Urine is an important nitrogen source for plants irrespective of vegetation composition in an Arctic tundra: Insights from a ¹⁵N-enriched urea tracer experiment

• Published in: The Journal of ecology Vol. 106; no. 1; pp. 367 - 378
• Main Authors: Barthélémy, Hélène, Stark, Sari, Michelsen, Anders, Olofsson, Johan
• Format: Journal Article
• Language: English
• Published: Oxford John Wiley & Sons Ltd 01.01.2018; Blackwell Publishing Ltd
• Subjects: above‐ and belowground linkages; aerial parts; Aquatic plants; Arctic region; Arctic tundra; bioactive properties; Biological activity; botanical composition; canopy; Composition; Cryptogams; Ecosystems; Flowers & plants; graminoids; Grazing intensity; Herbivores; Immobilization; Lichens; Litter; microbial N biomass; Microorganisms; mosses and liverworts; N labelling; Nitrogen; nitrogen cycle; Nitrogen isotopes; nutrient cycling; Organic matter; Plant cover; plant nutrient uptake; Plant roots; Plant-herbivore interactions; Plants; reindeer; roots; Shrubs; Soil; stable isotopes; Taiga & tundra; Taxa; Tracers; Tundra; Urea; Urine; vascular plants; Vegetation; Arctic region
• ISSN: 0022-0477; 1365-2745
• DOI: 10.1111/1365-2745.12820

1. Mammalian herbivores can strongly influence nitrogen (N) cycling and herbivore urine could be a central component of the N cycle in grazed ecosystems. Despite its potential role for ecosystem productivity and functioning, the fate of N derived from urine has rarely been investigated in grazed ecosystems. 2. This study explored the fate of ¹⁵N-enriched urea in tundra sites that have been either lightly or intensively grazed by reindeer for more than 50 years. We followed the fate of the ¹⁵N applied to the plant canopy, at 2 weeks and 1 year after tracer addition, in the different ecosystem N pools. 3. ¹⁵N-urea was rapidly incorporated in cryptogams and in above-ground parts of vascular plants, while the soil microbial pool and plant roots sequestered only a marginal proportion. Furthermore, the litter layer constituted a large sink for the ¹⁵N-urea, at least in the short term, indicating a high biological activity in the litter in the first phases of organic matter 15 layer and high immobilization decomposition. 4. Mosses and lichens still constituted the largest sink for 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. Despite large fundamental differences in their traits, deciduous and evergreen shrubs were just as efficient as graminoids in taking up the ¹⁵N-urea. The total recovery of ¹⁵N-urea was lower in the intensively grazed sites, suggesting that reindeer reduce ecosystem N retention. 5. Synthesis. The rapid incorporation of the applied ¹⁵N-urea indicates that arctic plants can take advantage of a pulse of incoming N from urine. In addition, δ¹⁵N values of all taxa in the heavily grazed sites converged towards the δ¹⁵N values for urine, bringing further evidence that urine is an important N source for plants in grazed tundra ecosystems.