The fate of experimentally deposited nitrogen in mesocosms from two Canadian peatlands

In large regions of Europe and North America, peatlands have been exposed to elevated rates of atmospheric nitrogen (N) deposition. We investigated the fate of experimentally added N (NH4 15NO3) at two different N loads (1.2 and 4.7 g N m 2 yr 1) and water tables (1 and 32 cm) in intact cores from two peatlands, located in Central and Eastern Canada. The sites receive an estimated total N load of 0.6 g m 2 a 1 and 1.5 g m 2 yr 1, excluding nitrogen fixation. In all treatments, experimentally added nitrate (NO3 ) was fully (96–99%) and ammonium (NH4 +) mostly (81–97%) retained by the plant cover, mainly consisting of Sphagnum mosses, or in the unsaturated zone below. However, on average only 48% of the 15N were recovered from the plant cover, and substantial amounts were found in depth layers of 2–6 cm (21–46%) and 8–12 cm (1.4– 10.8%) below the moss surface. The amount of 15N retained also significantly decreased with a lower water table from 56F 9% to 40F10%. These findings document a substantial mobility of N, particularly during water table drawdown. Analysis of 15N by a sequential diffusion procedure revealed a transfer of 15N from NO3 into NH4 + and dissolved organic N (DON), but the contents of 15N in these pools accounted for less than 1% of the total N, natural background subtracted. The mass flux of dissolved 15N into the peat was small compared to the total mass flux of 15N. The accumulation of 15N in the bulk peat must have been caused by a mechanism that was not investigated, possibly by transport of particulate organic N.