The impact of snow goose herbivory on soil nutrients, microbial biomass, and plant re-growth

Background/Question/Methods Primary productivity in Arctic ecosystems is typically limited by the availability of nitrogen (N), with changes in N-availability affecting plant and microbial biomass. Soil microbes strongly influence the availability of nutrients to plants during the growing season, and indirectly control the productivity of higher trophic levels (herbivores), by influencing the amount and quality of available plant forage. Recent evidence suggests that grazed plants can have a positive influence on soil microbes and the availability of nutrients. When some plants are subjected to aboveground herbivory, they have been shown to exude greater amounts of labile organic carbon (C) into the rhizosphere, benefiting C-limited microbes. Increased microbial activity results in higher rates of net N mineralization, which may increase the availability of inorganic N for the growth of new photosynthetic plant tissue. To test the hypothesis that grazing stimulates microbial biomass and belowground nutrient availability, experimental plots were set up in 2006 and 2007 at two field sites on the Cape Churchill Peninsula, that have been exposed to different histories of grazing. One site is located in an intertidal salt marsh that has been intensively grazed each summer for over 40 years, by lesser snow geese (Chen caerulescens caerulescens). In contrast, the second site is located in a freshwater sedge meadow that only recently has been subjected to grazing, brought about by the loss of salt-marsh habitat, due to the destruction of intertidal vegetation by geese. Plots received one of four nutrient treatments (control, C (glucose), NP, and NPC), with or without the exclusion of geese. The grazing treatment at the freshwater site was simulated, due to lower goose densities, by clipping plants down to the shoot base. Soil was harvested after grazing, and microbial biomass was estimated using the chloroform-fumigation extraction method. Results/Conclusions At the freshwater site, plots that received an addition of C showed an increase in microbial biomass C (MBC). This result supports the notion that microbes are limited by the availability of C in these soils. At the salt marsh site MBC did not increase in C-addition plots, although previous work has demonstrated a C response over a shorter timescale. Results from both sites showed that grazing did not lead to an increase in MBC. Thus, the severe loss of plant photosynthetic tissue in grazed plots has no effect on soil microbial biomass in the rhizosphere.