Recovery of 15N-labelled nitrate applied to a small upland boreal forest catchment

Nitrate labelled with 15N was added for 2 years to a small Boreal Shield forested catchment to evaluate where new nitrogen (N) inputs are stored in this system in the short term. The landscape in the study catchment is a mosaic of Pinus banksiana Lamb. - Picea mariana (Mill.) BSP "forest islands" surrounded by "pillows" of lichens, mosses, and grasses on exposed bedrock surfaces. After the second year of N addition, 10% of the added 15N was found in aboveground biomass, 27% in organic horizons, 6% in forest island mineral soil, 3% in "soil" underneath pillows, and 16% was lost through runoff. Despite covering a smaller proportion of the catchment and having only slightly larger N stocks, forest islands stored more 15N (31%) than pillows (15%). Preferential routing of water towards forest islands and a greater potential for biotic and abiotic N immobilization are hypothesized to favor N retention in forest islands. Thirty-eight percent of the 15N could not be accounted for. The comparison of a mass-balance analysis with the 15N recovery suggests a missing sink for the 15N in forest islands, possibly woody detritus. In the short term (~2 years), organic horizons are the main sinks for elevated N inputs in the boreal forest.