Changes In Microbial Properties Research Articles

Contrasting effects of different organic amendments on the microbial responses to extreme temperature changes

Soil organic amendments can change microbial communities, potentially impacting their resilience to environmental stresses and, consequently, their transformation of soil carbon and nitrogen. While several studies have investigated the impact of a few organic amendments on microbial resilience to stresses in the form of absolute change in environmental conditions, the potential stress effects of different rates of change have received much less attention. The rate of change in environmental properties such as temperature may induce different microbial stress responses, given the reliance of microbial defensive mechanisms on time-dependent signalling and activation processes. In this research, we explored the relevance of the rate of temperature change as a microbial stressor and evaluated the potential use of organic amendments in steering soil ecological response. We monitored the changes in microbial properties and concentrations of different carbon and nitrogen fractions to temperature change treatments of 2.5 and 30 °C/d after the application of three commonly available rural organic amendments to a podzol soil in a laboratory incubation study. Results showed significant effects of the temperature treatments on bacterial and total microbial DNA concentrations, as well as on dissolvable and insoluble carbon and nitrogen ratios regardless of the organic amendment treatment. Temperature impacts on respiration rates, priming rates and the concentration of soil hot water extractable carbon and nitrogen differed depending on the organic amendment treatment. Results demonstrated that compost-amended soils were least sensitive to the temperature treatments, while grasses and fermented grasses treatments were generally more sensitive while showing opposite responses. Our findings indicate that organic amendments might be utilized to manipulate the impact of environmental stresses on soil carbon and nitrogen concentrations, and highlight the need for future research to investigate the relevant ecological properties and mechanisms.

Introducing N2-fixing tree species into Eucalyptus plantations promotes soil organic carbon sequestration in aggregates by increasing microbial carbon use efficiency

The conversion of a monoculture plantation into a mixed forest can improve soil properties and increase soil organic carbon (SOC) sequestration. However, the impacts of introducing N2-fixing tree species into Eucalyptus plantations on SOC composition and transformation, as well as the associated changes in microbial properties remain poorly understood. Here, we examined the SOC fractions (LPI-C, LPII-C, and RP-C) in different aggregates (>2.0 mm, 1–2 mm, 0.25–1 mm, and <0.25 mm) from 15-year-old plantations of pure Eucalyptus urophylla (PP) and mixed E. urophylla and Acacia mangium (an N2-fixing tree species) (MP). The results showed that the contents of SOC fractions and the proportion of recalcitrant C pool (fRP-C) in aggregates were significantly higher (P < 0.05) in MP than those in PP. Pearson correlation analysis indicated strong correlations among litter properties, soil properties, and microbial properties. N2-fixing species increased soil aggregate stability and N availability while altering litter quality and quantity; all of these factors strongly impacted microbial properties. Changes in nitrogen availability and microbial properties (e.g. carbon use efficiency) were the drivers of changes in SOC fractions. Our findings highlight the importance of introducing N2-fixing species in regulating soil microbial properties thereby increased SOC sequestration and stability in Eucalyptus plantations, which should be a relatively good option for future afforestation projects in the view of improving the carbon benefits of the ecosystem.

Priming of soil organic carbon mineralization and its temperature sensitivity in response to vegetation restoration in a karst area of Southwest China

Plant residue input alters native soil organic carbon (SOC) mineralization through the priming effect, which strongly controls C sequestration during vegetation restoration. However, the effects of different vegetation types on SOC priming and the underlying microbial mechanisms due to global warming are poorly understood. To elucidate these unknowns, the current study quantified soil priming effects using 13C-labeled maize residue amendments and analyzed the community structure and abundances in the soils of a vegetation succession gradient (maize field (MF), grassland (GL), and secondary forest (SF)) from a karst region under two incubation temperatures (15 °C and 25 °C). Results revealed that after 120 d of incubation, vegetation restoration increased the soil priming effects. Compared to MF, the priming effects of SF at 15 °C and 25 °C increased by 142.36 % and 161.09 %, respectively. This may be attributed to a high C/N ratio and low-N availability (NO3−), which supports the “microbial nitrogen mining” theory. Variations in soil priming were linked to changes in microbial properties. Moreover, with vegetation restoration, the relative abundance of Actinobacteria (copiotrophs) increased, while Ascomycota (oligotrophs) decreased, which accelerated native SOC decomposition. Co-occurrence network analysis indicated that the cooperative interactions of co-existing keystone taxa may facilitate SOC priming. Furthermore, structural equation modeling (SEM) indicated that changes in the priming effects were directly related to the fungal Shannon index and microbial biomass C (MBC), which were affected by soil C/N and NO3−. Warming significantly decreased soil priming, which may be attributed to the increase in microbial respiration (qCO2) and decreased MBC. The temperature sensitivity (Q10) of SOC mineralization was higher after residue amendment, but significant differences were not detected among the vegetation types. Collectively, our results indicated that the intensity of priming effects was dependent on vegetation type and temperature. Microbial community alterations and physicochemical interactions played important roles in SOC decomposition and sequestration.

Changes in nutritional composition, volatile organic compounds and antioxidant activity of peach pulp fermented by lactobacillus

Fermentation with lactobacillus offers a natural means to modify nutritional and flavor properties of foods, and strains have crucial effects on it. In this study, peach pulp was fermented by five commercial probiotics (L. plantarum 21802, L. brevis 6239, L. pentosus 21798, L. alimentarius 21852 and L. pentosus 21832) to screen out a strain with higher adaptability ability and polyphenol bio-transformation capacity. Then, dynamics changes in microbial properties, nutritional composition, volatile organic compounds and DPPH radical-scavenging activity of peach pulp fermented by selected strain were studied and compared with the control sample. HPLC, LC-ESI-MS/MS and GC-MS were used for identification and quantification of phytochemistry. Results showed that the most efficient strain was L. plantaurm 21802 caused a significant (P < 0.05) increase in microbial population and total phenol content of peach pulp, which leads to an obvious decrease in fructose and glucose and a great increase in lactic acid content after 60 h fermentation. The malolactic conversion was completed within 12 h. Moreover, it caused an increase in β-carotene (14.82%), as well as chlorogenic acid (154.00%), vanillic acid (95.44%), epicatechin (27.99%), quercetin-3-O-glucoside (124.19%) and isorhamnetin (438.29%). Furthermore, lactobacillus fermentation resulting in the improvements of alcohols and aldehydes production, and the contents of four key aroma compounds (OVA>1), namely linalool, benzaldehyde, damascenone and eugenol, increased significantly (P < 0.05). These findings evidenced the potential use of L.plantarum 21802 in peach pulp fermentation for improving the nutritional and functional properties, and proved a possibility of peach fruit as a novel material for functional plant-based fermentation beverages.

Effects of heavy metals on bacterial community structure in the rhizosphere of Salsola collina and bulk soil in the Jinchuan mining area

It is important to understand the changes in microbial properties of plant rhizospheres to elucidate the interaction between heavy metals and plant communities. In this study, high throughput sequencing method was used to analyze the effects of heavy metals on rhizosphere microbial community structure of the autochthonous plant Salsola collina in the Jinchuan mining area. The operational taxonomic units (OTU) number and species diversity, and richness of the bacterial community in the rhizosphere of Salsola collina were higher than those of the bulk soil. Proteobacteria, Actinobacteria, and Firmicutes were the major bacterial phyla, and Salsola collina, Kaistobacter, Citrobacter, Acinetobacter and Pseudomonas were the main genera. Principal component analysis (PCA) and unweighted pair group method using arithmetic average (UPGMA) cluster analysis showed that there were significant differences in bacterial community structure between site and between rhizosphere and bulk soil. The moisture content, total nitrogen, phosphorus, magnesium, nickel, chromium, lead, and zinc contents, and pH had significant effects on species richness of the bacterial community, with total phosphorus content having a significant effect on species diversity. The finding of this study could be used as an important guideline for bioremediation of heavy metals in mining area.

High microbial diversity stabilizes the responses of soil organic carbon decomposition to warming in the subsoil on the Tibetan Plateau

Soil microbes are directly involved in soil organic carbon (SOC) decomposition, yet the importance of microbial biodiversity in regulating the temperature sensitivity of SOC decomposition remains elusive, particularly in alpine regions where climate change is predicted to strongly affect SOC dynamics and ecosystem stability. Here we collected topsoil and subsoil samples along an elevational gradient on the southeastern Tibetan Plateau to explore the temperature sensitivity (Q10 ) of SOC decomposition in relation to changes in microbial communities. Specifically, we tested whether the decomposition of SOC would be more sensitive to warming when microbial diversity is low. The estimated Q10 value ranged from 1.28 to 1.68, and 1.80 to 2.10 in the topsoil and subsoil, respectively. The highest Q10 value was observed at the lowest altitude of forests in the topsoil, and at the highest altitude of alpine meadow in the subsoil. Variations in Q10 were closely related to changes in microbial properties. In the topsoil the ratio of gram-positive to gram-negative bacteria (G+:G-) was the predominant factor associated with the altitudinal variations in Q10 . In the subsoil, SOC decomposition showed more resilience to warming when the diversity of soil bacteria (both whole community and major groups) and fungi was higher. Our results partly support the positive biodiversity-ecosystem stability hypothesis. Structural equation modeling further indicates that variations in Q10 in the subsoil were directly related to changes in microbial diversity and community composition, which were affected by soil pH. Collectively our results provide compelling evidence that microbial biodiversity plays an important role in stabilizing SOC decomposition in the subsoil of alpine montane ecosystems. Conservation of belowground biodiversity is therefore of great importance in maintaining the stability of ecosystem processes under climate change in high-elevation regions of the Tibetan Plateau.

Responses of soil WEOM quantity and quality to freeze–thaw and litter manipulation with contrasting soil water content: A laboratory experiment

Future climatic change is likely to increase the occurrence of soil freeze–thaw (FT) events in high latitude and/or high altitude zones, which can substantially influence the dynamics of dissolved organic matter (DOM) released into the soils. However, it is not clear how the quantity and quality of soil DOM respond to changing FT patterns under different soil moisture and litter manipulation conditions in northern temperate forest stands. In this study, litter-amended and non-amended forest soils were incubated for 360 days at three soil moisture levels (30%, 60%, and 90% water-filled pore space) and three intensities of FT disturbance (low, high, and none). We quantified heterotrophic respiration, enzyme activity, microbial biomass, and water-extractable organic matter (WEOM) as a proxy for DOM in soils during the incubation experiment. The quality of WEOM was characterized by biodegradability, UV absorbance and parallel factor analysis modelling of fluorescence excitation emission matrices. Concentrations of water-extractable organic carbon (WEOC) and biodegradable WEOC declined continuously in all treatments over the 360-day incubation period. The dominant component of fluorescent WEOM shifted from humic- and fulvic- like components during the first 108 days of incubation, to protein-like components of microbial origin, characterized by high aromaticity, at the end of the incubation period. Litter amendment, FT disturbance, and their interaction increased WEOC concentrations in soils during the early 108-day incubation period, particularly in soils with low moisture and high FT intensity, but these increases disappeared after 252 days incubation at 15 °C. Litter-derived fluorescent WEOM in soils without FT disturbance was dominated by protein-like components after 14 days of incubation, but these were replaced by humic- and fulvic-like components after 108 days. This replacement effect was weaker in soils with FT disturbance, which we attribute to changes in microbial properties, including enzyme activity, microbial biomass and activity.

The effect of moisture on soil microbial properties and nitrogen cyclers in Mediterranean sweet orange orchards under organic and inorganic fertilization

Water shortage and soil degradation are common environmental stressors encountered in the Mediterranean area. We evaluated how different soil moisture levels, dependent on distance from drip irrigation points, impact on the biological, chemical and physical properties of citrus soil under organic and inorganic fertilization. We measured soil physicochemical properties, basal soil respiration, soil microbial biomass carbon, soil microbial community structure (phospholipid fatty acid assay), bacterial load (16S rRNA gene abundance), enzymatic activities (urease, dehydrogenase, β-glucosidase and acid phosphatase) and abundance of microbial nitrogen cyclers (quantitative PCR). A field experiment was established in an orange orchard (Citrus sinensis) in southeast Spain and eighteen soil samples were taken from each plot to compare the impacts of soil moisture: near (wet, w) or away (dry, d) from drip-irrigation points, in plots with inorganic fertilizers under intensive ploughing (PI) or organic fertilization (OA). The results showed that changes in microbial properties and soil microbial indexes were strongly associated with soil moisture content under both organic and inorganic fertilization, and with organic carbon content. Soil moisture influenced soil aggregation, basal soil respiration, phosphatase activity, bacterial and fungal load (PLFAs) and the abundances of bacterial N cycling genes, including nifH (nitrogen fixation) nirS/K and nosZ genes (denitrification) and amoA-B (bacterial nitrification). The potential for N fixation and denitrification, two microbial processes that are crucial for determining the amount of N in the soil, were improved by increased soil moisture in the proximity of the drip irrigation. Soil OC and total N, which are higher under organic fertilization than under inorganic fertilization, were also shown to be highly correlated with the abundance of the N cycling genes. By controlling irrigation doses and applying organic amendments, it may be possible to increase the microbial abundance and function in soil and support greater fertility of soils.

Non-target effects of pretilachlor on microbial properties in tropical rice soil.

The use of herbicides has been questioned in recent past for their non-target effects. Therefore, we planned to study the effect of pretilachlor on growth and activities of microbes in tropical rice soil under controlled condition at National Rice Research Institute, Cuttack, India. Three pretilachlor treatments, namely, recommended dose at 600ga.i. ha(-1) (RD), double the recommended dose at 1200ga.i. ha(-1) (2RD), and ten times of the recommended dose at 6000ga.i. ha(-1) (10RD) along with control, were imposed. The initial residue (after 2h of spray) deposits in soil were 0.174, 0.968, and 3.35μgg(-1) for recommended, double the recommended, and ten times of the recommended doses, respectively. No residue in soil was detected in RD treatment on day 45. The half life values were 16.90, 17.76, and 36.47days for RD, 2RD, and 10RD treatments, respectively. Application of pretilachlor at 10RD, in general, had significantly reduced the number of bacteria, actinomycetes, fungi, nitrogen fixers, and microbial biomass carbon. Pretilachlor at RD did not record any significant changes in microbial properties compared to control. The results of the present study thus indicated that pretilachlor at RD can be safely used for controlling grassy weeds in rice fields.

Changes in microbial properties and community composition in acid soils receiving wastewater from concentrated animal farming operations

The increasingly intensified animal industry in recent decades has resulted in the discharge of a large amount of wastewater with high concentrations of organic matter and nutrients into the ambient environment, which influences soil properties. In this study, we applied a multi-parameter approach to investigate changes in soil microbial properties and community compositions from three acid soil sites that differed in land-use patterns and histories of receiving wastewater. Wastewater had been applied to the sites for 2–20 years. Compared to controls, soil pH, EC and total nutrients were significantly higher in soils receiving wastewater, as well as average increases of 149mgkg−1 for microbial biomass carbon and 0.19mg CO2–Ckg−1h−1 for basal respiration; whereas the metabolic quotient and the ratio of saturated to monounsaturated phospholipid fatty acids decreased by 13% to 31%, and 32% to 61%, respectively. Soil microbial communities of all sites changed with the impact of wastewater application and showed significant increases in bacteria, especially Gram-negative bacteria. The differences in microbial metabolic profiles from all sites were reduced by wastewater application. Soil pH and EC were the two most important factors controlling microbial community composition under wastewater application. These results suggested that wastewater application could reduce stress on acid soil microorganisms by providing more organic carbon and nutrients, and through neutralization of soil acidity.

Changes in Biological Properties in Soil Amended with Rock Phosphate and Waste Mica Enriched Compost using Biological Amendments and Chemical Fertilizers Under Wheat-Soybean Rotation

The main aim of this research work is to prepare an enriched compost using rice straw mixed with rock phosphate, waste mica and Aspergillus awamori and to study their effect on changes in microbial properties in soils with and without chemical fertilizers under wheat-soybean rotation. Data revealed that significant increase in microbial biomass carbon (MBC), dehydrogenase activity, phosphatase activities, and microbial biomass phosphorus (MBP) in soil were maintained in enriched compost than ordinary compost after both the crops. Significant increase in MBC, dehydrogenase activity, phosphatase activities, and MBP were found in surface soil. The maximum microbial activities were observed in the treatment receiving 50% recommended dose of fertilizer (RDF) + enriched compost at 5 t ha−1 indicating that integrated use of chemical fertilizers and enriched compost significantly improved the biological properties of soil under wheat–soybean rotation thereby enhanced soil fertility and crop production.

Changes of microbial properties in (near-) rhizosphere soils after phytoextraction by Sedum alfredii H: A rhizobox approach with an artificial Cd-contaminated soil

The ultimate goal of soil remediation is to restore soil health. Soil microbial parameters are considered to be effective indicators of soil health. The aim of this study was to determine the effects of phytoextraction on microbial properties through the measurement of soil microbial biomass carbon, soil basal respiration and enzyme activities. For this purpose, a pre-stratified rhizobox experiment was conducted with the Cd hyperaccumulator Sedum alfredii H. for phytoextraction Cd from an artificial contaminated soil (15.81mgkg−1) under greenhouse conditions. The plant and soil samples were collected after growing the plant for three and six months with three replications. The results indicated that the ecotype of S. alfredii H. originating from an ancient silver mining site was a Cd-hyperaccumulator as it showed high tolerance to Cd stress, the shoot Cd concentration were as high as 922.6mgkg−1 and 581.9mgkg−1 at the two samplings, and it also showed high BF (58.4 and 36.8 after 3 and 6 months growth), and TF (5.8 and 5.1 after 3 and 6 months growth). The amounts of Cd accumulated in the shoots of S. alfredii reached to an average of 1206μgplant−1 after 6 months growth. Basal respiration, invertase and acid phosphatase activities of the rhizosphere soil separated by the shaking method were significantly higher (P<0.01) than that of the near-rhizosphere soil and the unplanted soil after 3 months growth, so were microbial biomass carbon, urease, invertase and acid phosphatase activities of the rhizosphere soil after 6 months growth. Acid phosphatase activity of the 0–2mm sub-layer rhizosphere soil collected by the pre-stratified method after 3 months growth was significantly higher (P<0.05) than that of other sub-layer rhizosphere soils and bulk soil, and so were microbial biomass carbon, basal respiration, urease, invertase and acid phosphatase activities of the 0–2mm sub-layer rhizosphere soil after 6 months growth. It was concluded that phytoextraction by S. alfredii could improve soil microbial properties, especially in rhizosphere, and this plant poses a great potential for the remediation of Cd contaminated soil.

Biochemical properties and microbial community structure of five different soils after atrazine addition

Atrazine is one of the most used herbicides worldwide; however, consequences of its long-term agricultural use are still unknown. A laboratory study was performed to examine changes in microbial properties following ethylamino-N-15-atrazine addition, at recommended agronomic dose, to five acidic soils from Galicia (NW Spain) showing different physico-chemical characteristics, as well as atrazine application history. Net N mineralization was observed in all soils, with nitrate being the predominant substance formed. The highest values were detected in soils with low atrazine application history. From 2% to 23% of the atrazine-N-15 was found in the soil inorganic-N pool, the highest values being detected after 9 weeks in soils with longer atrazine application history and lower indigenous soil N mineralization. The application of atrazine slightly reduced the amount of soil N mineralized and microbial biomass at short term. Soluble carbohydrates and beta-glucosidase and urease activity decreased with incubation time, but were not significantly affected by the single application of atrazine. Microbial community structure changed as consequence of both soil type and incubation time, but no changes in the phospholipid fatty acid (PLFA) pattern were detected due to recent atrazine addition at normal doses. The saturated 17- to 20-carbon fatty acids had higher relative abundance in soils with a longer atrazine history and fungal biomass, as indicated by the PLFA 18:2 omega 6,9, decreased with the incubation time. The results suggested that the PLFA pattern and soil N dynamics can detect the long-term impact of repeated atrazine application to agricultural soils.

Changes in microbial properties after manure, lime, and bentonite application to a heavy metal-contaminated mine waste

Organic materials improve soil properties conducive to plant growth and may be necessary to stabilize lead (Pb) and zinc (Zn) mine wastes. Composted beef manure was applied to mine wastes at two sites at 45 and 269 Mg ha −1 with and without lime and bentonite. Switchgrass ( Panicum virgatum) was established as a vegetative cover. Compost significantly increased waste nutrient content, pH, and available water, and decreased heavy metal availability. The largest total phospholipid fatty acid (PLFA) increase was in the high compost treatment, with values of 80.3 and 30.2 (site A) and 84.9 and 23.1 (site B) μmol kg −1 soil for treated and control, respectively. However, high rates of compost (269 Mg ha −1) were generally required to increase bacterial and fungal communities beyond the control. The high compost-treated waste exhibited increased arylsulfatase, phosphatase, and β-glucosidase enzyme activities. Observed increases in pH may have resulted in lowered enzyme activity in the low compost treatment. Results suggest that an organic matter addition above 45 and up to 269 Mg ha −1 may be needed to support and sustain microbial activity and biomass in mine waste materials, at least over the two year period evaluated.

Changes in microbial properties and nutrient dynamics in bagasse and coir during vermicomposting: Quantification of fungal biomass through ergosterol estimation in vermicompost

In this experiment, different microorganisms viz., Trichoderma viridae, Aspergillus niger and Bacillus megaterium were inoculated in bagasse and coir with the objective to study their effect on nutrient dynamics and microbial properties, specially effect on fungal status in these waste materials. Fungal biomass (FBC) was calculated from the ergosterol content in the vermicompost samples. Inoculation of B. megaterium registered comparatively higher TP content in the final stabilized product. Vermicomposting increased microbial biomass carbon (MBC) and nitrogen (MBN) content in bagasse and coir. Microbial biomass carbon to nitrogen ratio (MBC/ MBN) was comparatively narrower in fungi inoculated vermicomposts and FBC/MBC ratio was increased up to 11.69 from 9.51 of control during vermicomposting.

Microbiological parameters as indicators of compost maturity

The objectives of this study were to determine the changes of microbial properties of pig manure collected from pens with different management strategies and composted using different turning and moisture regimes; relate their association with humification parameters and compost temperature; and identify the most suitable microbial indicators of compost maturity. Six different microbial parameters, including total bacterial count, oxygen consumption rate, ATP content, dehydrogenase activity, and microbial biomass C and N, along with humification parameters [humic acid (HA), fulvic acid (FA) and HA : FA ratio] and compost temperature were monitored during composting. Significant positive correlations were found between temperature and microbial properties, including O2 consumption rate, ATP content, dehydrogenase activity, and microbial biomass N. The humification parameters also showed significant correlations with microbial properties of the manure compost. For instance, HA contents of pig manures was positively correlated with total aerobic heterotrophs, and microbial biomass N and C; and negatively correlated with O2 consumption rate, ATP content, and dehydrogenase activity. Among the six microbial parameters examined, dehydrogenase activity was the most important factor affecting compost temperature and humification parameters. Composting strategies employed in this study affected the speed of composting and time of maturation. If the moisture content is maintained weekly at 60% with a 4-day turning frequency, the pig manure will reach maturity in 56 days. The composting process went through predictable changes in temperature, microbial properties and chemical components despite differences in the initial pig manure and composting strategies used. Among the six microbial parameters used, dehydrogenase activity is the most suitable indicator of compost maturity. Compared with respiration rate, ATP content and microbial biomass procedures, dehydrogenase activity is the simplest, quickest, and cheapest method that can be used to monitor the stability and maturity of composts. The results presented here show that microbial parameters can be used in revealing differences between composts and compost maturity. The statistical relationship established between humification parameters and microbial parameters, particularly dehydrogenase activity, demonstrates that it is possible to monitor the composting process more easily and rapidly by avoiding longer and more expensive analytical procedures.

Development of soil microbial properties in topsoil layer during spontaneous succession in heaps after brown coal mining in relation to humus microstructure development

A wide spectrum of soil microbial parameters (microbial biomass, respiration, direct counts of bacteria, fungal mycelium length, cellulose decomposition and FDA activity) was studied in the chronosequence of 1- to 41-year-old plots of spontaneous succession on post-mining sites near the town of Sokolov. At these sites, soil chemical properties and the role of humus micromorphology (driven by soil fauna activity) were recorded, and their relation to changes in microbial properties was evaluated. Growth of herbs and grasses occurred only occasionally on 3- to 14-year-old plots. Shrubs ( Salix caprea) covered a plot 15–25 years old, and 26- to 41-year-old plots were forested mainly with Populus tremuloides and Betula spp. Organic matter content increased, while pH decreased during succession. Organic matter accumulation seems to be the main factor affecting the development of the soil microbial community. All investigated parameters, with the exception of mycelium length, correlated positively with soil organic carbon. Most of the parameters measured at 30- to 40-year-old forested sites were of comparable magnitude to undisturbed habitats. Respiration per unit of microbial biomass (metabolic quotient) decreased with increasing succession age. The microbial community was strongly affected by humus form created by the activity of soil invertebrates. A rapid increase in all microbial parameters, again with the exception of mycelium length, was observed when moder developed under shrub cover. A shift from moder, typical of intermediate succession stages, to mull in later succession stages was accompanied by an increase in bacterial numbers and a decrease in microbial biomass, respiration and FDA. These changes correspond to: (i) a decrease in organic carbon content in topsoil layer, caused by earthworm-mediated soil mixing and (ii) a decrease in the availability of soil organic matter.

Changes in microbial properties associated with long-term arsenic and DDT contaminated soils at disused cattle dip sites

This work examined the effect of long-term arsenic (As) and DDT contamination on soil microbial properties at 11 cattle dip sites in northern New South Wales, Australia. Total As in the surface (0–10 cm) soils from these sites ranged from 34 to 2941 mg As kg −1 soil and hexane-extractable DDT concentrations ranged between 2.9 and 7673 mg DDT kg −1 soil. The concentrations of water and oxalate-extractable As were positively correlated with total As. Oxalate-extractable As was more strongly correlated ( r 2=0.87) with total As than water-extractable As ( r 2=0.34). A weak positive relationship was observed between the level of nutrient (organic carbon and nitrogen) and microbial biomass C ( r 2=0.61 and 0.45, respectively). There was a highly significant difference between the microbial properties of polluted and unpolluted sites ( P<0.001). In comparison to unpolluted soils, fungal counts, microbial biomass C, and respiration were dramatically reduced ( P<0.05) in polluted soils. However, the bacterial population between polluted and unpolluted soils were not different ( P<0.05). The results of this study suggested that (a) long-term contamination of soils adjacent to former cattle dipping sites by As and DDT adversely affected soil microbial properties with the fungal populations being the most sensitive and (b) there was little regeneration of microbiota despite 25 years of field ageing of the soils.

Carbon mineralization and microbial activity in a field site trial used for 14 C turnover experiments over a period of 30 years

Long-term experiments on different crop management systems provide essential information about turnover of soil organic matter and changes in microbial properties over a period of time. A long-term field site trial, which was established in 1967 near Vienna, Austria, to document the fate of 14C-labelled manure (straw and farmyard) under different crop management systems (crop rotation, spring wheat and bare fallow), was investigated. Soil samples were taken in 1997 and separated into size fractions (>250 μm, 250–63 μm, 63–2 μm, 2–0.1 μm and <0.1 μm) after aggregate dispersion using low-energy sonication. Organic C, total N and 14C content were measured in the bulk soil and the size fractions and microbial properties were analysed in the bulk soil. Additionally, C mineralization in bulk soil samples was monitored at 20 °C over a period of 28 days, and subsequently 14C-CO2 content was analysed. The distribution of organic C and N within the size fractions was similar between crop rotation and spring wheat; the highest amounts of organic C and N were found in the clay-sized fraction. The amounts of C and N were significantly smaller in the bare fallow, which was depleted of organic matter in the coarse-sized fractions. 14C distribution differed significantly from unlabelled C distribution, labelled C was accumulated in the silt-sized fraction, indicating weak humification of the applied manure C. The highest rate of C mineralization was measured in the crop rotation and spring wheat, whereas the emission rate of the bare fallow was about 40% lower. The higher 14C:C ratio of the bulk soil in comparison to the emitted CO2 indicated that labelled C compounds still remained mineralizable after a period of 30 years. Microbial properties showed a great difference between crop management systems and bare fallow, particularly regarding urease and xylanase activity.

Microbial activities during composting of spent pig-manure sawdust litter at different moisture contents

The changes in microbial properties, including total aerobic heterotrophs, O 2-consumption rate, ATP content, dehydrogenase activity and microbial biomass C and N of the spent pig-manure sawdust litter were examined during further compositing. The effects of three moisture levels, 50% (pile A), 60% (pile B) and 70% (pile C), on the composting process were also evaluated. Piles A and B had very similar trends of change in temperature and microbial properties during the composting period but pile C was significantly different. Temperatures in the first two piles increased to a peak of 64–69°C by day 4, while that of pile C rose to a lower peak (58°C) on day 7. The high moisture content (about 70%) of pile C led to early cooling of the pile and decreased the production of microbial activity and biomass. Although water was added frequently to maintain the moisture content of each pile, it was difficult in practice to maintain the moisture content of pile C at 70%, since water leaked out from the pile. Therefore, a moisture content of between 50 and 60% can be considered as the optimal moisture level for further composting of the spent litter. In general, the total aerobic heterotrophs, O 2 consumption rate and ATP content of all piles increased dramatically during the thermophilic stage of composting, but then decreased slowly and were maintained at lower levels at the end of the composting process. Stability of microbial properties was observed at day 60, indicating that two months is enough to convert spent litter to a mature compost. Temperature was found to be correlated with ATP content, dehydrogenase activity and oxygen consumption rate, and so these parameters could be used to indicate microbial activity and degradation of the spent pig-manure sawdust litter.