Showing papers by "Richard D. Bardgett published in 1999"

Plant removals in perennial grassland: vegetation dynamics, decomposers, soil biodiversity, and ecosystem properties

Abstract: The consequences of permanent loss of species or species groups from plant communities are poorly understood, although there is increasing evidence that individual species effects are important in modifying ecosystem properties. We conducted a field experiment in a New Zealand perennial grassland ecosystem, creating artificial vegetation gaps and imposing manipulation treatments on the reestablishing vegetation. Treatments consisted of continual removal of different subsets or “functional groups” of the flora. We monitored vegetation and soil biotic and chemical properties over a 3-yr period. Plant competitive effects were clear: removal of the C3 grass Lolium perenne L. enhanced vegetative cover, biomass, and species richness of both the C4 grass and dicotyledonous weed functional groups and had either positive or negative effects on the legume Trifolium repens L., depending on season. Treatments significantly affected total plant cover and biomass; in particular, C4 grass removal reduced total plant biomass in summer, because no other species had appropriate phenology. Removal of C3 grasses reduced total root biomass and drastically enhanced overall shoot-to-root biomass ratios. Aboveground net primary productivity (NPP) was not strongly affected by any treatment, indicating strong compensatory effects between different functional components of the flora. Removing all plants often negatively affected three further trophic levels of the decomposer functional food web: microflora, microbe-feeding nematodes, and predaceous nematodes. However, as long as plants were present, we did not find strong effects of removal treatments, NPP, or plant biomass on these trophic groupings, which instead were most closely related to spatial variation in soil chemical properties across all trophic levels, soil N in particular. Larger decomposer organisms, i.e., Collembola and earthworms, were unresponsive to any factor other than removal of all plants, which reduced their populations. We also considered five functional components of the soil biota at finer taxonomic levels: three decomposer components (microflora, microbe-feeding nematodes, predaceous nematodes) and two herbivore groups (nematodes and arthropods). Taxa within these five groups responded to removal treatments, indicating that plant community composition has multitrophic effects at higher levels of taxonomic resolution. The principal ordination axes summarizing community-level data for different trophic groups in the soil food web were related to each other in several instances, but the plant ordination axes were only significantly related to those of the soil microfloral community. There were time lag effects, with ordination axes of soil-associated herbivorous arthropods and microbial-feeding nematodes being related to ordination axes representing plant community structure at earlier measurement dates. Taxonomic diversity of some soil organism groups was linked to plant removals or to plant diversity. For herbivorous arthropods, removal of C4 grasses enhanced diversity; there were negative correlations between plant and arthropod diversity, presumably because of negative influences of C4 species in the most diverse treatments. There was evidence of lag relationships between diversity of plants and that of the three decomposer groups, indicating multitrophic effects of altering plant diversity. Relatively small effects of plant removal on the decomposer food web were also apparent in soil processes regulated by this food web. Decomposition rates of substrates added to soils showed no relationship with treatment, and rates of CO2 evolution from the soil were only adversely affected when all plants were removed. Few plant functional-group effects on soil nutrient dynamics were identified. Although some treatments affected temporal variability (and thus stability) of soil biotic properties (particularly CO2 release) throughout the experiment, there was no evidence of destabilizing effects of plant removals. Our data provide evidence that permanent exclusion of plant species from the species pool can have important consequences for overall vegetation composition in addition to the direct effects of vegetation removal, and various potential effects on both the above- and belowground subsystems. The nature of many of these effects is driven by which plant species are lost from the system, which depends on the various attributes or traits of these species.

Plant species and nitrogen effects on soil biological properties of temperate upland grasslands

Abstract: 1. The aim was to assess the extent to which the microbial biomass and activity, and community structure of fertilized upland grasslands are directly related to changes in soil N availability or indirectly related to individual plant species effects caused by changes in plant species composition and dominance. We investigated the short-term interactive effects of dominant plant species (Lolium perenne, Agrostis capillaris, Holcus lanatus and Festuca rubra) and nitrogen (N) amendment using an N-limited upland grassland soil. 2. In soils planted with different grass species, soil microbial biomass, and to some extent microbial activity, were determined by temporal changes in plant productivity. Variations in the way that individual plants influenced soil microbial biomass and activity were highly inconsistent over time, and largely independent of N-additions and differences in plant productivity. At the final sample date, those grass species which co-dominate the total plant biomass of intermediate fertility (H. lanatus) and semi-improved grasslands (A. capillaris and F. rubra) had a beneficial effect on the soil microbial biomass. In contrast, the dominant plant species of improved grasslands, L. perenne, had zero or a negative effect on soil microbial biomass. Two plant species (A. capillaris and H. lanatus) increased the proportion of fungi relative to bacteria in the soil microbial community, relative to the unplanted control soil and the other plant species. Lolium perenne and A. capillaris reduced the evenness of microbial PLFAs, suggesting negative effects of these plant species on the diversity of the soil microbial community. 3. The addition of N had no consistent effect on measures of soil microbial biomass or activity, but significantly altered the structure of the microbial community in favour of fungi. The lack of effects of N-addition on microbial biomass and activity were despite the finding that nitrogen addition reduced root biomass in all plant species and increased rhizosphere acidity. 4. The results suggest that in the short term, the abundance and activity of soil micro-organisms in upland grasslands are regulated more by plant species traits than by a direct effect of nitrogen. These effects are likely to be related to variations amongst plant species in root exudation patterns and/or efficiency of nutrient aquisition. 5. Our study provides evidence that the functional characteristics of dominant plant species are important determinants of soil biological properties, and hence ecosystem functioning in temperate upland grasslands.

The measurement of soil fungal:bacterial biomass ratios as an indicator of ecosystem self-regulation in temperate meadow grasslands

Abstract: There is much interest in the development of agricultural land management strategies aimed at enhancing reliance on ecosystem self-regulation rather than on artificial inputs such as fertilisers and pesticides This study tested the usefulness of measures of soil microbial biomass and fungal:bacterial biomass ratios as indicators of effective conversion from an intensive grassland system, reliant mainly on fertilisers for crop nutrition, to a low-input system reliant mainly on self-regulation through soil biological pathways of nutrient turnover Analysis of soils from a wide range of meadow grassland sites in northern England, along a gradient of long-term management intensity, showed that fungal:bacterial biomass ratios (measured by phospholipid fatty acid analysis; PLFA) were consistently and significantly higher in the unfertilised than the fertilised grasslands There was also some evidence that microbial biomass, measured by chloroform fumigation and total PLFA, was higher in the unfertilised than in the fertilised grasslands It was also found that levels of inorganic nitrogen (N), in particular nitrate-N, were significantly higher in the fertilised than in the unfertilised grasslands However, microbial activity, measured as basal respiration, did not differ between the sites A field manipulation trial was conducted to determine whether the reinstatement of traditional management on an improved mesotrophic grassland, for 6 years, resulted in similar changes in the soil microbial community It was found that neither the cessation of fertiliser applications nor changes in cutting and grazing management significantly affected soil microbial biomass or the fungal:bacterial biomass ratio It is suggested that the lack of effects on the soil microbial community may be related to high residual fertility caused by retention of fertiliser N in the soil On the basis of these results it is recommended that following the reinstatement of low-input management, the measurement of a significant increase in the soil fungal:bacterial biomass ratio, and perhaps total microbial biomass, may be an indicator of successful conversion to a grassland system reliant of self-regulation

Seasonal changes in soil microbial communities along a fertility gradient of temperate grasslands

Abstract: This study aimed to: (1) determine whether soil microbial communities along a gradient from intensive (fertilized) to low-input (unfertilized) grassland management, shift in their composition as shown by an increase in the abundance of fungi relative to bacteria and (2) whether these shifts in soil microbial communities vary depending on season. At all sample dates soil microbial biomass-C and -N, and the total abundance of phospholipid fatty acids (PLFA) were highest in unfertilized, undrained treatments and lowest in fertilized and drained grassland. Similarly, microbial activity, measured as CO2-C respiration, was found to be at its lowest in the most intensively managed grassland. Measures of microbial biomass showed a high degree of seasonality, having summer maxima and winter minima. In contrast, PLFA measures had spring maxima and autumn minima. Seasonal and management differences were also observed within the microbial community. PLFA profiles revealed that most individual fatty acids were highest in the unfertilized treatments, and lowest in fertilized grassland. The fungal-to-bacterial biomass ratio was also highest in the unfertilized and lowest in the fertilized soils, suggesting that higher microbial biomass in former were more due to the growth of fungi than bacteria. As with total PLFA, the abundance of individual fatty acids showed a spring maximum and an autumn minimum. Seasonal differences in PLFA patterns were shown to be related to soil mineral-N and soil moisture contents. Factors controlling shifts in microbial community structure between sample dates and sites are discussed in relation to other studies. A critical assessment of the different measures of microbial biomass is also given. Overall, the findings of this study support the thesis that fungi play a more significant role in soil biological processes of low-input, unfertilized grasslands, than in intensively managed systems.

Experimental evidence that soil fauna enhance nutrient mineralization and plant nutrient uptake in montane grassland ecosystems

Abstract: This microcosm study is concerned with understanding those factors which regulate ecosystem processes of nutrient cycling and plant productivity in a montane grassland ecosystem. We examined the effects of different groups of soil fauna, namely bacterial-feeding nematodes and Collembola, on nutrient mineralization (N and P) in an acid, organic soil taken from a montane grassland in the Peak District National Park, United Kingdom. We also examined whether faunal influences on nutrient release, a measure of nutrient mineralization, resulted in changes in nutrient uptake and biomass production of an indigenous montane grass species (Nardus stricta (L.)). We found that in the presence of Collembola, and when nematodes and Collembola were combined, N mineralization, nutrient leaching and shoot N contents of N. stricta was significantly increased relative to a defaunated control. We also found that net P mineralization and leaching increased (although not significantly) in the presence of both nematodes and Collembola, resulting in a significant increase in shoot P content of N. stricta. The presence of nematodes alone, which were largely bacterial-feeders, had no effect on the mineralization of N or P, or shoot nutrient content. We suggest that differences in the effect of faunal treatments on nutrient mineralization are related to the feeding strategies of the added fauna, and to their consequent effect on the size of the soil microbial biomass. The treatments that increased N mineralization and plant nutrient content (N and P) also significantly reduced plant growth (shoot and root). We suggest that high NH4+–N concentrations in the soil solution of Collembola treatments inhibited the growth of N. stricta and that the growth of other grassland species may benefit from this improvement in nutrient availability.

The influence of nematodes on below-ground processes in grassland ecosystems

Abstract: This review summarises recent information on beneficial roles that soil nematodes play in the cycling of carbon and other plant nutrients in grassland ecosystems. In particular, we focus on the role of the two dominant functional groups of nematodes, namely the microbial- and root-feeders, and how their activities may enhance soil ecosystem-level processes of nutrient cycling and, ultimately, plant productivity in managed and unmanaged grassland ecosystems. We report recent experiments which show that low amounts of root herbivory by nematodes can increase the allocation of photoassimilate carbon to roots, leading to increased root exudation and microbial activity in the rhizosphere. The effects of these interactions on soil nutrient cycling and plant productivity are discussed. Evidence is presented to show that the feeding activities of microbial-feeding nematodes can enhance nutrient mineralization and plant nutrient uptake in grasslands, but that these responses are highly species-specific and appear to be strongly regulated by higher trophic groups of fauna (top-down regulation). We recommend that future studies of the roles of nematodes in grasslands ecosystems should consider these more complex trophic interactions and also the effects of species diversity of nematodes on soil ecosystem-level processes.

Below-ground herbivory promotes soil nutrient transfer and root growth in grassland

Abstract: Extremely little is known about the ecosystem-level implications of below-ground herbivory, which often represents the dominant form of consumption of primary productivity. We provide the first empirical evidence that low levels of below-ground herbivory may promote soil nutrient flux and root growth of both host plants and companion plants. Low levels of white clover (Trifolium repens L.) root infection by clover cyst nematodes (Heterodera trifolii Goffart) increased root growth by 141% and 219% in the host plant and the uninfected neighbouring grass (Lolium perenne L.), respectively. Root infection increased the size of the soil microbial biomass in the root zone and the transfer of 15N from the host plant to soil and the neighbouring grass. These data suggest that low amounts of below-ground herbivory may increase the transfer of plant carbon and nitrogen below-ground, leading to increases in root growth and soil nutrient recycling in grasslands. Presumably, such interactions will influence the competitive interactions between plant species, altering plant community structure in grasslands.

Below-ground microbial community development in a high temperature world.

Abstract: The response of above-ground plant and ecosystem processes to climate change are likely to be influenced by both direct and indirect effects of elevated temperature on soil biota and their activities. This study examined the effects of elevated atmospheric temperature on the development of the soil microbial community in a model terrestrial ecosystem facility. The model system was characterized by a soil of low nutrient availability, a condition that simulates most native terrestrial plant communities. The experiment was run over three plant generations, broadly mimicking the early stages of a plant succession, and showed that microbial biomass, measured using phospholipid fatty acid (PLFA) analysis, increased significantly in response to elevated temperature during the first generation only. This increase was unrelated to changes in plant productivity or soil C-availability, and was largely due to a direct effect of elevated temperature on fast-growing Gram-positive bacteria. Slow growing soil micoorganisms such as fungi and actinomycetes were unaffected by elevated temperature throughout the experimental period. Measures of microbial biomass, microbial respiration and N-mineralization were also unaffected by elevated atmospheric temperature over the three generations. The lack of effects on the soil microbial community is thought to be due to the fact that elevated temperature did not influence root biomass or soil C-availability. We suggest that the observed reductions in above-ground plant productivity, in response to elevated temperature, will become apparent in the longer term when litter decomposition pathways are more established. The temporal measures of PLFA and microbial biomass indicated that over the experimental period rapid initial changes occurred in most soil biological characteristics, followed by periods of stabilization during later plant succession. These changes were associated with increases in above-ground plant productivity and amounts of available C in the soil. In contrast, total microbial biomass declined during the last plant generation. Reductions in the diversity of PLFAs in later plant generations appeared to be associated with an increase in the proportion of fatty acids associated with fungi, relative to those from bacteria. These changes are likely to be related to increased competition for resources within the soil, and an associated reduction in N- and C-availability. These changes appear to be broadly consistent with those reported for other studies on the successional development of soil microbial and plant communities. Overall, our data suggest that elevated atmospheric temperature has little effect on the development of below-ground microbial communities and their activities in soils of low nutrient status.