus was grown in liquid medium culture containing either ammonium or nitrate as the sole source of N. Insoluble, or sparingly-soluble P (1000 mg P l−1) was supplied as calcium monohydrogen phosphate (CaHPO4), calcium orthophosphate (Ca3(PO4)2), crystalline ferric phosphate (FePO4·4H2O), crystalline aluminium phosphate (AlPO4), colloidal ferric phosphate or colloidal aluminium phosphate. The titratable acidity, pH and concentrations of organic acids and soluble phosphate were determined periodically during a 20 or 31 d incubation. Phosphate solubilisation was highest from CaHPO4 (475 mg P l−1), Ca3(PO4)2 (360 mg P l−1) and colloidal aluminium phosphate (207 mg P l−1). Phosphate solubilisation was generally higher when ammonium rather than nitrate was the sole source of N. Soluble phosphate concentrations in the culture medium were directly proportional to the titratable acidity and organic acid (principally gluconic acid) concentration and inversely related to pH. The main mechanism for phosphate solubilisation was acid production leading to a decrease in pH. Evidence from an abiotic study using HCl and gluconic acid to solubilise P also indicated that chelation of Al3+ by gluconic acid may have been a factor in the solubilisation of colloidal aluminium phosphate.

Estimation of the biomass of arbuscular mycorrhizal fungi in a linseed field

Abstract: Linseed was grown in field plots included in a long-term P fertilisation experiment (0, 15 or 30 kg P ha −1 yr −1 for 20 yr). Two months before sowing, half of each plot was applied with dazomet to prevent the formation of arbuscular mycorrhiza (AM). The biomass of different groups of micro-organisms was estimated 28, 51 and 72 d after sowing based on amounts of certain fatty acids extracted from the soil. Dazomet application strongly suppressed colonisation of the linseed roots by AM fungi throughout the experiment. In plots with no dazomet application, root colonisation by the AM fungi increased from harvests 1 to 3 as judged both from microscopical estimates and from quantitative analysis of the AM fungal indicative fatty acid 16:1ω5. These methods also revealed that AM formation was reduced in P-fertilised plots. The phospholipid fatty acid (PLFA) 16:1ω5 decreased in dazomet-treated soil, and it was assumed that the PLFA 16:1ω5 remaining in treated soil originated from bacteria. The biomass of the extraradical AM mycelium could then be estimated by multiplying the difference in PLFA 16:1ω5 between dazomet treated and nontreated soils by a conversion factor. This calculation indicated that the biomass of the extraradical mycelium of AM fungi was about 10 times as high as the biomass of intraradical mycelium and that the extraradical mycelium constituted the largest fraction of the soil microbial biomass. Dazomet application also decreased the biomass of saprophytic fungi in the soil as indicated by the amount of PLFA 18:2ω6,9, while analyses of bacteria-specific fatty acids indicated that the bacterial biomass in the soil was not affected by either dazomet or P application.

Nitrogen availability effects on carbon mineralization, fungal and bacterial growth, and enzyme activities during decomposition of wheat straw in soil

Abstract: Our objectives were (1) to investigate the response of wheat ( Triticum aestivum L.) straw decomposition to increasing nitrogen availability (0.5, 0.8, 1.2 and 1.9% N of straw dry matter) in a microcosm experiment at controlled temperature (15°C) and moisture (−10 kPa), (2) to elucidate underlying mechanisms at the microbial-community level, and (3) to develop a model according to the measurements. Concentrations of available N (straw N and soil inorganic N) below 1.2% of straw dry matter significantly reduced the rate of carbon mineralization from straw residues and the growth of total soil microbial biomass (chloroform fumigation-extraction). The negative effects appeared shortly after the pool of soil mineral N had been depleted. The N effect on the microbial community was mainly caused by reduced fungal growth (ergosterol), while total bacterial biomass (epifluorescence microscopy) was not significantly affected. However, number of cellulase-producing, colony-forming bacterial units decreased with decreasing N availability. In straw-amended soil, decreasing N concentrations reduced activities of exocellulase, endocellulase and hemicellulase, while in unamended control soil the effects were opposite. We developed a model in which three fractions of straw residues, determined by proximate analysis (Van Soest), and a humus pool were assumed to decay according to first-order rate kinetics. In accordance with the microbial measurements, the microbial community was subdivided into three groups consuming readily decomposable, structural and humus materials respectively. When holocellulose decay rate was modified as a function of N availability, the model simulated N effects on C mineralization and microbial biomass growth very well. Our experiment showed that wheat straw mineralization may be retarded at N concentrations well above those frequently found after grain harvest in the field and that effects of N availability should be taken into account when modelling C and N turnover in agricultural soils. Moreover, the trial exemplified a situation where studies of functional groups of decomposer organisms were required to explain and model satisfactorily differences observed at the process level.

Rapid response of soil microbial communities from conventional, low input, and organic farming systems to a wet/dry cycle

Abstract: Soil microbial communities may be strongly influenced by agricultural practices which change the soil environment. One such practice is the use of organic amendments and cover crops which increase carbon availability to microorganisms. Another is irrigation which, in California’s hot, rain-free growing season, can cause severe wet/dry cycles. We investigated (i) long-term diAerences in amounts of organic inputs using soils from organic, low input, and conventional farming systems, and (ii) diAerences in severity of soil drying following irrigation, using soil from two depths, 0‐3 and 3‐15 cm. All soils were air-dried and re-wetted, and we measured short-term changes in microbial biomass carbon (MBC), dissolved organic carbon (DOC), respiration, and phospholipid ester-linked fatty acid (PLFA) composition before and for 27 h after re-wetting. Respiration rates were fit to a two-first-order-component model. Carbon respired from the more slowly utilized C pool of the two-component model, MBC, and DOC increased with increasing amounts of organic inputs, and PLFA composition of the organic and conventional soils clearly diAered in their mole percentages of numerous fatty acids when analyzed by principal components analysis and redundancy analysis. Despite these diAerences, the response of microbial communities in the three farming systems to soil drying and re-wetting was similar. For example, the relative increase in MBC following soil re-wetting did not diAer among the farming system soils. In contrast, the relative increase in MBC after re-wetting was greater, and the respiratory response to soil re-wetting was more rapid in the surface (0‐3 cm) than deeper (3‐15 cm) layer. Higher ratios of cyclopropyl fatty acids to their precursors suggested greater stress to bacteria in the deep than surface layer, and these ratios declined more rapidly after re-wetting in the deep than surface layer. This study suggested that adaptation to wet/dry cycles by surface microorganisms had occurred during the 3-month growing season, leading to changes in both microbial process rates and community composition. # 1999 Published by Elsevier Science Ltd. All rights reserved.

Changes in enzyme activities and microbial biomass of tallgrass prairie soil as related to burning and nitrogen fertilization

Abstract: Microbial biomass and enzyme activities are affected by management practices and can be used as sensitive indicators of ecological stability. Microbial biomass C (MBC), microbial biomass N (MBN) and eight enzyme activities involved in the cycling of C, N, P and S were studied in the surface (0–5 cm) of an Irwin silty clay loam soil (fine, mixed, mesic, Pachic Arguistoll) in a tallgrass prairie ecosystem. Treatments of annual spring burning and N fertilization were initiated in 1986 and encompassed: (1) unburned–unfertilized, (2) burned–unfertilized, (3) burned–fertilized, and (4) unburned–fertilized. Activities of dehydrogenase, β-glucosidase, urease, deaminase, denitrifying enzyme, acid phosphatase, alkaline phosphatase, and arylsulfatase were assayed. Long-term burning and N fertilization of the tallgrass prairie soil reduced MBC and MBN relative to the unburned–unfertilized treatment. The effects of burning and N fertilization varied among the enzymes and the time of sampling. Long-term burning significantly (P

Wet–dry cycles affect dissolved organic carbon in two California agricultural soils

Abstract: During California's hot, dry summers, irrigated soils are subjected to frequent wet–dry cycles and surface layers dry to near air–dry conditions between irrigations. We investigate whether wet–dry cycles enhance soil dissolved organic carbon (DOC) concentrations. This research follows up on previous observations of higher DOC concentrations in the surface (0–2 cm) than deeper (2–15 cm) soil layer late in the growing season, even when soils were moist throughout the profile. We also investigate whether DOC contents correspond to other measures of C available to microorganisms. All measurements were made on soils stabilized at −0.03 MPa water potential for 48 h at 25°C to avoid the initial pulse of microbial activity which follows re-wetting of dry soils. After 3 months during the summer field season, DOC concentrations increased 2.5-fold in the surface 0–2 cm layer and 1.20 to 1.35-fold in the 2–15 cm layer in soils under both organic (N inputs of cover crop and manure) and conventional (inorganic N inputs) management for irrigated tomatoes. In microcosms exposed to wet–dry cycles for 3 months, DOC concentrations increased by 70%, while in microcosms maintained at −0.03 MPa for 3 months DOC remained constant. The increase in DOC in both field and microcosm soils exposed to wet–dry cycles indicates that wet–dry cycles contribute to higher background DOC contents. The greater DOC increase in the field than microcosms may be due to evaporation causing upward movement of water and concentrating DOC at the soil surface, or to greater C availability in the field due to the presence of plant roots. Respiration and microbial biomass C (MBC) remained constant or declined slightly in both soil layers and microcosm treatments over the growing season, counter to the trends in DOC concentration. Therefore DOC contents measured under moist soil conditions do not appear to consistently indicate C availability to microorganisms. The percentage of labile DOC, as measured by a bioassay, declined in the surface layer of the organic field soil and in organic and conventional soils in both microcosm treatments over the 3 month experiment, possibly indicating that roots were a continuing source of labile DOC in the lower field layers. Reflecting the higher organic inputs to the organic than conventional soil, DOC, MBC and respiration rates were 2–2.5 times higher in the organic than conventional soil throughout the experiments, however the percentage of labile DOC was approximately twice as high in the conventional soil as in organic soil.

Earthworm humic matter produces auxin-like effects on Daucus carota cell growth and nitrate metabolism

Abstract: While earthworms are known to improve plant growth by improving the structure of the soil, recent work has suggested that earthworms also produced humic substances endowed with hormone-like activity. Suspensions of Daucus carota (carrot) cells were treated with auxin derivatives (2,4-D=2,4-dichlorophenoxyacetic acid, IAA=indole-3-acetic acid and NAA=1-naphthylacetic acid) and a humic substance of low molecular weight (HEf), obtained from the faeces of the earthworms Nicodrilus caliginosus and Allolobophora rosea, which have auxin-like activity. HEf, at a concentration of 200 μg C l−l, caused an increase in carrot cell growth, determined by packed cell volumes, similar to that of 2,4-D, and morphological changes induced by HEf were similar to those induced by IAA. Humic matter and the tested auxin derivatives stimulated carrot cell glutamate dehydrogenase (GDH), glutamine synthetase (GS) and malic dehydrogenase (MDH) to various extents. When carrot cells were treated with HEf or IAA, GDH and MDH isoenzymes separated by isoelectrofocusing showed similar polymorphism. The addition of HEf or auxin derivatives to carrot cells caused an increase in free amino acids belonging to the oxaloacetate and α-ketoglutarate pathways. Our results showed that HEf affects nitrate metabolism, has auxin-like activity, was strongly acidic and had elevated peptide, carbohydrate and aliphatic C contents. Our work has confirmed that the effective materials in the HEf were highly acidic and of low molecular weight. We conclude that, in addition to the already well established modes of action, earthworms are important in humogenesis because their faeces contained humic substances that can influence plant growth via physiological effects.

Experimental evidence that soil fauna enhance nutrient mineralization and plant nutrient uptake in montane grassland ecosystems

Abstract: This microcosm study is concerned with understanding those factors which regulate ecosystem processes of nutrient cycling and plant productivity in a montane grassland ecosystem. We examined the effects of different groups of soil fauna, namely bacterial-feeding nematodes and Collembola, on nutrient mineralization (N and P) in an acid, organic soil taken from a montane grassland in the Peak District National Park, United Kingdom. We also examined whether faunal influences on nutrient release, a measure of nutrient mineralization, resulted in changes in nutrient uptake and biomass production of an indigenous montane grass species (Nardus stricta (L.)). We found that in the presence of Collembola, and when nematodes and Collembola were combined, N mineralization, nutrient leaching and shoot N contents of N. stricta was significantly increased relative to a defaunated control. We also found that net P mineralization and leaching increased (although not significantly) in the presence of both nematodes and Collembola, resulting in a significant increase in shoot P content of N. stricta. The presence of nematodes alone, which were largely bacterial-feeders, had no effect on the mineralization of N or P, or shoot nutrient content. We suggest that differences in the effect of faunal treatments on nutrient mineralization are related to the feeding strategies of the added fauna, and to their consequent effect on the size of the soil microbial biomass. The treatments that increased N mineralization and plant nutrient content (N and P) also significantly reduced plant growth (shoot and root). We suggest that high NH4+–N concentrations in the soil solution of Collembola treatments inhibited the growth of N. stricta and that the growth of other grassland species may benefit from this improvement in nutrient availability.

Dependence of mineralization of soil organic matter on temperature and moisture

Abstract: It is widely believed that increases in ambient temperature due to global climatic change will decrease the organic matter content of soils and increase the emission of greenhouse gases from them. These effects, which are due to stimulation of the decomposition of fresh and humified organic matter, are predicted to be most pronounced in temperate regions between the 40th and 70th parallels. To investigate the possible implications of this for organic matter mineralization in soils of Galicia (northwest Spain, north of the 40th parallel), in this work samples of O and Ah layers taken at various times of the year (to account for seasonal effects) were subjected to incubation experiments. The results indicate that the effects of soil temperature and moisture content on C- and N-mineralization can be modelled by simple equations, the equation for CO2 evolved being more reliable than that for N-mineralized. The effects of interactions between soil moisture and temperature on the mineralization of organic matter were also modelled, using multiple regression to fit an equation including a term for their product to the results. The equation for N-mineralized explained 30–40% of the variance in this parameter, while that for CO2 evolved explained almost 80% of its variance. The latter equation allowed prediction of the effects of climatic change on respiration processes.

Nitrogen transformations in soil under Pinus sylvestris, Picea abies and Betula pendula at two forest sites

Abstract: Microbial biomass N, net ammonification, net nitrification, denitrification potential, numbers of nitrifiers and the pH-dependency of nitrification were measured from the humus layer and mineral soil layers in adjacent stands of Scots pine ( Pinus sylvestris L.), Norway spruce ( Picea abies (L.) Karst.) and silver birch ( Betula pendula L.). The trees had been established at two forest sites of different fertility approximately 60 years ago. The aim was to see whether the microbial and chemical characteristics of the soils differed under different tree species. The soil pH(H 2 O) varied from 3.8 to 5.0 and was lowest in spruce soil at both sites in all soil layers. Microbial biomass N, ammonification, nitrification and denitrification all differed in soils of pine, spruce and birch. The flush of N from fumigation varied from 36 to 67 μg N cm −3 fresh soil in the humus layer, and from 13 to 50 μg cm −3 in the mineral soil layers. Denitrification potential with added nitrate was 2–29 ng N 2 O-N cm −3 soil h −1 in the humus layer and 0–28 ng in the mineral soil layers. Both tended to be lowest under spruce and highest under birch, at the fertile site in all soil layers and at the less fertile site in the humus layer. In the mineral soil layers of the fertile site and in the humus layer and upper mineral soil layer of the less fertile site the content of mineral N was highest under birch. Different populations of nitrifiers existed in the soils, regarding numbers, activity and pH-dependency. Only the nitrifier community in pine humus layer from the fertile site was adapted to acidic (pH 4.1) conditions. In an aerobic soil suspension the cumulative nitrate production of it was 32 μg cm −3 soil in three weeks, compared to negligible production in the other soils. When the pH of the suspensions was raised to 6.0, all soils from the fertile site produced nitrate, but the production at the less fertile site was still negligible. Higher C-to-N ratios probably explained the low nitrification activity and numbers of nitrifiers at the less fertile site. Thus, there were differences in N transformations under pine, spruce and birch, but the changes depended also on the fertility of the site.

Land-use change: effects on soil carbon, nitrogen and phosphorus pools and fluxes in three adjacent ecosystems.

Abstract: Changes in land use can affect soil organic matter contents and fertility and also atmospheric CO 2 concentrations and global warming through soil respiration We compared total and microbial C, N and P pools and C and N metabolism in sandy loam soils (Typic Udivitrands) under indigenous broadleaf-podocarp forest, grazed introduced pasture and 19-yr old Pinus radiata D Don forest (planted on previous pasture) in New Zealand Total and microbial C and N declined consistently with profile depth (except for total N in L and FH samples), and in comparable depths of mineral soil (to 20 cm) tended to be lower in the pine than in the other systems Total P, organic P and extractable inorganic P concentrations at comparable depths were, in contrast, lowest in the indigenous forest Microbial P concentrations did not differ significantly between the different systems Microbial C-to-microbial N ratios differed little among soil profile depths and ecosystems In 0–10 cm depth mineral soil, CO 2 -C production, metabolic quotients ( q CO 2 values) and net N mineralization were all highest in the pasture samples Net nitrification was high in the pine and pasture samples, but much lower in the indigenous forest samples; nitrate-N was, however, consistently present in streamwater from all three ecosystems Changes in total C and microbial C and N pools on an area basis to 20 cm depth mineral soil were greatest after conversion of the indigenous forest to pasture; total N contents were, however, as high in the pasture as in the forest and net N mineralization was highest in the pasture On this area basis, changes in total C contents were small after conversion of pasture to pines, although the distribution within the soil profile did differ considerably between the pine and pasture systems

Potential C and N mineralization and microbial biomass from intact and increasingly disturbed soils of varying texture

Abstract: Potential C and N mineralization and soil microbial biomass C were determined following disturbance (i.e. drying and sieving) pretreatments in five soils varying in texture (30‐350 mg clay g ˇ1 soil) from the southern Piedmont USA. Soil disturbance by drying (i.e. rewetting following drying at 558C for 72 h) of intact soil cores resulted in a flush of C mineralization (70% to 2.5-fold greater) during 0‐3 d of incubation, but was not significantly diAerent during 3‐10 and 10‐24 d periods compared with field-moist-intact soil cores. Soil disturbance by sieving resulted in greater C mineralization earlier than later in the incubation and led to significant immobilization of N of surface soil where respiration was highest. Increasing soil disturbance through smaller sieve openings resulted in a 10‐60% greater flush of C mineralization that may have been due to disruption of macroaggregates, which protected soil organic C. With a conditioning period of 10 d following rewetting of dried soil, soil microbial biomass C was unaAected by drying or extent of sieving. Soil texture (i.e. clay content) did not interact with disturbance eAects. Immobilization of N was predominant in surface soils (0‐40 mm) of this bermudagrass pasture, where mineralizable C was very high. Carbon mineralization during 0‐3 d was highly related (r 2 =0.9620.04) to C mineralization during 0‐24 d, basal soil respiration and soil microbial biomass C, although increasing soil disturbance (i.e. drying and extent of sieving) altered these relationships in a predictable manner. I conclude that dried and coarsely sieved soil compares favorably to field-moist-intact soil cores for estimating soil microbial biomass and potential activity in landscapes scoured by various degrees of erosion. Published by Elsevier Science Ltd.

Chitin-mediated changes in bacterial communities of the soil, rhizosphere and within roots of cotton in relation to nematode control

Abstract: Changes in microbial communities associated with nematode control were studied by comparing population numbers of fungi and bacteria in the soil and in internal root tissues (endorhiza) in non-amended and chitin-amended soils Addition of chitin to soil at 1% (w/w) eliminated plant-parasitic nematodes in a first planting of cotton cv `Rowden' and significantly reduced Meloidogyne incognita infestation in a second planting, confirming long-term nematode suppressiveness induced by this organic amendment The chitin amendment was associated with an increase in fungal and bacterial populations, especially those with chitinolytic activity The bacterial communities of soil, rhizosphere and endorhiza were assessed by examining the taxonomic diversity of recoverable bacteria based on identification with fatty acid analysis of sample sizes of 35 soil and rhizosphere bacteria and 25 endophytic bacteria All major bacterial species which formed at least 2% of the total population in non-amended soils and rhizospheres also occurred with chitin amendment In contrast, chitin-amended soils and rhizospheres yielded several species which were not found without chitin amendment, including Aureobacterium testaceum, Corynebacterium aquaticum and Rathayibacter tritici Burkholderia cepacia was recovered from both amended and non-amended soils and rhizospheres, but it was most abundant with chitin amendment at the end of the first cotton planting Soil was probably the major source for bacterial endophytes of cotton roots, since nearly all endophytic bacteria were also found in the soil or rhizosphere However, two dominant genera in the soil and rhizosphere, Bacillus and Arthrobacter, were not detected as endophytes Chitin amendment exhibited a further specific influence on the endophytic bacterial community; Phyllobacterium rubiacearum was not a common endophyte following chitin amendment, even though chitin amendment stimulated its populations in non-planted soil Burkholderia cepacia, found in similar numbers in the soil of both treatments, was the dominant endophyte in plants grown in chitin-amended soil but rarely colonized cotton roots grown in non-amended soil These results indicate that application of an organic amendment can lead to modifications of the bacterial communities of the soil, rhizosphere and endorhiza

Soil organic matter and nutrient dynamics in a Kenyan nitisol under long-term fertilizer and organic input management

Abstract: Three soil fertility management practices relevant to smallhold farming systems in the East African Highlands were tested in an 18 yr-old experiment on a humic nitisol (Kikuyu Red Clay) under a maize-bean rotation. These practices were the addition of mineral fertilizers (120 kg N and 52 kg P ha ˇ1 yr ˇ1 ), application of cattle manure (10 t ha ˇ1 yr ˇ1 ) and retention of maize stover. Eight treatments arranged as a 2 2 2 factorial were examined for their eAects on crop yield, soil organic matter (SOM) fractions and soil chemical properties. Total crop yields of maize and beans ranged between 1.4 t ha ˇ1 yr ˇ1 when maize stover was retained without external inputs to 6.0 t ha ˇ1 yr ˇ1 when stover was retained and fertilizers and manure applied. Soil organic C contents to a depth of 15 cm ranged between 23.6 t ha ˇ1 (14.3 mg g ˇ1 ) with combined addition of mineral fertilizers and stover removal to 28.7 t ha ˇ1 (17.4 mg g ˇ1 ) with chemical fertilization, manuring and stover retention. DiAerences in particulate organic matter and microbial biomass among treatments were proportionately larger than changes in total soil organic carbon. All land managements resulted in an overall decline in SOM over time and the greatest average rate of loss, 557 kg C ha ˇ1 yr ˇ1 , was observed with fertilization and crop residue removal. Addition of manure and retention of maize stover reduced this loss by 49%. Carbon balances suggest that particulate organic matter is more eAciently restocked by manure than maize stover. Particulate organic matter is a key fraction for understanding soil fertility changes in humic nitisols of the Kenyan Highlands and has potential for use as an indicator of soil quality. # 1999 Elsevier Science Ltd. All rights reserved.

Phosphatase and arylsulphatase activities in wetland soils: annual variation and controlling factors

Abstract: Phosphatase and arylsulfatase activities were measured in three wetland soils (bog, fen and swamp) in North Wales, UK over 12 months. The fen site (85–176 nmol g −1 min −1 ) showed the highest phosphatase activity of the three, whilst there was little difference between the arylsulphatase activities of the fen (4–14 nmol g −1 min −1 ) and the swamp (5–16 nmol g −1 min −1 ). For both enzymes, the lowest activity was observed in the bog site (20–61 nmol g −1 min −1 for phosphatase; 1–3 nmol g −1 min −1 for arylsulphatase). Hydrogen ion concentration was a dominant controlling factor for the phosphatase activities in all sites. Waterlogging and low temperature seem to restrict enzyme activities in the fen and the swamp sites, as both factors showed significant correlations with the enzyme activities. No temporal relationships between the enzyme activities and the inorganic nutrient concentrations were detected. However, a negative relationship between phosphatase activity and phosphate content was discernable, when compared on a spatial basis.

Amino acid biodegradation and its potential effects on organic nitrogen capture by plants

Abstract: It has been reported that plant roots can directly utilise soil organic-N in the form of amino acids without prior mineralisation by the soil's microbial biomass. To critically assess this, however, requires a knowledge of microbial amino acid-N turnover times in soil. The effects of soil type, depth and temperature on the uptake and partitioning of a mixture of 15 14C-labelled amino acids by the soil's microbial biomass was therefore studied in 10 contrasting soil types. The results indicated that the degradation of amino acids was soil dependent but that the mean half-life in topsoils at 18°C was 1.7±0.6 h, whilst in subsoils the mean half-life was 12.2±3.3 h. On average 34% of the amino acid-C was respired as CO2 whilst 66% was utilised for new cell biomass. Amino acid decomposition increased with soil temperature, however, rapid rates of amino acid uptake and assimilation were also observed at 5°C (mean half-life in topsoil=2.9±1.5 h). Little correlation was observed between amino acid half-life and either microbial yield, soil arginase activity or organic matter content (r2<0.40), however, decomposition did appear to be weakly related with soil respiration. The high concentration of amino acids used here (5 mM) was intended to simulate amino acid release after root cell lysis. For previously reported lower concentrations in the bulk soil solution, half lives can be predicted to be even less based on microbial amino acid transport kinetics. The significance of this previously overlooked microbial decomposition of amino acids in the utilisation of organic N by plants is discussed.

Changes in microbial biomass and community composition, and soil carbon and nitrogen pools after incorporation of rye into three California agricultural soils

Abstract: The effects of long-term agricultural management on active soil organic matter (SOM) and short-term microbial C and N dynamics were investigated. Short-term changes in chemical and biological variables after incorporating fresh rye shoots were measured in intact soil cylinders from three contrasting agricultural systems. Two of the soils were from organic or conventional 4-yr rotations which had been in place for 6 yr as part of the University of California at Davis Sustainable Agriculture Farming Systems (SAFS) project and the third was from a double-cropped, intensive vegetable production system in the Salinas Valley of California. Microbial biomass (MB) and respiration, numbers of organisms in several trophic groups, soil inorganic N, dissolved organic C and recoverable rye were measured before and during the 6 weeks following rye incorporation. Active soil organic matter, expressed as the ratios of microbial biomass C or N to total soil C or N, respectively, appeared to be related to long-term management. These ratios increased in proportion to increased organic inputs and reduced tillage or periods of fallow. In all soils, MBC increased and decreased rapidly following rye incorporation, but MBN was fairly constant. Significant differences among the soils in MBC and MBN were maintained over the 6 week experiment. Following rye incorporation, fluorescein diacetate (FDA) active counts of bacteria and bacterial-feeding nematodes increased rapidly, whereas changes in FDA active fungal hyphal lengths and fungal-feeding nematodes were less pronounced. The rates of rye decomposition, respiration and net N mineralization were highest the first week after incorporation, coincident with increases in MBC and numbers of active bacteria in all three soils. There were significant differences among soils in numbers of organisms in the trophic groups on some sample dates, but changes in soil respiration and inorganic N and the rate of rye decomposition remained similar in all three soils. The SAFS organic soil had a somewhat lower ratio of bacterial to fungal biomass and lower ratio of respiration to MBC throughout the experiment than the SAFS conventional soil. Despite long-term differences in agricultural management and differences in active SOM contents among the three soils, the rates of rye decomposition and C and N mineralization were similar. Rye incorporation produced a short-term burst of microbial growth and activity of similar magnitude in all three soils although the initial MB contents in the three soils were different. Variations among the soils in FDA active counts of fungi and numbers of bacterial- and fungal-feeding nematodes indicated that microbial community composition was more responsive to rye incorporation than were changes in soil C and N pools.

Microbial respiration, biomass, biovolume and nutrient status in burrow walls of Lumbricus terrestris L. (Lumbricidae)

Abstract: Chemical characteristics and microbial activity were studied in burrow walls of the anecic earthworm species Lumbricus terrestris and in control soil of a lime ( Tilia cordata ), oak ( Quercus robur ) and beech ( Fagus sylvatica ) forest. Samples were taken in June and October at distances of 0–4 (drilosphere), 8–12 and 50–60 (control soil) mm from earthworm burrows. The following variables were measured: organic C, total N, moisture, pH, basal respiration, microbial biomass (SIR method), fungal and bacterial volume (epifluorescence microscopy) and nutrient (C, N and P) limitation of microbial growth. Organic C and total N contents increased in the burrow walls by factors of 1.8–3.5 and 1.3–2.2, respectively, compared to the control soil. The moisture content and pH (up to 1.2 units) was higher. Basal respiration, microbial biomass and bacterial volume in the drilosphere exceeded those in control soil significantly by factors of 3.7–9.1, 2.3–4.7 and 2.1–5.4, respectively. Changes in fungal volume with vicinity to burrows differed between forest sites. Fungal volume was increased significantly by factors of 1.9–3.4 in the earthworm burrow walls in the oak and beech forest, but was similar to that in control soil in the lime forest. Microbial growth in the control soil was limited by N in the oak forest and by N and P in the lime and beech forest. The nutrient status of the microflora changed little in vicinity to burrows. However, microbial N and P demand in earthworm burrow walls exceeded that in soil. The specific respiration ( q O 2 ) was increased and the growth response to nutrient additions was faster in the burrow walls suggesting that the microbial community in the burrow walls contains a larger fraction of metabolically-active microorganisms, adapted to continuous resource additions by earthworm faeces and mucus. Enrichment in organic matter, but also other mechanisms, particularly the activity of microbivorous soil animals, are presumably responsible for the formation of a specific microbial community in earthworm burrow walls. It is concluded that L. terrestris burrow walls are stable microhabitats which sustain a large and active microbial community and are likely to play an important role in the soil system by regulating microbial-mediated chemical processes.

Methane flux from irrigated rice fields in relation to crop growth and N-fertilization

Abstract: The effects of N-fertilization on rice plant growth (number of tillers, shoot and root biomass, root volume and porosity, grain yield) and their relationship with methane flux was investigated in three irrigated varieties of Oryza sativa L. (Sarju-52, Malviya-36 and Pant Dhan-4). The study design consisted of (a) control (unfertilized) vegetated, (b) fertilized vegetated, (c) control (unfertilized) bare, and (d) fertilized bare plots; laid down in a completely randomized block design in triplicate. Urea was applied in (b) and (d) in three split doses at a rate of 40, 30 and 30 kg N ha−1 at the time of transplanting, active tillering and grain filling stages of crop. The field was submerged before transplanting and the water depth ranged from 6.7 to 23.9 cm in response to rainfall. Every 10 d, crop growth and CH4 flux were measured from d 9 to 115 after rice transplanting. Sarju-52 and Pant Dhan-4 were similar in phenological stages but different than Malviya-36. Results showed that there were significant differences in all the growth variables measured for all the rice varieties due to growth period and fertilization. Variety×treatment, variety×growth period and treatment×growth period interactions were significant for all growth variables. Maximum CH4 flux from control (vegetated) plots was observed at the flowering stage (65 d after transplanting in Sarju-52 and Pant Dhan-4 and 76 d after transplanting in Malviya-36) and ranged from 10.79 to 14.20 mg m−2 h−1. In vegetated fertilized plots, maximum CH4 emission was observed 10 d later than in the vegetated control plots and ranged from 14.43 to 20.20 mg m−2 h−1. These values were from 7- to 12.3-fold higher than bare (unfertilized) plots. All growth variables, except mean shoot and root biomass, showed strong positive relationships with seasonal CH4 emission. It was concluded that the CH4 source strength was dependent on the rice variety under cultivation, its phenology, growth variables and soil fertilization.

Methane production and oxidation potentials in relation to water table fluctuations in two boreal mires

Abstract: We studied the response of methane production and oxidation potentials in a minerotrophic and an ombrotrophic mire to water table fluctuations. In profiles where water table had not varied, the water-saturated layers showed significant potentials while the unsaturated layers did not. The production potentials in the saturated layers below water level ranged from 0.1 to 2.4 m gC H4 h ˇ1 (g d.w.) ˇ 1 and oxidation potentials (first order reaction rate constants) betweenˇ0.010 andˇ0.120 h ˇ1 (g d.w.) ˇ 1 . In profiles with constant water level, the maximal production potential occurred 20 cm and maximal oxidation potential 10 cm below water level. When water table varied only a little, production potentials slightly increased towards the autumn. After a water level draw-down, in the profiles from the dry microsites, production and oxidation potentials were detected in layers that had been unsaturated up to 6 weeks. The maximal oxidation zone was shifted downwards during low water periods. In a wet microsite, a 2 week period of unsaturation eliminated the production potentials and decreased the oxidation potentials. After a rise in the water level, the potentials were reactivated more rapidly in the wet than in the dry microsites. # 1999 Elsevier Science Ltd. All rights reserved.