Grazing decreases N partitioning among coexisting plant species
(English)Manuscript (preprint) (Other academic)
Grazing by large herbivores plays a key role in shaping ecosystem functions by influencing nutrient cycling and plant community composition. This study investigates the long-term effects of grazing on resource acquisition by plant species. We explored differences in the natural δ15N signatures in plant, microbial and soil N pools, and examined mycorrhizal colonization in two tundra sites that have been lightly and intensively grazed by reindeer for more than 60 years. Study species included the mycorrhizal dwarf shrubs, Betula nana, Vaccinium myrtillus and Empetrum hermaphroditum, a mycorrhizal grass, Deschampsia flexuosa and a non-mycorrhizal sedge, Carex bigelowii. There were large variations in δ15N among coexisting plant species in the lightly grazed sites. This variation was dramatically reduced in the heavily grazed sites. At an individual species level, δ15N was higher in E. hermaphroditum and lower in C. bigelowii in the heavily grazed sites. Mycorrhizal colonization in B. nana and E. hermaphroditum roots were also lower at the heavily grazed sites. The δ15N signatures of the total soil N pool and of the microbial N pools were higher in the heavily grazed sites. Since the strong δ15N differentiation among plant species has been interpreted as a result of plants with different mycorrhizal types using different sources of soil nitrogen, we suggest that the lower variation in δ15N in heavily grazed sites indicate a lower niche differentiation in nitrogen uptake among plants. The reduced mycorrhiza-mediated nitrogen uptake by some of the species, a shift towards a more mineral nutrition due to higher nutrient turnover, and uptake of labile nitrogen from dung and urine in the heavily grazed sites could all contribute to the changes in plant δ15N. We conclude that herbivores have the potential to influence plant nutrient uptake and provide the first data suggesting that herbivores decrease nutrient partitioning on the basis of chemical N forms among plant species. Reduced niche complementarity among species is potentially important for estimates of the effects of herbivory on plant nutrient availability and species coexistence.
Above- belowground linkages, Arctic tundra, Microbial N biomass, Mycorrhizal colonization, Nutrient cycling, Plant-herbivore interactions, Plant nutrient uptake, Ungulate Grazing
Research subject biology
IdentifiersURN: urn:nbn:se:umu:diva-120190OAI: oai:DiVA.org:umu-120190DiVA: diva2:927066