Ongoing research

My current research focuses on the role of soil organisms in plant community dynamics and ecosystem functioning and how plant-soil organism interactions are affected by environmental changes. A focal theme is the role of historical contingency in plant and soil community dynamics. Assembly history, i.e. the order of species arrival, can have persistent effects on both plant and soil community composition and, hence, potentially has important consequence for plant-soil organism interactions, soil ecosystem processes and above-belowground interactions.

The contribution of plant-soil feedbacks and legacies to plant performance can depend on competitive pressure from neighboring species as well as on environmental factors subject to climate change. Importantly, plant-soil feedbacks and legacies involve interactions with the abiotic environment as well as interactions with the biotic soil component. Biotic plant-soil feedback include interactions with root-associated soil organisms (pathogens, mutualists) as well as interactions with soil organisms involved in the decomposition of organic compounds (litter, root exudates).

Studying plant and soil communities under environmetnal changes and the consequences for ecosystem functioning inherently demands a strategic combination of testing individual responses of component taxa, their interactions and community effects, both under controlled (lab, greenhouse) and more natural conditions (open-top chamber, field). Such research requires novel analytical techniques and state-of-the-art multivariate statistics and/or the construction of theoretical models.

Currently, I am involved in the Old-Field Community, Climate, and Atmosphere Manipulation Experiment (OCCAM), a joint project of Oak Ridge National Laboratory and the University of Tennessee in Knoxville. In the OCCAM experiment we test effects of multiple climate change factors on the structure and function of a constructed old-field ecosystem.Simultaneously operating mechanisms, interlinked by negative and positive feedbacks, and varying in the spatial and temporal scales on which they act, challenges our ability to understand climate change effects on plant community dynamics and (soil) ecosystem functioning. One of the hypothesis I tested in the OCCAM experiment is that direct effects of climate change on soil ecosystem functioning and effects through changes in individual plant function depend on concurrent shifts in plant community composition.

Other experiments that I recently conducted or that are currently in progress include 1) a greenhouse experiment testing the role of time intervals between species arrivals under contrasting levels of resource availability in plant community assembly and ecosystem functioning, 2) a growth chamber experiment testing the importance of effects of climate change on soil properties for plant performance and plant-competitive interactions. The latter is a follow-up of the OCCAM experiment. Further, I am involved in the Oak Ridge FACE experiment and in analyzing long-term data-sets from the Walker Branch Watershed.

Previous research

My dissertation research at the Department of Multitrophic Interactions (Netherlands Institute of Ecology (NIOO-KNAW)) and Wageningen University, which was conducted within the framework of a multidisciplinary project on ‘soils in transition…’, aimed at disentangling the interplay between plant and soil community assembly in secondary succession on former agricultural land.

Successional patterns of soil and plant communities
Living soil organisms are reliable bio-indicators, as they provide a good reflection of ecological services and functioning of the soil ecosystem. I particularly focused on the taxonomic and trophic structure of soil nematodes communities. Nematodes are the most numerous soil organisms in agricultural and grassland soils and they display high taxonomic and functional richness. In a chronosequence of former agricultural fields, I showed that plant and soil nematode communities do not necessarily develop in parallel towards the same reference system and that successful restoration of plant communities does not imply successful restoration of soil communities. Furthermore, I showed that successional soil diversity patterns depend both on the spatial scale of the sampling regime and the group of organisms considered. Successional changes in the soil nematode community were mainly due to gradual shifts in dominance patterns in response to altered environmental conditions, while successional changes in the soil mite community depended most on colonization from local species pools.

Plant-soil feedbacks
In microcosm experiments, I studied the role of biotic plant-soil feedbacks in successional  plant community dynamics. Biotic plant-soil feedback can be defined as the effect of a plant species on soil biological properties and the subsequent effect on its own performance. My work suggests that, irrespective of abiotic soil conditions, negative plant-soil feedback effects enhance plant community succession in early stages, while positive feedback effects stabilize succession and promotes plant community diversity in later stages. Explicitly, I showed that micro-mediated plant-soil feedback effects enhance replacement of early-successional plant species by mid-successional plant species. As a key result of my dissertation work, I showed that early-successional plant-soil feedback effects provide biotic legacies, which influence dominance patterns of later-successional plant species.

Restoration of species-rich grasslands
Soil organisms can strongly affect competitive interactions and successional replacements of grassland plant species. In collaboration with NGO’s and co-workers from the Netherlands Institute of Ecology (NIOO-KNAW) and Utrecht University, I conducted two field experiments testing the potential of soil community manipulations as management strategy in restoration of species-rich grasslands. In the Assel experiment I tested the interactive effects of carbon amendments (to induce microbial N-immobilization), top soil removal and seed addition on plant and soil community development. The results show that seed addition is more important than soil fertility reduction measures in restoration of plant communities. Apparently, initial stages of secondary vegetation succession are determined by plant species arrival rather than by abiotic factors. In the Lievelde experiment, I tested the hypothesis that simultaneous introduction of later-successional plants and soil organism enhances plant community development towards the late-successional ‘target system’. I did not find support for this hypothesis. Most likely, unfavourable soil conditions at the restoration site limited successful establishment of the soil organisms.

ORNL's Ecosystem Science and Climate Change Research (Read more)