Seasonal dynamics of N in two Sphagnum moss species and the underlying peat treated with 15NH415NO3

Uptake of N-15 labelled NH4NO3 by two Sphagnum mosses on a raised bog in north east Scotland was measured at different times of the year. In a field experiment, fortnightly additions of NH4NO3 at natural abundance, equivalent to 3 g N m(-2) yr(-1), were made over 14 months to cores of Sphagnum capillifolium occupying hummocks and S. recurvum colonizing hollows. Pre-harvested cores were treated with (NH4NO3)-N-15-N-15 two weeks before harvesting and N-15 abundance determined for the total N in the moss, inorganic and dissolved organic N (DON) in the moss water and extractable inorganic, organic and microbial N in the underlying peat. The proportion of added N-15 taken up by the mosses two weeks after each addition averaged 72% and ranged between 11 and 100%, tending to be least during October when the rising water table reached the surface, particularly for S. recurvum. A small proportion of the N-15 was detected in the moss water as NH4+ (0.01%) and as DON (0.03%) and on occasions a large proportion remained unaccounted for. In waters from S. capillifolium, DON was proportional to the amount of inorganic N added, but this was not the case for S. recurvum. Little or no N-15 was detected in the underlying peat partly because of the large size and variability of the NH4+, DON and microbial N pools.