Soil microbes, resource availability and plant herbivory in Arctic coastal ecosystems

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The lack of essential nutrients, such as carbon (C), and nitrogen (N) in natural systems, limits the growth of microorganisms and plants, which indirectly affects animal productivity. This is especially evident in cold Arctic soils where soil microbial decomposition proceeds slowly in wet peaty soils in which sedges are a prime food source for geese. However, peak microbial biomass in Arctic and alpine soils is realized in late winter, not summer, in snow-covered soils. The essential nutrients to drive growth of these organisms appear to be released from the decomposition of senescent plant tissue over the winter. The build-up of cold-tolerant microorganisms declines sharply at spring thaw concurrent with declines of pools of soluble soil nutrients. The onset of plant growth and the establishment of a summer microbial community is thought to be partly dependent on these pools and nutrients released from the decomposition of the winter microbes. The controls on this transition from one microbial community to another and the type of microorganisms involved are poorly understood. This proposal is based on a series of field perturbation experiments designed to determine the roles of temperature, nutrient availability and plant growth on the activity of different groups of microorganisms at this winter-spring boundary. The wider significance of this study is that Arctic geese feed extensively on sedges growing in these marshes, and any disruption of the nutrient supply to these forage plants, associated with early incipient spring thaws (climate change) and the loss of nutrients in melt water before plant growth has started, could adversely affect the productivity of all trophic levels. In a related study, the effect of simulated herbivory of sedges on potential carbon limitation of soil microbial growth in summer will be examined. Herbivory changes plant litter input and possibly organic acid exudation from roots that together may alter soil microbial activity as a result of changes in the availability of soil carbon, nitrogen and phosphorus for growth. These perturbations and faecal inputs may be expected to affect soil nutrient dynamics and therefore the potential forage quality and availability in these grazed systems.