Heterotrophic respiration and nitrogen mineralisation in soils of Norway spruce, Scots pine and silver birch stands in contrasting climates

Different tree species are often associated with different soil properties. Earlier studies have shown that Norway spruce (Picea abies (L.) Karst.) and Scots pine (Pinus sylvestris L.), the two dominant tree species in Fennoscandia, often generate soils with larger carbon (C) and nitrogen (N) pools than silver birch (Betula pendula Roth.). Consequently, we hypothesised that spruce and pine would create soils with slower turnover rates than birch. To test this, C and N pools and C and N mineralisation rates were determined in different soil layers (humus, 0-10 cm, 10-20 cm mineral soil) at two sites with contrasting climatic conditions. One site (Tonnersjoheden) was located in the temperate zone in SW Sweden and one (Kivalo) in the north boreal zone in N Finland. At both sites, experimental plots with the three tree species had been established more than 50 years before the study. Samples from the different soil layers were incubated at 15 C in the laboratory for 30 days, and C and N mineralisation rates were determined. In addition, earthworm abundance was estimated at Tonnersjoheden but not at Kivalo (no sign of bioturbation). At Tonnersjoheden, soil C and N pools (g C or N m(-2)) were ranked spruce > pine > birch. C mineralisation rate (mg CO2-C g(-1) C d(-1)) was higher in the birch plots than in the other plots, but because of larger C pools in the spruce plots, field C mineralisation (g CO2-C m(-2) year(-1)) was higher for spruce than for pine and birch. Field net N mineralisation (80-90 kg N ha(-1) year(-1)) did not differ significantly between tree species, but nitrification rates (mu g NO3-N g-1 C d-1) in the topsoil were higher in the birch plots than in the other plots. The birch plots had larger populations of earthworms and a higher degree of bioturbation than any of the coniferous plots, which probably explains the higher turnover rate of birch soil organic matter (SOM). At Kivalo, C and N soil pools were significantly larger in spruce than in birch plots, and C mineralisation rate was higher in birch and spruce humus than in pine humus. Net N mineralisation rate and annual field net N mineralisation (<4 kg N ha-1 year-1) were estimated to be very low, with no effect of tree species. Thus, the hypothesis of a 'birch effect' was supported at Tonnersjoheden, but only partly at Kivalo. The main difference seemed to be that the earthworms at Tonnersjoheden accelerated SOM decomposition under birch, whereas earthworm stimulation was negligible at Kivalo, probably because of climate-related limitations. (C) 2012 Elsevier B.V. All rights reserved.