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 by admiring deciduous tree species, silver birch (Betula pendula) and woollen birch (B. pubescens), in managed Norway spruce (Picea abies) stands. Pure spruce and mixed spruce-birch stands were examined at four sites in southern and central Sweden. Soil macroarthropods and enchytraeids were sampled in litter and soil. In the uppermost 5 cm of soil humus we determined microbial biomass and microbial respiration; we estimated the rate of C, N and P mineralisation under laboratory conditions. The densities of Coleoptera, Diptera and Collembola were larger in mixed stands than in spruce stands. Soil fauna composition differed between mixed and spruce stands (as revealed by redundancy analysis). Staphyliniidae, Elateridae, Cecidiomyidae larvae and Onychiuridae were the families that increased most strongly in mixed stands. There were no differences in microbial biomass and microbial respiration, nor in the C, N and P mineralisation rates, between mixed and spruce stands. However, within mixed stands microbial biomass, microbial activity and C mineralisation were approximately 15% higher under birch trees than under spruce trees. We propose that the presence of birch leaf litter was likely to be the most important factor causing differences in soil fauna composition. Birch may also influence the quality and the decomposition rate of humus in mixed stands. However, when the proportion of birch trees is Low, the short-term (decades) effect of this species on decomposition is likely to be small in mixed stands on acid forest soils.