Impacts of tree species admixing on soil carbon and soil properties in boreal conifer-dominated forests: Evidence from multiple datasets

Increasing the proportion of broadleaved species in conifer-dominated forests can enhance forest multifunctionality and resilience. However, the effects of admixing on soil C stock development and soil properties in Fennoscandia are not well known because forest management has long been dominated by conifer monocultures. Here, we combined field data from multiple, existing studies with contemporary statistical methods. We applied non-parametric permutation-based inference to study the effects of tree species identity and species admixing on soil carbon (C) stocks, and key soil properties in mixed stands, and in single-species birch ( Betula spp.), pine ( Pinus sylvestris ), and spruce ( Picea abies ) stands. Data showed higher soil C stocks in the organic horizon under birch monocultures than under spruce monocultures. Tree diversity experiments indicated on average higher soil C stocks under birch-spruce mixtures, but the other datasets did not show significant differences in C stocks between mixed and conifer stands. In soil inventory, broadleaved-conifer mixed stands had higher pH, concentrations of N, and exchangeable Mg and K in the organic horizon than conifer monocultures. In addition, birch monocultures were characterized by a greater abundance of fine roots in deeper soil layers relative to mixed stands and conifer monocultures. Furthermore, C mineralisation rates in the organic horizon were higher in birch monocultures compared with pine monocultures, indicating enhanced microbial activity and accelerated organic matter turnover. Our results point to positive effects of birch admixture on several soil properties relevant to soil fertility and functioning, highlighting important implications for the management of mixed-species forests in Fennoscandia.