Relations of stable soil organic carbon with forest management and soil fungal guilds and genera in urban Picea abies dominated forests in Southern Finland

Boreal forests store a large proportion of global carbon. A large proportion of the boreal forest carbon stock is in soil organic carbon. Release of carbon from soils is mostly controlled by soil microbial activity. The cool climate retards enzymatic activity of decomposers and enables slow-turnover of carbon due to decreased decomposition, which can result in long term storage of carbon up to hundreds of years. Changes in environmental covariates, such as temperature and nutrient availability can affect soil microbial community structure and thus have an impact on quality and quantity of organic carbon in forest soils. Urban forests in Southern Finland are mostly used for recreational purposes, but are still affected by anthropogenic activity, such as forest management, trampling and fragmentation, all of which can affect soil abiotic properties and soil microbial communities.

The goal for this thesis was threefold: (1) How does forest management and related changes in forest structure affect stable soil organic carbon, fungal necro and biomass stocks in urban forests? (2) What is the relationship between different functional soil fungi groups and stable soil organic carbon in urban forests? And (3) what is the relationship between stable soil organic carbon and the abundance of soil fungi genera in urban forests? These questions were studied by comparing 74 Norway spruce (Picea abies) dominated urban forest stands in Southern Finland with varying management histories. Data was collected by field work and consisted of living and dead tree measurements, soil samples and GPS location data. Data was analysed with linear mixed models, generalized linear mixed models and hierarchical model of species communities.

The results show that intermediate management can decrease and larger carbon to nitrogen ratio increase the amount of major components of soil organic carbon; fungal bio- and necromass. Increased ratio of conifer to broadleaf trees decreased the proportion of stable soil organic carbon. Increases in the proportion of stable soil organic carbon can also decrease richness of ectomycorrhizal and soil saprotroph fungi but increase richness of some saprotrophic fungi. Richness of 20 % studied soil fungi genera also decreased when the proportion of stable soil organic carbon increased, while richness of none of the 50 genera increased. This study shows that soil fungi guilds and genera have connections with stable soil organic carbon. Effects on soil carbon and soil fungi communities should be considered when making land use- and forest management decisions in urban boreal forests.