Substrate utilization patterns of desert soil microbial communities in response to xeric and mesic conditions

The Negev Desert is characterized by low soil moisture and organic matter content and an unpredictable rainfall amount, dispersion, and intensity. Water and nutrient availability are, therefore, the major limiting factors of biological activity in arid and semi-arid ecosystems. Plants have developed different ecophysiological adaptations in order to cope with the harsh conditions in this xeric environment, e.g., excretion of salt (Reaumuria negevensis) and chemical compounds (Artemisia sieberi) through the leaves. Microorganisms constitute a major part of these ecosystems' total biomass, and are diverse members of the soil food web, being primarily responsible for breaking down complex organic compounds, which are then recycled. They are also known to be very sensitive to abiotic changes and can time their activity to the environmental conditions.Soil samples were collected monthly from a 0 to 10 cm depth, under the canopies of A. sieberi, Noaea mucronata, and R. negevensis. Samples collected from inter-shrub spaces served as control. CO₂ evolution, microbial biomass, microbial functional diversity, and the physiological profile of the community, were determined by MicroResp™ analysis. A significant difference was found between the two dry periods in most of the examined parameters. The values of water, organic matter content, and total soluble nitrogen were higher in soil samples collected in the vicinity of R. negevensis than in samples collected in the vicinity of N. mucronata, A. sieberi, and the open area. A similar trend was found in CO₂ evolution, microbial biomass, and H' values, in which soil samples collected beneath the canopies of N. mucronata and R. negevensis and from open area had higher values during the wet periods (which were characterized by a mesic environment) and in samples collected beneath the A. sieberi in the wet 2006 and dry 2007 periods.     

Composition of the microbial communities in the mineral soil under different types of natural forest

Phospholipid fatty acid (PLFA) patterns were used to describe the composition of the soil microbial communities under 12 natural forest stands including oak and beech, spruce-fir-beech, floodplain and pine forests. In addition to the quantification of total PLFAs, soil microbial biomass was measured by substrate-induced respiration and chloroform fumigation-extraction. The forest stands possess natural vegetation, representing an expression of the natural site factors, and we hypothesised that each forest type would support a specific soil microbial community. Principal component analysis (PCA) of PLFA patterns revealed that the microbial communities were compositionally distinct in the floodplain and pine forests, comprising azonal forest types, and were more similar in the oak, beech and spruce-fir-beech forests, which represent the zonal vegetation types of the region. In the nutrient-rich floodplain forests, the fatty acids 16:1ω5, 17:0cy, a15:0 and a17:0 were the most prevalent and soil pH seemed to be responsible for the discrimination of the soil microbial communities against those of the zonal forest types. The pine forest soils were set apart from the other forest soils by a higher abundance of PLFA 18:2ω6,9, which is typical of fungi and may also indicate ectomycorrhizal fungi associated with pine trees, and high amounts of PLFA 10Me18:0, which is common in actinomycetes. These findings suggest that the occurrence of azonal forest types at sites with specific soil conditions is accompanied by the development of specific soil microbial communities. The study provides information on the microbial communities in undisturbed forest soils which may facilitate interpretation of data derived from managed or even damaged or degraded forests.     

Experimental warming effects on the microbial community of a temperate mountain forest soil

Soil microbial communities mediate the decomposition of soil organic matter (SOM). The amount of carbon (C) that is respired leaves the soil as CO₂ (soil respiration) and causes one of the greatest fluxes in the global carbon cycle. How soil microbial communities will respond to global warming, however, is not well understood. To elucidate the effect of warming on the microbial community we analyzed soil from the soil warming experiment Achenkirch, Austria. Soil of a mature spruce forest was warmed by 4 °C during snow-free seasons since 2004. Repeated soil sampling from control and warmed plots took place from 2008 until 2010. We monitored microbial biomass C and nitrogen (N). Microbial community composition was assessed by phospholipid fatty acid analysis (PLFA) and by quantitative real time polymerase chain reaction (qPCR) of ribosomal RNA genes. Microbial metabolic activity was estimated by soil respiration to biomass ratios and RNA to DNA ratios. Soil warming did not affect microbial biomass, nor did warming affect the abundances of most microbial groups. Warming significantly enhanced microbial metabolic activity in terms of soil respiration per amount of microbial biomass C. Microbial stress biomarkers were elevated in warmed plots. In summary, the 4 °C increase in soil temperature during the snow-free season had no influence on microbial community composition and biomass but strongly increased microbial metabolic activity and hence reduced carbon use efficiency.     

Comparison of phospholipid fatty acid (PLFA) and total soil fatty acid methyl esters (TSFAME) for characterizing soil microbial communities

Phospholipid fatty acid (PLFA) and total soil fatty acid methyl esters (TSFAME), both lipid-based approaches used to characterize microbial communities, were compared with respect to their reliable detection limits, extraction precision, and ability to differentiate agricultural soils. Two sets of soil samples, representing seven crop types from California's Central Valley, were extracted using PLFA and TSFAME procedures. PLFA analysis required 10 times more soil than TSFAME analysis to obtain a reliable microbial community fingerprint and total fatty acid content measurement. Although less soil initially was extracted with TSFAME, total fatty acid (FA) content g⁻¹ soil (DW) was more than 7-fold higher in TSFAME- versus PLFA-extracted samples. Sample extraction precision was much lower with TSFAME analysis than PLFA analysis, with the coefficient of variation between replicates being as much as 4-fold higher with TSFAME extraction. There were significant differences between PLFA- and TSFAME-extracted samples when biomarker pool sizes (mol% values) for bacteria, actinomycetes, and fungi were compared. Correspondence analysis (CA) of PLFA and TSFAME samples indicated that extraction method had the greatest influence on sample FA composition. Soil type also influenced FA composition, with samples grouping by soil type with both extraction methods. However, separate CAs of PLFA- and TSFAME extracted samples depicted strong differences in underlying sample groupings. Recommendations for the selection of extraction method are presented and discussed.     

Functional resilience of soil microbial communities depends on both soil structure and microbial community composition

The effects of soil structure and microbial community composition on microbial resistance and resilience to stress were found to be interrelated in a series of experiments. The initial ability of Pseudomonas fluorescens to decompose added plant residues immediately after a copper or heat stress (resistance) depended significantly on which of 26 sterile soils it was inoculated into. Subsequent studies showed that both the resistance and subsequent recovery in the ability of P. fluorescens to decompose added plant residues over 28 days after stress (resilience) varied significantly between a sandy and a clay-loam soil. Sterile, sandy and clay-loam soil was then inoculated with a complex microbial community extracted from either of the soils. The resulting microbial community structure depended on soil type rather than the source of inoculum, whilst the resistance and resilience of decomposition was similarly governed by the soil and not the inoculum source. Resilience of the clay-loam soil to heat stress did not depend on the water content of the soil at the time of stress, although the physical condition of the soil when decomposition was measured did affect the outcome. We propose that soil functional resilience is governed by the physico-chemical structure of the soil through its effect on microbial community composition and microbial physiology.     

Cattle grazing increases microbial biomass and alters soil nematode communities in subtropical pastures

This study focused on examining the impacts of cattle grazing on belowground communities and soil processes in humid grasslands. Multiple components in the soil communities were examined in heavily grazed and ungrazed areas of unimproved and improved bahiagrass (Paspalum notatum Flugge) pastures in south-central Florida. By using small (1-m×1-m) sampling plots, we were able to detect critical differences in nematode communities, microbial biomass, and mineralized C and N, resulting from the patchy grazing pattern of cattle. Soil samples were collected on three occasions between June 2002 and June 2003. Microbial C and N were greater (P?0.01) in grazed than in ungrazed plots on all sampling dates. Effects of grazing varied among nematode genera. Most genera of colonizer bacterivores were decreased (P?0.10) by grazing, but more persistent bacterivores such as Euteratocephalus and Prismatolaimus were increased, as were omnivores and predators. Higher numbers of persisters indicated that grazing resulted in a more structured nematode community. Some herbivores, particularly Criconematidae, were decreased by grazing. Abundance of omnivores, predators, and especially fungivores were strongly associated with C mineralization potential. Strong correlation of microbial C and N with nematode canonical variables composed of five trophic groups illustrates important links between nematode community structure and soil microbial resources. Including the analysis of nematode trophic groups with soil microbial responses reveals detection of grazing impact deeper into the hierarchy of the decomposition process in soil, and illustrates the complexity of responses to grazing in the soil foodweb. Although highly sensitive to grazing impacts, small-scale sampling could not be used to generalize the overall impact of cattle grazing in large-scale pastures, which might be determined by the intensity and grazing patterns of various stocking densities at the whole pasture level.     

Seasonal and long-term resource-related variations in soil microbial communities in wheat-based rotations of the Canadian prairie

Like all other living organisms, microorganisms depend on nutrients, carbon and energy. Since microorganisms are central to most soil processes, the sustainable management of agricultural soils may need to consider the impact of soil fertility management on the soil microbial community. We tested the hypothesis that different rates of N and P fertilizers, and cropping frequency (modifying C input to soil) influence the size, structure and physiological condition of soil microbial populations residing in the plough layer (top 7.5 cm). For this study, we used a 37-yr old long-term wheat-based rotation experiment located in the semiarid Brown soil zone of Saskatchewan. The experiment included (1) four input treatments: (i) no N or (ii) no P fertilizer application to wheat (Triticum aestivum L.) grown in fallow-wheat-wheat (F-W-W) rotations, and (iii) recommended rates of both N and P fertilizer applied to fallow-wheat (F-W) and (iv) to F-W-W; (2) two rotation phases: fallow and wheat-after-fallow; and (3) four sampling times: 8 June, 4 July, 5 August and 16 September 2003. Increased partitioning into storage lipids of the arbuscular mycorrhizal fungi (AMF) fatty acid methyl ester (FAME) biomarker 16:1ω5 (P=0.04), suggested the accumulation of storage material under low soil N availability. Discriminant analysis detected modifications in soil microbial community structure due to cropping frequency (P=0.001) and sampling time, the effect of which was different in the fallow (P<0.0001) and wheat-after-fallow (P<0.0001) phases of the rotations. Correlation analysis of soil variables conducted in plots growing wheat revealed a dual effect of plants, which stimulated active soil microbial biomass (SMB), possibly through the release of soluble extractable C (C_sol−ext) in soil and, at the same time, SMB competed with wheat for soil water and N. The 37 y of different nutrient input treatments had no effect upon the active soil microbial biomass according to PLFA measurements, despite changes in soil resource-related variables (soil water potential, soil PO₄-P and NO₃-N fluxes, and C_sol−ext concentrations) (P?0.003). The biomass of each of three microbial populations monitored was lowest on 4 July, when the amounts of the soil resources monitored were average, and greatest on 5 August, when N, P and soil moisture availability was lowest. The temporal effect on the biomass of microbial populations seemed unrelated to variation in nutrient or water availability. We conclude that the soil microbial community is adaptable to a wide range of soil conditions. We propose therefore that the occurrence of sudden and dramatic events, such as a heavy rainfall on a dry soil, is the most important determinant of seasonal variation in active soil microbial biomass.     

The spectral quality of light influences the temporal development of the microbial phenotype at the arable soil surface

The uppermost zone of soil represents the primary interface between the above- and below-ground compartments of terrestrial ecosystems and is functionally important since it affects water infiltration, gaseous exchange, erosion processes and the habitat for surface and near-surface dwelling fauna. Two microcosm-scale experiments were conducted to investigate the development of microbial communities in the uppermost centimetre of an arable soil surface following a physical disturbance event, and to determine the effects of the spectral wavebands of light upon such development. Following establishment of freshly disturbed surfaces, subsequently exposed in the field, phenotypically distinct communities determined by PLFA analysis were apparent after eight weeks. Community structure subsequently diverged with respect to depth and time over a further 24 weeks, following consistent trajectories in principal component ordinations. Microbial biomass concentrations increased over time and were greater in the uppermost surface layer (c. 1 mm) compared to deeper layers (up to 11 mm), hypothesised to be driven by the development of photoautotrophs in the extreme surface, where chlorophyll a concentrations increased by an order-of-magnitude over the experimental period. The waveband of light reaching the soil surface, controlled experimentally by the use of selective filters, had a profound effect upon these microbiological properties. In the absence of either light or photosynthetically active radiation (PAR), chlorophyll a concentrations were negligible. Restricting admission of UV-A to the surface resulted in a fourfold increase in chlorophyll a concentration at the extreme surface, and significantly greater concentration in the sub-surface layer compared to the +UV-A treatment. In the surface and sub-surface layers, biomass C concentrations were greatest where UV-A was restricted, and least where PAR was restricted. Biomass C decreased significantly with depth where UV-A was restricted, but increased with depth where PAR was restricted. The waveband of light reaching the soil surface did not affect biomass in the deeper layers. The phenotypic community structure was affected by both the admission and restriction of UV-A, but only in the community present in the extreme surface layers, and principally in relation to the relative proportions of the PLFAs 16:0, 14:0 and 16:1ω7c. Otherwise, the community phenotype was relatively insensitive to the waveband of light reaching the soil surface. This research demonstrates that it is effectively the uppermost 1 mm that is the biotically distinct phase of the arable soil surface, with the presence and waveband of light being an important governing factor that influences its development and phenotypic properties.     

Development of soil microbial communities during tallgrass prairie restoration

Soil microbial communities were examined in a chronosequence of four different land-use treatments at the Konza Prairie Biological Station, Kansas. The time series comprised a conventionally tilled cropland (CTC) developed on former prairie soils, two restored grasslands that were initiated on former agricultural soils in 1998 (RG₉₈) and 1978 (RG₇₈), and an annually burned native tallgrass prairie (BNP), all on similar soil types. In addition, an unburned native tallgrass prairie (UNP) and another grassland restored in 2000 (RG₀₀) on a different soil type were studied to examine the effect of long-term fire exclusion vs. annual burning in native prairie and the influence of soil type on soil microbial communities in restored grasslands. Both 16S rRNA gene clone libraries and phospholipid fatty acid analyses indicated that the structure and composition of bacterial communities in the CTC soil were significantly different from those in prairie soils. Within the time series, soil physicochemical characteristics changed monotonically. However, changes in the microbial communities were not monotonic, and a transitional bacterial community formed during restoration that differed from communities in either the highly disturbed cropland or the undisturbed original prairie. The microbial communities of RG₉₈ and RG₀₀ grasslands were also significantly different even though they were restored at approximately the same time and were managed similarly; a result attributable to the differences in soil type and associated soil chemistry such as pH and Ca. Burning and seasonal effects on soil microbial communities were small. Similarly, changing plot size from 300 m² to 150 m² in area caused small differences in the estimates of microbial community structure. In conclusion, microbial community structure and biochemical properties of soil from the tallgrass prairie were strongly impacted by cultivation, and the microbial community was not fully restored even after 30 years.     

Frequent addition of wheat straw residues to soil enhances carbon mineralization rate

In many ecosystems, residues are added frequently to soil, in the form of root turnover and litter fall. However, in most studies on residue decomposition, residues are added once and there are few studies that have investigated the effect of frequent residue addition on C mineralization and N dynamics. To close this knowledge gap, we mixed mature wheat residue (C/N 122) into soil at a total rate of 2% w/w once at the start (R1×), every 16 days (R4×), every 8 days (R8×) or every 4 days (R16×). Un-amended soil served as control. All treatments were mixed every 4 days. Soil respiration was measured continuously over the 80-day incubation. Inorganic N, K₂SO₄-extractable C and N, chloroform-labile C and N (as an estimate of microbial biomass C and N), soil pH and microbial community composition were assessed every 16 days. Increasing frequency of residue addition increased C mineralization per g residue. Compared to R1×, cumulative respiration per g residue at the end of the incubation (day 80) was increased by 57, 82 and 92% in R4×, R8× and R16×, respectively. The largest differences in soil respiration per g residue occurred in the first 30 days. Despite large increases in cumulative respiration, frequent residue addition did not affect inorganic N or K₂SO₄-extractable N concentrations, chloroform-labile C and N or soil pH. Compared to the control, all residue treatments resulted in increases in chloroform-labile C and N and soil pH but decreased inorganic and K₂SO₄-extractable N. Microbial community composition was affected by residue addition, however there were no consistent differences among residue treatments. It is concluded that experiments with single residue additions may underestimate residue decomposition rates in the field. The increased C mineralization caused by frequent residue additions does not appear to be due to an increased microbial biomass or changes in microbial community composition, but rather to increased C mineralization per unit biomass.     

Interactions between microbial community structure and the soil environment found on golf courses

Approximately 0.6% of the total UK land surface is occupied by golf courses, but little investigation into the biological properties of the soil under this type of amenity turf has been reported. The soil microbiota has a significant role within the majority of nutrient cycles. In order to analyse how golf course management affects the soil microbial community, an investigation of the phenotypic microbial community structure using phospholipid fatty acid (PLFA) analysis was carried out. Principal component analysis of PLFA biomarkers indicated that there were consistent relationships between the tees, fairways and greens and the soil microbial community structure. No conclusive mechanism could be demonstrated in one-way analysis with corresponding physical parameters (P>0.05 in all cases). Cannonical correlation analysis (CCA) using 28 PLFA biomarkers concurrently with 9 physicochemical parameters showed a highly significant relationship on different playing surfaces at all of the golf courses surveyed (P<0.01). The construction and maintenance of specific areas of a golf course, irrespective of geographical location, closely reflect the physicochemical status of the soil microbial habitat.     

Exploring soil microbial communities and soil organic matter: Variability and interactions in arable soils under minimum tillage practice

This study describes an integrated approach (1) to monitor the quantity and quality of water extractable organic matter (WEOM) and size, structure and function of microbial communities in space (depth) and time, and (2) to explore the relationships among the measured properties. The study site was an arable field in Southern Germany under integrated farming management including reduced tillage. Samples of this Eutric Cambisol soil were taken in July 2001, October 2001, April 2002 and July 2002 and separated into three depths according to the soil profile (0–10 cm, 10–28 cm and 28–40 cm). For each sample, the quantity and quality (humification index, HIX) of water extractable organic matter (WEOM) were measured concomitantly with soil enzyme activities (alkaline phosphatase, β-glucosidase, protease) and microbial community size (C_mic). Furthermore, microbial community structure was characterised based on the fingerprints of nucleic acids (DNA) as well as phospholipid fatty acids (PLFA). We observed strong influences of sampling date and depth on the measured parameters, with depth accounting for more of the observed variability than date. Increasing depth resulted in decreases in all parameters, while seasonal effects differed among variants. Principal component (PC) analysis revealed that both DNA and PLFA fingerprints differentiated among microbial communities from different depths, and to a smaller extent, sampling dates. The majority of the 10 PLFAs contributing most to PC 1 were specific for anaerobes. Enzyme activities were strongly related to C_mic, which was depending on water extractable organic carbon and nitrogen (WEOC and WEON) but not to HIX. HIX and WEOM interact with the microbial community, illustrated by (1) the correlation with the number of PLFA peaks (community richness), and (2) the correlations with community PC analysis scores.     

Soil microbial communities resistant to changes in plant functional group composition

The soil community is an often ignored part of research which links plant biodiversity and ecosystem functioning despite their influence on numerous functions such as decomposition and nutrient cycling. Few consistent patterns have been detected that link plant and soil community composition. We used a removal experiment in a northern Canadian grassland to examine the effects of plant functional group identity on soil microbial community structure and function. Plant functional groups (graminoids, legumes and forbs) were removed independently from plots for five growing seasons (2003-2007) and in the fifth year effects on the soil microbial community were examined using substrate-induced respiration (SIR - a measure of metabolic diversity) and phospholipid fatty acid analysis (PLFA - a measure of microbial community composition). Removal treatments were also crossed with both a fertilizer treatment and a fungicide treatment to determine if effects of functional group identity on the soil community were context dependent. Plant functional group identity had almost no effect on the soil microbial community as measured by either SIR or PLFA. Likewise, soil properties including total carbon, pH, moisture and nutrients showed a limited response to plant removals in the fifth year after removals. We found a direct effect of fertilizer on the soil community, with fertilized plots having decreased metabolic diversity, with a decreased ability to metabolize amino acids and a phenolic acid, but there was no direct soil microbial response to fungicide. We show that in this northern Canadian grassland the soil microbial community is relatively insensitive to changes in plant functional group composition, and suggest that in northern ecosystems, where plant material is only slowly incorporated into the soil, five growing seasons may be insufficient to detect the impact of a changing plant community on the soil microbes.     

Soil microbial communities in (sub)alpine grasslands indicate a moderate shift towards new environmental conditions 11 years after soil translocation

A change in environmental conditions may result in altered soil microbial communities in alpine grasslands but the extent and direction of the change is largely unknown. The aim of our study was to investigate (i) differences in soil microbial communities across an elevation gradient of (sub)alpine grassland soils in the Swiss Alps, and (ii) the long-term effect of translocation of soil cores from a higher to a lower elevation site. The translocation of undisturbed soil cores from a high alpine site (2525 m asl) to a subalpine site near the timberline (1895 m asl) induced an effective artificial warming of 3.3 °C. We hypothesized that after longer than a decade, soil microbial community in translocated cores would differ from that at the original site but resemble the community at the new site. Results from soil phospholipid fatty acid (PLFA) analysis confirm significant differences in microbial communities between sites and a shift in total microbial biomass (TMB) and proportional distribution of structural groups in the translocated cores towards the lower elevation community. Patterns related to translocation were also observed as shifts in the fractional biomass of ectomycorrhizal and arbuscular fungi, and in relative contents of several structural groups. Hence, soil microbial community activity and diversity indicate a moderate shift towards new site conditions after 11 years and therefore, our data suggest slow responses of microbial communities to environmental changes in alpine soils.     

The effect of electrokinetics on soil microbial communities

There is growing interest in the potential of applying an electric field to soil to move and stimulate the degradation of contaminants, but we know little of the impact of this approach on exposed microbial communities. The effect of electrokinetics (3.14 A m⁻²) on soil bacterial and fungal communities was studied using soil cartridge microcosms (13 cm×5.4 cm×5.9 cm). After 27 days of electrokinetics, a zone of low pH (<4) was detected close to the anode. Soil exposed to electrokinetics and immediately adjacent to the anode demonstrated an increase in carbon substrate utilisation potential (≤290%) and microbial respiration rates. The diversity and structure of the bacterial community showed little response to electrokinetics, with the exception of soil close to the anode. Here, an increase in the percentage of Gram-positive species isolated was identified, most notably of Bacillus megaterium. Overall, the only detectable response of the microbial community was observed in soil immediately adjacent to the anode. The results of this study provide evidence that the application of electrokinetics has no serious negative effect on ‘soil microbial health’, thus endorsing its validity as a viable soil remediation technology.     

Land-use intensification and agroforestry in the Kenyan highland: Impacts on soil microbial community composition and functional capacity

This study investigates microbial communities in soil from sites under different land use in Kenya. We sampled natural forest, forest plantations, agricultural fields of agroforestry farms, agricultural fields with traditional farming and eroded soil on the slopes of Mount Elgon, Kenya. We hypothesised that microbial decomposition capacity, biomass and diversity (1) decreases with intensified cultivation; and (2) can be restored by soil and land management in agroforestry. Functional capacity of soil microbial communities was estimated by degradation of 31 substrates on Biolog EcoPlates™. Microbial community composition and biomass were characterised by phospholipid fatty acid (PLFA) and microbial C and N analyses. All 31 substrates were metabolised in all studied soil types, i.e. functional diversity did not differ. However, both the substrate utilisation rates and the microbial biomass decreased with intensification of land use, and the biomass was positively correlated with organic matter content. Multivariate analysis of PLFA and Biolog EcoPlate™ data showed clear differences between land uses, also indicated by different relative abundance of PLFA markers for certain microorganism groups. In conclusion, our results show that vegetation and land use control the substrate utilisation capacity and microbial community composition and that functional capacity of depleted soils can be restored by active soil management, e.g. forest plantation. However, although 20–30 years of agroforestry farming practises did result in improved soil microbiological and chemical conditions of agricultural soil as compared to traditional agricultural fields, the change was not statistically significant.     

Organic compounds that reach subsoil may threaten groundwater quality; effect of benzotriazole on degradation kinetics and microbial community composition

Toxic compounds in soils threaten groundwater quality in two ways: as potential contaminants themselves, and by retarding the microbial degradation of other organic compounds, thus enhancing their deep penetration. Benzotriazole (BTA) is a chemical with versatile industrial applications, used in large quantities worldwide, and represents a potential threat to the environment due to its apparent toxicity and recalcitrance. When used as an additive in aircraft deicing/antiicing fluid on airports, substantial spills of these mixtures and jet fuel will inevitably reach the soil. We have investigated the subsoil (1-2 m depth) microbial degradation and growth on four relevant organic substrates found in airport run-off (acetate, formate, glycol and toluene) in the presence of concentrations of BTA which can be found in airport run-off. Monitoring CO₂ evolution showed growth-dependent degradation rates for all substrates (sigmoid CO₂ accumulation curves), which were significantly affected by BTA. The mineralization of acetate was only moderately retarded and only by the highest BTA concentration used (400 mg l⁻¹ in soil solution); formate and glycol mineralization was substantially retarded at 200 mg l⁻¹, and toluene mineralization already at 10 mg l⁻¹ BTA. Mass balances (fraction of added C recovered as CO₂) suggested that the microbial growth yield (g biomass-C formed per g substrate C) was severely reduced with increasing concentrations of BTA. The analysis of phospholipid fatty acids (PLFA) demonstrated that Gram-negative bacteria were dominating among the organisms growing on all four substrates. The total amount of PLFA increased with approximately 1000 pmol PLFA g⁻¹ soil in response to a dose of 0.93 μmol glycol-C g⁻¹ soil, but this increase was gradually reduced with increasing BTA concentrations. This was in agreement with C mass balances based on CO₂ measurements, verifying that BTA severely reduced the growth yields. The response of individual PLFA's to BTA and substrates demonstrated that non-growing organisms were largely unaffected (i.e. the PLFA's of which the absolute amounts did not increase in response to substrates were not affected by BTA), whereas those which were growing on the added substrates were uniformly reduced by BTA (all the PLFA's which increased in response to the substrates were negatively affected by BTA). The results suggest that BTA functions as an uncoupler, i.e. a substance that reduces the yield of ATP per mole of substrate used, or that the defence mechanisms represent a large energy burden to all microbial cells.     

Contrasting soil microbial responses to fertilizers and herbicides in a canola-barley rotation

The combination of high input costs and low commodity prices is forcing some farmers to consider reducing crop inputs like seed, fertilizer and herbicides. In a field trial in which different canola (Brassica napus L.) and barley (Hordeum vulgare L.) inputs were subtracted from a full package, or added to an empty package, we studied the effects of full or reduced fertilizer and herbicide inputs on soil microbiological characteristics at two sites from 2005 to 2008. The full package consisted of a high-yielding crop variety seeded at an optimum rate, with fertilizers and herbicides applied at recommended rates. The empty package consisted of a less expensive, low-yielding crop variety seeded at a low rate, with no fertilizer or herbicide applied. Between these two extremes were treatments in which fertilizers or herbicides were applied at 50% of recommended rates or not at all. Each treatment was repeated year after year in the same plot, i.e., treatment effects were cumulative. Fertilizer effects on soil microbial biomass C (MBC), β-glucosidase enzyme activity and bacterial functional diversity (based on community-level physiological profiles) were usually positive. Reduced fertilizer application rates reduced the beneficial fertilizer effects. Significant herbicide effects on soil microbiological properties occurred less often, were smaller in magnitude than fertilizer effects, and were mostly negative. Reduced herbicide rates reduced the deleterious herbicide effects. These significant fertilizer and herbicide effects were observed in canola more than barley, and mostly in the final year of the study, indicating the cumulative nature of treatment effects over time. Therefore, repeated applications of agricultural inputs like fertilizers and herbicides can have more significant effects on soil biology and biological processes than single applications indicate.     

Residue chemistry and microbial community structure during decomposition of eucalypt, wheat and vetch residues

Previous studies have shown that residue chemistry and microbial community structure change during decomposition, however little is known about the relationship between C-chemistry and microbial community structure. To address this knowledge gap, we studied C-chemistry and microbial community structure during the decomposition of eucalypt, wheat and vetch residues with and without additional inorganic N. Bags containing ground residues of eucalypt, wheat, and vetch were buried in sand microcosms after inoculation with a diverse microbial community. Respiration was measured over an incubation period of 150 days. At different times during incubation, total C and N of the residues were analysed and residue carbon chemistry was determined by ¹³C-NMR (nuclear magnetic resonance) spectroscopy. Microbial communities were assessed by phospholipid fatty acid (PLFA) analyses.Results indicated that during decomposition, residue C-chemistry and microbial community composition changed over time and differed between residue types. Changes in microbial community structure were associated with changes in residue C-chemistry, mainly the relative content of aryl-C and O-alkyl-C. Addition of N increased cumulative respiration, altered C-chemistry during decomposition, particularly in high C/N residues (wheat and eucalypt), and changed microbial succession leading to an earlier establishment of a stable microbial community structure. N addition to eucalypt and wheat reduced the decomposition of aryl-C compounds.     

Microbial community phenotypic profiles change markedly with depth within the first centimetre of the arable soil surface

The nature of the first few millimetres of a soil horizon strongly affects water infiltration rates, generation of run-off, and soil detachment. Whilst much is known about the physics and erosion of soil surfaces at this scale, little is known about their microbiology, particularly in temperate arable systems. This investigation aimed to discover whether any early colonisation stages of microbiotic crusts exist within the soil surface of temperate arable systems. The phenotypic structure of the microbial community was measured by means of phospholipid fatty acid analysis (PLFA) in soils sampled from the surface of arable fields that had been either cultivated 4 weeks previously or left undisturbed for 4-6 months. Within the top circa 1 mm of the soil that had been undisturbed for 6 months or more, distinct microbial communities were found to be present, which were statistically significantly different from the communities found in subsequent depths to circa 10 mm, where differences between communities were less pronounced. The PLFA responsible for the majority of the variation seen between depths was 16:0, the proportion of which was shown to decrease with depth. This was not the case in the recently cultivated soils, where communities were more homogeneous with respect to depth.