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

• Published in: Journal of Ecology
• Main Authors: Barthelemy, Hélène, Stark, Sari, Michelsen, Anders, Olofsson, Johan
• Other Authors: Aerts, Rien, Stiftelsen Oscar och Lili Lamms Minne, Danmarks Grundforskningsfond
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2017
• Subjects: Arctic; Tundra
• Online Access: http://dx.doi.org/10.1111/1365-2745.12820; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2F1365-2745.12820; https://besjournals.onlinelibrary.wiley.com/doi/pdf/10.1111/1365-2745.12820

Abstract 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. This study explored the fate of 15 N‐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 15 N applied to the plant canopy, at 2 weeks and 1 year after tracer addition, in the different ecosystem N pools. 15 N‐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 15 N‐urea, at least in the short term, indicating a high biological activity in the litter layer and high immobilization in the first phases of organic matter decomposition. Mosses and lichens still constituted the largest sink for the 15 N‐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 15 N‐urea. The total recovery of 15 N‐urea was lower in the intensively grazed sites, suggesting that reindeer reduce ecosystem N retention. Synthesis . The rapid incorporation of the applied 15 N‐urea indicates that arctic plants can take advantage of a pulse of incoming N from urine. In addition, δ 15 N values of all taxa in the heavily grazed sites converged towards the δ 15 N values for urine, bringing further evidence that urine is an important N source for plants in grazed tundra ecosystems.