Upland Grassland Soil Research Articles

Acidification of previously limed upland pastures – An overlooked flood risk factor?

In low-lying land, the impact of agriculture on flooding has focussed on soil compaction, whilst in the uplands there has been more interest in the influence of afforestation. The potential effect of acidification of previously limed upland grassland soils on this risk has been overlooked. The marginal economics of upland farms has led to inadequate lime application on these grasslands. In Wales, UK, agronomic improvement of upland acid grasslands with liming was widespread in the last century. The extent and topographical distribution of this land use in Wales was estimated and these characteristics were mapped in four catchments studied in more detail. Then 41 sites on improved pastures within the catchments were sampled, where lime had not been applied for periods of between two and 30 years; unimproved acid pastures adjacent to five of these sites were also sampled. Soil pH, organic matter, infiltration rates and earthworm populations were recorded. Grasslands at risk of acidification without maintenance liming were estimated to cover almost 20 % of upland Wales. The majority of these grasslands were located on steeper slopes (gradients >7o) where any reduction in infiltration would promote surface runoff and limit rainwater retention. The extent of these pastures varied markedly between the four study catchments. There was a 6-fold reduction in infiltration rates between high and low pH soils, and this trend was correlated with reductions in anecic earthworm abundance. The vertical burrows of these earthworms are important for infiltration and no such earthworms were present in the most acidic soils. Recently limed soils had infiltration rates similar to those of unimproved acid pastures. Soil acidification has the potential to exacerbate flood risk but further research is needed to assess the extent of any impact. Modelling of catchment specific flood risk should include the extent of upland soil acidification as an additional land use factor.

Upland grassland habitats and agri‐environment schemes change soil microarthropod abundance

Abstract Agricultural intensification is a major driver of biodiversity loss, with the implementation of Agri‐Environment Schemes (AESs) being a widespread policy designed to prevent further loss, and to maintain or restore ecosystem health. Upland grassland soils are disproportionately impacted by intensification including drainage, artificial fertiliser use, compaction and erosion while the effect of AES management on below‐ground microarthropods, which mediate a range of ecosystem processes, is largely unknown. This study tested the effects of AES management of upland grasslands on microarthropod communities using a large‐scale factorial field experiment. The study focused on microarthropods with distinct taxa‐specific responses observed. Oribatid mite abundance was associated with wetter, more extensive pastures with native grasses characteristic of semi‐improved grassland managed by AESs, whereas Collembola abundance was associated with drier, more intense, reseeded (Lolium perenne dominated) pastures characteristic of conventionally managed improved grasslands. Differences in taxa responses may be driven by life‐history traits and resilience to disturbance. There was no net effect of management, habitat or their interaction on total microarthropod family diversity or abundance. Synthesis and Applications. Results from this study suggest that in floristically simple upland agroecosystems the impact of agri‐environment measures on below‐ground soil biota are context‐dependent (differing between contrasting grassland types), and taxa‐specific, rather than leading to general increases in biodiversity per se, and the responses of soil microarthropods are driven by environmental variation caused by overall management of grassland fields. Measures should aim to increase the availability of areas of semi‐improved grassland, with native plant mixtures, adjacent to improved grasslands, to maximise the habitats available to soil microarthropods.

Upland Soil Cluster Gamma dominates methanotrophic communities in upland grassland soils

Aerobic methanotrophs in upland soils consume atmospheric methane, serving as a critical counterbalance to global warming; however, the biogeographic distribution patterns of their abundance and community composition are poorly understood, especial at a large scale. In this study, soils were sampled from 30 grasslands across >2000 km on the Qinghai-Tibetan Plateau to determine the distribution patterns of methanotrophs and their driving factors at a regional scale. Methanotroph abundance and community composition were analyzed using quantitative PCR and Illumina Miseq sequencing of pmoA genes, respectively. The pmoA gene copies ranged from 8.2 × 105 to 1.1 × 108 per gram dry soil. Among the 30 grassland soil samples, Upland Soil Cluster Gamma (USCγ) dominated the methanotroph communities in 26 samples. Jasper Ridge Cluster (JR3) was the most dominant methanotrophic cluster in two samples; while Methylocystis, cluster FWs, and Methylobacter were abundant in other two wet soil samples. Interestingly, reanalyzing the pmoA genes sequencing data from existing publications suggested that USCγ was also the main methanotrophic cluster in grassland soils in other regions, especially when their mean annual precipitation was <500 mm. Canonical Analysis of Principal Coordinates including all soil samples indicated that the methanotrophic community composition was significantly correlated with local environmental factors, among which mean annual precipitation and pH showed the strongest correlations. Variance partitioning analysis showed that environmental factors and spatial distance were significant factors affecting the community structure of methanotrophs, and environmental properties were more important factors. Collectively, these findings indicate that atmospheric methane may be mainly oxidized by USCγ in upland soils. They also highlight the key role of water availability and pH in determining the abundance and community profiles of grassland soil methanotrophs.

Soil organic carbon stock and fractional distribution in upland grasslands

Upland grassland soils are an important terrestrial carbon (C) store and provide vital ecosystem services such as climate regulation. The C stocks in these soils are subject to changes due to management activities. In this study, we compared soil organic C (SOC) stocks and fractions under traditional hay meadow and conventional (silage or permanent) pasture management regimes at two upland grassland locations within the Yorkshire Dales, northern England, United Kingdom. Stocks of SOC in the top 15cm of the grassland soils were determined from bulk soil samples and in soil fractions after a combination of physical and chemical fractionation. Results showed that these upland grasslands stored significant amounts of organic C in the top 15cm of their soils, ranging from 58.93±3.50 to 100.69±8.64Mgha−1. Overall, there was little contrast between the sites except that soil C stock was significantly higher under the Nidderdale silage pasture that received the highest nitrogen (N) inputs, possibly due to enhanced vegetation growth and subsequent litter return to the soil. No effect of soil parent material was observed on soil C stock due to liming-induced increase in the pH of Nidderdale soils and neither management activities nor soil parent materials affected the distribution of soil C into different fractions. In all sites, about 70% of the SOC stock was protected in stable soil fractions, specifically in the mineral soil mass, indicating a potential for the grasslands to contribute in climate change mitigation. However, due to the rapid turnover of the labile C pool and especially in a changing climate, it is recommended that further research be carried out in order to understand the turnover rates of labile C pools and whether management activities alter the distribution between the decomposition products. This will help to better understand whether rapid turnover of labile C will negate the likely benefits of stable C pools to climate change mitigation.

Plant soil interactions alter carbon cycling in an upland grassland soil

Soil carbon (C) storage is dependent upon the complex dynamics of fresh and native organic matter cycling, which are regulated by plant and soil-microbial activities. A fundamental challenge exists to link microbial biodiversity with plant-soil C cycling processes to elucidate the underlying mechanisms regulating soil carbon. To address this, we contrasted vegetated grassland soils with bare soils, which had been plant-free for 3 years, using stable isotope (13C) labeled substrate assays and molecular analyses of bacterial communities. Vegetated soils had higher C and N contents, biomass, and substrate-specific respiration rates. Conversely, following substrate addition unlabeled, native soil C cycling was accelerated in bare soil and retarded in vegetated soil; indicative of differential priming effects. Functional differences were reflected in bacterial biodiversity with Alphaproteobacteria and Acidobacteria dominating vegetated and bare soils, respectively. Significant isotopic enrichment of soil RNA was found after substrate addition and rates varied according to substrate type. However, assimilation was independent of plant presence which, in contrast to large differences in 13CO2 respiration rates, indicated greater substrate C use efficiency in bare, Acidobacteria-dominated soils. Stable isotope probing (SIP) revealed most community members had utilized substrates with little evidence for competitive outgrowth of sub-populations. Our findings support theories on how plant-mediated soil resource availability affects the turnover of different pools of soil carbon, and we further identify a potential role of soil microbial biodiversity. Specifically we conclude that emerging theories on the life histories of dominant soil taxa can be invoked to explain changes in soil carbon cycling linked to resource availability, and that there is a strong case for considering microbial biodiversity in future studies investigating the turnover of different pools of soil carbon.

Cattle impact on composition of archaeal, bacterial, and fungal communities by comparative fingerprinting of total and extracellular DNA

The aim of the study was to evaluate the impact of cattle overwintering husbandry on composition of upland grassland soil archaeal, bacterial, and fungal communities by comparative fingerprinting (SSU rRNA denaturing gradient gel electrophoresis (DGGE)) of total (tDNA) and extracellular DNA (eDNA) extracted from three differently impacted soils (severely, moderately, and non-impacted soil) and cattle excrements. Cattle excrements carried a significant amount of viable microorganisms and eDNA, and as a result of its high external returns, the amounts of extractable eDNA in soil increased with cattle impact, being positively correlated with soil microbial biomass and activity. The soil eDNA fraction (2.8 to 5.7 μg g−1 dw) significantly contributed to the soil metagenome, representing 18–31 % of soil tDNA. The largest shift in soil community structure was observed for Archaea, followed by fungi and bacteria, indicating that soil bacteria possess the highest resilience to cattle-induced changes. Cattle excrements showed more diverse bacterial than archaeal and fungal communities. The specific DGGE bands of cattle excrements were also observed in community profiles of cattle-impacted soils, confirming the effect of cattle husbandry on composition of soil microbial communities, probably as a result of introduced rumen-borne microbes. Similar changes were also reflected in the eDNA-derived DGGE profiles, suggesting a higher contribution of bacteria to soil extracellular metagenome than Archaea and fungi. This study provided first evidences about the extracellular mobilome in cattle-impacted soils, carrying also genetic information of Archaea, and its susceptibility to impact by outdoor cattle husbandry.

Multivariate analysis of soils: microbial biomass, metabolic activity, and bacterial‐community structure and their relationships with soil depth and type

AbstractA multivariate statistical approach based on a large data set of abiotic and biotic variables was used to classify four contrasting‐land‐use soils. Soil samples were collected at increasing depth from a calcareous agricultural soil, a temperate upland grassland soil, a moderately acidic agricultural soil, and an acidic pine forest soil. Analytical investigations were carried out by using a combination of conventional physical, chemical, and biochemical methods coupled with denaturing gradient gel electrophoresis (DGGE) community fingerprinting of PCR‐amplified 16S rRNA gene‐coding fragments from soil‐extracted total‐community DNA. The data set of soil physical, chemical, and biochemical variables was reduced in dimensionality by means of a principal‐component‐analysis (PCA) procedure. Compositional shifts in soil bacterial‐community structure were analyzed through a clustering algorithm that allowed identifying six main bacterial‐community clusters. DGGE fingerprinting clusters were further analyzed by discriminant analysis (DA) using extracted PCA components as explanatory variables. Soil organic matter–related pools (TOC, TN) and functionally related active pools (microbial biomass C and N, K2SO4‐extractable C) significantly decreased with soil depth, and resulted statistically linked to one other and positively related to enzymatic activities (acid phosphatase, arylsulfatase, β‐glucosidase, dehydrogenase, hydrolysis of fluorescein diacetate) and silt content. Besides organic‐C gradients, pedogenetic‐driven physico‐chemical properties, and possibly soil thermal and moisture regimes seemed to play a key role in regulating size and energetic ecophysiological status of soil microbial communities. DGGE analysis showed that contrasting horizons were conducive to the dominance of particular bacterial ribotypes. DA revealed that the bacterial‐community structure was mainly influenced by organic matter–related variables (TOC, TN, CEC, Cflush, Nflush, Extr‐C), chemical properties such as pH, CaCO3, and EC, together with textural properties. Results indicate that, beyond land use or plant cover, pedogenetic‐driven physico‐chemical conditions changing with soil type and depth are the key factors regulating microbial size and activity, and determining the genetic structure of bacterial community.

Responses of Ammonia-Oxidising Bacterial Communities to Nitrogen, Lime, and Plant Species in Upland Grassland Soil

Agricultural improvement of seminatural grasslands has been shown to result in changes to plant and microbial diversity, with consequences for ecosystem functioning. A microcosm approach was used to elucidate the effects of two key components of agricultural improvement (nitrogen addition and liming) on ammonia-oxidising bacterial (AOB) communities in an upland grassland soil. Plant species characteristic of unimproved and improved pastures (A. capillarisandL. perenne) were planted in microcosms, and lime, nitrogen (NH4NO3), or lime plus nitrogen added. The AOB community was profiled using terminal restriction fragment length polymorphism (TRFLP) of theamoAgene. AOB community structure was largely altered byNH4NO3addition, rather than liming, although interactions between nitrogen addition and plant species were also evident. Results indicate that nitrogen addition drives shifts in the structure of key microbial communities in upland grassland soils, and that plant species may play a significant role in determining AOB community structure.

Vegetation Affects the Relative Abundances of Dominant Soil Bacterial Taxa and Soil Respiration Rates in an Upland Grassland Soil

Plant-derived organic matter inputs are thought to be a key driver of soil bacterial community composition and associated soil processes. We sought to investigate the role of acid grassland vegetation on soil bacterial community structure by assessing bacterial diversity in combination with other soil variables in temporally and spatially distinct samples taken from a field-based plant removal experiment. Removal of aboveground vegetation resulted in reproducible differences in soil properties, soil respiration and bacterial diversity. Vegetated soils had significantly increased carbon and nitrogen concentrations and exhibited higher rates of respiration. Molecular analyses revealed that the soils were broadly dominated by Alphaproteobacterial and Acidobacterial lineages, with increased abundances of Alphaproteobacteria in vegetated soils and more Acidobacteria in bare soils. This field-based study contributes to a growing body of evidence documenting the effect of soil nutrient status on the relative abundances of dominant soil bacterial taxa, with Proteobacterial taxa dominating over Acidobacteria in soils exhibiting higher rates of C turnover. Furthermore, we highlight the role of aboveground vegetation in mediating this effect by demonstrating that plant removal can alter the relative abundances of dominant soil taxa with concomitant changes in soil CO(2)-C efflux.

Synthetic sheep urine alters fungal community structure in an upland grassland soil

Abstract Agricultural improvement (fertilisation, liming, intensification of grazing) of acidic upland pastures results in loss of indigenous flora and notable changes in microbial community structure. Such practices have recently raised concerns regarding the possible impacts on natural ecosystem biodiversity and functioning. The effects of synthetic sheep urine (SSU) and plant species on fungal community structure in upland grassland microcosms were investigated. Plant species typical of agriculturally unimproved ( Agrostis capillaris ) and improved ( Lolium perenne ) pastures were treated with low, medium or high concentrations of SSU, with harvests carried out 10 d and 50 d after SSU application. Root biomass was negatively affected by SSU addition whereas shoot biomass did not display any significant change. Fungal richness (number of operational taxonomic units) was negatively correlated with SSU concentration ( p p Multi-dimensional scaling plots revealed significant changes in fungal community composition, depending on concentration of SSU and plant species type, while canonical correspondence analysis also emphasised the importance of interacting environmental variables. In addition, SIMPER analyses supported the finding that shifts in fungal community composition under different SSU and plant treatments had occurred. Overall, while SSU appeared to be influential in determining fungal community structure, community changes were largely driven by interacting environmental factors. This study contributes to our understanding of the potential implications of intensified farming, in particular increased pressure from grazing animals, on fungal community structure in semi-natural grassland systems.

Impact of sheep urine deposition and plant species on ammonia-oxidizing bacteria in upland grassland soil

The effects of different concentrations of synthetic sheep urine and plant species on ammonia-oxidizing bacterial (AOB) communities in an upland grassland soil were investigated using a microcosm approach. Plant species characteristic of unimproved and improved agricultural pastures (Agrostis capillaris and Lolium perenne, respectively) were planted in soil microcosms, and different levels of synthetic sheep urine were applied, with harvests 10 and 50 days following urine application. Shifts in the community structure of the AOB were investigated using terminal restriction fragment length polymorphism of amoA amplicons. Species richness and diversity were significantly altered by synthetic sheep urine addition and time depending on plant species type. Principal coordinate analysis revealed that AOB community structure was largely dependent on interactions between sheep urine deposition, plant species, and time after urine application, while significant changes in AOB structure were also revealed by similarity percentage analysis. The results of this study suggested that high levels of sheep urine, combined with floristic changes that are characteristic of agricultural intensification, can contribute to temporal and spatial changes in the structure of key bacterial communities in upland grassland soil. Changes in AOB community structure could potentially affect important soil processes, such as nitrification, with subsequent implications for nutrient cycling in agricultural systems.

Phosphate Addition and Plant Species Alters Microbial Community Structure in Acidic Upland Grassland Soil

Agricultural improvement (addition of fertilizers, liming) of seminatural acidic grasslands across Ireland and the UK has resulted in significant shifts in floristic composition, soil chemistry, and microbial community structure. Although several factors have been proposed as responsible for driving shifts in microbial communities, the exact causes of such changes are not well defined. Phosphate was added to grassland microcosms to investigate the effect on fungal and bacterial communities. Plant species typical of unimproved grasslands (Agrostis capillaris, Festuca ovina) and agriculturally improved grasslands (Lolium perenne) were grown, and phosphate was added 25 days after seed germination, with harvesting after a further 50 days. Phosphate addition significantly increased root biomass (p < 0.001) and shoot biomass (p < 0.05), soil pH (by 0.1 U), and microbial activity (by 5.33 mg triphenylformazan [TPF] g(-1) soil; p < 0.001). A slight decrease (by 0.257 mg biomass-C g(-1) soil; p < 0.05) in microbial biomass after phosphate addition was found. The presence of plant species significantly decreased soil pH (p < 0.05; by up to 0.2 U) and increased microbial activity (by up to 6.02 mg TPF g(-1) soil) but had no significant effect on microbial biomass. Microbial communities were profiled using automated ribosomal intergenic spacer analysis. Multidimensional scaling plots and canonical correspondence analysis revealed that phosphate addition and its interactions with upland grassland plant species resulted in considerable changes in the fungal and bacterial communities of upland soil. The fungal community structure was significantly affected by both phosphate (R = 0.948) and plant species (R = 0.857), and the bacterial community structure was also significantly affected by phosphate (R = 0.758) and plant species (R = 0.753). Differences in microbial community structure following P addition were also revealed by similarity percentage analysis. These data suggest that phosphate application may be an important contributor to microbial community structural change during agricultural management of upland grasslands.

Effect of Sheep Urine Deposition on the Bacterial Community Structure in an Acidic Upland Grassland Soil

The effect of the addition of synthetic sheep urine (SSU) and plant species on the bacterial community composition of upland acidic grasslands was studied using a microcosm approach. Low, medium, and high concentrations of SSU were applied to pots containing plant species typical of both unimproved (Agrostis capillaris) and agriculturally improved (Lolium perenne) grasslands, and harvests were carried out 10 days and 50 days after the addition of SSU. SSU application significantly increased both soil pH (P < 0.005), with pH values ranging from pH 5.4 (zero SSU) to pH 6.4 (high SSU), and microbial activity (P < 0.005), with treatment with medium and high levels of SSU displaying significantly higher microbial activity (triphenylformazan dehydrogenase activity) than treatment of soil with zero or low concentrations of SSU. Microbial biomass, however, was not significantly altered by any of the SSU applications. Plant species alone had no effect on microbial biomass or activity. Bacterial community structure was profiled using bacterial automated ribosomal intergenic spacer analysis. Multidimensional scaling plots indicated that applications of high concentrations of SSU significantly altered the bacterial community composition in the presence of plant species but at different times: 10 days after application of high concentrations of SSU, the bacterial community composition of L. perenne-planted soils differed significantly from those of any other soils, whereas in the case of A. capillaris-planted soils, the bacterial community composition was different 50 days after treatment with high concentrations of SSU. Canonical correspondence analysis also highlighted the importance of interactions between SSU addition, plant species, and time in the bacterial community structure. This study has shown that the response of plants and bacterial communities to sheep urine deposition in grasslands is dependent on both the grass species present and the concentration of SSU applied, which may have important ecological consequences for agricultural grasslands.

Isotopic detection of recent photosynthate carbon flow into grassland rhizosphere fauna

In this study, we measured the incorporation of recent photosynthate-C inputs into active rhizosphere fauna (earthworms, enchytraeids, mites and collembolans) in an upland grassland soil under natural environmental conditions. This was achieved by means of a 13CO 2 pulse-chase experiment made during the growing season, followed by a 20-day dynamic sampling of soil fauna for 12C/ 13C analysis by IRMS. The effect of post- 13C labelling defoliation (cutting) on fauna 12C/ 13C ratios was also examined. Results showed that earthworms made up over 93% of the extracted fauna biomass, while mites, collembolans and enchytraeids together accounted for less than 7%. All fauna groups showed evidence of tracer 13C in their tissues within a week of 13CO 2 pulse labelling in both control and cut treatments. Cutting significantly increased the amount of tracer 13C entering the organisms ( P = 0.0002 ). Similarly, the fauna group also had a significant effect ( P = 0.0001 ). Time did not have any effect on fauna 13C content between groups as differences were only significant at the last sampling occasion. The interaction time×animal group, however, had a significant effect ( P = 0.0054 ). Collembolans accounted for most of the tracer 13C measured within the fauna biomass, i.e. mean±standard deviation of 44.78±12.75% and 44.29±14.69% of fauna 13C in control and cut treatments, respectively. Mites and earthworms contained between 22.13% and 28.45%, and enchytraeids less than 6% of the tracer 13C. We conclude that, during the growing season, there was a rapid incorporation of recent photosynthate-C into rhizosphere mesofauna. This carbon transfer was most significantly increased by defoliation in mites and collembolans ( P < 0.01 ). These results provide evidence that soil foodweb carbon dynamics are not solely underpinned by detrital decomposition but are also affected by short-term plant rhizodeposition patterns.

Seasonal influences on fungal community structure in unimproved and improved upland grassland soils

Seasonal and management influences on the fungal community structure of two upland grassland soils were investigated. An upland site containing both unimproved floristically diverse (U4a) and improved mesotrophic (MG7b) grassland types was selected. Samples from both grassland types were taken at five times in one year. Soil fungal community structure was assessed using fungal automated ribosomal intergenic spacer analysis (ARISA), a DNA-profiling approach. A grassland management regime was found to strongly affect fungal community structure, with fungal ARISA profiles from unimproved and improved grassland soils differing significantly. The number of fungal ribotypes found was higher in unimproved than improved grassland soils, providing evidence that improvement may reduce the suitability of upland soil as a habitat for specific groups of fungi. Seasonal influences on fungal community structure were also noted, with samples taken in autumn (October) more correlated with change in ribotype profiles than samples from other seasons. However, seasonal variation did not obscure the measurement of differences in the fungal community structure that were due to agricultural improvement, with canonical correspondence analysis indicating grassland type had a stronger influence on fungal profiles than did season.

The abundance, richness and functional role of soil meso- and macrofauna in temperate grassland—A case study

This paper reviews the abundance and species richness, and factors that cause these parameters to vary, of mesofauna and macrofauna in an upland grassland soil studied intensively under the NERC Soil Biodiversity Programme. The concept that competitive exclusion does not act within soil communities is reviewed in light of these findings, which are placed in the context of the wider literature relating to earthworm, enchytraeid, collembolan and mite diversity, and factors that influence these, with particular reference to land management. The second half of the paper reviews laboratory and field 13C-tracer studies, carried out under the programme, that assess linkages between specific biota and ecosystem processes. The concept of functional redundancy in soil food webs is discussed in the context of these studies. We conclude that competitive interactions occur most widely amongst soil macrofauna, and that competitive exclusion amongst mesofauna is potentially limited by both fine-scale spatial heterogeneity and predation. The most profound impacts of soil fauna on soil properties at Sourhope appeared to be due to the presence of macrofauna in soil communities. There was also evidence for functional redundancy at the species level amongst soil biota, but this was dependent upon which ecosystem process was measured. We conclude that it is likely that functional redundancy at the species level occurs most widely in species rich faunal groups with generalist feeding behaviour.

Relationships between biodiversity and soil structure and function: Evidence from laboratory and field experiments

This paper offers new insights into the structure and carbon dynamics of an upland organo-mineral soil using a range of complementary research methods including soil micromorphology and laser sizing of water-stable aggregates to quantify the development of aggregates and pore space and the spatial location of carbon additions. Examination of soil thin sections revealed that the organic horizons and uppermost organo-mineral horizon are dominated by excrement from meso- and macrofauna, a finding which highlights the importance of soil fauna in reprocessing organic material. Much of this excrement had fused into undifferentiable forms, but some could be associated specifically with enchytraeids and earthworms. The specific objective of the study was to determine the extent to which alterations in soil faunal diversity and activity result in changes to the soil structure. This was done through a series of experiments at different scales in which faunal communities were manipulated by liming to increase soil pH, additions of nitrogen to increase faunal activity and additions of fauna of different sizes. Induced changes in soil fauna did not produce marked and immediate responses in either void space or water-stable aggregation; changes induced by liming and additions of fauna were small and often statistically not significant. When the location of newly photosynthesised C was traced using pulse labelling with 13CO 2 it was found that there was a nucleated distribution with low concentrations of new C found in between 0 and 40% of aggregates derived from faunal excrement. Bacterial numbers on excrement were also not consistently high, although some excrement was colonised more densely, probably a result of more recent formation. This upland grassland soil is very largely the product of faunal activity over the course of its development. However, it is highly resistant to change, as current faunal activity appears to have a relatively small impact on its structure, and thus contains many elements which have functional redundancy.

Functional resilience of microbial communities from perturbed upland grassland soils to further persistent or transient stresses

The microbial functioning of soils following perturbation was assessed at a temperate upland grassland site, maintained by the Soil Biodiversity and Ecosystem Function Programme at Sourhope Research Station, Scotland. Published results indicated that the soil microbial communities were resilient to these initial perturbations; in this paper we tested whether they were equally resilient to a subsequent perturbation. Soil samples were taken from field plots receiving treatments that represented different forms of perturbation, viz. reseeding, application of sewage-sludge, biocide or nitrogen plus lime, and a non-perturbed control. Functional resilience following further perturbation comprising a transient heat or persistent copper perturbation was assessed over 28 days, by monitoring the short-term decomposition of added plant residues. Bacterial community structure was assessed by DGGE separation of eubacterial 16S rDNA PCR products. PCR-DGGE did not distinguish any significant difference ( P > 0.05 ) between the bacterial communities of soils under different treatments, showing differences only between treated soils and the untreated, control soils. Two days after the application of stresses, functional capability differed in soils under different treatments. Soil samples from all the treated plots were less resilient to heat stress than soil from control plots. The initial reduction in decomposition following the addition of copper differed between treatments, but function had not recovered in any of the Cu-amended soils within 28 days. Soil resilience varied according to the type and duration of stress applied, microbial activity, soil characteristics and treatment regimes. The initial resistance of function to stress was not predictive of recovery of function over time.

Spatial heterogeneity in the relocation of added 13C within the structure of an upland grassland soil

A pulse of 13CO 2 was added to the above ground vegetation in an upland grassland to determine the effects of faunal diversity on the flux of carbon to the surface horizons of the soil. Faunal diversity was manipulated by liming and biocide treatments for three years prior to the pulse addition. The relocation of 13C within roots and rhizosphere soil was determined by analysis of samples of bulk soil and of specific features identified on soil thin sections on four dates after the addition of the 13CO 2 pulse. Analysis of bulk soils showed only a small enrichment in 13C and no significant effects of the treatments. Analysis by isotope ratio mass spectrometry of the products of in situ laser combustion of root material and aggregates formed from faunal excrement showed that the distribution of the newly photosynthesised 13C is very localised, with large spatial variability in soil and root δ 13C at scales of less than 1 mm. δ 13C values ranged from the natural abundance level of around −28‰ to −4.9‰ in roots and to −8.4‰ in aggregates. The small pulse and large spatial variability masked any effects of the liming and biocide treatments in these soils. However, the variability in the relocation of newly photosynthesised carbon may help to explain the large spatial variability found in bacterial numbers at the sub-mm scale within soils and emphasises the importance of the accessibility of substrates to decomposers in undisturbed structured soils.

Diversity and function of decomposer fungi from a grassland soil

Despite the substantial interest to ecologists of the relationship between species diversity and ecosystem functioning, little is known about how the high species richness of decomposer (saprotrophic) fungi and their relative frequencies of occurrence influence the decomposition of organic matter. Three experiments were conducted to test the ability of culturable saprotrophic fungal isolates to utilise a range of artificial and more natural substrates that occur in organic matter, with the aims of (1) characterising the functional potential of ‘common’ and ‘occasional’ taxa in an upland grassland soil and (2) determining whether there was a high degree of apparent functional redundancy in these communities. ‘Function’ was defined as the ability of a fungal isolate to utilise broad categories of substrates (e.g. sugars, cellulose, lignin) that occur in organic matter and which change in proportion during decomposition. The terms ‘common/abundant/frequent’ and ‘occasional/infrequent’ usually referred here to the frequencies of occurrence of taxa estimated using Warcup soil plates. Accepting the difficulties of sampling fungi in soil, this appeared to be the most useful isolation method to produce a general picture of the microfungal community with an estimate of frequency of occurrence for every taxon obtained, and to provide cultures for use in function tests. The influence of this technique on the interpretation of the results is discussed. Forty-eight fungal isolates, obtained from an upland grassland in Roxburghshire, UK, were selected to cover the most ‘abundant’ taxa and a range of ‘occasionals’. Pure cultures of anamorphic fungi and members of the Zygomycota, Ascomycota and Basidiomycota were tested. Although there was apparently a high degree of functional redundancy (equivalence) in assemblages of culturable decomposer fungi, with ‘frequent’ and ‘infrequent’ taxa largely utilising the same substrates, the ‘infrequent’ taxa played important roles in decomposition. ‘Infrequent’ microfungi tested were potentially more active in decomposition than the ‘frequent’ taxa, i.e. several had a higher overall activity, were able to utilise a wider range of substrates and were more combative than the ‘abundant’ taxa. When ‘abundant’ and ‘occasional’ taxa from the same putative guild were inoculated together on grass litter, there was slight evidence of ‘positive’ indirect effects on decomposition and cellulose degradation. Some ‘negative’ effects on lignin degradation, probably as a result of combat, were observed. It is possible that the ‘occasional’ taxa increased the temporal resilience of the ecosystem process of decomposition, and were ‘waiting in the wings’ to replace the abundant taxa. Nevertheless, greater functional diversity could be associated with the uncultured taxa not studied here.