Moss-nitrogen input to boreal forest soils: Tracking N-15 in a field experiment

Cyanobacteria living epiphytically on mosses in pristine, unpolluted areas fix substantial amounts of atmospheric nitrogen (N) and therefore represent a primary source of N in N-limited boreal forests. However, the fate of this N is unclear, in particular, how the fixed N-2 enters the soil and becomes available to the ecosystem. In this study, we applied N-15-ammonium chloride (N-15-NH4Cl) onto carpets of the feather moss Pleurozium schreberi and traced the N-15 label into green (living) and brown (senescent) moss and into the upper soil layer over time. Further, we placed filters between moss and soil to assess the role of moss-associated fungi for N-transfer to the soil. The experiment was conducted at endpoints of a N-2 fixation gradient in Northern Sweden. Feather moss retained the applied N in the green moss parts for up to 1 year and no increase of excess N-15 was found in the brown moss parts or in the soil within that same time frame. The filter treatment did not alter the N-15-distribution in moss or soil. Nitrogen retention in the moss was similar regardless of position along the N-2 fixation gradient. Our results suggest that mosses represent a short-term inorganic N sink and that transfer of N to the soil is not facilitated by fungal hyphae. (C) 2014 Elsevier Ltd. All rights reserved.