Predictability of plant-soil feedback

ABSTRACT In my thesis project I studied the role of soil biota as possible drivers of linkages between plant community diversity and plant productivity. My study was carried out in the framework of a large grassland biodiversity experiment in Jena, the so-called Jena Experiment. In chapter 1 I explain how soil biota may exert control over plant community productivity by recycling organic material and by intimately interacting with plant roots, either acting as antagonists to plants or as plant growth-promoting symbionts. Reciprocal interactions between plant and soil communities are an important component of so-called ‘plant-soil feedbacks’ (PSFs). In the PSF loop, plant community composition drives changes in belowground communities and abiotic conditions, which can subsequently alter plant community composition and productivity. Such PSF interactions have been proposed to play a major role in plant community composition and functioning. In the second chapter I review studies that use an experimental approach of inoculating live soils into sterilized background soils to study the effects of root symbionts on plant growth. I demonstrate that we make many assumptions when translating results of controlled studies to natural systems. I propose that we should continuously and carefully consider these assumptions and aim for rigid hypothesis testing by cross-talking between different levels of ecological realism. In chapter 3 I test how plant traits relate to PSF using a 49 grassland plant species of the Jena Experiment. First, I grew individuals of all species for two months in sterilized soil inoculated with field soil. In the subsequent feedback phase, I grew all plant species for 6 weeks in sterilized soil inoculated with (I) species-specific inoculum (conspecific conditioned soil), (II) sterilized species-specific inoculum, or (III) a mixture of all 49 species-specific inoculums (mixed conditioned soil). Subsequently I compared biomass production in conspecific conditioned soil to biomass production in sterilized soil (PSFsterilized) and in mixed conditioned soil (PSFmixed). Species with increasing specific root length (SRL) were increasingly susceptible to antagonistic interactions in conspecific conditioned soil (i.e. they had strong negative PSFsterilized), while thick-rooted plants had both positive PSFsterilized and high colonization rates of arbuscular mycorrhizal fungi (AMF). Finally, I showed that species ranking of PSFmixed was similar to species ranking of PSFsterilized, indicating that plants with increasingly negative net interactions in conspecific conditioned soil increasingly benefit from growing in mixed conditioned soil. With these findings, I made a first important step in placing PSFs in plant ecological strategy frameworks: high SRL is typical for plants that adopt a ‘fast’ growth strategy, characterized by fast resource acquisition but poor defense against antagonists and little reliance on AMF. In chapter 4, I test the relation between phylogenetic relatedness and the feedback effect of one (soil conditioning) plant species to another (responding) plant species. This is named indirect PSF. I grew eleven focal plant species, chosen to represent plants that had negative, neutral and positive PSFsterilized, in soils that were conditioned by conspecifics and soils conditioned by three to four other species with a varying degree of phylogenetic relatedness to the focal plant species. I found that plant species with negative PSF had no different or slightly better growth when growing in soil conditioned by plant species with larger phylogenetic distance to the focal plant. In contrast, plant species with neutral PSF grew less well, and species with positive PSF even worse, in soil conditioned by plant species with increasing phylogenetic distance to the focal plant. I conclude that the effect of phylogenetic relatedness on PSF interactions between plant species may depend on the tendency of the focal plant species to develop detrimental or beneficial interactions with soil microbes. In chapter 5, I use the PSFmixed values of chapter 3 in a correlational analysis to test how short-term PSFs relate to longer-term species’ performances in the field, using established monocultures and species-rich (60 species) plant communities of the Jena Experiment. Based on some recently published studies I expected that plants with more negative PSFmixed would benefit most from growing in mixtures; these plant species were expected to overyield most in mixed plant communities. However, opposite to the expectation, plant species with the most negative PSF produced least biomass in the 60-species plant communities, whereas plant performance in monoculture was not related to its short-term PSF. I conclude that species-specific overyielding was positively related to species-specific PSF, and that community overyielding was mostly driven by plant species with a neutral to positive PSF. Finally, in chapter 6 I examine the role of quality and quantity of plant biomass in driving nematode feeding group abundance and diversity. I found strong positive effects of both plant species- and plant functional group-richness on abundances of plant feeding, bacterial feeding and fungal feeding nematodes, as well as omnivores, but not for predators. Structural equation modeling (SEM) analysis showed that the positive effect of plant diversity on the abundance of microbial feeding nematodes (fungal plus bacterial feeders) could not be explained by increased microbial biomass. Similarly, the abundance of plant feeding nematodes was not driven by the higher plant biomass in species rich plant communities. Instead, increased plant biomass explained the positive relation between plant species richness and the abundance of microbial feeding nematodes, while for plant feeding nematodes, increased C to N ratio of aboveground plant biomass appeared to explain the positive relation between the abundance of plant feeding nematodes and plant species and functional group richness. Importantly, the density of plant feeding nematodes per unit root biomass decreased with increasing plant diversity, indicating a root feeder dilution effect. I conclude that plant diversity does not explain nematode community composition primarily by simple bottom-up relations, but that other aspects, such as quality of resource and microhabitats quality, may play a role as well.