Norway spruce monocultures in the boreal region are vulnerable to the effects of climate change and unfavourable in terms of soil fertility. Introducing broadleaved tree species to these forests may increase not only the resilience of the forest ecosystem to climate change but also enhance soil productivity and carbon (C) stock. We studied how grey alder, having symbiosis with N₂-fixing Frankia, and birch affect soil nitrogen (N) and carbon pools as an admixture in Norway spruce stands in Southern Finland.

Study sites were three 40–60-year-old Norway spruce (Picea abies (L.) Karst.) -dominated stands with both grey alder (Alnus incana) and birch (both Betula pendula and Betula pubescens) as an admixture and a 20-year-old spruce stand with an admixture of grey alder. The forest type was a relatively fertile Oxalis acetosella – Vaccinium myrtillus type (OMT) on the two older (60 yr) sites and a slightly less fertile Vaccinium myrtillus type (MT) on the two younger sites (20–40 yr), applying the Finnish forest type classification. We took the soil samples at a 50–100 cm distance from 3–8 stems of the different tree species for the determinations of soil C and N stocks from all sites and for additional characterisation of organic matter only from the 60-year OMT sites. Soil diffusive N fluxes were measured using in situ microdialysis sampling and the subsequent laboratory analyses of plant-available N compounds.

On average, forest floor N stock was larger under the canopy of alder versus birch or spruce. C-to-N ratios of forest floor and topmost 10 cm mineral soil layer were lower under alder versus spruce. Soil C stock was affected by tree species only at the 40-year MT site, where alder had a higher forest floor C stock than birch or spruce. Differences in diffusive N fluxes between tree species were non-significant, and we observed inconsistent trends at different sites. C mineralisation rate tended to be lower under alder versus spruce on the two 60-year OMT sites, and the amount of microbial biomass N was lower under alder versus birch. Microbial biomass C-to-N ratio and forest floor thickness were lower under birch than spruce on one of the 60-year OMT sites.

The results point towards complex interactions and dynamics between tree species in mixed forests. Although we observed tree-species-induced spatial variation in soil properties, the distribution of above- and belowground litter and root activities in mixed stands reduces the differences between tree species. We found tree species to affect N stocks and C-to-N ratios most strongly, alder altering soil properties of spruce stands more than birch.