NITROGEN AND PHOSPHORUS STORES IN PEATLANDS DRAINED FOR FORESTRY IN FINLAND

Nitrogen and phosphorus concentrations were determined for surface pea t samples for 78 sites on both undrained and drained pine mires. The o ldest areas had been drained 55 years earlier. Generally, the N and P stores in a 0 - 50 cm peat layer increased with increasing drainage ag e in oligo-ombrotrophic sites, and remained unchanged in meso-oligotro phic sites, even if large quantities were bound up in the increasing t ree stand biomasses. This was mainly caused by post-drainage subsidenc e of the mire surface and the consequent compaction, which increased t he peat bulk densities and brought new stores from deeper peat layers into the layer under observation. Measured stand volumes and published values for N and P concentrations of tree stand compartments were use d to estimate the amounts of N and P bound up in the tree stands. The estimates of N and P in the tree stands of the oldest drainage areas w ere considerable (ca. 400 kg N and 40 kg P ha-1 in the meso-oligotroph ic and ca. 300 kg N and 30 kg P ha-1 in the oligo-ombrotrophic sites), but were small when compared to the stores in the peat (ca. 10 000 kg N and 400 kg P ha-1 in the meso-oligotrophic and ca. 7000 kg N and 30 0 kg P ha-1 in the oligo-ombrotrophic sites in the 0-50 cm layer).