土壤微生物群落结构与多样性对农田集约化管理程度降低的响应

Using a scheme of agricultural fields with progressively less intensive management （deintensification）, different management practices in six agroecosystems located near Goldsboro, NC, USA were tested in a large-scale experiment, including two cash-grain cropping systems employing either tillage （CT） or no-tillage （NT）, an organic farming system （OR）, an integrated cropping system with animals （IN）, a successional field （SU）, and a plantation woodlot （WO）. Microbial phospholipid fatty acid （PLFA） profiles and substrate utilization patterns （BIOLOG ECO plates） were measured to examine the effects of deintensification on the structure and diversity of soil microbial communities. Principle component analyses of PLFA and BIOLOG data showed that the microbial community structure diverged among the soils of the six systems.Lower microbial diversity was found in lowly managed ecosystem than that in intensive and moderately managed agroecosystems, and both fungal contribution to the total identified PLFAs and the ratio of microbial biomass C/N increased along with agricultural deintensification. Significantly higher ratios of C/N （P 〈 0.05） were found in the WO and SU systems, and for fungal/bacterial PLFAs in the WO system （P 〈 0.05）. There were also significant decreases （P 〈 0.05） along with agricultural deintensification for contributions of total bacterial and gram positive （G＋） bacterial PLFAs.Agricultural deintensification could facilitate the development of microbial communities that favor soil fungi over bacteria.     

Soil texture affects soil microbial and structural recovery during grassland restoration

Many biotic and abiotic factors influence recovery of soil communities following prolonged disturbance. We investigated the role of soil texture in the recovery of soil microbial community structure and changes in microbial stress, as indexed by phospholipid fatty acid (PLFA) profiles, using two chronosequences of grasslands restored from 0 to 19 years on silty clay loam and loamy fine sand soils in Nebraska, USA. All restorations were formerly cultivated fields seeded to native warm-season grasses through the USDA’s Conservation Reserve Program. Increases in many PLFA concentrations occurred across the silty clay loam chronosequence including total PLFA biomass, richness, fungi, arbuscular mycorrhizal fungi, Gram-positive bacteria, Gram-negative bacteria, and actinomycetes. Ratios of saturated:monounsaturated and iso:anteiso PLFAs decreased across the silty clay loam chronosequence indicating reduction in nutrient stress of the microbial community as grassland established. Multivariate analysis of entire PLFA profiles across the silty clay loam chronosequence showed recovery of microbial community structure on the trajectory toward native prairie. Conversely, no microbial groups exhibited a directional change across the loamy fine sand chronosequence. Changes in soil structure were also only observed across the silty clay loam chronosequence. Aggregate mean weighted diameter (MWD) exhibited an exponential rise to maximum resulting from an exponential rise to maximum in the proportion of large macroaggregates (>2000 μm) and exponential decay in microaggregates (<250 μm and >53 μm) and the silt and clay fraction (<53 μm). Across both chronosequences, MWD was highly correlated with total PLFA biomass and the biomass of many microbial groups. Strong correlations between many PLFA groups and the MWD of aggregates underscore the interdependence between the recovery of soil microbial communities and soil structure that may explain more variation than time for some soils (i.e., loamy fine sand). This study demonstrates that soil microbial responses to grassland restoration are modulated by soil texture with implications for estimating the true capacity of restoration efforts to rehabilitate ecosystem functions.     

玉米幼苗对芘污染土壤微生物活性及多样性的影响

通过盆栽土培试验研究了玉米幼苗生长期间对芘污染土壤微生物活性及多样性的影响。结果表明,玉米加快了土壤中芘的降解,提高了芘在土壤中的降解速率。试验期间,根际土中可提取态芘含量显著低于非根际土,根际土微生物生物量碳、微生物熵、多酚氧化酶和脱氢酶活性均高于非根际土,代谢熵低于非根际土。脂肪酸（FAME）分析结果表明,与非根际土相比,芘污染玉米根际土微生物群落结构发生了显著的变化,主要表现在真菌特征脂肪酸以及真菌/细菌的比值显著升高,细菌和GN^-细菌特征脂肪酸显著降低,且这种效应随着培养时间的推移在P〈0.01水平显著。根际土和非根际土中丛枝菌根真菌、GN^＋细菌和放线菌特征脂肪酸差异随着培养时间的延长逐渐加大,45 d时其差异均在P〈0.05水平显著。     

Shifting microbial community structure across a marine terrace grassland chronosequence, Santa Cruz, California

Changes in the biomass and structure of soil microbial communities have the potential to impact ecosystems via interactions with plants and weathering minerals. Previous studies of forested long-term (1000s - 100,000s of years) chronosequences suggest that surface microbial communities change with soil age. However, significant gaps remain in our understanding of long-term soil microbial community dynamics, especially for non-forested ecosystems and in subsurface soil horizons. We investigated soil chemistry, aboveground plant productivity, and soil microbial communities across a grassland chronosequence (65,000-226,000 yrs old) located near Santa Cruz, CA. Aboveground net primary productivity (ANPP) initially increased to a maximum and then decreased for the older soils. We used polar lipid fatty acids (PLFA) to investigate microbial communities including both surface (<0.1 m) and subsurface (≥0.2 m) soil horizons. PLFAs characteristic of Gram-positive bacteria and actinobacteria increased as a fraction of the microbial community with depth while the fungal fraction decreased relative to the surface. Differences among microbial communities from each chronosequence soil were found primarily in the subsurface where older subsurface soils had smaller microbial community biomass, a higher proportion of fungi, and a different community structure than the younger subsurface soil. Subsurface microbial community shifts in biomass and community structure correlated with, and were likely driven by, decreasing soil P availability and Ca concentrations, respectively. Trends in soil chemistry as a function of soil age led to the separation of the biological (≤1 m depth) and geochemical (>1 m) cycles in the old, slowly eroding landscape we investigated, indicating that this separation, commonly observed in tropical and subtropical ecosystems, can also occur in temperate climates. This study is the first to investigate subsurface microbial communities in a long-term chronosequence. Our results highlight connections between soil chemistry and both the aboveground and belowground parts of an ecosystem.     

Soil Microbial Community Diversity and Its Relationships with Geochemical Elements under different Farmlands in Shouguang,China

The aims of this study were to investigate soil microbial community characteristics and their interrelationships with soil geochemistry under different farmlands in Shouguang, China. The traditional dilution plate counts, BIOLOG system, and fatty acid methyl ester (FAME) analyses were used to assess microbial populations, substrate utilization, and fatty acid profiles. The number of aerobic heterotrophic bacteria varied significantly among untilled land, maize, and mungbean fields. The amounts of actinomycetes, fungal fatty acids, and Gram-positive/Gram-negative bacteria ratios varied greatly among celery, tomato, and aubergine fields. In the tomato field, the soil microbial community characteristics were significantly different from other fields. Principal component analysis of BIOLOG and FAME data revealed differences in the catabolic capability and fatty acid profiles of soil microbial communities among different farmlands. Spearman correlation analyses showed that in these sand clay soils of Shouguang, microbial communities in different farmlands were closely correlated with soil geochemical elements, moisture, and organic matter.     

Using landscape and depth gradients to decouple the impact of correlated environmental variables on soil microbial community composition

Simultaneously assessing shifts in microbial community composition along landscape and depth gradients allows us to decouple correlations among environmental variables, thus revealing underlying controls on microbial community composition. We examined how soil microbial community composition changed with depth and along a successional gradient of native prairie restoration. We predicted that carbon would be the primary control on both microbial biomass and community composition, and that deeper, low-carbon soils would be more similar to low-carbon agricultural soils than to high carbon remnant prairie soils. Soil microbial community composition was characterized using phospholipid fatty acid (PLFA) analysis, and explicitly linked to environmental data using structural equations modeling (SEM). We found that total microbial biomass declined strongly with depth, and increased with restoration age, and that changes in microbial biomass were largely attributable to changes in soil C and/or N concentrations, together with both direct and indirect impacts of root biomass and magnesium. Community composition also shifted with depth and age: the relative abundance of sulfate-reducing bacteria increased with both depth and restoration age, while gram-negative bacteria declined with depth and age. In contrast to prediction, deeper, low-C soils were more similar to high-C remnant prairie soils than to low-C agricultural soils, suggesting that carbon is not the primary control on soil microbial community composition. Instead, the effects of depth and restoration age on microbial community composition were mediated via changes in available phosphorus, exchangeable calcium, and soil water, together with a large undetermined effect of depth. Only by examining soil microbial community composition shifts across sites and down the soil column simultaneously were we able to tease apart the impact of these correlates environmental variables.     

Shrub-interspace dynamics alter relationships between microbial community composition and belowground ecosystem characteristics

In desert ecosystems, belowground characteristics are influenced chiefly by the formation and persistence of “shrub-islands of fertility” in contrast to barren plant interspaces. If soil microbial communities are exclusively compared between these two biogeochemically distinct soil types, the impact of characteristics altered by shrub species, especially soil C and N, are likely to be overemphasized and overshadow the role of other characteristics in structuring microbial composition. To determine how belowground characteristics influence microbial community composition, and if the relative importance of these characteristics shifts across the landscape (i.e., between and within shrub and interspace soils), changes in microbial communities across a 3000-year cold desert chronosequence were related to 27 belowground characteristics in surface and subsurface soils. When shrub and interspace communities in surface and subsurface soils were combined across the entire chronosequence, communities differed and were primarily influenced by soil C, NO₃⁻ concentrations, bulk density, pH, and root presence. Within shrub soils, microbial communities were shrub species-specific, especially in surface soils, highlighting differences in soil characteristics created by specific shrub species and/or similarity in stresses structuring shrub species and microbial communities alike. Microbial communities in shrub soils were not influenced by soil C, but by NO₃⁻ and NH₄⁺ concentrations, pH, and silt in surface soils; and Cl, P, soil N, and NO₃⁻ concentrations in subsurface soils. Interspace soil communities were distinct across the chronosequence at both depths and were strongly influenced by dune development. Interspace communities were primarily associated with soil stresses (i.e., high B and Cl concentrations), which decreased with dune development. The distribution of Gram-positive bacteria, Actinobacteria, and fungi highlighted community differences between and within shrub and interspace soils, while Gram-negative bacteria were common in all soils across the chronosequence. Of the 27 belowground characteristics investigated, 13 separated shrub from interspace communities, and of those, only five emerged as factors influencing community composition within shrub and interspace soils. As dunes develop across this cold desert chronosequence, microbial community composition was not regulated primarily by soil C, but by N and P availability and soil stresses in shrub soils, and exclusively by soil stresses in interspace soils.     

Olive mill wastewater effects on the microbial communities as studied in the field of olive trees by analysis of fatty acid signatures

The aim of this work was to study the effects of spreading olive mill wastewater (OMW) on the soil surface of an olive grove on the soil microbial communities. Analyses of ester-linked fatty acid methyl esters (EL-FAME) were used to assess variations in the soil microbial community structure following land spreading of OMW. Our data provide evidence that agronomic application of OMW has important effects on soil microbial community. Bacteria were relatively more reduced by these treatments than fungi and actinomycetes as revealed by an increased index of fungal/bacterial FAME and actinomycetes/bacterial FAME. Specific FAME markers indicated a significant reduction in the Gram-positive bacteria. However, the relative proportion of the Gram-negative bacteria was not significantly different after agronomic application of OMW. The ratios of cyclopropyl/monoenoic precursors decreased and the total monounsaturated/total saturated fatty acids increased in the OMW amended soils, suggesting that the microbes inhabiting the control soil are more carbon limited than the OMW amended soils. The changes in the FAME pattern of the soil organisms possibly were related (i) to an altered substrate quantity, that is the availability of substrates after the treatments, (ii) the complex nature of OMW which also contains high molecular-mass recalcitrant polyphenols.     

Characterization of redox-related soil microbial communities along a river floodplain continuum by fatty acid methyl ester (FAME) and 16S rRNA genes

Redox states affect substrate availability and energy transformation, and, thus, play a crucial role in regulating soil microbial abundance, diversity, and community structure. We evaluated microbial communities in soils under oxic, intermittent, and anoxic conditions along a river floodplain continuum using fatty acid methyl ester (FAME) and 16S rRNA genes-based terminal-restriction fragment length polymorphism (T-RFLP) bacterial fingerprints. In all the soils tested, microbial communities clustered according to soil redox state by both evaluation techniques. Bacteria were dominant components of soil microbial communities, while mycorrhizal fungi composed about 12% of the microbial community in the oxic soils. Gram-positive bacteria consisted >10% of the community in all soils tested and their abundance increased with increasing soil depth when shifting from oxic to anoxic conditions. In the anoxic soils, Gram-positive bacteria composed about 16% of the total community, suggesting that their growth and maintenance were not as sensitive to oxygen supply as for other microbes. In general, microorganisms were more abundant and diverse, and distributed more evenly in the oxic layers than the anoxic layers. The decrease in abundance with increasing oxygen and substrate limitation, however, was considerably more drastic than the decrease in diversity, suggesting that growth of soil microorganisms is more energy demanding than maintenance. The lower diversity in the anoxic than the oxic soils was attributed primarily to the differences in oxygen availability in these soils.     

Effects of heavy metal contamination and remediation on soil microbial communities in the vicinity of a zinc smelter as indicated by analysis of microbial community phospholipid fatty acid profiles

Heavy metal contamination in an area immediately surrounding a zinc smelter has resulted in destruction of over 485 hectares of forest. The elevated levels of heavy metals in these soils have had significant impacts on the population size and overall activity of the soil microbial communities. Remediation of these soils has resulted in increases in indicators of biological activity and viable population size, which suggest recovery of the microbial populations. Questions remain as to how the metal contamination and subsequent remediation at this site have impacted the population structure of the soil microbial communities. In the current study, microbial communities from this site were analyzed by the phospholipid fatty acid (PLFA) procedure. Principal component analysis of the PLFA profiles indicated that there were differences in the profiles for soils with different levels of metal contamination, and that soils with higher levels of metal contamination showed decreases in indicator PLFAs for mycorrhizal fungi, Gram-positive bacteria, fungi, and actinomycetes. PLFA profiles for remediated sites indicated that remediated soils showed increases in indicator PLFAs for fungi, actinomycetes, and Gram-positive bacteria, compared to unremediated metal contaminated soils. These data suggest a change in the population structure of the soil microbial communities resulting from metal contamination and a recovery of several microbial populations resulting from remediation.     

Functional and structural responses of bacterial and fungal communities from paddy fields following long-term rice cultivation

Purpose

Rice paddy soils undergo pedogenesis driven by periodic flooding and drainage cycles that lead to accumulation of organic matter and the stratification of nutrients and oxygen in the soil profile. Here, we examined the effects of continuous rice cultivation on microbial community structures, enzyme activities, and chemical properties for paddy soils along a chronosequence representing 0–700 years of rice cropping in China.

Materials and methods

Changes in the abundance and composition of bacterial and fungal communities were characterized at three depths (0–5, 5–10, and 10–20 cm) in relation to organic carbon, total nitrogen, dissolved organic carbon, microbial biomass carbon/nitrogen, and activities of acid phosphatase, invertase, and urease.

Results and discussion

Both soil organic carbon and total nitrogen increased over time at all three depths, while pH generally decreased. Microbial abundance (bacteria and fungi) and invertase and urease activity significantly increased with the duration of rice cultivation, especially in the surface layer. Fungal abundance and acid phosphatase activity declined with depth, whereas bacterial abundance was highest at the 5–10-cm soil depth. Profiles of the microbial community based on PCR-DGGE of 16S rRNA indicated that the composition of fungal communities was strongly influenced by soil depth, whereas soil bacterial community structures were similar throughout the profile.

Conclusions

Soil bioactivity (microbial abundance and soil enzymes) gradually increased with organic carbon and total nitrogen accumulation under prolonged rice cultivation. Microbial activity decreased with depth, and soil microbial communities were stratified with soil depth. The fungal community was more sensitive than the bacterial community to cultivation age and soil depth. However, the mechanism of fungal community succession with rice cultivation needs further research.

     

Spatial analysis reveals differences in soil microbial community interactions between adjacent coniferous forest and clearcut ecosystems

Knowledge of how forest management influences soil microbial community interactions is necessary for complete understanding of forest ecology. In this study, soil microbial communities, vegetation characteristics and soil physical and chemical properties were examined across a rectangular 4.57 × 36.58 m sample grid spanning adjacent coniferous forest and clearcut areas. Based on analysis of soil extracted phospholipid fatty acids, total microbial biomass, fungi and Gram-negative bacteria were found to be significantly reduced in soil of the clearcut area relative to the forest. Concurrent with changes in microbial communities, soil macroaggregate stability was reduced in the clearcut area, while no significant differences in soil pH and organic matter content were found. Variography indicated that the range at which spatial autocorrelation between samples was evident (patch size) was greater for all microbial groups analyzed in the clearcut area. Overall, less spatial structure could be resolved in the forest. Variance decomposition using principal coordinates of neighbor matrices spatial variables indicated that soil aggregate stability and vegetation characteristics accounted for significant microbial community spatial variation in analyses that included the entire plot. When clearcut and forest areas were analyzed separately, different environmental variables (pH in the forest area and soil organic matter in the clearcut) were found to account for variation in soil microbial communities, but little of this variation could be ascribed to spatial interactions. Most microbial variation explained by different components of microbial communities occurred at spatial scales other than those analyzed. Fungi accounted for over 50% of the variation in bacteria of the forest area but less than 11% in the clearcut. Conversely, AMF accounted for significant variation in clearcut area, but not forest, bacteria. These results indicate broadly disparate controls on soil microbial community composition in the two systems. We present multiple lines of evidence pointing toward shifts in fungi functional groups as a salient mechanism responsible for qualitative, quantitative and spatial distribution differences in soil microbial communities.     

Effects of cropping systems under no‐till agriculture on arbuscular mycorrhizal fungi in Argentinean Pampas

Here, we compare arbuscular mycorrhizal fungi (AMF) communities and fatty acids in soils under different no‐till (NT) agricultural managements over two seasons in two consecutive years. Two NT practices with different agricultural managements were compared: crop rotation (CR) and soya bean monoculture (MC). Soils of natural grasslands (NGs) were used as a reference. Treatments were tested along a regional gradient (four geographical locations) across a 400‐km transect of the Argentinean Pampas. We identified 46 morphospecies. Several morphospecies occurred abundantly at all soils; others appeared to be restricted to specific situations. At the regional scale, CR maintained the same richness levels of AMF spores, whereas MC showed less richness, when compared with the NG. Although AMF spore density was clearly affected by cropping practices in the four locations, we could observe some changes in AMF species richness, and similar diversity under agricultural and natural soils. Fatty acid concentrations (whole‐cell, phospholipid and neutral lipid fatty acids) revealed differences between soil managements and showed similar patterns of variation in all locations. Spore density positively correlated with all soil lipids fractions. The results suggest that AMF spore communities and fatty acids in soils are suitable indicators of soil management involving different levels of crop rotation. Spore richness measured at a regional scale proved to be sensitive to different NT agricultural managements. Moreover, certain morphospecies could be good bioindicators for NT practices based on cropping systems on the Argentinean Pampas.     

Rhizosphere microbial communities and organic acids secreted by aluminum-tolerant and aluminum-sensitive soybean in acid soil

This study aimed to investigate the correlation between organic acids secreted by two soybeans genotypes, BX10 [aluminum (Al) tolerant] and BD2 (Al sensitive) and rhizosphere microbial communities in acid soil. The organic acids secreted by BX10 and BD2 were significantly different at each growth stage. Both fungi/bacteria and gram-negative bacteria/gram-positive bacteria ratio values were affected by the two soybean genotypes at different growth periods. Compared with BD2, phospholipid fatty acid of BX10 showed higher Shannon diversity at the seedling and flowering stages, but had lower Shannon diversity at the pod-setting stage. Redundancy analysis and canonical correspondence analysis revealed that the organic acids including tartaric acid, lactic acid, and citric acid significantly affected rhizosphere bacterial communities. Sequence analysis indicated that uncultured Acidobacterium, Chloroflexi, and actinomycete enriched in BD2, whereas some uncultured bacteria enriched in BX10. The two soybean genotypes exhibit distinct rhizosphere microbial communities; root organic acid exudates may affect composition of microbial communities of rhizosphere soil: tartaric acid may negatively affect rhizosphere bacteria at the seedling stage, lactic acid may positively affect rhizosphere actinomycetes at the flowering stage, and succinic acid may stimulate fungi at the pod-setting stage.     

Influence of long-term fertilisation and crop rotation on changes in fungal and bacterial residues in a tropical rice-field soil

Amino sugars, as a microbial residue biomarker, are highly involved in microbial-mediated soil organic matter formation. However, accumulation of microbial biomass and responses of bacterial and fungal residues to the management practices are different and poorly characterized in rice soils. The objectives of this study were to evaluate the effects of mineral fertiliser (MIN), farmyard manure (FYM) and groundnut oil cake (GOC) on crop yield and co-accumulation of microbial residues and microbial biomass under rice-monoculture (RRR) and rice–legume–rice (RLR) systems. In the organic fertiliser treatments and RLR, rice grain yield and stocks of soil and microbial nutrients were significantly higher than those of the MIN treatment and RRR, respectively. The increased presence of saprotrophic fungi in the organic fertiliser treatments and RRR was indicated by significantly increased ergosterol/C_mic ratio and extractable sulphur. In both crop rotation systems, the long-term application of FYM and GOC led to increased bacterial residues as indicated by greater accumulation of muramic acid. In contrast, the higher fungal C/bacterial C ratio and lower ergosterol/C_mic ratio in the MIN treatment, is likely caused by a shift within the fungal community structure towards ergosterol-free arbuscular mycorrhizal fungi (AMF). The organic fertiliser treatments contributed 22 % more microbial residual C to soil organic C compared to the MIN treatment. Our results suggest that the negative relationship between the ratios ergosterol/C_mic and fungal C/bacterial C encourages studying responses of both saprotrophic fungi and AMF when assessing management effects on the soil microbial community.     

A soil microbial community structural-functional index: the microscopy-based total/active/active fungal/bacterial (TA/AFB) biovolumes ratio

In most studies of fungal–bacterial communities in soils, single-value indices such as fumigation–extraction (FE) of microbe-derived organic carbon, measures of specific microbial cell chemical constituents, or activity-related measures have been used. These widely used single value indices, however, do not provide information on the physical structure of the filamentous fungal and bacterial community in a soil. The filamentous fungi, considered as indeterminate organisms, have a variable and changing hyphal network, most of which is devoid of cytoplasm. To meet this need for a direct integrated measure of the physical characteristics of the indeterminate fungi and their associated bacteria, a microscopy-based microbial biovolumes ratios approach is suggested. To provide this information, the total and active biovolumes of both the filamentous fungi and bacteria are assessed by microscopy. To normalize these responses, the ratio of total to active (TA) fungal plus bacterial biovolumes is divided by the ratio of the active fungal to bacterial biovolume (AFB), to yield the total/active/active fungal/bacterial (TA/AFB) biovolumes ratio. This approach has been used to analyze data from recently-cultivated early successional (ES) and uncultivated late successional (LS) sites at a shortgrass steppe of northeastern Colorado, where control plots were compared with those receiving mineral nitrogen amendments, using samples taken during the summer of 1995. The TA/AFB ratio index showed distinct and significant decreases in response to soil disturbance which reflected the decreased hyphal lengths present in these disturbed soils. These changes were not detected by the use of FE-based extractable carbon measurements. The TA/AFB ratio also showed significant positive correlations with indices of plant community development and mineral nitrogen, especially in the plots not amended with N. This TA/AFB ratios index should be able to provide information on the physical structure of the indeterminate filamentous fungi and associated soil bacteria for use in the assessment of soil quality, health and resiliency.     

Net microbial amino sugar accumulation process in soil as influenced by different plant material inputs

Identifying the impact of plant material inputs on soil amino sugar synthesis may advance our knowledge of microbial transformation processes in soils. In a 12-week laboratory microcosm incubation, 1, 2, 4, and 6% (w/w) soybean leaf or maize stalk were initially added to soil, respectively, whereas soil without plant addition was used as a control. The results showed that adding organic materials to the soil led to a net accumulation of amino sugars, because of greater microbial synthesis. The ratios of glucosamine to galactosamine and of glucosamine to muramic acid, two indicators differentiating the relative contribution to soil organic matter of fungi and bacteria, showed substantial variance across the gradient of substrate addition. Our results suggest that the amount of nutrients in a given substrate is the primary attribute determining microbial net accumulation of soil amino sugars, especially in the relatively short term, whereas the composition of nutrients might be more important in the relatively long term when nutrients are not sufficient. The use of the two ratios (glucosamine to galactosamine and glucosamine to muramic acid) reflects different dynamics of galactosamine and muramic acid during the decomposition of organic substrates in soils. Muramic acid, compared with galactosamine, is more likely to accumulate in the soil active organic fraction under abundant nutrient conditions, whereas it would be decomposed along with active organic matter when the nutrients are scarce and remain in minor quantities in the clay fraction without being attacked by microbes.     

Decoupling plant-growth from land-use legacies in soil microbial communities

Differences in soil microbial communities between ex-arable and undisturbed soils are often assumed to reflect long-term legacies of agricultural practices. Ex-arable soils, however, are commonly dominated by different plant species than undisturbed soils making it difficult to separate the importance of land-use and plant-growth legacies. In a system where non-native plants dominate ex-arable soils, we decoupled land-use (ex-arable, undisturbed) and plant-growth (native, non-native) effects on soil microbial communities using a factorial sampling design. Soils were removed from 14 sites that formed a 52-year chronosequence of agricultural abandonment. Microbial abundance and composition were measured using whole-soil phospholipid fatty acid analyses and microbial activity was measured in a subset of samples using sole-carbon-source utilization analyses. We found that both non-native-cultivated and ex-arable soils were independently associated with lower microbial abundance and diversity than native and undisturbed soils. We also found a correlation between microbial abundance and age-since-agricultural abandonment in ex-arable/non-native-cultivated soils suggesting that non-native plant effects accumulate over time. Microbial activity was consistent with microbial abundance; microbial communities in non-native-cultivated, ex-arable soils were slow to respire most carbon sources. Our data suggests that agricultural practices create soil conditions that favor non-native plant growth and non-native plants maintain these conditions. Potential mechanisms explaining how non-natives create soils with small microbial communities and how small microbial communities may benefit non-natives are discussed.     

Drying and rewetting effects on soil microbial community composition and nutrient leaching

The effects of a dry-rewetting event (D/RW) on soil microbial properties and nutrient release by leaching from two soils taken from adjacent grasslands with different histories of management intensity were studied. These were a low-productivity grassland, with no history of fertilizer application and a high-productivity grassland with a history of high fertilizer application, referred to as unimproved and improved grassland, respectively. The use of phospholipid fatty acid analysis (PLFA) revealed that the soil of the unimproved grassland had a significantly greater microbial biomass, and a greater abundance of fungi relative to bacteria than did the improved grassland. Soils from both grasslands were maintained at 55% water holding capacity (WHC) or dried to 10% WHC and rewetted to 55% WHC, and then sampled on days 1, 3, 9, 16, 30 and 50 after rewetting. The D/RW stress significantly reduced microbial biomass carbon (C), fungal PLFA and the ratio of fungal-to-bacterial PLFA in both soils. In contrast, D/RW increased microbial activity, but had no effect on total PLFA and bacterial PLFA in either soil. Microbial biomass nitrogen (N) was reduced significantly by D/RW in both soils, but especially in those of the improved grassland. In terms of nutrient leaching, the D/RW stress significantly increased concentrations of dissolved organic C and dissolved organic N in leachates taken from the improved soil only. This treatment increased the concentration of dissolved inorganic N in leachate of both soils, but this effect was most pronounced in the improved soil. Overall, our data show that D/RW stress leads to greater nutrient leaching from improved than from unimproved grassland soils, which have a greater microbial biomass and abundance of fungi relative to bacteria. This finding supports the notion that soils with more fungal-rich communities are better able to retain nutrients under D/RW than are their intensively managed counterparts with lower fungal to bacterial ratios, and that D/RW can enhance nutrient leaching with potential implications for water quality.     

Recovery of soil microbial community structure after fire in a sagebrush‐grassland ecosystem

Recovery of the soil microbial community after fire in a sagebrush‐grassland ecosystem was examined using a chronosequence of four sites ranging in time since fire from 3–39 years. The successional stage communities examined included Recent Burn (3 years since fire, ysf), Establishment (7 ysf), Expansion (21 ysf), and Mature (39 ysf). Aboveground standing plant biomass increased with time since disturbance to the Mature stage where sagebrush became dominant over herbaceous species. Phospholipid fatty acid (PLFA) analysis was used to characterize the microbial community structure. Soil microbial community productivity generally appeared to be similar to the Mature site soil (39 ysf) within 7 years of fire. Diversity of PLFAs detected in soils, at both depths, increased from a low value of 29 at the Recent site to a high of 37 at the Establishment site and then decreased again to 31 at the Mature stage site. Canonical variates analysis indicated important disparities in microbial community structure at the four sites. Greatest disparities were observed in microbial community structure between the Recent and Establishment stages but greater similarity between the Recent stage and the sagebrush dominated Mature stage. This study emphasizes both short‐term and long‐term changes in the belowground community and suggests that soil microbial communities are highly resilient to disturbances after prescribed fire. Copyright © 2010 John Wiley & Sons, Ltd.