Effect of organic amendment amount on soil nematode community structure and metabolic footprints in soybean phase of a soybean-maize rotation on Mollisols

It has been well documented that organic amendment affects soil nematode community structure. However, little is known about the effect of organic amendment amount on soil nematodes. To assess the effect of the amount of organic amendments on soil nematode community structure and metabolic activity, the community composition, abundance, and metabolic footprints of soil nematodes were determined in a long-term field experiment with various amounts of organic amendment in Northeast China. Fertilization treatments included an unfertilized control (CK), chemical fertilizer without manure amendment (OM₀), manure applied at 7.5 Mg ha^-1 plus chemical fertilizer (OM₁), and manure applied at 22.5 Mg ha^-1 plus chemical fertilizer (OM₂). A total of 46 nematode genera were found. Treatments with the largest amount of organic amendment had the smallest number of plant parasite genera (5), but a largest number of dominant genera (7). Soil nematodes, bacterivores, and fungivores were the most abundant in OM₂, followed by OM₁, and the lowest in OM₀ and CK. Organic amendment increased the enrichment index (EI), and the large amount of organic amendment increased the metabolic footprints of bacterivore (Baf) and fungivore (Fuf) and enrichment footprint (Ef). The relationships between Baf (or Fuf) and the increases in soil organic carbon (ΔSOC) and total nitrogen (ΔTN) were stronger than those of bacterivore (or fungivore) abundance with ΔSOC and ΔTN, except for the relationship between bacterivore abundance and ΔSOC. The EI and Ef were positively correlated with ΔSOC and ΔTN. These findings suggest that the amount of organic amendment affects soil nematode activity and function at entry levels in soil food web, and that metabolic footprints of soil nematodes may be better indicators than their abundances in assessing their relationships with soil nutrients.     

Cover crops alter the soil nematode food web in banana agroecosystems

Cover crops are increasingly being used in agriculture, primarily for weed or erosion management. The addition of cover crops increases the primary productivity of the system and diversifies basal resources for higher trophic levels. How increases in the quality and quantity of basal resources affect bottom-up and top-down control remains a key question in soil food web ecology. We evaluated the response of the nematode community to the introduction of cover crops between rows of a banana plantation. We measured changes in nematode food web structure and inferred the prevalence of bottom-up and top-down effects on the abundance of phytophagous nematodes (i.e., plant-feeding and root-hair-feeding species) 1.5 years after plots with cover crops (Poaceae or Fabaceae species) or bare soil were established. The addition of a cover crop greatly affected the structure and the abundance of the soil nematode community 1.5 years after planting. The abundance of all trophic groups except for plant-feeding nematodes tended to increase with the addition of cover crops. The Shannon–Weaver diversity index and the enrichment index increased with the addition of cover crops, indicating that opportunistic, bacterivorous and fungivorous nematodes benefited from the added resources. Plant-feeding nematodes were least abundant in plots with Poaceae cover crops, while bacterivorous, omnivorous, and root-hair-feeding nematodes were more abundant with Fabaceae cover crops than with bare soil, indicating that cover crop identity or quality greatly affected soil food web structure. Bottom-up effects on all trophic groups other than plant-feeding nematodes were evident with Poaceae cover crops, suggesting an top-down control of plant-feeding nematodes by omnivorous nematodes. Conversely, plant-feeding nematodes were evidently not suppressed in Fabaceae cover crops, perhaps because bottom-up effects on omnivorous nematodes were weaker (hence, top-down control by omnivorous nematodes was weaker), and because Fabaceae cover crops probably served as good hosts for some plant-feeding nematodes.     

Structure of nematode communities in forest soils of southern British Columbia: relationships to nitrogen mineralization and effects of clearcut harvesting and fertilization

The effects of clearcut harvesting and fertilization of clearcuts on the structure of nematode communities were assessed at three sites in the southern interior of British Columbia. The Shannon-Weaver and Simpson indices of diversity were greater in clearcuts than in forests. The diversity of the bacterivorous and fungivorous trophic groups were not significantly different between clearcuts and forests, whereas the diversity of omnivorous and predacious nematodes was lower in clearcuts than in forests. The nematode maturity index (MI) was lower in clearcuts than in forests. Several ratios describing the prevalence of enrichment opportunists (the absolute abundance of Rhabditida, the relative abundance of Rhabditida, and the ratio of Aphelenchida/fungivorous nematodes) were greater in clearcuts than in forests. Nematode diversity tended to be lower in fertilizer treatments than in controls within clearcuts, but the effects were not consistent across sample dates. The MI was lower in fertilizer treatments than in controls. All ratios representing the relative abundance of enrichment opportunists were greater in fertilizer treatments than in controls. The MI was negatively correlated with N mineralization in data sets representing forests and fertilized and non-fertilized plots within clearcuts. None of the diversity indices were correlated with N mineralization. The absolute abundance of Rhabditida and the relative abundance of Rhabditida were both consistently positively correlated with N mineralization in the data set representing fertilized and non-fertilized plots within clearcuts, and the absolute abundance of Rhabditida was positively correlated with N mineralization in forests.     

Effects of organic,low input,conventional management practices on soil nematode community under greenhouse conditions

A comparative study of organic, low input, conventional vegetable greenhouse systems was conducted to assess the effect of management practices on the soil nematode community. Bacterivores were the most dominant trophic group in all three systems with a mean proportion of over 80%, followed by omnivore-carnivores. In general, organic management practices increased the abundance of total nematodes, bacterivores, fungivores, and omnivore-carnivores in comparison with low input and conventional management practices. Though inhibitory effects of plant feeders were found in organic and low input systems, these effects were more evident in organic systems. However, small differences were observed in the composition of trophic groups and fauna analysis. All three systems displayed enriched soil conditions and structured food webs. We inferred that the bottom-up effect resulting from organic input in the soil food web may play a more important role than the disruption effects under our high input greenhouse conditions. The Shannon index (H′) and genus dominance (λ) suggested that in greenhouse conditions, excessive manure input would cause a decrease in nematode diversity but increase the dominance, particularly for enrichment opportunists. We concluded that management practices under greenhouse conditions were more influential on nematode biomass (including trophic groups) than community structure.     

Spatial patterns of variation in the composition and structure of nematode communities in relation to different microhabitats: a case study of Quercus dalechampii Ten. forest

Variability in the spatial distribution of nematode communities in relation to the structural heterogeneity of the environment was studied in nine different microhabitats within a relatively small area of a natural oak forest in Bulgaria. Maturity and diversity indices, trophic structure and the distribution of colonizer-persister groups were applied to analyze the quality of substrate and ecological processes involved from a functional point of view. Two main groups of nematode communities, below- and above-ground, were distinguished in terms of the location of the microhabitats. Our results indicated a higher percentage similarity between nematode communities inhabiting microhabitats with a higher resemblance in substrate structure, and abiotic and biotic conditions than between microhabitats with more dissimilar microenvironmental conditions. The application of Detrended Correspondence Analysis helped to reveal two ecological gradients. The first one was from microhabitats characterized by smaller fluctuations in microclimatic conditions and nutrient supply to microhabitats with more adverse abiotic conditions and dynamics of food resources. Along this gradient from below- to above-ground microhabitats, the proportion of general opportunists (cp 2 taxa) increased, whereas the diversity, MI and the proportions of persisters (cp (3-5) taxa), decreased. Along the second gradient a gradual decrease in the decomposition rate within above-ground microhabitats was revealed, which was indicated by the proportion of enrichment opportunists (cp 1 taxa). The nematode communities of decaying wood had the most specific cp groups' distribution characterized by a high proportion of enrichment opportunists (colonizers). Each microhabitat has developed nematode communities with a characteristic trophic structure that was related to the relative importance of primary production and decomposition processes occurring within the microhabitat. The nematode communities of mosses growing on soil, stones and tree trunks had similar trophic structure dominated by bacterial-feeding nematode taxa. Our results supported the role of nematode communities as potential indicators of environmental conditions.     

Nematode communities of grazed and ungrazed semi-natural steppe grasslands in Eastern Austria

Soil nematode communities were investigated at eight semi-natural steppe grasslands in the National Park Seewinkel, eastern Austria. Four sites were moderately grazed by horses, cattle and donkeys, four were ungrazed. Nematodes were sampled on four occasions from mineral soil, and their total abundance, diversity of genera, trophic structure and functional guilds were determined. Altogether 58 nematode genera inhabited the grasslands, with Acrobeloides, Anaplectus, Heterocephalobus, Prismatolaimus, Aphelenchoides, Aphelenchus, Tylenchus and Pratylenchus dominating. Mean total abundance at sites was 185–590 individuals per 100 g soil. Diversity indices did not separate communities well, but cluster analysis showed distinct site effects on nematode generic structure. Within feeding groups the relative proportion of bacterial-feeding nematodes was the highest, followed by the fungal- and plant-feeding group. Omnivores and predators occurred in low abundance. The maturity indices and plant parasite indices were characteristic for temperate grasslands, but the abundance of early colonizers (c-p 1 nematodes) was low. A high density of fungal-feeding c-p 2 families (Aphelenchoidae, Aphelenchoididae) resulted in remarkably high channel index values, suggesting that decomposition pathways are driven by fungi. Nematode community indices of all sites pointed towards a structured, non-enriched soil food web. At most sites, grazing showed little or no effect on nematode community parameters, but total abundance was higher at ungrazed areas. Significant differences in the percentage of omnivorous nematodes, the sum of the maturity index, the number of genera and Simpson's index of diversity were found at one long-term grazed pasture, and this site was also separated by multi-dimensional scaling (MDS).     

Urbanization alters the functional composition,but not taxonomic diversity,of the soil nematode community

We evaluated the response of riparian forest soil nematode community structure to the physico-chemical environment associated with urban land use. Soils were sampled seasonally between December 2000 and October 2002 along an urban—rural transect in Asheville, North Carolina. We characterized the taxonomic (to genus) and functional composition (trophic groups) of the nematode community of forest soils, as well as several nematode ecological indicators (maturity index, channel index, weighted faunal index). The diversity of nematode genera was not affected by urban land use. However, there tended to be functional differences in the nematode communities along the land use gradient. The urban soils tended to have lower abundances of predatory and omnivorous nematodes. Differences in channel index scores indicated that there was less fungal dominance in the soil food webs of the urban soils. Our results indicate that the functional composition of the soil food web is an important component of soil biodiversity that can be affected by land use practices. This study was conducted in a relatively small city; hence the influence of pollutants on the soil environment was not as great as in larger cities. Correspondingly, the impact on the soil nematode community was not very severe. The utilization of the nematode community assemblage as an indicator of soil conditions should be further explored in urban places of differing magnitudes of environmental effects.     

The impact of clearcutting and partial harvesting systems on population dynamics of soil nematodes in coastal Douglas-fir forests

Soil nematodes were used as bioindicators to compare possible differences in soil ecological conditions in Douglas-fir forests harvested by clearcutting and by two partialharvesting systems, namely shelterwood and extended rotation. An unlogged forest block was used as a control treatment. The pre- and post-harvest comparisons were based on total abundance of the nematodes, their trophic structure, and relative abundance of the nematode taxa found in the four blocks. The nematode and soil data were obtained bimonthly over a year. Pre-harvest nematode abundance and taxonomic composition were similar at the four blocks. Post-harvest data were obtained 6-7 months after the harvesting treatments were instituted. The post-harvest abundance at the three logged blocks was reduced to varying levels as compared to the control block, and analysis of variance showed that the severity of impact of the harvesting treatments on nematode abundance ranked as: clearcut shelterwood > extended rotation unlogged control. However, the taxonomic and trophic structures of the nematode community, which consisted of 48 taxa in these forests, were only slightly altered by any of the harvesting treatments as shown by maturity and diversity indices and by multivariate analysis.     

Effects of organic mulches on soil microfauna in the root zone of apple: implications for nutrient fluxes and functional diversity of the soil food web

A variety of organic materials (e.g. composts, paper recycling wastes, hay) can be used as in-row mulches in perennial horticultural cropping systems such as high density apple orchards. As organic materials with variable properties, such mulches can be expected to have variable effects on structure of the soil food web and mineralisation of N and P in the root zone. The overall objectives of this study were to: (1) assess the effects of a selection of organic mulches on the abundance of protozoa and nematode trophic groups; (2) use the model of Hunt et al. [Biol. Fertil. Soils 3 (1987) 393] to assess the implications of changes in microfaunal abundance for microbial turnover and N mineralisation; and (3) determine effects of the mulches on nematode indicators of increased microbial production/turnover and functional diversity of the soil food web. Organic mulch treatments commenced in 1994 and included shredded office paper, municipal biosolids, shredded paper applied over municipal biosolids, shredded paper applied over municipal compost, alfalfa hay, and black polyethylene fabric. The control was conventional tree-row weed management with glyphosate. Biosolids and municipal compost treatments were re-applied in 1997. Protozoan abundance was determined in 1998, 1999 and 2000. Nematode community structure was assessed in 1998, 1999, and twice in 2000. Nematode community parameters evaluated included: abundance of bacterivorous, fungivorous, omnivorous and predacious nematodes; abundance of the root-lesion nematode, Pratylenchus penetrans; absolute and relative abundances of enrichment opportunist nematodes (Rhabditidae+Diplogasteridae+Panagrolaimidae); Simpson’s diversity; evenness; and the indices of nematode community enrichment (EI) and structure (SI) described by Ferris et al. [Appl. Soil Ecol. 18 (2001) 13]. Measurements of the abundance of enrichment opportunists and the EI were evaluated as indicators of enhanced nutrient fluxes. Diversity and the SI were evaluated as indicators of changes in functional diversity of the soil food web.The abundance of protozoa and bacterivorous nematodes, and estimated fluxes of N and P through the microfauna, were greater under all combinations of biosolids or municipal compost and shredded paper than under the control and plastic mulch. The abundance of enrichment opportunist nematodes and the EI were also consistently greater under combinations of biosolids or municipal compost and shredded paper. The abundance of enrichment opportunists and EI were both also correlated with leaf P, providing additional evidence to support the use of these parameters as indicators of enhanced turnover of microbial biomass and nutrients. The SI was greatest under shredded paper and shredded paper applied over municipal compost, and least under municipal biosolids and alfalfa hay. Population densities of P. penetrans were reduced under shredded paper mulch relative to the control and biosolids alone.     

‘David and Goliath’ of the soil food web – Flagellates that kill nematodes

Nematodes and flagellates are important bacterial predators in soil and sediments. Generally, these organisms are considered to be competitors for bacterial food. We studied the interaction among flagellates and nematodes using axenic liquid cultures amended with heat-killed bacteria as food and showed for the first time that a small and common soil flagellate (Cercomonas sp.) is able to attack and kill the much larger nematode Caenorhabditis elegans. The killing process is not caused by soluble metabolites but requires direct contact between the flagellate cells and the nematode surface and occurs rapidly (within a few hours) at high flagellate density. At lower flagellate density, adult nematodes sometimes avoid attachment of flagellates, feed on them and become the dominant bacterial predator. Considering that bacterial feeders affect bacterial communities differently, and that one bacterial feeder can control the abundance of another, suggests a new perspective on how bacterial diversity and trophic interactions are linked in the soil food web.     

Abundance,diversity and connectance of soil food web channels along environmental gradients in an agricultural landscape

Soil food webs respond to anthropogenic and natural environmental variables and gradients. We studied abundance, connectance (a measure of the trophic interactions within each channel), and diversity in three different channels of the soil food web, each comprised of a resource-consumer pair: the microbivore channel (microbes and their nematode grazers), the plant–herbivore channel (plants and plant-feeding nematodes), and the predator–prey channel (predatory nematodes and their nematode prey), and their associations with different gradients in a heterogeneous agricultural landscape that consisted of intensive row crop agriculture and grazed non-irrigated grasslands in central California. Samples were taken at three positions in relation to water channels: water’s edge, bench above waterway, and the adjacent arable or grazed field. Nematode communities, phospholipid fatty acid (PLFA) biomarkers, and soil properties (NH₄⁺-N, NO₃⁻-N, total N, total C, pH, P, bulk density and soil texture) were measured, and riparian health ratings were scored. Environmental variables were obtained from publicly-available data sources (slope, elevation, available water capacity, erodability, hydraulic conductivity, exchangeable cation capacity, organic matter, clay and sand content and pH).The abundance and richness in most food web components were higher in grazed grasslands than in intensive agricultural fields. Consumers contributed less than their resources to the abundance and richness of the community in all channels. The association between richness and abundance for each component was strongest for the lowest trophic links (microbes, as inferred by PLFA) and weakest for the highest (predatory nematodes). The trophic interactions for the predator–prey and plant–herbivore channels were greater in the grassland than in the cropland. Fields for crops or grazing supported more interactions than the water’s edge in the plant–herbivore and microbivore channels. Connectance increased with the total richness of each community. Higher connectance within the microbivore and predator–prey soil food web channels were associated with soil NO₃⁻-N and elevation respectively, which served as surrogate indicators of high and low agricultural intensification.     

东北黑土农业生态系统线虫多样性研究

The diversity and distribution patterns of soil nematode communities in phaeozem agroecosystems of Northeast China were assessed to evaluate nematode taxonomic diversity and functional diversity in relation to climatic condition and soil characteristics in human modified landscape.Along the latitudinal gradient,soil samples were collected from north (Hailun) to south (Gongzhuling) down to a depth of 100 cm with intervals of 0-20,20-40,40-60,60-80,and 80-100 cm.The nematode abundance and taxonomic diversity (generic richness) were lower at Hailun than at other sites,and higher values of evenness were observed at Hailun and Harbin than at Dehui and Gongzhuling.Nematode faunal analysis revealed that soil food web at Hailun was successionally more mature or structured,and the environment little disturbed,while at Harbin and Gongzhuling,the soil food web was degraded with stressed environment.The environmental variables relevant in explaining the patterns of nematode distribution and diversity in phaeozem agroecosystems,using canonical correspondence analysis (CCA),were the mean annual temperature,total nitrogen,electrical conductivity,mean annual precipitation,and other soil properties.Among these variables,the mean annual temperature was a relatively important factor,which could explain 29.05% of the variations in nematode composition.     

Strong impacts of belowground tree inputs on soil nematode trophic composition

Trees have a key role in determining the composition of soil biota via both above and belowground resource-based mechanisms, and by altering abiotic conditions. We conducted an outdoor mesocosm experiment to investigate the relative impact of above and belowground tree inputs on soil nematode trophic composition, and examine whether tree-driven impacts differed between contrasting species (birch and pine). For both species, we created a factorial design of litter addition and root presence treatments. The litter addition treatment was equivalent to natural levels of litterfall; tree saplings were planted in mesocosms for the root presence treatment and an unplanted control treatment was established that had no litter or root inputs. Litter addition had a limited impact on soil nematode community composition: it primarily decreased omnivore and predatory nematode abundance in birch but had few other effects on the nematode community. By contrast, root presence markedly altered nematode community composition through changes in a range of trophic groups. For both birch and pine, there were significant increases in total, fungivore and predatory nematode abundance in root presence treatments, and furthermore, total and fungivore abundances were positively related to root biomass. Root presence of these contrasting tree species also had a distinctive impact on some specific nematode trophic groups; pine roots promoted bacterivore abundance while birch roots promoted root-hair feeding nematode abundance. These findings suggest strong bottom-up effects of belowground tree inputs, and indicate that particular components of the nematode community may be affected differently by resource quantity and quality. Consequently, we suggest that, in the short-term, belowground rather than aboveground tree inputs have a strong impact on soil food web structure and complexity.     

Abundance, diversity and connectance of soil food web channels along environmental gradients in an agricultural landscape

Soil food webs respond to anthropogenic and natural environmental variables and gradients. We studied abundance, connectance (a measure of the trophic interactions within each channel), and diversity in three different channels of the soil food web, each comprised of a resource-consumer pair: the microbivore channel (microbes and their nematode grazers), the plant–herbivore channel (plants and plant-feeding nematodes), and the predator–prey channel (predatory nematodes and their nematode prey), and their associations with different gradients in a heterogeneous agricultural landscape that consisted of intensive row crop agriculture and grazed non-irrigated grasslands in central California. Samples were taken at three positions in relation to water channels: water’s edge, bench above waterway, and the adjacent arable or grazed field. Nematode communities, phospholipid fatty acid (PLFA) biomarkers, and soil properties (NH₄⁺-N, NO₃⁻-N, total N, total C, pH, P, bulk density and soil texture) were measured, and riparian health ratings were scored. Environmental variables were obtained from publicly-available data sources (slope, elevation, available water capacity, erodability, hydraulic conductivity, exchangeable cation capacity, organic matter, clay and sand content and pH).The abundance and richness in most food web components were higher in grazed grasslands than in intensive agricultural fields. Consumers contributed less than their resources to the abundance and richness of the community in all channels. The association between richness and abundance for each component was strongest for the lowest trophic links (microbes, as inferred by PLFA) and weakest for the highest (predatory nematodes). The trophic interactions for the predator–prey and plant–herbivore channels were greater in the grassland than in the cropland. Fields for crops or grazing supported more interactions than the water’s edge in the plant–herbivore and microbivore channels. Connectance increased with the total richness of each community. Higher connectance within the microbivore and predator–prey soil food web channels were associated with soil NO₃⁻-N and elevation respectively, which served as surrogate indicators of high and low agricultural intensification.     

Spatial variations in the trophic structure of soil animal communities in boreal forests of Pechora-Ilych Nature Reserve

Soil animal communities and detrital food webs are spatially compartmentalized. In old-growth boreal forests the dynamics of dominating plant species forms a considerable heterogeneity of edaphic conditions in the soil layer. We demonstrate a strong difference in total and relative abundance of main trophic groups of soil macrofauna in four microsites, i.e. under tree crowns, in gaps, in mounds and in pits created by fallen spruce trees. The variation in the functional structure of soil animal communities is likely related to different availability of key energy resources (leaf litter, roots and root deposits) in the microsites studied. However, results of the stable isotope analysis suggest that mobile litter-dwelling predators occupy very similar trophic positions in different microsites. The compartmentalization of soil invertebrate communities caused by the vegetation-induced mosaic of edaphic conditions seemingly does not lead to spatial isolation of local food webs that are integrated at the top trophic levels.     

Birch invasion of heather moorland increases nematode diversity and trophic complexity

To determine whether successional changes in plant communities may influence belowground community structure, we quantified nematode abundance, trophic structure and diversity along two separate chronosequences from heather moorland to birch woodland in the Scottish uplands. Tree invasion markedly altered plant community composition, and hence litter inputs, both directly, and indirectly through changes in understorey species. In turn, these changes in detrital inputs were reflected in consistent changes in nematode community structure. Nematode abundance increased from moorland to birch woodland, with moorland soils being dominated by a few taxa, notably root-hair and fungal feeders, compared to the more diverse composition of the birch woodland soils. Trophic structure was altered through an increase in the abundance of bacterial feeding relative to fungal-feeding nematodes, and an increase in the abundance of predatory nematodes. The increase in predators during the succession from moorland to woodland was associated with an increase in soil pH, highlighting that not only changes in the plant community, but also changes in soil properties associated with tree invasion may influence soil nematodes. Nematode diversity increased from moorland to birch woodland, with nematode richness being positively related to both plant species richness and soil pH. These results suggest that trees may control soil community structure through the manipulation of resources and the soil physico-chemical environment, promoting greater nematode diversity and trophic complexity.     

Effects of agricultural management on nematode–mite assemblages: Soil food web indices as predictors of mite community composition

Biological indicators based on abundances of soil organisms are powerful tools for inferring functional and diversity changes in soils affected by agricultural perturbations. Field plots, combining organic and conventional practices with no tillage, conservation tillage and standard tillage maintained different nematode assemblages and soil food webs. Soil food web indices based on nematode assemblages were reliable predictors of the trophic composition of functional characteristics of soil mite assemblages. Bacterial-feeding and predatory nematodes, together with predatory mites, were abundant in the organic-no till treatments and were associated with high values of the Enrichment and the Structure Index based on nematode assemblages. Conventional-Standard tillage treatments had high abundances of fungal- and plant-feeding nematodes and algivorous mites, associated with high values of the Basal and Channel Index. This study validates the hypothesis that nematode-based soil food web indices are useful indicators of other soil organisms such as mites, with similar functional roles and environmental sensitivities.     

The influence of seabird nutrient enrichment and grazing on the structure and function of island soil food webs

Marine inputs from seabirds (in the form of guano) to terrestrial coastal communities play an important role in supporting aboveground food webs. However, little is known about the importance of seabird-derived nutrient inputs for belowground food webs and their function relative to other factors that regulate belowground communities. Here, we tested the relative importance of nutrient enrichment from seabirds and grazing, a known driver of belowground properties, in determining the structure and function of the soil food web in an island system. This was tested by measuring the size and composition of the microbial community, the abundance of nematode feeding groups and rates of decomposition and net nitrogen (N) mineralisation in soil samples collected from grazed and ungrazed plots at coastal and inland locations, representing sites of high and low seabird influence respectively, on the Isle of May in the Firth of Forth, east Scotland. We found that proximity to seabird breeding colonies, and associated greater input of seabird-derived N, stimulated the size of the soil microbial biomass and the abundance of bacteria relative to fungi in the soil microbial community relative to inland areas that received significantly less N. Despite this, proximity to seabird colonies had no detectable effect on rates of decomposition or N-mineralisation. The short-term removal of mammalian grazers, in the form of rabbits, had only limited effects on the structure of the soil food web, mainly affecting the abundance of bactivorous nematodes which were greater in grazed than ungrazed situations. However, cessation of grazing did impact significantly on rates of N-mineralisation and decomposition, which were higher and lower in grazed than ungrazed situations respectively. In conclusion, our study provides evidence that allochthonous nutrient inputs from seabirds have significant impacts on the composition of the soil microbial community, and that these effects outweigh short-term effects of grazers as a driver of soil food web structure in the island system studied. Overall, our results indicate the important roles that natural sources of N and grazing play as drivers of soil food webs and their function.