Soil organisms and carbon, nitrogen and phosphorus mineralisation in Norway spruce and mixed Norway spruce - Birch stands

We examined how soil organisms and C, N and P mineralisation are affected b y admiring deciduous tree species, silver birch (Betula pendula) and woolle n birch (B. pubescens), in managed Norway spruce (Picea abies) stands. Pure spruce and mixed spruce-birch stands were examined at four sites in southe rn and central Sweden. Soil macroarthropods and enchytraeids were sampled i n litter and soil. In the uppermost 5 cm of soil humus we determined microb ial biomass and microbial respiration; we estimated the rate of C, N and P mineralisation under laboratory conditions. The densities of Coleoptera, Di ptera and Collembola were larger in mixed stands than in spruce stands. Soi l fauna composition differed between mixed and spruce stands (as revealed b y redundancy analysis). Staphyliniidae, Elateridae, Cecidiomyidae larvae an d Onychiuridae were the families that increased most strongly in mixed stan ds. There were no differences in microbial biomass and microbial respiratio n, nor in the C, N and P mineralisation rates, between mixed and spruce sta nds. However, within mixed stands microbial biomass, microbial activity and C mineralisation were approximately 15% higher under birch trees than unde r spruce trees. We propose that the presence of birch leaf litter was likel y to be the most important factor causing differences in soil fauna composi tion. Birch may also influence the quality and the decomposition rate of hu mus in mixed stands. However, when the proportion of birch trees is Low, th e short-term (decades) effect of this species on decomposition is likely to be small in mixed stands on acid forest soils.