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Effect of herbivory on the fate of added 15N-urea in a grazed Arctic tundra
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. (Arcum)
Arctic Center, University of Lapland Rovaniemi, Finland.
Department of Biology, Terrestrial Ecology, University of Copenhagen 2. 4Center for Permafrost (CENPERM), University of Copenhagen.
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. (Arcum)
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Mammalian herbivores can strongly influence nitrogen cycling and herbivore urine could be an important component of the nutrient cycle in grazed ecosystems. Despite its potential role for ecosystem productivity and soil processes, the distribution of N from urine in the different ecosystem compartments is poorly understood. This study investigates the fate of 15N enriched urea applied above the plant canopy in two tundra sites either heavily or lightly grazed by reindeer for the last 50 years. We explored the fate of the 15N in the different ecosystem N pools at 2 weeks and 1 years following tracer addition. We hypothesized that cryptogams would take up most N under light grazing, but graminoids most N under heavy grazing. The 15N-urea was rapidly incorporated in cryptogams and aboveground parts of vascular plants, while the soil microbial pool and plant roots sequestered only a marginal proportion of the labelled N applied. Hence, urine addition supports a higher primary production in tundra since most of the nutrients released from urine could be assimilated by the aboveground components with little N reaching the belowground compartments. Mosses and lichens still constituted the largest sink of the 15N-urea 1 year after tracer addition at both levels of grazing intensity demonstrating their large ability to capture and retain N  from urine. Deciduous and evergreen shrubs were just as efficient as graminoids in taking up the 15N-urea. The total recovery of the labelled urea was lower in the heavily grazed sites, suggesting that reindeer reduce the N retention in the system. Rapid incorporation of the applied 15N-urea indicates that arctic plants can take advantage of a pulse of incoming N in the form of urea, which supports a higher primary production. However, whether urine also maintains a high production of forage plants depend on plant community composition, since most urea was recovered in non-forage plants for reindeer.

Keyword [en]
Above-belowground linkages, Arctic tundra, Cryptogams, Microbial N biomass, Nutrient cycling, Plant-herbivore interactions, nutrient uptake, Grazing intensity, urine, 15N labelling
National Category
Ecology
Research subject
biology
Identifiers
URN: urn:nbn:se:umu:diva-120177OAI: oai:DiVA.org:umu-120177DiVA: diva2:927064
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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