Plants impact soil processes and organisms via leaf and root litter inputs, rooting patterns, root processes related to nutrient foraging and rhizodeposition, or through a modification of microclimatic conditions. Since plant species and genotypes differ in their effects on soil, changes in plant diversity can influence the composition and functioning of soil organism communities. Soil processes have been hypothesized to be less responsive to plant diversity changes than aboveground processes, but very few studies have directly compared the relative responsiveness of above- and belowground processes to plant diversity within the same experimental setting. 

Our researches in the Satakunta forest diversity experiments aim to evaluate the effects of both tree species and genetic diversity on different soil parameters :

• litter decomposition
• microbial processes
• soil organisms
• soil chemistry

Litter decomposition

Litter decomposition is a key process for elemental cycling and ecosystem functioning. Decomposers depend on plant litter as their main source of energy and nutrients and therefore are strongly affected by litter quality. Litter quality vary among plant species and numerous studies have shown that mixing litter from different species generally increases decomposition. These positive effects of litter mixing are likely due to a higher chemical heterogeneity within mixtures improving resource complementarity to decomposers. Although genetic variation is likely to cause less variation in litter quality than variation among species, several studies have shown that plant genetic diversity may have significant impact on litter decomposition as well. In addition to the effects of  interactions between litter species in mixtures, tree diversity may also affect decomposition through long-term effects of trees on soil microenvironment and communities, but theses long-term effects are still poorly studied. 

We are investigating the effects of tree species and genetic diversity on litter decomposition and the mechanisms behind these effects by separating the effects of both litter composition and long-term effects of trees on soil.

Soil organisms and microbial processes

Effects of plant diversity on soil organism communities remain controversial. Most of existing studies were short-term and assessed the impact of plant diversity on soil processes and organisms in grasslands while its effects in forests remains largely unexplored.

We examined the effects of tree species diversity in the Satakunta tree species diversity experiment on soil microbial biomass and taxonomic richness of soil biota using a DNA metabarcoding approach which allows to identify the major eukaryote groups directly from soil with roughly species-level resolution. Using this method, we have shown that soil nutrients, pH and relative abundance of different tree species have stronger effect on diversity of soil biota than tree species richness per se.

We also collected data on ground-dwelling arthropods using pitfall traps at the Satakunta tree species diversity experiment. These data were included in a meta-analysis on the effects of tree species diversity on abundance of five main groups of predatory arthropods (ants, spiders, carabids, staphylinids and opilionids). Overall, the diversity of tree species did not affect abundances of predators with the exception of staphylinids, which were more abundant in mixed stands than in monocultures. Preference for stands containing particular tree species was clear in the case of opilionids, carabids and staphylinids. Therefore, the identity of tree species within stands may be more important to ground dwelling predators than the number of different tree species per stand.

Currently, we are investigating how tree species and genetic diversity influence soil microbial biomass and respiration, microbial community level physiological profiling (using MicroRespTM technique) and potential nitrification.