Active Bacterial Biomass Research Articles

The accelerated enzymatic biodegradation and COD removal of petroleum hydrocarbons in the SCR using active bacterial biomass capable of in-situ generating peroxidase and biosurfactants

The enzyme-accelerated biodegradation of total petroleum hydrocarbons (TPH) was investigated in a sequencing continuous-inflow reactor (SCR) at different operational parameters of H2O2/TPH ratio, initial TPH concentration and hydraulic retention time (HRT). The optimum H2O2/TPH mass ratio was determined to be 0.35 at which the complete TPH removal of inlet TPH concentrations up to 4g/L at HRT of 24h, corresponding to the loading rate of 4kgTPH/m3.d, was attained. The average COD removal efficiency at this loading rate was 96.7%. With increasing the inlet TPH concentration from 1 to 2.5g/L, the biomass bacterial activity as dehydrogenase activity (DHA) increased from 7.5 to 27.1μgTF/gbiomass.d and remained almost unchanged with further increase of TPH concentration. The peroxidase activity (PA) remained high between 382 and 410U/gbiomass. In addition, the complete removal of 1g/L TPH (88.7% COD removal) was observed at HRT of as small as 4h (corresponding to the loading rate of 6kgTPH/m3.d) under optimum H2O2/TPH mass ratio. With the decrease of HRT from 24h to 4h at the constant TPH concentration of 1g/L the value of DHA remained between 24.4 and 28.4μgTF/gbiomass.d while the PA value increased from 287.9 to 394.4U/gbiomass. Total production of biosurfactants was 131mg/L (38mg/L rhamnolipid and 93mg/L surfactin) when the SCR was operated at TPH loading rate of 6kg/m3.d.Finally, the enhanced enzymatic biodegradation of TPH by using diverse microbial consortia capable of in-situ production of peroxidase and biosurfactant generation in the SCR is a very efficient and promising technique for accelerated biodegradation and COD removal of petroleum hydrocarbons.

Bacterial diversity and active biomass in full-scale granular activated carbon filters operated at low water temperatures

Granular activated carbon (GAC) filtration enhances the removal of natural organic matter and micropollutants in drinking water treatment. Microbial communities in GAC filters contribute to the removal of the biodegradable part of organic matter, and thus help to control microbial regrowth in the distribution system. Our objectives were to investigate bacterial community dynamics, identify the major bacterial groups, and determine the concentration of active bacterial biomass in full-scale GAC filters treating cold (3.7–9.5°C), physicochemically pretreated, and ozonated lake water. Three sampling rounds were conducted to study six GAC filters of different operation times and flow modes in winter, spring, and summer. Total organic carbon results indicated that both the first-step and second-step filters contributed to the removal of organic matter. Length heterogeneity analysis of amplified 16S rRNA genes illustrated that bacterial communities were diverse and considerably stable over time. α-Proteobacteria, β-Proteobacteria, and Nitrospira dominated in all of the GAC filters, although the relative proportion of dominant phylogenetic groups in individual filters differed. The active bacterial biomass accumulation, measured as adenosine triphosphate, was limited due to low temperature, low flux of nutrients, and frequent backwashing. The concentration of active bacterial biomass was not affected by the moderate seasonal temperature variation. In summary, the results provided an insight into the biological component of GAC filtration in cold water temperatures and the operational parameters affecting it.

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.

Chemistry and Microbial Functional Diversity Differences in Biofuel Crop and Grassland Soils in Multiple Geographies

We obtained soil samples from geographically diverse switchgrass (Panicum virgatum L.) and sorghum (Sorghum bicolor L.) crop sites and from nearby reference grasslands and compared their edaphic properties, microbial gene diversity and abundance, and active microbial biomass content. We hypothesized that soils under switchgrass, a perennial, would be more similar to reference grassland soils than sorghum, an annual crop. Sorghum crop soils had significantly higher NO3−-N, NH4+-N, SO42−-S, and Cu levels than grassland soils. In contrast, few significant differences in soil chemistry were observed between switchgrass crop and grassland soils. Active bacterial biomass was significantly lower in sorghum soils than switchgrass soils. Using GeoChip 4.0 functional gene arrays, we observed that microbial gene diversity was significantly lower in sorghum soils than grassland soils. Gene diversity at sorghum locations was negatively correlated with NO3−-N, NH4+-N, and SO42−-S in C and N cycling microbial gene categories. Microbial gene diversity at switchgrass sites varied among geographic locations, but crop and grassland sites tended to be similar. Microbial gene abundance did not differ between sorghum crop and grassland soils, but was generally lower in switchgrass crop soils compared to grassland soils. Our results suggest that switchgrass has fewer adverse impacts on microbial soil ecosystem services than cultivation of an annual biofuel crop such as sorghum. Multi-year, multi-disciplinary regional studies comparing these and additional annual and perennial biofuel crop and grassland soils are recommended to help define sustainable crop production and soil ecosystem service practices.

Carbon fluxes in the pelagic ecosystem of the Gulf of Trieste (Northern Adriatic Sea)

By measuring a broad suite of physical, chemical, and biological parameters coupled with experiments on grazing efficiency of mesozoo-, microzoo- and heteronano-plankton we were able to depict the seasonal trophic status of the pelagic system in the Gulf of Trieste over a period of 8 years from 1998 to 2005. In winter and spring, primary production exceeded respiration, the autotrophic fraction biomass was higher than the heterotrophic biomass. Moreover, predation on microphytoplankton and autotrophic nanoplankton largely structured the ecosystem and bacterial carbon production accounted for <50% of primary production. The ratio of primary production/respiration was higher than 1 in winter and spring suggesting that pelagic ecosystem was autotrophic whereas in summer and in autumn the ratio was lower than 1 suggesting a shift towards net heterotrophic status. Carbon export was possible in winter and in autumn, and the few data from the sediment trap supported the theoretical rates. Thus since spring most of the C fixed by photosynthesis remained segregated in the surface layer and possibly it was exported to the bottom through grazer fecal pellets. In summer the system was dominated by heterotrophic picoplankton, which showed the highest production rate. In this scenario we hypothesize that the DOC produced during the winter-spring period can sustain a high and active bacterial biomass that is the primary energy source for the whole system. Picoplankton communities were heavily grazed by microzooplankton and heteronano-plankton, moreover predation rates of mesozooplankton on microzooplankton were particularly high in summer. Despite the high variability typical of the coastal areas, the pelagic ecosystem during these 8 years has shifted seasonally from a nutrient-excited state (winter–spring) to a background state (summer–autumn) as it has been observed from open-ocean ecosystem. Understanding the dynamic and the magnitude of this variability-shift is rather compelling in order to give guidance in managing the Gulf area in the context of CO2 sequestration mitigation programs (carbon export downward flux) as well as for fishery economy.

Direct quantification of bacterial biomass in influent, effluent and activated sludge of wastewater treatment plants by using flow cytometry

A rapid multi-step procedure, potentially amenable to automation, was proposed for quantifying viable and active bacterial cells, estimating their biovolume using flow cytometry (FCM) and to calculate their biomass within the main stages of a wastewater treatment plant: raw wastewater, settled wastewater, activated sludge and effluent. Fluorescent staining of bacteria using SYBR-Green I + Propidium Iodide (to discriminate cell integrity or permeabilisation) and BCECF-AM (to identify enzymatic activity) was applied to count bacterial cells by FCM. A recently developed specific procedure was applied to convert Forward Angle Light Scatter measured by FCM into the corresponding bacterial biovolume. This conversion permits the calculation of the viable and active bacterial biomass in wastewater, activated sludge and effluent, expressed as Volatile Suspended Solids (VSS) or particulate Chemical Oxygen Demand (COD). Viable bacterial biomass represented only a small part of particulate COD in raw wastewater (4.8 ± 2.4%), settled wastewater (10.7 ± 3.1%), activated sludge (11.1 ± 2.1%) and effluent (3.2 ± 2.2%). Active bacterial biomass counted for a percentage of 30–47% of the viable bacterial biomass within the stages of the wastewater treatment plant.

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.

Influence of irrigated agriculture on soil microbial diversity

Organic carbon (C), bacterial biomass and structural community diversity were measured in Southern Idaho soils with long term cropping histories. The soils tested were native sagebrush vegetation (NSB), irrigated moldboard plowed crops (IMP), irrigated conservation – chisel – tilled crops (ICT) and irrigated pasture systems (IP). Organic C concentration in soils decreased in the order NSB 0–5 cm > IP 0–30 cm = ICT 0–15 cm > IMP 0–30 cm > NSB 5–15 cm = NSB 15–30 cm. Active bacterial, fungal and microbial biomass correlated with soil C as measured by the Walkely Black method in positive curvilinear relationships ( r 2 = 0.93, 0.80 and 0.76, respectively). Amplicon length heterogeneity (LH-PCR) DNA profiling was used to access the eubacterial diversity in all soils and at all depths. The Shannon–Weaver diversity index was used to measure the differences using the combined data from three hypervariable domains of the eubacterial 16S rRNA genes. Diversity was greatest in NSB 15–30 cm soil and lowest in the IMP soil. With the exception of IMP with the lowest diversity index, the samples highest in C (NSB 0–5 cm, IP 0–30 cm, ICT 0–15 cm) reflected lower diversity indices. However, these indices were not significantly different from each other. ICT and IP increase soil C and to some extent increase diversity relative to IMP. Since soil bacteria respond quickly to environmental changes, monitoring microbial communities may be one way to assess the impact of agricultural practices such as irrigation and tillage regimes.

Potential impacts of climate change on nitrogen transformations and greenhouse gas fluxes in forests: a soil transfer study

AbstractRelatively little research has been conducted on how climate change may affect the structure and function of arid to semiarid ecosystems of the American Southwest. Along the slopes of the San Francisco Peaks of Arizona, USA, I transferred intact soil cores from a spruce‐fir to a ponderosa pine forest 730 m lower in elevation to assess the potential impacts of climate change on soil N cycling and trace gas fluxes. The low‐elevation site has a mean annual soil temperature about 2.5°C higher than the high‐elevation site. Net rates of N transformations and trace gas fluxes were measured in high‐elevation soil cores incubated in situ and soil cores transferred to the low‐elevation site. Over a 13‐month period, volumetric soil water content was similar in transferred soil cores relative to soil cores incubated in situ. Net N mineralization and nitrification increased over 80% in transferred soil cores compared with in situ soil cores. Soil transfer significantly increased net CO2 efflux (120%) and net CH4 consumption (90%) relative to fluxes of these gases from soil cores incubated in situ. Soil net N2O fluxes were relatively low and were not generally altered by soil transfer. Although the soil microbial biomass as a whole decreased in transferred soil cores compared with in situ soil cores after the incubation period, active bacterial biomass increased. Transferring soil cores from the low‐elevation to the high‐elevation site (i.e. simulated global cooling) commonly, but not consistently, resulted in the opposite effects on soil pools and processes. In general, soil containment (root trenching) did not significantly affect soil measurements. My results suggest that small increases in mean annual temperature can have large impacts on soil N cycling, soil–atmosphere trace gas exchanges, and soil microbial communities even in ecosystems where water availability is a major limiting resource.

SOIL BIOTA CAN CHANGE AFTER EXOTIC PLANT INVASION: DOES THIS AFFECT ECOSYSTEM PROCESSES?

Invasion of the exotic annual grass Bromus tectorum into stands of the native perennial grass Hilaria jamesii significantly reduced the abundance of soil biota, especially microarthropods and nematodes. Effects of invasion on active and total bacterial and fungal biomass were variable, although populations generally increased after 50+ years of invasion. The invasion of Bromus also resulted in a decrease in richness and a species shift in plants, microarthropods, fungi, and nematodes. However, despite the depauperate soil fauna at the invaded sites, no effects were seen on cellulose decomposition rates, nitrogen mineralization rates, or vascular plant growth. When Hilaria was planted into soils from not-invaded, recently invaded, and historically invaded sites (all currently or once dominated by Hilaria), germination and survivorship were not affected. In contrast, aboveground Hilaria biomass was significantly greater in recently invaded soils than in the other two soils. We attributed the Hilaria respons...

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.

RESPONSE OF MAJOR SOIL DECOMPOSERS TO LANDSLIDE DISTURBANCE IN A PUERTO RICAN RAINFOREST

To understand the relationship between soil biota and soil disturbance, we sampled an upper and a lower transect within each of two landslides and their adjoining forests, during both the wet and dry season in Puerto Rico. We found that the distribution of earthworms and soil microbes (e.g., fungi and bacteria)showed considerable spatial difference in these tropical landslides. We also found that endogenic earthworms (Pontoscolex corethrurus) occurred in all habitats (upper landslide 9.5 ± 4 No. m−2, lower landslide 43.8 ± 11 No. m−2, upper forest 35.7 ± 8 No. m−2, and lower forest 30.5 ± 14 No. m−2), but anecic earthworms (Amynthas rodericensis) were only found in the undisturbed forests (3.4 ±0.6 No. m−2). Total bacterial and fungal biomasses were significantly higher in the forests than in the landslides. Active bacterial and fungal biomasses were significantly higher in the lower landslide area than in the upper landslide area. For all sampled soil parameters there was a dominance of microsite variation within landslides compared with seasonal changes or differences between landslides and adjacent forests. Earthworm density and biomass correlated positively with leaf litter, light-carbon fraction, and total bacteria and negatively with fine roots, suggesting that earthworm abundance and composition in landslides were regulated by carbon pools. Earthworm abundance and community structure as well as active and total fungal and bacterial biomass may reflect soil disturbance history and soil development processes over geological time in the Puerto Rican rainforest.

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.

The influence of plant species, fertilization and elevated CO2 on soil aggregate stability

We tested the effects of plant species, fertilization and elevated CO2 on water-stable soil aggregation. Five annual grassland species and a plant community were grown in outdoor mesocosms for 4 years, with and without NPK fertilization, at ambient or elevated atmospheric CO2 concentrations. Aggregate stability (resistance of aggregates to slaking) in the top 0.15 m of soil differed among plant species. However, the more diverse plant community did not enhance aggregate stability relative to most monocultures. Species differences in aggregate stability were positively correlated with soil active bacterial biomass, but did not correlate with root biomass or fungal length. Plant species did not affect aggregate stability lower in the soil profile (0.15–0.45 m), where soil biological activity is generally decreased. Elevated CO2 and NPK fertilization altered many of the factors known to influence aggregation, but did not affect water-stable aggregation at either depth, in any of the plant treatments. These results suggest that global changes will alter soil structure primarily due to shifts in vegetation composition.