Showing papers by "Richard D. Bardgett published in 2003"

Herbivore-mediated linkages between aboveground and belowground communities

Abstract: Understanding how terrestrial ecosystems function requires a combined aboveground–belowground approach, because of the importance of feedbacks that occur between herbivores, producers, and the decomposer subsystem. In this paper, we identify several mechanisms by which herbivores can indirectly affect decomposer organisms and soil processes through altering the quantity and quality of resources entering the soil. We show that these mechanisms are broadly similar in nature for both foliar and root herbivory, regardless of whether they operate in the short term as a result of physiological responses of individual plants to herbivore attack or long-term following alteration of plant community structure by herbivores and subsequent changes in the quality of litter inputs to soil. We propose that a variety of possible mechanisms is responsible for the idiosyncratic nature of herbivore effects on soil biota and ecosystem function; positive, negative, or neutral effects of herbivory are possible depending upon the balance of these different mechanisms. However, we predict that positive effects of herbivory on soil biota and soil processes are most common in ecosystems of high soil fertility and high consumption rates, whereas negative effects are most common in unproductive ecosystems with low consumption rates. The significance of multiple-species herbivore communities is also emphasized, and we propose that if resource use complementarity among herbivore species or functional groups leads to greater total consumption of phytomass, and thus greater net herbivory, then both positive and negative consequences of increasing herbivore diversity for belowground properties and processes are theoretically possible. Research priorities are highlighted and include a need for comparative studies of herbivore impacts on above- and belowground processes across ecosystems of varying productivity, as well as a need for experimental testing of the influence of antiherbivore defense compounds on complex multitrophic interactions in the rhizosphere and the significance of multiple herbivore species communities on these plant–soil interactions.

Soil microbes compete effectively with plants for organic‐nitrogen inputs to temperate grasslands

Abstract: Although agricultural grassland soils have inherently high rates of net nitrogen (N) mineralization, they often have soil concentrations of soluble organic N that are comparable to inorganic N. We set out to examine in situ the significance of organic N for plant nutrition in grasslands of differing management intensity and soil fertility. Using in situ dual-labeling techniques (glycine-2-13C-15N) we measured preferential uptake of amino-acid N vs. inorganic N [(15NH4)2SO4] in early and late season in low-productivity Agrostis capillaris–Festuca ovina grassland and in agriculturally improved, high-productivity Lolium perenne-dominated grassland. The dominant soluble-N form differed greatly between grasslands. Inorganic N (especially nitrate N) dominated the soluble N pool of the highly productive improved grassland whereas amino acid N was the dominant soluble N form in the low-productivity unimproved grassland. There was no difference in the amount of 15N taken up by plants from the two N forms in either grassland. However, our data indicate that amino-acid N was taken up directly by plants of both grasslands and that more N was captured in this way by plants of low-productivity grassland where amino acids were the dominant soluble N form in soil. Our data from both grasslands also indicate significant microbial competition for added 15N from both N sources, but especially in the low-productivity grassland where the bulk of 15N added was sequestered by the microbial biomass. A significantly greater amount of added 15N was captured by the microbial biomass in the unimproved than in the improved grassland, and substantially more 15N was detected in the microbial biomass than in plant tissue in the unimproved grassland. On the basis of our findings, we predict that subsequent microbial turnover and release of this N into the plant–soil system is the major pathway for plant N capture in these temperate grasslands. Microbial sequestration of added N might be an important mechanism of N retention in these grasslands, especially in the low-productivity systems where microbial N sink strength is greater and organic matter slowly accumulates.

Relationships at the aboveground-belowground interface: plants, soil biota and soil processes

Abstract: Interactions at the aboveground-below ground interface provide important feedbacks that regulate ecosystem processes. Organisms within soil food webs are involved in processes of decomposition and nutrient mineralization, and their abundance and activity have been linked to plant ecophysiological traits such as species identity and the quality and quantity of plant tissue. We tested aboveground-below ground diversity relationships in a naturally developed plant community of native tallgrass prairie by taking soil samples from beneath naturally established grass tillers of chosen characteristics (e.g., homogeneous vs. heterogeneous plant combinations or C-4 vs. C-3 photosynthetic pathway) without imposing any disturbances to existing plant-soil relationships. The goal of this study was to elucidate the consequences, for soil microbiota (microflora phospholipid fatty acids, protozoa, and nematode functional groups) and for C and N mineralization, of plant community properties such as species richness, resource quality, resource heterogeneity, species identity, and presence of exotics. None of the biotic or abiotic soil variables was related to plant resource heterogeneity. Protozoa were not responsive to any of the plant community traits. Some bacterial and nematode groups were affected by plant characteristics specific to a particular plant species, but no uniform pattern emerged. Invasive and native plants generally were similar with respect to soil variables tested in this study. The lack of clear responses of soil variables to plant community traits indicates that idiosyncratic effects dominate both at the plant and soil biotic level and that generalized plant and soil diversity effects are hard to predict.

Soil microbial community, fertility, vegetation and diversity as targets in the restoration management of a meadow grassland

Abstract: 1. The enhancement of biodiversity in meadow grassland, an environmental aim of European agricultural policy, requires definition of appropriate management regimes and the rate at which they enhance biodiversity and change ecosystem properties. We describe vegetation changes in a 10-year trial on mesotrophic grassland that was previously agriculturally improved, plus change in the soil microbial community and fertility, important factors that influence biodiversity. 2. Management treatments were three hay-cut dates, plus two mineral fertilizer, two seed addition and two farmyard manure (FYM) applications. Treatment combinations included the traditional management regime (21 July hay-cut date, no mineral fertilizer, autumn grazing with cattle, spring grazing with sheep), modern variants of this (14 June hay-cut date, mineral fertilizer) and exceptional historic variants (1 September hay-cut date). 3. Sowing seed increased species richness and, in the absence of fertilizer and FYM, produced a plant community similar to Geranium sylvaticum–Anthoxanthum odouratum grassland. The greatest cover of sown species was found in seeded treatments, cut for hay on 21 July, in the absence of mineral fertilizer. The target plant community (MG3b grassland) was most rapidly achieved with a 21 July hay cut. Initial decrease in Ellenberg fertility scores only persisted in the 21 July and 1 September cut dates when mineral fertilizer was absent. Soil phosphate was lowest in the joint absence of mineral fertilizer and FYM. 4. There were few treatment effects on the soil microflora. Bacterial biomass was reduced when FYM was applied with the 14 June cut date, but increased when FYM was applied with the 1 September cut date. Fungal biomass decreased when mineral fertilizer was applied. 5. Increased species richness, primarily through an increase in legumes, stress-tolerant and stress-tolerant ruderal plant strategists, was associated with an increase in soil fungi and the abundance of fungi relative to bacteria. All these were associated with seed addition to unfertilized plots cut on 21 July, in the absence of FYM, indicating a functional role for individual species. 6. Synthesis and applications. The enhancement of biodiversity in meadow grassland is a long-term (> 10-year) secondary succession, most rapidly achieved in the absence of mineral fertilizer by cutting for hay in mid-July and autumn grazing with cattle. The sowing of key functional species, i.e. legumes and Rhinanthus minor, was important in facilitating the staged colonization of other sown species.

Effect of density and species richness of soil mesofauna on nutrient mineralisation and plant growth

Abstract: The aim of this study was to test the relative importance of changes in density and species richness of soil mesofauna as determinants of nutrient mineralisation and plant growth The experiment was carried out using microcosms containing a mixture of plant litter and soil in which seedlings of Lolium perenne were planted, and a range of combinations of levels of density and species richness of microarthropods added Over the duration of the experiment, nutrient release, measured as concentrations of NO3--N and total N in leachates, increased significantly with increasing microarthropod density, but decreased with increasing species richness Leachate concentrations of NH4+-N, dissolved organic N and C (DON and DOC) were not affected by the faunal treatments Soil respiration, a measure of microbial activity, decreased with increasing density of microarthropods, whereas microbial biomass was not affected by microarthropods Increasing density of soil animals had a negative effect on the shoot biomass of L perenne while the effect of species richness was positive Neither the species richness nor density of soil microarthropods was found to significantly influence root biomass We conclude that variations in animal density had a greater influence on soil nutrient mineralisation processes than did species richness Possible reasons for these opposing effects of animal density and diversity on soil N mobilization are discussed

Inter-specific variability in organic nitrogen uptake of three temperate grassland species.

Abstract: We tested the inter-specific variability in the ability of three dominant grasses of temperate grasslands to take up organic nitrogen (N) in the form of amino acids in soils of differing fertility. Amino acid uptake was determined by injecting dual labeled glycine-2-C-13-N-15 into the soil, and then measuring the enrichment of both C-13 and N-15 in plant tissue after 50 hours. We found enrichment of both C-13 and N-15 in root and shoot material of all species in both soils, providing first evidence for direct uptake of glycine. We show that there was considerable inter-specific variability in amino acid uptake in the low fertility soil. Here, direct uptake of amino acid was greater in the grass Agrostis capillaris, which typically dominates low fertility grassland, than Lolium perenne, which inhabits more fertile sites. Direct uptake of amino acid for Holcus lanatus was intermediate between the above two species. Unlike in the low fertility soil, there was no difference in uptake of either C-13 or N-15 by grasses in the high fertility soil, where uptake of mineral N is thought to be the major mechanism of N uptake of these grasses. Overall, our findings may contribute to our understanding of differences in competitive interactions between grasses in soils of different fertility status.

How browsing by red deer impacts on litter decomposition in a native regenerating woodland in the Highlands of Scotland

Abstract: Herbivores can indirectly affect ecosystem productivity and processes such as nutrient cycling and decomposition by altering the quantity and quality of resource inputs into the decomposer subsystem. Here, we tested how browsing by red deer impacts on the decomposition of, and nutrient loss from, birch leaf litter (Betula pubescens), and tested whether effects of browsing on these measures were direct, via alteration of the quality of leaf litter, or indirect through long term impacts of deer browsing on soil biological properties. This was tested in a microcosm experiment using soil and litter taken from inside and outside three individual fenced exclosures located at Creag Meagaidh National Nature Reserve, Scotland. We found that litter of un-browsed trees decomposed faster than that from browsed trees, irrespective of whether soil was sourced from inside or outside exclosures. These findings suggest that effects of browsing on litter quality, rather than on soil biological properties, are the key determinant of enhanced decomposition in un-browsed areas of this ecosystem. Despite this, we found no consistent impact of browsing on litter C:N, a key indicator of litter quality; however, the rate of litter decomposition was linearly and negatively related to litter C:N when analysed across all the sites, indicating that this measure, in part, contributed to variation in rates of decomposition in this ecosystem. Our findings indicate that herbivores impact negatively on rates of decomposition in this ecosystem, ultimately retarding nutrient cycling rates, and that these effects are, in part, related to changes in litter quality.

Soil functioning in natural and planted woodlands on slate waste

Abstract: Quarries and minesites are examples of extreme disturbance. Frequently, materials that form soil are scarce and ecological restoration is easier to achieve than productive land. Establishment of soil microbial function is critical to ecological restoration and a key objective of the study. An organic fertilizer containing a mix of sewage and paper sludges was designed to promote soil functioning during the revegetation of slate waste. We compared soil formation under naturally established birch trees (Betula pubescens) on slate waste with that of container-grown seedling birches of local provenance in slate waste amended with either organic or mineral fertilizer. Hypotheses that an organic nutrient source would lead to more rapid establishment of microbial communities and nutrient cycling than mineral fertilizer, and result in a substrate biochemically comparable to that under naturally established revegetation, were supported.