Active Fungal Biomass Research Articles

Litter Inhibitory Effects on Soil Microbial Biomass, Activity, and Catabolic Diversity in Two Paired Stands of Robinia pseudoacacia L. and Pinus nigra Arn.

Research Highlights: Plant cover drives the activity of the microbial decomposer community and affects carbon (C) sequestration in the soil. Despite the relationship between microbial activity and C sequestration in the soil, potential inhibition of soil microbial activity by plant cover has received little attention to date. Background and Objectives: Differences in soil microbial activity between two paired stands on soil at a very early stage of formation and a common story until afforestation, can be traced back to the plant cover. We hypothesized that in a black locust (Robinia pseudoacacia L.) stand the high-quality leaf litter of the tree, and that of the blackberry (Rubus fruticosus L.) understory had an inhibitory effect on soil microbial community resulting in lower mineralization of soil organic matter compared to the paired black pine (Pinus nigra Arn.) stand. Materials and Methods: We estimated potential mineralization rates (MR), microbial (MB), and active fungal biomass (AFB) of newly-shed litter, forest floor, and mineral soil. We tested the effects of litters’ water extracts on soil MR, MB, AFB and its catabolic response profile (CRP). Results: Newly-shed litter of black locust had higher MR than that of blackberry and black pine; MR, MB, and AFB were higher in forest floor and in mineral soil under black pine than under black locust. Water extracts of black locust and blackberry litter had a negative effect on the amount, activity of microorganisms, and CRP. Conclusions: The results demonstrate the potential for black locust and blackberry litter to have a marked inhibitory effect on decomposer microorganisms that, in turn, reduce organic matter mineralization with possible consequences at the ecosystem level, by increasing C sequestration in mineral soil.

Soil aggregation and soil organic matter in conventionally and organically farmed Austrian Chernozems / Bodenaggregation und organische Substanz in konventionell und biologisch bewirtschafteten österreichischen Tschernosemböden

Summary In order to study the soil aggregate distributions and soil organic matter (SOM), we sampled top- and subsoils in four intensively farmed croplands (two organic (Org-OB and Org-LA), and two conventional (Con-OB and Con-LA)) on Haplic Chernozems located in Marchfeld in the east of Vienna (Austria). Soil structure and SOM quantity, quality and distribution between free and occluded particulate organic matter and aggregate size fractions (<20 µm, 20-250 µm, 250-5000 µm) were studied by following a density fractionation procedure with low-energy ultrasound treatment. The relation of the soil physicochemical (e.g., particle size distribution, pH, organic carbon, total nitrogen) and biological properties (e.g., fungal biomass, active fungi) with stable soil aggregate size fractions and SOM was studied. The mean weight diameter (MWD) showed no significant difference between all studied sites and was between 3.8 mm and 10.0 mm in topsoils and between 6.7 mm and 11.9 mm in subsoils. In topsoils, the contents of calcium-acetate-lactate (CAL)-extractable P, active fungal biomass, dithionite-extractable Fe and sand were significantly positively correlated with the amount of the macroaggregates and with the MWD. We observed that most soil organic carbon, depending on soil texture, was stored in the microaggregate size classes <20 µm and 20-250 µm.

Impact of organic and conventional peach and apple production practices on soil microbial populations and plant nutrients

This work expects to supplement information on soil biological processes, particularly for fruit production systems in a semiarid region, by comparing soil microbial diversity and density, and nutrient concentration in soil as well as plant tissues in organic and conventional production systems for different varieties of apple and peach. Organic and conventional practices were compared by taking soil samples and analyzing soil microbial populations and plant and soil nutrients using several laboratory procedures. Significantly higher active and total fungal biomass, flagellate, and Actinobacteria populations and plant nutrients, P and Cu in plant tissues and OM, P, and S in soils were observed in organic compared to the conventional practices, irrespective of crops and varieties. In peach, protozoa and nematode populations were significantly higher in organic than in conventional soils, but not in apple. Organic fruit production practices harbored both greater microbial activity and higher concentrations of some plant and soil nutrients and are anticipated to promote better soil health and productivity than conventional practices. However, introduction and/or enhancement of certain microbes, especially mycorrhizae in both production systems, expected to increase soil health and productivity, are suggested. Use of ranges of microbial population to estimate soil health and management strategy is proposed.

Effect of arsenic contamination on bacterial and fungal biomass and enzyme activities in tropical arsenic-contaminated soils

The importance of assessing the impacts of soil arsenic (As) contamination on microbial properties lay on the fact that microbes are instrumental in nutrient cycling and are therefore indicators of soil quality. In this study, soil chemical extraction methods were used to extract labile and freely exchangeable As (water-soluble As and sodium bicarbonate-extractable As), amorphous/crystalline Fe and Mn oxide-bound As (acid ammonium oxalate-extractable As and hydroxylamine hydrochloride-extractable As), and their impacts on microbial biomass (microbial biomass C, total bacterial and fungal biomass, active bacterial and fungal biomass), enzyme activities representing four major soil biogeochemical cycles, i.e., C (β-glucosidase activity), N (urease activity), P (acid phosphomonoesterase activity), S (acryl-sulfatase activity), and microbial activity (fluorescein diacetate hydrolysis and dehydrogenase activity) were investigated in As-contaminated soils of Ambagarh Chauki block, Chhattisgarh, Central India. The results revealed that the majority of the As in soils resided in the Fe/Mn oxide-bound fraction. The microbial biomass C, total and active fungal biomass, and enzyme activities were significantly inhibited by all the forms of As. However, water-soluble As, even though occupying only a small portion of the total As (0.9–2.9 %), exerted the greatest impact. Interestingly, total and active bacterial biomass was not significantly affected by As toxicity, suggesting their resistance to As. Urease activity was not affected by As pollution.

High Polyacrylamide Application Rates Do Not Affect Eubacterial Structural Diversity

Anionic polyacrylamide (PAM) is a linear, water-soluble anionic polymer that is widely used for erosion control and water quality protection. There has been an issue whether this formulation could possibly have negative effects on soil microbial diversity by altering microbial binding to soil particles or to one another and thus restricting their mobility and role in carbon and nutrient cycling. We conducted an 8-year study annually applying ultra-high rates of PAM to soil and then monitored impacts on soil bacterial diversity. In July and August, we measured active soil bacterial and fungal biomass and microbial diversity in soils receiving 0 (control), 2,691, and 5,382 kg active ingredient PAM ha−1. Active microbial biomass in soil was 19–33 % greater in the untreated control than soil treated with 2,691 or 5,382 kg of active ingredient PAM ha−1. Active bacterial biomass in soil was 21–31 % greater in the control treatment than in soil treated with 2,691 or 5,382 kg of active ingredient PAM ha−1 in August, but not July. Active fungal biomass in soils was 38–50 % greater in the control treatment than soil treated with 2,691 or 5,382 kg of active ingredient PAM ha−1 in July, but not August. Molecular methods were used to access the bacterial diversity, richness, and evenness in an agricultural soil that received 0 (control), 2,691, and 5,382 kg of active ingredient PAM ha−1. We found that although soil receiving these massive PAM application rates and prolonged exposure may reduce active bacterial and fungal biomass, PAM application did not substantially or consistently affect bacterial structural diversity, richness, or evenness in this agricultural soil.

Fungi, bacteria and soil pH: the oxalate–carbonate pathway as a model for metabolic interaction

The oxalate-carbonate pathway involves the oxidation of calcium oxalate to low-magnesium calcite and represents a potential long-term terrestrial sink for atmospheric CO(2). In this pathway, bacterial oxalate degradation is associated with a strong local alkalinization and subsequent carbonate precipitation. In order to test whether this process occurs in soil, the role of bacteria, fungi and calcium oxalate amendments was studied using microcosms. In a model system with sterile soil amended with laboratory cultures of oxalotrophic bacteria and fungi, the addition of calcium oxalate induced a distinct pH shift and led to the final precipitation of calcite. However, the simultaneous presence of bacteria and fungi was essential to drive this pH shift. Growth of both oxalotrophic bacteria and fungi was confirmed by qPCR on the frc (oxalotrophic bacteria) and 16S rRNA genes, and the quantification of ergosterol (active fungal biomass) respectively. The experiment was replicated in microcosms with non-sterilized soil. In this case, the bacterial and fungal contribution to oxalate degradation was evaluated by treatments with specific biocides (cycloheximide and bronopol). Results showed that the autochthonous microflora oxidized calcium oxalate and induced a significant soil alkalinization. Moreover, data confirmed the results from the model soil showing that bacteria are essentially responsible for the pH shift, but require the presence of fungi for their oxalotrophic activity. The combined results highlight that the interaction between bacteria and fungi is essential to drive metabolic processes in complex environments such as soil.

Humus macro-morphology and soil microbial community changes along a 130-yr-old Fagus sylvatica chronosequence

We aimed to characterize humus macro-morphology and the associated soil microbial community within the unmodified litter (OL), the fragmented and humified layers (FH) and the organo-mineral (A) layer along a beech (Fagus sylvatica L.) forest chronosequence with four stand age-classes (15-, 65-, 95-, 130-yr-old) in Normandy, France. Humus macro-morphology was described with 36 quantitative and semi-quantitative variables. We measured microbial biomass N (Nmic), microbial N quotient (Nmic-to-Nt), fungal ergosterol, bacterial and fungal DNA using 16S and 18S rDNA real-time qPCR and evaluated the potential metabolic profile of heterotrophic bacteria within each soil layer and stand age-class. The log-transform ergosterol/fungal DNA ratio (EFR index) was used as an indicator related to active fungal biomass and the fungal/bacterial (F/B) ratio was calculated from qPCR results. There was a shift from mull (mainly dysmull) to moder humus forms along the chronosequence. While the Nmic did not change significantly, the Nmic-to-Nt decreased along the chronosequence in the OL layer. Ergosterol content increased in FH and A layers and the F/B ratio increased in the FH layer with increasing beech forest age. The EFR index was significantly higher in the OL and A layers of the oldest stands, whereas the highest EFR index in the FH layer occurred in the 15-yr-old stands. The functional diversity of heterotrophic bacteria was greater within OL and FH layers of 130-yr-old stands, but highest in the A layer of 15-yr-old stands while the Average Well Color Development remained stable for all soil layers. We found significant correlations between macro-morphology and microbial variables, especially between FH-based morphology and fungal biomass. Our main results are that beech forest maturation is accompanied by (1) an increase in fungal biomass in the FH layers and, (2) an increase in heterotrophic bacteria functional diversity in the organic layers. We have identified key macro-morphology variables that are good predictors of the structural and functional profile of the soil microbial community during beech forest development.

Suitability of two types of organic wastes for the growth of sclerophyllous shrubs on limestone debris: A mesocosm trial

This study investigated whether overlaying organic wastes directly on limestone debris allowed the growth of sclerophyllous shrubs; the aim was to explore the feasibility of rehabilitation of sites destroyed by quarrying activity. In an open air mesocosm experiment two types of organic material were compared: compost from municipal wastes (C) and a mixture of compost and poultry manure added with wheat husk (C-PW). Mesocosms were pots (1 m diameter, 60 cm height) containing limestone debris covered by the organic material. Seven mesocosms with C and seven mesocosms with C-PW were planted with sclerophyllous shrubs ( Laurus nobilis L., Phillyrea angustifolia L. and Quercus ilex L.). The substrates were characterised in terms of chemical and physical parameters, microbial activity and biomass, and total and active fungal biomass. Shrub photosynthetic performance and growth were evaluated. Over the whole experimental period, organic matter mineralization was higher in C-PW. Microbial biomass and respiration were higher in C-PW than in C but after one year no statistically significant difference between the two substrates occurred. Fungal mycelium was a minor fraction of the microbial community in both types of substrates and decreased dramatically after setting up the mesocosms. The metabolic quotient was higher in C suggesting more stressful conditions as compared to C-PW. Both substrates allowed shrub growth; however photosynthetic rates and the increase of plant size were higher on C-PW than on C. The results demonstrated that, as compared to only compost, the mixture of compost and poultry manure added with wheat husk is a substrate more suitable to both microbial processes and plant growth. Therefore a plan to revegetate quarries based on the use of organic wastes as a substrate for sclerophyllous shrubs could be feasible and, what is more, helpful to mitigate the environmental impact of organic wastes disposal.

Ergosterol and Water Changes in Tricholoma matsutake Soil Colony during the Mushroom Fruiting Season

The purpose of this study is to understand spatio-temporal changes of active fungal biomass and water in Tricholoma matsutake soil colonies during the mushroom fruiting season. The active fungal biomass was estimated by analyzing ergosterol content at four different points within four replicated locations in a single circular T. matsutake colony at Ssanggok valleyin the Sogri Mt. National Park in Korea during 2003 to 2005. The four points were the ahead of the colony, the front edge of the colony and 20 cm and 40 cm back from the front edge of the colony. Ergosterol content was 0.0 to 0.7 μg per gram dried soil at the ahead, 2.5 to 4.8 μg at the front edge, 0.5 to 1.8 μg at the 20 cm back and 0.3 to 0.8 pg at the 40 cm back. The ergosterol content was very high at the front edge where the 7. matsutake hyphae were most active. However, ergosterol content did not significantly change during the fruiting season, September to October. Soil water contents were lower at the front edge and 20 cm back from the front edge of the colony than at the ahead and 40 cm back during the fruiting season. Soil water content ranged from 12 to 19% at the ahead, 10 to 11% at the edge, 9 to 11% at the 20 cm back and 11 to 15% at the 40 cm back. Our results suggest that the active front edge of the T. matsutake soil colony could be managed in terms of water relation and T. matsutake eetomyeorrhizal root development.

Soil carbon sequestration and changes in fungal and bacterial biomass following incorporation of forest residues

Sequestering carbon (C) in forest soils can benefit site fertility and help offset greenhouse gas emissions. However, identifying soil conditions and forest management practices which best promote C accumulation remains a challenging task. We tested whether soil incorporation of masticated woody residues alters short-term C storage at forested sites in western and southeastern USA. Our hypothesis was that woody residues would preferentially stimulate soil fungal biomass, resulting in improved C use efficiency and greater soil C storage. Harvest slash at loblolly pine sites in South Carolina was masticated (chipped) and either (1) retained on the soil surface, (2) tilled to a soil depth of 40 cm, or (3) tilled using at least twice the mass of organics. At comparative sites in California, live woody fuels in ponderosa pine stands were (1) masticated and surface applied, (2) masticated and tilled, or (3) left untreated. Sites with clayey and sandy soils were compared in each region, with residue additions ranging from 20 to 207 Mg ha −1. Total and active fungal biomass were not strongly affected by residue incorporation despite the high input of organics. Limited response was also found for total and active bacterial biomass. As a consequence, fungal:bacterial (F:B) biomass ratios were similar among treatments at each site. Total soil C was elevated at one California site following residue incorporation, yet was significantly lower compared to surface-applied residues at both loblolly pine sites, presumably due to the oxidative effects of tilling on soil organic matter. The findings demonstrated an inconsequential effect of residue incorporation on fungal and bacterial biomass and suggest a limited potential of such practices to enhance long-term soil C storage in these forests.

Impact of fungal and bacterial biocides on microbial induced water repellency in arable soil

Fungi are thought to affect soil slaking resistance and bypass flow by inducing low levels of water repellency on soil pore surfaces. This was investigated in a laboratory study on arable field soil by selectively inhibiting fungi and/or bacteria with biocides and adding ground barley shoot substrate (2 mg g − 1 soil) to half of the samples. The fungicide Captan and the bactericide Bronopol were used alone and in combination at a concentration of 1 mg g − 1 , resulting in different levels of microbial respiration and active fungal biomass (ergosterol) after 8 days of incubation. Repellency was measured on the surface and on an internal fracture of the soil cores. Wet–dry cycles were applied to some samples to investigate the persistence of water repellency. Substrate addition caused the greatest increase to water repellency after incubation, suggesting that microbial activity affects repellency. The drying surface of the soil had greater repellency than the internal fracture surface. No clear trends could be found between the level of fungal biomass and repellency; this was most evident in soils treated with bactericide and substrate. Despite having the highest fungal biomass ( P < 0.01), more than 300% greater than the no-biocide control, these soils did not develop greater water repellency ( P > 0.05). Wetting and drying cycles caused significant changes to repellency and structure. Ethanol sorptivity indicates pore structure and its value changed least in the soil that had the greatest fungal biomass (i.e. bactericide treated with substrate), indicating that fungal populations protected soil structures from slaking stresses. Given the poor relationship found between fungal biomass and water repellency, these results suggest that other microbial or physico-chemical processes were involved. Our findings do not support previous research that reported a close link between water repellency and fungal biomass. However, given the strong evidence of fungal-produced hydrophobins and the close links between soil structural stability, repellency and fungal biomass found in field studies, the effectiveness of the biocides used in this study or the influence of different fungal species could have influenced the results and thus requires greater investigation.

The influence of high application rates of polyacrylamide on microbial metabolic potential in an agricultural soil

Water soluble anionic polyacrylamide (PAM) is a highly effective erosion preventing and infiltration enhancing polymer, when applied at rates of 1–10 g m −3 in furrow irrigation water. PAM greatly reduces sediment, nutrients, pesticides and coliform bacteria in irrigation runoff. There has been some concern about the potential for PAM accumulation to affect microbial ecology. We ran a long-term study applying massive quantities of PAM to soil and monitored its impact on soil microbial potential. In June, July and August, we measured active soil bacterial and fungal biomass and microbial diversity in soils receiving 0 (control), 2691 and 5382 kg active ingredient (ai) PAM ha −1. Active bacterial biomass in soil was 20–30% greater in the control treatment than in soil treated with 2691 or 5382 kg ai PAM ha −1 in June and August, but not July. Active fungal biomass in soils was 30–50% greater in the control treatment than soil treated with 2691 or 5382 kg ai PAM ha −1 in June and July, but not August. Active microbial biomass in soil was 27–48% greater in the untreated control than soil treated with 2691 or 5382 kg ai PAM ha −1 except in June. Whole soil fatty acid profiles showed no discernible change in the soil microbial community due to either of the PAM treatments at any sampling time. Analysis of nutritional characteristics using Biolog GN plates, however, yielded an apparent separation of the non-amended control soils from those plots receiving the high PAM application rate in June, but not in July or August. In contrast, comparisons of the three sampling times by both the fatty acid and Biolog analyses indicated that the microbial metabolic potential present in June were different from those sampled in July and August. Although PAM application to soil or irrigation water in some cases may reduce active bacterial and fungal biomass it does not seem to appreciably affect the soil microbial metabolic potential.

Soil CO2 efflux and fungal and bacterial biomass in a plantation and a secondary forest in wet tropics in Puerto Rico

We examined the effects of root and litter exclusion on the rate of soil CO2 efflux and microbial biomass using trenching and tent separation techniques in a secondary forest (SF) and a pine (Pinus caribaea Morelet) plantation in the Luquillo Experimental Forest in Puerto Rico. Soil surface CO2 efflux was measured using the alkali trap method at 12 randomly-distributed locations in each treatment (control, root exclusion, litter exclusion, and both root and litter exclusion) in the plantation and the SF, respectively. We measured soil CO2 efflux every two months and collected soil samples at each sampling location in different seasons to determine microbial biomass from August 1996 to July 1997. We found that soil CO2 efflux was significantly reduced in the litter and root exclusion plots (7-year litter and/or root exclusion) in both the secondary forest and the pine plantation compared with the control. The reduction of soil CO2 efflux was 35.6% greater in the root exclusion plots than in the litter exclusion plots in the plantation, whereas a reversed pattern was found in the secondary forest. Microbial biomass was also reduced during the litter and root exclusion period. In the root exclusion plots, total fungal biomass averaged 31.4% and 65.2% lower than the control plots in the plantation and the secondary forest, respectively, while the total bacterial biomass was 24% and 8.3% lower than the control plots in the plantation and the secondary forest, respectively. In the litter exclusion treatment, total fungal biomass averaged 69.2% and 69.7% lower than the control plots in the plantation and the secondary forest, respectively, while the total bacterial biomass was 48% and 50.1% lower than the control plots in the plantation and the secondary forest, respectively. Soil CO2 efflux was positively correlated with both fungal and bacterial biomass in both the plantation the secondary forest. The correlation between soil CO2 efflux and active fungal biomass was significantly higher in the plantation than in the secondary forest. However, the correlation between the soil CO2 efflux and both the active and total bacterial biomass was significantly higher in the secondary forest than in the plantation in the day season. In addition, we found soil CO2 efflux was highly related to the strong interactions among root, fungal and bacterial biomass by multiple regression analysis (R2 > 0.61, P < 0.05). Our results suggest that carbon input from aboveground litterfall and roots (root litter and exudates) is critical to the soil microbial community and ecosystem carbon cycling in the wet tropical forests.

Chronic nitrogen enrichment affects the structure and function of the soil microbial community in temperate hardwood and pine forests

We examined how chronic nitrogen (N) enrichment of pine and hardwood forest stands has affected the relative abundance, functional capacity, and activity of soil bacteria and fungi. During Fall 2002 we collected one soil core (5.6 cm diameter; organic horizon plus 10 cm of mineral soil) from each of four 5 m×5 m subplots within the control, low N (5 g N m −2 per year), and high N (15 g N m −2 per year) plots in both the hardwood and pine stands at the Chronic Nitrogen Amendment Study at Harvard Forest. The samples were analyzed for total and active bacterial and fungal biomass, microbial catabolic response profiles, the activities of cellulolytic and ligninolytic enzymes, and total, labile and microbially derived organic carbon (C). Live, fine roots were also collected from the control and low N pine plots and analyzed for ectomycorrhizal fungal community composition and diversity. Active fungal biomass was 27–61% and 42–69% lower in the fertilized compared to control plots in the hardwood and pine stands, respectively. Active bacterial biomass was not greatly affected by N additions, resulting in significantly lower fungal:bacterial biomass ratios in the N-treated plots. This shift in microbial community composition was accompanied by a significant reduction in the activity of phenol oxidase, a lignin-degrading enzyme produced by white-rot fungi. In the pine stand, ectomycorrhizal fungal community diversity was lower in the low N-treated plot than in the control plot. Differences in ectomycorrhizal community structure were also detected between control and fertilized pine plots, including a reduction in those species with the highest relative frequencies in the control community. Finally, N enrichment altered the pattern of microbial substrate use, with the relative response to the addition of carboxylic acids and carbohydrates being significantly lower in the N-treated plots, even after the data were normalized to account for differences in microbial biomass. These patterns are consistent with lower decomposition rates and altered N cycling observed previously at this site.

Chronic nitrogen enrichment affects the structure and function of the soil microbial community in temperate hardwood and pine forests

We examined how chronic nitrogen (N) enrichment of pine and hardwood forest stands has affected the relative abundance, functional capacity, and activity of soil bacteria and fungi. During Fall 2002 we collected one soil core (5.6 cm diameter; organic horizon plus 10 cm of mineral soil) from each of four 5m×5 m subplots within the control, low N (5 g N m−2 per year), and high N (15 g N m−2 per year) plots in both the hardwood and pine stands at the Chronic Nitrogen Amendment Study at Harvard Forest. The samples were analyzed for total and active bacterial and fungal biomass, microbial catabolic response profiles, the activities of cellulolytic and ligninolytic enzymes, and total, labile and microbially derived organic carbon (C). Live, fine roots were also collected from the control and low N pine plots and analyzed for ectomycorrhizal fungal community composition and diversity. Active fungal biomass was 27–61% and 42–69% lower in the fertilized compared to control plots in the hardwood and pine stands, respectively. Active bacterial biomass was not greatly affected by N additions, resulting in significantly lower fungal:bacterial biomass ratios in the N-treated plots. This shift in microbial community composition was accompanied by a significant reduction in the activity of phenol oxidase, a lignin-degrading enzyme produced by white-rot fungi. In the pine stand, ectomycorrhizal fungal community diversity was lower in the low N-treated plot than in the control plot. Differences in ectomycorrhizal community structure were also detected between control and fertilized pine plots, including a reduction in those species with the highest relative frequencies in the control community. Finally, N enrichment altered the pattern of microbial substrate use, with the relative response to the addition of carboxylic acids and carbohydrates being significantly lower in the N-treated plots, even after the data were normalized to account for differences in microbial biomass. These patterns are consistent with lower decomposition rates and altered N cycling observed previously at this site.

Influence of Irrigated Agriculture on Soil Carbon and Microbial Community Structure

Increasing the amount of carbon (C) in soils is one method to reduce the concentration of carbon dixoide (CO2) in the atmosphere. We measured organic C stored in southern Idaho soils having long-term cropping histories that supported native sagebrush vegetation (NSB), irrigated moldboard plowed crops (IMP), irrigated conservation-chisel-tilled crops (ICT), and irrigated pasture systems (IP). The CO2 emitted as a result of fertilizer production, farm operations, and CO2 lost via dissolved carbonate in irrigation water, over a 30-year period, was estimated and used to calculate net C fixation. Organic C in ecosystems decreased in the order IP>ICT>IMP> NSB. In February 2001, active fungal, bacterial, and microbial biomass was greater in IP soils than all other soils. Active fungal, bacterial, and microbial biomass was least in ICT soils at the 15–30-cm depth than all other soils. In August 2001, active bacterial biomass was greater in IMP soils than IP, ICT, and NSB soils. Active fungal biomass was greater in IP soils than all other soils. Whole-soil fatty acid profiles differed among management regimes and sampling dates and, to a lesser extent, with soil depth. FAME profiles from the NSB soils were distinct from the agricultural treatments and contained greater amounts of total fatty acids than the other treatments. The IMP and ICT soils yielded fatty acid profiles that were similar to each other, although those at the 15–30-cm depth were distinct from all other treatment-depth combinations. The IP FAME profiles suggest that arbuscular mycorrhizal fungi are more common in these soils than soils from the other treatments. Differences in carbon substrate utilization patterns (BIOLOG) among treatments were more variable and less pronounced that FAME results. In general, irrigated arid soils can both increase C storage while increasing microbial biomass and changing microbial diversity.

Microbial and Microfaunal Community Dynamics in Artificial and Lumbricus terrestris (L.) Burrows

Macropore formation and litter incorporation are two results of earthworm [Lumbricus terrestris (L.)] activities that can influence trophic dynamics inside burrows. Thus, mesocosms were constructed to examine changes in microbial biomass and microfaunal communities inside artificial compared with earthworm burrows. Four treatments were established: (i) no worms (CTRL); (ii) unlined, artificial burrows (ARTF); (iii) corn (Zea mays L.) leaf litter‐lined, artificial burrows (LEAF); and (iv) Lumbricus terrestris (L.) burrows (WORM). There were no consistent differences in community structures between unlined, artificial burrows and control soils during a 16‐wk incubation. In contrast, protozoan numbers were elevated throughout the experiment in LEAF and WORM. A succession of nematode abundances occurred in LEAF, with plant parasitic and Tylenchid nematode numbers peaking at 5 wk, followed by high bacterivorous and fungivorous nematode numbers. In WORM, bacterivorous nematode numbers and active bacterial biomass were elevated for 1 and 3 wk, respectively, before declining. Active fungal biomass increased in WORM, whereas fungivorous nematodes were inhibited in earthworm burrows. While litter incorporation appeared to accelerate the rate of trophic interactions in artificial burrows, the effects of earthworms appeared to transcend that of litter translocation into soil, with earthworms differentially selecting for particular food web dynamics.

Microbial and Microfaunal Community Dynamics in Artificial and (L.) Burrows

Macropore formation and litter incorporation are two results of earthworm [Lumbricus terrestris (L.)] activities that can influence trophic dynamics inside burrows. Thus, mesocosms were constructed to examine changes in microbial biomass and microfaunal communities inside artificial compared with earthworm burrows. Four treatments were established: (i) no worms (CTRL); (ii) unlined, artificial burrows (ARTF); (iii) corn (Zea mays L.) leaf litter-lined, artificial burrows (LEAF); and (iv) Lumbricus terrestris (L.) burrows (WORM). There were no consistent differences in community structures between unlined, artificial burrows and control soils during a 16-wk incubation. In contrast, protozoan numbers were elevated throughout the experiment in LEAF and WORM. A succession of nematode abundances occurred in LEAF, with plant parasitic and Tylenchid nematode numbers peaking at 5 wk, followed by high bacterivorous and fungivorous nematode numbers. In WORM, bacterivorous nematode numbers and active bacterial biomass were elevated for 1 and 3 wk, respectively, before declining. Active fungal biomass increased in WORM, whereas fungivorous nematodes were inhibited in earthworm burrows. While litter incorporation appeared to accelerate the rate of trophic interactions in artificial burrows, the effects of earthworms appeared to transcend that of litter translocation into soil, with earthworms differentially selecting for particular food web dynamics.

Soil fungal community structure in a temperate upland grassland soil

Alterations in soil microfungal community structure across a transect between a semi-natural upland grassland and an agriculturally improved enclosure were assessed using an indirect measurement of active fungal biomass (ergosterol), together with a nucleic acid approach, terminal restriction fragment length polymorphism (TRFLP), which was compared to a commonly used but less sensitive community fingerprinting technique, denaturing gradient gel electrophoresis (DGGE). These techniques indicated that there was no reduction in numbers of fungal ribotypes across the floristic transect, despite decreased floristic diversity and a reduction of more than two-fold in ergosterol concentration. Although there were no differences in ribotype number, there was a decrease in diversity and an increase in dominance in only one of the transitional areas. The highest degree of variability within fungal communities was also found in this transitional area, with 84% of ribotypes only being detected in one of three replicates. Comparison of the two fungal community fingerprinting approaches indicated that TRFLP (26–33 ribotypes) was more sensitive for monitoring alterations in fungal community structure than DGGE (13–18 ribotypes). Using a measurement of the relative percentage of each ribotype within communities, a decrease in abundance of prominent ribotypes of the natural grassland soil fungal community was indicated together with an emergence of previously undetected ribotypes towards the improved area. This may have important implications for ecosystem stability or productivity, particularly if agricultural inputs to managed grasslands are suspended.

Physiological characteristics (SDH and ALP activities) of arbuscular mycorrhizal colonization as affected by Bacillus thuringiensis inoculation under two phosphorus levels

The effect of Bacillus thuringiensis ( B.t.) inoculation on plant growth and on the intra- and extraradical mycorrhizal development of lettuce roots colonized by Glomus mosseae or Glomus intraradices was examined in an inert, soil-less substrate. Histochemical determination of succinate dehydrogenase (SDH) and alkaline phosphatase (ALP) activities which indicate active fungal metabolism was carried out at two phosphorus (P) levels. The presence of B.t. increased extra- and intraradical colonization [measured as frequency (% F), intensity (% I) and percentage of arbuscules (%A)] for both arbuscular mycorrhizal fungi (AMF) rather than plant growth or nutrition regardless P level. Under the lowest level of P fertilization, B.t. enhanced to a similar extent the extra- and intraradical development of both endophytes, but the proportion of fungal tissue showing SDH or ALP was increased in G. intraradices-colonized plants. [SDH: 458% (M) and 512% (A); ALP: 358% (M) and 300% (A)]. P supply decreased G. intraradices colonization to a higher extent than G. mosseae. Nevertheless, the totality of G. intraradices structures developed in P-amended medium showed intraradical o extraradical activity, while in G. mosseae-colonized roots, SDH and ALP activities highly decreased relative to fungal tissue determined by TB staining as affected by P. Our results show that bacterial inoculation compensates the negative effect of P on the intraradical fungal growth and vitality. P amendment reduced in a higher extent G. intraradices infection intensity (non-vital and vital staining) and G. mosseae activity (ALP staining). Thus, big differences in the proportion of SDH-active infection showing ALP activity in mycelium developed by each endophyte were noted at the highest P level. Physiological plant parameters such as photosynthetic activity did not explain specific changes on each arbuscular-mycorrhizal fungus as affected by P or B.t. inoculation. The increased extraradical mycelium development and metabolic fungal activity as a result of B.t. inoculation positively affected N and P plant content and photosynthetic rate in G. intraradices-colonized plants under the lowest P conditions. In general, the increased metabolically active fungal biomass in co-inoculated plants was irrespective of P level and was not related to the P plant uptake from the inert soil-less substrate. These results show the bacterial effect increasing the physiological and metabolic status of AM endophytes, which not only confirms but also extends previous findings on arbuscular mycorrhizae-bacteria interactions. The present study emphasizes the ecological and practical importance of rhizosphere free-living bacteria as mycorrhizae-helper microorganisms.