Diversity relations of plants and soil microbes

Soil biota are critical to terrestrial ecosystem functioning. Plant and soil communities are engaged in dynamic interactions, however, plant species richness is declining worldwide and little is known about what this means for soil biodiversity. In this thesis, I investigated the relative impact of plant community changes (plant species richness and functional group identity) on soil microbial communities compared to abiotic factors. I showed that soil abiotic properties within the same field alter community composition of bacteria, mycorrhizal and non-mycorrhizal fungi, protists, and archaea more strongly than plant species richness or plant functional group identity. Among the abiotic factors measured, soil texture (the proportions of sand, silt, and clay) explained most of the variation in community composition of all the microbial groups analysed. I also showed that minor variation near neutral soil pH does not have a substantial impact on microbial community composition. Despite the considerable impact of abiotic soil properties, I found that the richness of fungi in the bulk soil, root-associated fungi and root-associated fungal saprophytes increased with plant species richness. However, I found no such response of fungal plant pathogens, arbuscular mycorrhizal fungi (AMF), bacteria, protists, or nematodes with respect to plant species richness. In contrast, plant functional group identity altered richness and abundance of soil biota with marked difference between plant communities with and without legumes. The community composition of fungi in soil and roots, as well as of nematodes in soil correlated with the percentage cover of grasses and forbs. In a next step, I aimed to separate effects of plant species richness from those of plant traits on the community composition of soil organisms. I found that the community weighted mean of specific morphology and development-related plant traits explained more variation in soil community composition than plant functional group or plant species richness. For the group of soil biota examined, I show that compositional changes in the plant community have the strongest effect on the richness and abundance on fungal communities, a smaller effect on nematode communities, and the least effect on bacterial communities. I examined which microbial and nematode groups of the soil food web were most sensitive to changes in plant community composition. I found that plant trait composition affected the abundance of soil microbes and nematodes from the green pathway (living plant-based) in the soil foodweb more than from the brown pathway (dead plant-based) of the soil foodweb. In addition, differences in plant trait composition had the strongest impact on the abundance of lowest trophic position organisms in the soil food web. In the last part of my thesis, I tested the functional consequences of plant seedlings being linked to the mycorrhizal network in soil. In contrast to the original expectations, my results show that established mycorrhizal networks did not promote the performance of seedlings. Instead, I found biomass reduction of seedlings that were connected to a mycorrhizal network and this was independent of successional stage, plant functional group identity and previously reported negative or positive plant-soil feedback interactions. In addition, increased levels of AMF colonisation correlated with decreased seedling performance.