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Grazing decreases N partitioning among coexisting plant species
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. (Arcum)
Arctic Center, University of Lapland, Rovaniemi, Finland.
Department of Biological and Environmental Science, University of Jyväskylä, Finland.
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. (Arcum)
(English)Manuscript (preprint) (Other academic)
Abstract [en]

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.

Keyword [en]
Above- belowground linkages, Arctic tundra, Microbial N biomass, Mycorrhizal colonization, Nutrient cycling, Plant-herbivore interactions, Plant nutrient uptake, Ungulate Grazing
National Category
Ecology
Research subject
biology
Identifiers
URN: urn:nbn:se:umu:diva-120190OAI: oai:DiVA.org:umu-120190DiVA: diva2:927066
Available from: 2016-05-10 Created: 2016-05-10 Last updated: 2016-06-30
In thesis
1. Herbivores influence nutrient cycling and plant nutrient uptake: insights from tundra ecosystems
Open this publication in new window or tab >>Herbivores influence nutrient cycling and plant nutrient uptake: insights from tundra ecosystems
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Reindeer appear to have strong positive effects on plant productivity and nutrient cycling in strongly nutrient-limited ecosystems. While the direct effects of grazing on vegetation composition have been intensively studied, much less is known about the indirect effect of grazing on plant-soil interactions. This thesis investigated the indirect effects of ungulate grazing on arctic plant communities via soil nutrient availability and plant nutrient uptake.

At high density, the deposition of dung alone increased plant productivity both in nutrient rich and nutrient poor tundra habitats without causing major changes in soil possesses. Plant community responses to dung addition was slow, with a delay of at least some years. By contrast, a 15N-urea tracer study revealed that nutrients from reindeer urine could be rapidly incorporated into arctic plant tissues. Soil and microbial N pools only sequestered small proportions of the tracer. This thesis therefore suggests a strong effect of dung and urine on plant productivity by directly providing nutrient-rich resources, rather than by stimulating soil microbial activities, N mineralization and ultimately increasing soil nutrient availability. Further, defoliation alone did not induce compensatory growth, but resulted in plants with higher nutrient contents. This grazing-induced increase in plant quality could drive the high N cycling in arctic secondary grasslands by providing litter of a better quality to the belowground system and thus increase organic matter decomposition and enhance soil nutrient availability. Finally, a 15N natural abundance study revealed that intense reindeer grazing influences how plants are taking up their nutrients and thus decreased plant N partitioning among coexisting plant species.

Taken together these results demonstrate the central role of dung and urine and grazing-induced changes in plant quality for plant productivity. Soil nutrient concentrations alone do not reveal nutrient availability for plants since reindeer have a strong influence on how plants are taking up their nutrients. This thesis highlights that both direct and indirect effects of reindeer grazing are strong determinants of tundra ecosystem functioning. Therefore, their complex influence on the aboveground and belowground linkages should be integrated in future work on tundra ecosystem N dynamic.

Place, publisher, year, edition, pages
Umeå: Umeå University, 2016. 36 p.
Keyword
Reindeer grazing, large herbivores, nutrient cycling, plant nutrient uptake, soil nutrient availability, arctic plant ecology, soil microbial communities, 15N stable isotopes, plant-soil interactions, plant quality, dung and urine.
National Category
Ecology
Research subject
biology, Environmental Science; biology
Identifiers
urn:nbn:se:umu:diva-120191 (URN)978-91-7601-456-1 (ISBN)
Public defence
2016-06-03, Lilla Hörsalen, KBC (KB3A9), Umeå, 13:00 (English)
Opponent
Supervisors
Available from: 2016-05-12 Created: 2016-05-10 Last updated: 2016-06-30Bibliographically approved

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Barthelemy, HélèneOlofsson, Johan
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