Showing papers by "Richard D. Bardgett published in 2007"

Preferences for different nitrogen forms by coexisting plant species and soil microbes.

Abstract: The growing awareness that plants might use a variety of nitrogen (N) forms, both organic and inorganic, has raised questions about the role of resource partitioning in plant communities. It has been proposed that coexisting plant species might be able to partition a limited N pool, thereby avoiding competition for resources, through the uptake of different chemical forms of N. In this study, we used in situ stable isotope labeling techniques to assess whether coexisting plant species of a temperate grassland (England, UK) display preferences for different chemical forms of N, including inorganic N and a range of amino acids of varying complexity. We also tested whether plants and soil microbes differ in their preference for different N forms, thereby relaxing competition for this limiting resource. We examined preferential uptake of a range of 13C15N-labeled amino acids (glycine, serine, and phenylalanine) and 15N-labeled inorganic N by coexisting grass species and soil microbes in the field. Our data show that while coexisting plant species simultaneously take up a variety of N forms, including inorganic N and amino acids, they all showed a preference for inorganic N over organic N and for simple over the more complex amino acids. Soil microbes outcompeted plants for added N after 50 hours, but in the long-term (33 days) the proportion of added 15N contained in the plant pool increased for all N forms except for phenylalanine, while the proportion in the microbial biomass declined relative to the first harvest. These findings suggest that in the longer-term plants become more effective competitors for added 15N. This might be due to microbial turnover releasing 15N back into the plant-soil system or to the mineralization and subsequent plant uptake of 15N transferred initially to the organic matter pool. We found no evidence that soil microbes preferentially utilize any of the N forms added, despite previous studies showing that microbial preferences for N forms vary over time. Our data suggest that coexisting plants can outcompete microbes for a variety of N forms, but that such plant species show similar preferences for inorganic over organic N.

Heterotrophic microbial communities use ancient carbon following glacial retreat

Abstract: When glaciers retreat they expose barren substrates that become colonized by organisms, beginning the process of primary succession. Recent studies reveal that heterotrophic microbial communities occur in newly exposed glacial substrates before autotrophic succession begins. This raises questions about how heterotrophic microbial communities function in the absence of carbon inputs from autotrophs. We measured patterns of soil organic matter development and changes in microbial community composition and carbon use along a 150-year chronosequence of a retreating glacier in the Austrian Alps. We found that soil microbial communities of recently deglaciated terrain differed markedly from those of later successional stages, being of lower biomass and higher abundance of bacteria relative to fungi. Moreover, we found that these initial microbial communities used ancient and recalcitrant carbon as an energy source, along with modern carbon. Only after more than 50 years of organic matter accumulation did the soil microbial community change to one supported primarily by modern carbon, most likely from recent plant production. Our findings suggest the existence of an initial stage of heterotrophic microbial community development that precedes autotrophic community assembly and is sustained, in part, by ancient carbon.

Long-Term Consequences of Grazing and Burning on Northern Peatland Carbon Dynamics

Abstract: Using a 50-year-old field experiment, we investigated the effects of the long-term land management practices of repeated burning and grazing on peatland vegetation and carbon dynamics (C). Plant community composition, C stocks in soils and vegetation, and C fluxes of CO2, CH4 and DOC, were measured over an 18-month period. We found that both burning and grazing reduced aboveground C stocks, and that burning reduced C stocks in the surface peat. Both burning and grazing strongly affected vegetation community composition, causing an increase in graminoids and a decrease in ericoid subshrubs and bryophytes relative to unburned and ungrazed controls; this effect was especially pronounced in burned treatments. Soil microbial properties were unaffected by grazing and showed minor responses to burning, in that the C:N ratio of the microbial biomass increased in burned relative to unburned treatments. Increases in the gross ecosystem CO2 fluxes of respiration and photosynthesis were observed in burned and grazed treatments relative to controls. Here, the greatest effects were seen in the burning treatment, where the mean increase in gross fluxes over the experimental period was greater than 40%. Increases in gross CO2 fluxes were greatest during the summer months, suggesting an interactive effect of land use and climate on ecosystem C cycling. Collectively, our results indicate that long-term management of peatland has marked effects on ecosystem C dynamics and CO2 flux, which are primarily related to changes in vegetation community structure.

The influence of below-ground herbivory and defoliation of a legume on nitrogen transfer to neighbouring plants.

Abstract: 1. Both foliar and root herbivory can alter the exudation of carbon from plant roots, which in turn can affect nitrogen availability in the soil. However, few studies have investigated the effects of herbivory on N fluxes from roots, which can directly increase N availability in the soil and uptake by neighbouring plants. Moreover, the combined effects of foliar and root herbivory on N fluxes remains unexplored. 2. We subjected the legume white clover (Trifolium repens L.) to defoliation (through clipping) and root herbivory (by an obligate root-feeding nematode, Heterodera trifolii Goggart) to examine how these stresses individually, and simultaneously, affected the transfer of T. repens-derived N to neighbouring perennial ryegrass (Lolium perenne L.) plants using 15N stable-isotope techniques. We also examined the effects of defoliation and root herbivory on the size of the soil microbial community and the growth response of L. perenne. 3. Neither defoliation nor root herbivory negatively affected T. repens biomass. On the contrary, defoliation increased root biomass (34%) and total shoot production by T. repens (100%). Furthermore, defoliation resulted in a fivefold increase in T. repens-derived 15N recovered in L. perenne roots, and increased the size of the soil microbial biomass (77%). In contrast, root herbivory by H. trifolii slightly reduced 15N transfer from T. repens to L. perenne when T. repens root 15N concentration was included as a covariate, and root herbivory did not affect microbial biomass. Growth of L. perenne was not affected by any of the treatments. 4. Our findings demonstrate that defoliation of a common grassland legume can substantially increase the transfer of its N to neighbouring plants by directly affecting below-ground N fluxes. These finding require further examination under field conditions but, given the prevalence of N-limitation of plant productivity in terrestrial ecosystems, increased transfer of N from legumes to non-N-fixing species could alter competitive interactions, with implications for plant community structure.

Temporal variability in plant and soil nitrogen pools in a high-Arctic ecosystem.

Abstract: This study determined temporal variability in N pools, both aboveground and belowground, across two contrasting plant communities in high-Arctic Spitsbergen, Svalbard (78°N). We measured N pools in plant material, soil microbial biomass and soil organic matter in moist (Alopecurus borealis dominated) and dry (Dryas octopetala dominated) meadow communities at four times during the growing season. We found that plant, microbial and dissolved inorganic and organic N pools were subject to significant, but surprisingly low, temporal variation that was controlled primarily by changes in temperature and moisture availability over the short growing season. This temporal variability is much less than that experienced in other seasonally cold ecosystems such as alpine tundra where strong seasonal partitioning of N occurs between plant and soil microbial pools. While only a small proportion of the total ecosystem N, the microbial biomass represented the single largest of the dynamic N pools in both moist and dry meadow communities (3.4% and 4.6% of the total ecosystem N pool, respectively). This points to the importance of soil microbial community dynamics for N cycling in high-Arctic ecosystems. Microbial N was strongly and positively related to soil temperature in the dry meadow, but this relationship did not hold true in the wet meadow where other factors such as wetter soil conditions might constrain biological activity. Vascular live belowground plant parts represented the single largest plant N pool in both dry and moist meadow, constituting an average of 1.6% of the total N pool in both systems; this value did not vary across the growing season or between plant communities. Overall, our data illustrate a surprisingly low growing season variability in labile N pools in high-Arctic ecosystems, which we propose is controlled primarily by temperature and moisture.

Preferences for uptake of different nitrogen forms by co-existing plant species and soil microbes in temperate grasslands.

Abstract: The growing awareness that plants might use a variety of nitrogen (N) forms, both organic and inorganic, has raised questions about the role of resource partitioning in plant communities. It has been proposed that coexisting plant species might be able to partition a limited N pool, thereby avoiding competition for resources, through the uptake of different chemical forms of N. In this study, we used in situ stable isotope labeling techniques to assess whether coexisting plant species of a temperate grassland (England, UK) display preferences for different chemical forms of N, including inorganic N and a range of amino acids of varying complexity. We also tested whether plants and soil microbes differ in their preference for different N forms, thereby relaxing competition for this limiting resource. We examined preferential uptake of a range of 13C15N-labeled amino acids (glycine, serine, and phenylalanine) and 15N-labeled inorganic N by coexisting grass species and soil microbes in the field. Our data show that while coexisting plant species simultaneously take up a variety of N forms, including inorganic N and amino acids, they all showed a preference for inorganic N over organic N and for simple over the more complex amino acids. Soil microbes outcompeted plants for added N after 50 hours, but in the long term (33 days) the proportion of added 15N contained in the plant pool increased for all N forms except for phenylalanine, while the proportion in the microbial biomass declined relative to the first harvest. These findings suggest that in the longer term plants become more effective competitors for added 15N. This might be due to microbial turnover releasing 15N back into the plant–soil system or to the mineralization and subsequent plant uptake of 15N transferred initially to the organic matter pool. We found no evidence that soil microbes preferentially utilize any of the N forms added, despite previous studies showing that microbial preferences for N forms vary over time. Our data suggest that coexisting plants can outcompete microbes for a variety of N forms, but that such plant species show similar preferences for inorganic over organic N.