The fate of inorganic N additions to an ombrotrophic bog

Undisturbed peat cores (50 cm depth) were maintained in situ in a Sphagnum dominated ombrotrophic bog for four months. Solutions of NH4NO3 at four application rates were added at two week intervals, the total amount of N added over the experimental period was 0, 2.7, 8.1 and 26.8 kg N ha−1 (equivalent to 0, 10, 30 and 100 kg N ha−1 yr−1). On harvesting, cores were cut into 5 cm slices for analysis. Mineral-N was present in all N treatments predominantly as NH4-N with only trace amounts of NO3-N and there was no significant (p < 0.05) effect of treatment on either of the mineral N forms. Soluble organic N extracted with 0.5 M K2SO4 was present in greater amounts (a mean value across the treatments of 47.2 kg N ha−1) to 50 cm depth than inorganic-N (13.3 kg N ha-1) throughout the peat profile, and the microbial biomass N pool, determined by fumigation extraction, (113.7 kg N ha-1) was the largest of the fractions measured. All N pools varied significantly (p < 0.005) with depth and a greater value for the microbial N flush in the 20-25 cm layer than at other depths suggested that this was the zone where decomposition and immobilization were on average most active in this peat profile.