Total Soil Microbial Biomass Research Articles

Effect of distance and depth on microbial biomass and mineral nitrogen content under Acacia senegal (L.) Willd. trees

The relations between plants and soil biota involve positive and negative feedbacks between soil organisms, their chemical environment, and plants. Then, characterization of microbial community functioning is important to understand these relations. An experiment was conducted in a field system in the north of Senegal for two years (2005 and 2006) in order to investigate the effect of depth and distance from Acacia senegal tree stem on soil microbial biomass and inorganic-N content. Soils were sampled during dry season (April, T(0)) and wet season (August, T(1)) along transects (R(0), foot tree; R(/2,) approximately 0.50 m distance from the stem; and R, approximately 1 m distance from the stem) and at different layers: 0-25 cm, 25-50 cm and 50-75 cm of A. senegal trees rhizosphere. Total microbial biomass and inorganic-N content were negatively correlated to the distance from tree stem and the depth. The highest values of microbial biomass and mineral nitrogen were found at the foot tree (R(0)) and at 0-25 cm layer. Inorganic-N was mostly in nitrate form (NO(3)(-)) during the dry season. In contrast, during the wet season, inorganic-N was dominated by ammoniac form (NH(4)(+)). Soil total microbial biomass and inorganic-N (NH(4)(+)+NO(3)(-)) were negatively correlated. Our results suggest a positive influence of A. senegal rhizosphere on soil microbial biomass and inorganic-N content.

Tillage-induced changes in fungal and bacterial biomass associated with soil aggregates: A long-term field study in a subtropical rice soil in China

The composition and function of the soil microbial community can be strongly influenced by soil structure and tillage. Soil total microbial biomass C (MBC), microbial biomass N (MBN), and fungal and bacterial biomass associated with different soil aggregate-size fractions were measured for a long-term no-till subtropical rice soil ecosystem to determine how tillage shifts microbial community structure and to detect the spatial scale at which microorganisms are the most sensitive to disturbance. Surface soil (0–20cm) was fractionated into aggregate-sizes (>4.76mm, 4.76–2.0mm, 2.0–1.0mm, 1.0–0.25mm, 0.25–0.053mm, <0.053mm) under three tillage regimes. Soil MBC, MBN, fungal biomass and bacterial biomass were significantly higher under no tillage (NT) than conventional tillage (CT) and CT flooded paddy field (FPF) in whole soil (p<0.05). Microbial biomass C, N and fungal biomass were significantly higher under NT than CT and FPF for all aggregate sizes. No significant tillage effects on microbial biomass C:N ratio were observed, but analysis of soil aggregates revealed significant differences due to tillage. Microbial biomass C:N ratio averaged 8.8, 8.5, and 8.4 for CT, NT, and FPF, respectively, while the ratio was significantly higher under CT than NT for macroaggregates >1.0mm, indicating a tillage-induced N limitation in macroaggregates. The fungal:bacterial biomass ratio ranged from 0.5 to 4.5, and was highest in 4.76–2mm aggregates for all tillage regimes. Fungal:bacterial ratios were significantly higher for aggregate sizes >1.0mm (2.4) than for <1.0mm aggregates (0.7). Tillage effects on fungal over bacterial dominance were not significant for whole soil and most aggregate sizes. The NT regime increased microbial biomass, but this increase was proportional for both bacteria and fungi. The soil microbial biomass and community structure was likely controlled by particle size at the aggregate-scale (0.05–5.0mm), while tillage played a role in regulating the microbial community structure.

Assessment of soil quality using soil organic carbon and total nitrogen and microbial properties in tropical agroecosystems

Assessment of soil quality is an invaluable tool in determining the sustainability and environmental impact of agricultural ecosystems. The study was conducted to assess the quality of the soils under arable cultivation, locally irri-gated and non-irrigated, forestry plantations of teak (Tectona grandis Lin.) and gmelina (Gme- lina arborea Roxb.), and cashew (Anacardium occidentale Lin.) plantation agro ecosystems using soil organic carbon (SOC), soil total ni-trogen (STN) and soil microbial biomass C (SMBC) and N (SMBN) at Minna in the southern Guinea savanna of Nigeria. Soil samples were collected from soil depths of 0-5 cm and 5-10 cm in all the agro ecosystems and analyzed for physical, chemical and biological properties. All the agro ecosystems had similar loamy soil texture at both depths. The soils have high fer-tility status in terms of available phosphorus and exchangeable calcium, magnesium and po- tassium. The irrigated arable land had significantly (P 6.6 suggesting fungal domination in all the agroecosystems. The forestry plantation soils had higher SMBC and SMBN as a per-centage of SOC and STN respectively than the cultivated arable land soils. Burning for clearing vegetation and poor stocking of forestry planta-tions may impair the quality of the soil. The study suggests that the locally irrigated agro- ecosystem soil seems to be of better quality than the other agroecosystem soils.

Fungistasis and general soil biostasis – A new synthesis

In most soils, fungal propagules are restricted to a certain extent in their ability to grow or germinate. This phenomenon, known as soil fungistasis, has received considerable attention for more than five decades, mostly due to its association with the general suppression of soil-borne fungal diseases. Here, we review major breakthroughs in understanding the mechanisms of fungistasis. Integration of older fungistasis research and more recent findings from different biological and chemical disciplines has lead to the consensus opinion that fungistasis is most likely caused by a combination of microbial activities, namely withdrawal of nutrients from fungal propagules and production of fungistatic compounds. In addition, recent findings indicate that there are mechanistic links between these activities leading towards an integrated theory of fungistasis. Among the potentially fungistatic compounds volatiles have received particular attention. Whereas it has long been assumed that fungistasis is the result of the metabolic activity of the total soil microbial biomass, more recent research points at the importance of activities of specific components of the microbial community. These insights into fungistasis have also formed the basis for strategies to increase general soil suppression. Besides these basic and practical aspects of fungistasis, its impact on fungal ecology, in particular on fungal exploration strategies, is discussed. Finally, we take a closer look at plant–soil feedback experiments to demonstrate the occurrence of fungistasis-like phenomena and to suggest that fungistasis may be part of a much wider phenomenon: general soil biostasis.

Soil respiration rate and its sensitivity to temperature in pasture systems of dry-tropics

Tree clearing is a topical issue the world over. In Queensland, the high rates of clearing in the past were mainly to increase pasture production. The present research evaluates the impact of clearing on some soil biological properties, i.e. total soil respiration, root respiration, microbial respiration, and microbial biomass (C and N), and the response of soil respiration to change in temperature. In-field and laboratory (polyhouse) experiments were undertaken. For in-field studies, paired cleared and uncleared pasture plots were selected to represent three major tree communities of the region, i.e. Eucalyptus populnea, E. melanophloia, and Acacia harpophylla. The cleared sites were chosen to represent three different time-since-clearing durations (5, 11–13, and 33 years; n=18 for cleared and uncleared plots) to determine the temporal impact of clearing on soil biological properties. Experiments were conducted in the polyhouse to study in detail the response of soil respiration to changes in soil temperature and soil moisture, and to complement in-field studies for estimating root respiration. The average rate of CO2 emission was 964 g CO2/m2/yr, with no significant difference (P<0.05) among cleared and uncleared sites. Microbial respiration and microbial biomass were greater at uncleared compared with those at cleared sites. The Q 10-value of 1.42 (measured for different seasons in a year) for in-field measurements suggested a small response of soil respiration to soil temperature, possibly due to the limited availability of soil moisture and/or organic matter. However, results from the polyhouse experiment suggested greater sensitivity of root respiration to temperature change than for total soil respiration. Since root biomass (herbaceous roots) was greater at the cleared than at uncleared sites, and root respiration increased with an increase in temperature, we speculate that with rising ambient temperature and consequently soil temperature, total soil respiration in cleared pastures will increase at a faster rate than that in uncleared pastures.

Effects of soil type and farm management on soil ecological functional genes and microbial activities

Relationships between soil microbial diversity and soil function are the subject of much debate. Process-level analyses have shown that microbial function varies with soil type and responds to soil management. However, such measurements cannot determine the role of community structure and diversity in soil function. The goal of this study was to investigate the role of gene frequency and diversity, measured by microarray analysis, on soil processes. The study was conducted in an agro-ecosystem characterized by contrasting management practices and soil types. Eight pairs of adjacent commercial organic and conventional strawberry fields were matched for soil type, strawberry variety, and all other environmental conditions. Soil physical, chemical and biological analyses were conducted including functional gene microarrays (FGA). Soil physical and chemical characteristics were primarily determined by soil textural type (coarse vs fine-textured), but biological and FGA measures were more influenced by management (organic vs conventional). Organically managed soils consistently showed greater functional activity as well as FGA signal intensity (SI) and diversity. Overall FGA SI and diversity were correlated to total soil microbial biomass. Functional gene group SI and/or diversity were correlated to related soil chemical and biological measures such as microbial biomass, cellulose, dehydrogenase, ammonium and sulfur. Management was the dominant determinant of soil biology as measured by microbial gene frequency and diversity, which paralleled measured microbial processes.

Variation in total biological productivity and soil microbial biomass in rainfed agroecosystems: Impact of application of herbicide and soil amendments

Total biological productivity and soil microbial biomass are important characteristics to describe sustainable agroecosystems. We investigated the impact of herbicide, alone or in combination with soil amendments, on crop and weed productivity and microbial biomass in a rice ( Oryza sativa)–barley ( Hordeum vulgare)–summer fallow rotation in a tropical rainfed agroecosystem. Total net productivity (TNP) of crops was greater with herbicide (Butachlor) + chemical fertilizer (NPK), herbicide + farmyard animal manure, and herbicide + green manure ( Sesbania aculeata) than with herbicide + crop residue ( Triticum aestivum straw), herbicide only, and the control without amendment. Weed TNP was highest in the control, lowest in herbicide only, and intermediate in combined herbicide and amendment treatments except herbicide + crop residue. The sum of crop TNP and weed TNP was highest in herbicide + green manure and lowest in herbicide only. Microbial biomass C and N were greater with herbicide and amendment treatments than with herbicide only. Microbial biomass showed distinct temporal variation. Yearly plant input to soil had strong correlation with microbial biomass in the control and herbicide only treatments. In these agroecosystems, combining organic soil amendments with herbicide application could help sustain soil fertility and maintain higher total biological productivity.

The ecological engineering impact of a single tree species on the soil microbial community

Summary 1. Ecosystem engineering by a single species can have a cascading effect on many ecosystem processes. While the impact of above‐ground ecological engineers on soil chemical properties has been studied, few studies have assessed their impact on the soil microbial community, which is largely responsible for many ecosystem functions. 2. Utilizing a long‐term experiment where birch was planted on heather moorland 20 years ago, the engineering impact of a single tree species (Betula pubescens) on the soil microbial community was assessed using phospholipid fatty acid (PLFA) analysis and fungal polymerase chain reaction–denaturing gradient gel electrophoresis (PCR‐DGGE). Changes in the soil microbial community were then related to soil chemical and physical variables and tree performance variables. 3. Under birch, total microbial biomass (total PLFA) declined, species richness increased and the ratio of fungal : bacterial PLFA declined. The fungal PLFA marker increased with increasing organic matter and depth of the LFH and O soil horizons, characteristics associated with moorland soils. Bacterial PLFAs increased with increasing birch canopy cover. The fungal community of the birch plots was different from that in the heather plots and changes in the fungal community composition were related to the size of the birch trees in the plots. 4. Changes in the soil microbial community were also related to changes in mineralizable N. Mineralizable N was correlated with both decreasing total soil microbial biomass and decreasing fungal : bacterial ratio. 5. The durability of the engineering effects of birch was studied in a second experiment. Plots were established in first‐generation birch woodland that had developed on Calluna‐dominated moorland. The plots were cleared of birch and planted with heather. After 20 years, there was no difference in the community composition of the PLFAs. In contrast, there were significant differences in the fungal community composition as judged by DGGE analysis, with the fungal community in the felled birch plots being similar to the heather moorland. 6. Synthesis. This work demonstrates that addition of a single tree species to heather moorland results in changes in below‐ground soil microbial communities and in nutrient cycling.

Sensitivity of Soil Restoration Indicators during Páramo Succession in the High Tropical Andes: Chronosequence and Permanent Plot Approaches

Abstract The aim of this study was to analyze indicators of soil restoration during secondary succession in a heterogeneous valley in the high tropical Andes. A combination of chronosequence and permanent plot methods was used to detect changes in this heterogeneous matrix. Thirty‐six plots with different fallow times (1–9 years) and four noncultivated plots with natural vegetation (páramo) were sampled twice in a 3‐year interval (1996 and 1999). The following soil properties were determined: total C and N, pH, exchangeable bases, cation exchange capacity, and microbial biomass N (MB‐N). Using the chronosequence approach, successional increases in soil pH and Mg were detected, pointing to these variables as indicators of soil restoration during the fallow period. Comparing the noncultivated páramo with the fallow plots, a significant decrease in MB‐N was found, suggesting that this is a sensitive agricultural disturbance indicator. The permanent plot analysis failed to detect successional trends in any of the study variables, probably as a result of a lack of sensitivity of the indicators used within the 3‐year interval. Nevertheless, a strong acidification was detected by the permanent plot method when fallow plots were cultivated. We conclude that the size of important soil components such as total soil organic matter or microbial biomass is not a sensitive soil restoration indicator in these heterogeneous mountain systems but that other integrative variables such as pH could be more sensitive to successional changes in key soil processes (e.g., nitrification or nutrient losses).

Tree girdling provides insight on the role of labile carbon in nitrogen partitioning between soil microorganisms and adult European beech

Nitrogen (N) cycling in terrestrial ecosystems is complex since it involves the closely interwoven processes of both N uptake by plants and microbial turnover of a variety of N metabolites. Major interactions between plants and microorganisms involve competition for the same N species, provision of plant nutrients by microorganisms and labile carbon (C) supply to microorganisms by plants via root exudation. Despite these close links between microbial N metabolism and plant N uptake, only a few studies have tried to overcome isolated views of plant N acquisition or microbial N fluxes. In this study we studied competitive patterns of N fluxes in a mountainous beech forest ecosystem between both plants and microorganisms by reducing rhizodeposition by tree girdling. Besides labile C and N pools in soil, we investigated total microbial biomass in soil, microbial N turnover (N mineralization, nitrification, denitrification, microbial immobilization) as well as microbial community structure using denitrifiers and mycorrhizal fungi as model organisms for important functional groups. Furthermore, plant uptake of organic and inorganic N and N metabolite profiles in roots were determined. Surprisingly plants preferred organic N over inorganic N and nitrate (NO 3 −) over ammonium (NH 4 +) in all treatments. Microbial N turnover and microbial biomass were in general negatively correlated to plant N acquisition and plant N pools, thus indicating strong competition for N between plants and free living microorganisms. The abundance of the dominant mycorrhizal fungi Cenococcum geophilum was negatively correlated to total soil microbial biomass but positively correlated to glutamine uptake by beech and amino acid concentration in fine roots indicating a significant role of this mycorrhizal fungus in the acquisition of organic N by beech. Tree girdling in general resulted in a decrease of dissolved organic carbon and total microbial biomass in soil while the abundance of C. geophilum remained unaffected, and N uptake by plants was increased. Overall, the girdling-induced decline of rhizodeposition altered the competitive balance of N partitioning in favour of beech and its most abundant mycorrhizal symbiont and at the expense of heterotrophic N turnover by free living microorganisms in soil. Similar to tree girdling, drought periods followed by intensive drying/rewetting events seemed to have favoured N acquisition by plants at the expense of free living microorganisms.

Vertical Distribution and Activity of Soil Microbial Population in a Sandy Desert Ecosystem

The vertical distribution of a microbial community on a temporal and spatial (vertical) scale under the Tamarix aphylla salt-resistant plant was studied. The uniqueness of this plant is in the creation of "islands of salinity" beneath the plant, due to salt excretion and leaf shedding at the plant base. Our working hypothesis was that the activity and biomass of the soil microbial population and the ratio between bacterial biomass and fungal biomass will decrease toward the driest season. Soil samples were collected beneath and between plants under the T. aphylla canopy down to the 0- to 50-cm depth at 10-cm intervals. The substrate-induced respiration method was used to estimate the total soil microbial biomass (MB), CO2 evolution, metabolic quotient (qCO2) index, and MB of both the fungal and bacterial communities. A significant difference was observed (p < 0.05) in MB and qCO2 index between the different soil layers in the control samples, whereas there was no significant difference in the soil samples in the vicinity of the plant (p > 0.05) between the depths, due to the plant rhizosphere effect.

Soil microbial community composition as affected by restoration practices in California grassland

Agricultural practices have strong impacts on soil microbes including both the indices related to biomass and activity as well as those related to community composition. In a grassland restoration project in California, where native perennial bunchgrasses were introduced into non-native annual grassland after a period of intensive tillage, weeding, and herbicide use to reduce the annual seed bank, microbial community composition was investigated. Three treatments were compared: annual grassland, bare soil fallow, and restored perennial grassland. Soil profiles down to 80 cm in depth were investigated in four separate layers (0–15, 15–30, 30–60, and 60–80 cm) using both phospholipid ester-linked fatty acid (PLFAs) and ergosterol as biomarkers in addition to microbial biomass C by fumigation extraction. PLFA fingerprinting showed much stronger differences between the tilled bare fallow treatment vs. grasslands, compared to fewer differences between restored perennial grassland and annual grassland. The presence or absence of plants over several years clearly distinguished microbial communities. Microbial communities in lower soil layers were little affected by management practices. Regardless of treatment, soil depth caused a strong gradient of changing habitat conditions, which was reflected in Canonical Correspondence Analysis of PLFAs. Fungal organisms were associated with the presence of plants and/or litter since the total amount and the relative proportion of fungal markers were reduced in the tilled bare fallow and in lower layers of the grassland treatments. Total PLFA and soil microbial biomass were highly correlated, and fungal PLFA biomarkers showed strong correlations to ergosterol content. In conclusion, microbial communities are resilient to the grassland restoration process, but do not reflect the change in plant species composition that occurred after planting native bunchgrasses.

Networks of power and influence: the role of mycorrhizal mycelium in controlling plant communities and agroecosystem functioning

Extraradical mycelia of mycorrhizal fungi are normally the “hidden half” of the symbiosis, but they are powerful underground influences upon biogeochemical cycling, the composition of plant communities, and agroecosystem functioning. Mycorrhizal mycelial networks are the most dynamic and functionally diverse components of the symbiosis, and recent estimates suggest they are empowered by receiving as much as 10% or more of the net photosynthate of their host plants. They often constitute 20%–30% of total soil microbial biomass yet are undetected by standard measures of biomass used by soil scientists and agromomists. Mycorrhizal mycelia provide extensive pathways for carbon and nutrient fluxes through soil, often exceeding tens of metres per gram of soil. We consider the amounts of photosynthate “power” allocated to these mycelial networks and how this is used in fungal respiration, biomass, and growth and in influencing soil, plant, and ecosystem processes. The costs and functional “benefits” to plants linking to these networks are fungal specific and, because of variations in physiology and host specificity, are not shared equally; some plants even depend exclusively on these networks for carbon. We briefly assess the potential contribution of extraradical mycorrhizal mycelium to sustainable agriculture and maintenance of biodiversity and highlight technologies that promise new vistas and improved fine-scale resolution of the dynamic spatial and temporal functioning of these networks in soil.Key words: arbuscular mycorrhiza, ectomycorrhiza, extraradical mycelium, hyphal networks.

Microbial immobilisation of added nitrogen and phosphorus in constructed wetland buffer

The dynamics of soil microbial C, N and P in response to NO 3 − and PO 4 3− addition were investigated and quantified in a constructed wetland buffer zone divided into three experimental areas. The method used was fumigation–extraction method (FEM), which is commonly used to estimate total soil microbial biomass. N immobilisation in the microbial biomass during the first year after N and P additions ranged from 30.6 kg N ha −1 in the upper experimental area (0.2 ha), which received a high N and P addition, to 2.1 kg N ha −1 in the lower experimental area (0.3 ha) which got lower N and P additions, and 0.4 kg N ha −1 in the control area (0.3 ha). P immobilisation ranged from 13.8 kg P ha −1 in the upper experimental area to 2.8 kg P ha −1 in the lower experimental area and −2.3 kg P ha −1 in the control area. The immobilised proportions of N and P in the microbial biomass in the experimental areas were approximately 15% of the added N (90 kg N ha −1) and approximately 25% of the added P (30 kg P ha −1) during the first year after N and P addition in 1999. Microbial biomass is thus an important factor contributing to nutrient immobilisation in a constructed wetland buffer in transient conditions.

The potential of soil protein-based methods to indicate metal contamination

Soil metal contamination is an international problem and there is a recognised need for methods to detect the effects of such pollution. To this end, we investigated the use of soil protein extraction, quantification and expression as indicators of cadmium contamination of soil. A variety of methods to extract total soil protein were examined and the effects of cadmium and lime treatment on total soil protein, protein expression and microbial biomass were determined. A simple snap freeze protein extraction technique (using liquid nitrogen) was found to extract the most protein from soil samples compared to a bead beating method used commonly for DNA extraction from soil. Using the snap freeze method, cadmium (Cd) contamination was observed to significantly decrease total soil protein levels compared to an uncontaminated control soil (16 μg g −1 cf. 24 μg g −1) whilst liming significantly increased total soil protein in the control and contaminated treatments (35 and 22 μg g −1). These results were related to similar, but less pronounced, changes in microbial biomass in Cd contaminated and limed control soils. Limed Cd contaminated soils had similar amounts of total soil protein to unlimed control soils. The protein profiles (SDS-PAGE) of extracted soil protein from control and cadmium contaminated soils concentrated by both acetone precipitation and heating showed no obvious changes in protein expression between treatments. However, large amounts of presumably low molecular weight proteins (<21 kDa) were observed at the bottom of the protein profiles from the contaminated soils.

Accumulation of Microbial Biomass within Particulate Organic Matter of Aging Golf Greens

Microbial biomass (MB) is a key variable controlling soil organic matter dynamics in soil. Currently, there is little information on the amount and significance of MB in highly managed golf greens. Our objective was to determine the amount and distribution of MB within soil structural components of golf greens and its relationship to the location of organic substrates. During 1996, 47 greens were sampled from 12 golf courses within Nebraska (USA). Microbial biomass, determined as extractable lipid phosphate on field‐moist soils, increased linearly with age of green . In 1997 and 1999, selected greens were resampled and separated into mineral fraction (MF) and particulate organic matter (POM) fraction using a sodium metatungstate (NMT; ). Then, POM was separated into light (L‐POM) and heavy (H‐POM) fractions using NMT . Amount of MB of whole soil and POM was linearly related to green age (r2 = 0.76 and 0.68, respectively). Amount of MB in MF was not related to green age. The portion of total soil MB associated with POM increased significantly from 25.6% for an 8‐yr‐old green to 77.8% for a 28‐yr‐old green. Carbon in fulvic acid and humic acid increased with green age from 0.5 to 1.7 and 0.6 to 2.6 g kg−1 soil, respectively. As humus is a relatively stable form of soil organic matter, we hypothesized that humus accumulation within POM renders both POM and associated MB more resistant to degradation; thus, they accumulate.

Fungal-to-bacterial ratios in soils investigated for enhanced C sequestration

Fungi and bacteria govern most of the transformations and ensuing long-term storage of organic C in soils. We assessed the relative contributions of these two groups of organisms to the microbial biomass and activity of soils from five different ecosystems with treatments hypothesized to enhance soil C sequestration: (1) desert (an elevation gradient allowed comparison of soil developed in a cooler, moister climate with soil developed in a warmer, drier climate), (2) restored tallgrass prairie (land reverted to native prairie in 1979 and neighboring land farmed to row crops for ∼100 year), (3,4) two forest types (Douglas fir and loblolly pine, unfertilized control and N-fertilized plots), and (5) agricultural land (conventional- and no-till management systems). The selective inhibition technique, using captan (fungicide) and oxytetracycline hydrochloride (bactericide), was used to determine the activities (respiration) of fungi and bacteria in each of these soils and substrate-induced respiration was used to measure total active soil microbial biomass C. Phospholipid fatty acid analysis was used to determine the composition of the soil microbial biomass and determine if the activities and structure of the microbial communities were related. Differences in fungal-to-bacterial (F:B) activities between treatments at a site were greatest at the prairie sites. The restored prairie had the highest F:B (13.5) and high total C (49.9 g C kg −1 soil); neighboring soil farmed to corn had an F:B of 0.85 and total C of 36.0 g C kg −1 soil. Within the pairs of study soils, those that were tilled had lower fungal activities and stored C than those that were managed to native or no-till systems. In all pairs of soils, soils that had higher absolute fungal activities also had more total soil C and when two extreme cases were removed fungal activity was correlated with total soil C ( R 2=0.85). Thus, in this small set of diverse soils, increased fungal activities, more than F:B ratios, were associated with increased soil C. Practices that involved invasive land management decreased fungal activity and stored soil C compared to similar soils that were less intrusively managed.

Changes in microbial biomass C, N, and P and enzyme activities in soil incubated with the earthworms Metaphire guillelmi or Eisenia fetida

Earthworms are ubiquitous in soil and ingest large amounts of soil, organic matter and leaf litter. To assess changes in organic matter fractions after passage through the earthworm gut, we measured microbial biomass C, N and P and the fungal-to-bacterial ratios in worm-worked soil (WWS), obtained by incubating soil for 24 h with large numbers of the anecic earthworm Metaphire guillelmi (1:5 ratio of fresh weight worms: dry weight soil). Microbial biomass C, N and P were estimated by the fumigation–extraction methods, and fungal-to-bacterial ratios by selective inhibition using substrate induced respiration. Enzyme activities in the gut of M. guillelmi were also compared with an epigeic earthworm species Eisenia fetida. Activities of cellulase, protease, chitinase, acid and alkaline phosphatases, in gut, casts and uningested soil were measured. In WWS, microbial biomass decreased (130 μg C g −1 soil), and there was a concomitant increase of available nutrients (27 and 10 μg g −1 soil for ninhydrin-reactive N and inorganic P, respectively). There was no difference between the glucose-sensitive microbial biomass (MB) and the control but the respiratory quotient was greater (2.85±0.17 and 2.95±0.07 μg CO 2-C g −1 soil h −1 for WWS and control, respectively). The fungal-to-bacterial ratio was slightly higher in WWS than in uningested soil (1.61 vs. 1.35). Cellulase activity was greater in the gut of the epigeic earthworm than in that of the anecic one (152.8±18.7 vs. 18.9±1.3 μg glucose g −1 worm fw h −1); conversely, protease and phosphatase activities were significantly higher in gut of the anecic specie as opposed to the epigeic species. The activity of cellulolytic enzymes was slightly higher in casts than in soil; while activities of protease, acid (pH 6.5) and alkaline (pH 9.0) phosphatases were lower in earthworm casts than in the uningested soils (protease, acid and alkaline phosphatase activity were 29.5±1.7 mg tyrosine g −1 worm fw h −1, 570±2.9, 748±7.3 μg p-nitrophenol g −1 soil h −1 in soil and 17.8±2.0 mg tyrosine g −1 worm fw h −1, 327±26.7, 549±19.7 μg p-nitrophenol g −1 soil h −1 in casts, respectively). We conclude that micro-organisms are used by earthworms as a secondary food resource, and that passage through earthworm gut decreases the total soil MB and increase the active components of MB.

Estimating the active and total soil microbial biomass by kinetic respiration analysis

A model describing the respiration curves of glucose-amended soils was applied to the characterization of microbial biomass. Both lag and exponential growth phases were simulated. Fitted parameters were used for the determination of the growing and sustaining fractions of the microbial biomass as well as its specific growth rate (μmax). These microbial biomass characteristics were measured periodically in a loamy silt and a sandy loam soil incubated under laboratory conditions. Less than 1% of the biomass oxidizing glucose was able to grow immediately due to the chronic starvation of the microbial populations in situ. Glucose applied at a rate of 0.5 mg C g–1 increased that portion to 4–10%. Both soils showed similar dynamics with a peak in the growing biomass at day 3 after initial glucose amendment, while the total (sustaining plus growing) biomass was maximum at day 7. The microorganisms in the loamy silt soil showed a larger growth potential, with the growing biomass increasing 16-fold after glucose application compared to a sevenfold increase in the sandy loam soil. The results gained by the applied kinetic approach were compared to those obtained by the substrate-induced respiration (SIR) technique for soil microbial biomass estimation, and with results from a simple exponential model used to describe the growth response. SIR proved to be only suitable for soils that contain a sustaining microbial biomass and no growing microbial biomass. The exponential model was unsuitable for situations where a growing microbial biomass was associated with a sustaining biomass. The kinetic model tested in this study (Panikov and Sizova 1996) proved to describe all situations in a meaningful, quantitative and statistically reliable way.

Modifications of Soil Microbial Activity and Triticonazole Biodegradation by Pesticide Formulation Additives

AbstractPesticide seed coating represents an example where formulating agents can modify pesticide fate because they are jointly applied in the coating. The influence of additives used to formniate commercial pesticides on degradation in soil of triticonazole [5‐((4‐chlorophenyl) methylene)‐2,2‐dimethyl‐1‐(1H‐1,2,4‐1riazole‐1‐ylmethyl)cyclopentanol] was considered. The studied additives were three different surfactants, one antifoam, one polymer adhesive, and one solvent. Two doses of some formulating additives commonly included in the commercial formulations were used. The individual effects on overall microbial activity and on triticonazole degradation were monitored during laboratory incubations. Total microbial activity was monitored by respirometric measurements and total soil microbial biomass was determined by fumigation‐extraction. The 14C‐triticonazole behavior was characterized by extraction and monitoring of mineralization kinetics. Only the solvent (propylene glycol) and the nonionic micellar surfactant significantly increased the quantities of organic C mineralized. Triticonazole degradation was modified only after the incorporation of formulation additives at high doses (equivalent to 1 g C additive kg−1 soil). The triticonazole degradation increased with the level of available organic C in the case of the solvent, arguing for degradation by a cometabolic pathway. Degradation decreased with the incorporation of the antifoam and the nonionic and anionic (sodiumalkylnaphthalensulfate) surfactants. High concentrations of formulation additives can modify triticonazole behavior in soils, but at the low levels (equivalent to 0.5 g C additive kg−1 soil), degradation data of triticonazole as an active ingredient alone can be used to describe formulated triticonazole behavior.