Nitrogen mineralisation in podzol soils under boreal Scots pine and Norway spruce stands

N mineralisation was investigated in the mor humus layer of a podzol at a forested catchment area of Saarejarve Lake in Eastern Estonia. The investigated areas were pine ( Rhodococcum understorey) and spruce ( Vaccinium understorey) stands, which are permanent sample plots of an integrated monitoring network. The seasonal pattern of net N mineralisation was studied by incubating undisturbed cores of mor humus ( 0 - 8 cm) in buried polyethylene bags in situ. Samples were collected and incubated between July 1996 and April 1998. The period of incubation was approximately 1 month, except for wintertime when incubation lasted till thawing of ground (similar to 5 months). The amounts of mineral nitrogen formed during monthly incubations in vegetation period vary considerably (0.4-8.7 kg ha(-1)). About 70% of the variation of net ammonification could be explained by environmental factors temperature, initial moisture and pH. Ammonium was the dominant form of mineral nitrogen, which is typical for mor humus. The rate of nitrification was very low, and most of the annual net nitrification occurred during just one or two months ( May - June, October) depending on site and year. Measured annual net N mineralisation was 29.2 kg ha(-1) for the spruce stand and 23.6 kg ha(-1) for the pine stand. These measures were found to be in good accordance with other N-fluxes in the ecosystem.