Short- and long-term plant and microbial uptake of 15 N-labelled urea in a mesic tundra heath, West Greenland

Terrestrial animals are key elements in the cycling of elements in the Arctic where nutrient availability is low. Waste production by herbivores, in particular urine deposition, has a crucial role for nitrogen (N) recycling, still, it remains largely unexplored. Also, experimental evidence is biased toward short-term studies and Arctic regions under high herbivore pressure. In this study, we aimed to examine the fate of N derived from urine in a nutrient poor tundra heath in West Greenland, with historical low level of herbivory. We performed a pulse labelling with 15 N-urea over the plant canopy and explored ecosystem N partition and retention in the short-term (2 weeks and 1 year) and longer-term (5 years). We found that all vascular plants, irrespective of their traits, could rapidly take up N-urea, but mosses and lichens were even more efficient. Total 15 N enrichment was severely reduced for all plants 5 years after tracer addition, with the exception of cryptogams, indicating that non-vascular plants constituted a long-term sink of 15 N-urea. The 15 N recovery was also high in the litter suggesting high N immobilization in this layer, potentially delaying the nutrients from urine entering the soil compartment. Long-term 15 N recovery in soil microbial biomass was minimal, but as much as 30% of added 15 N remained in the non-microbial fraction after 5 years. Our results demonstrate that tundra plants that have evolved under low herbivory pressure are well adapted to quickly take advantage of labile urea, with urine having only a transient effect on soil nutrient availability.