Prof. dr. Martijn Bezemer

Biodiversity and Ecosystem function:

• Biodiversity is declining worldwide but the consequences of this loss are still not well understood. We study how changes in plant diversity affect the functioning of grassland ecosystems and how this influences aboveground and belowground multitrophic communities. We study these interactions in two field experiments, a series of old fields, and under controlled conditions in climate chambers and greenhouses. With the NIOO project group biodiversity we examined the impact and consequences of herbivore diversity in terrestrial and aquatic ecosystems (see project group webpage).

Long-term successional biodiversity experiment:

• In the field different seed mixtures (15 species, 4 species, 0 species) were sown in a recently abandoned field in 1996. The plots have not been weeded since, but this single sowing event has resulted in long-term differences in plant and soil communities. Since 1996 we have been collecting data on plant and nematode community composition and we now have a unique long-term dataset on how initial seed diversity affects the temporal dynamics of plant and soil communities during secondary succession. We also study ecosystem functioning, invasibility and community and ecosystem stability in the plots. Since the plots are not weeded and colonization is allowed this is a unique biodiversity experiment and the results show that biodiversity-ecosystem function relationships in unweeded, natural plant communities differ remarkably from those observed in controlled biodiversity experiments (see publications for details).

The succesional biodiversity field site

Annual harvest event

Classical biodiversity experiment:

• In 2008 we initiated a classical biodiversity grassland experiment with 70 plots with 1 to 9 plant species and where species diversity and composition are maintained by hand weeding. Here we also study biodiversity – ecosystem function relationships and this enables us to compare it to the successional biodiversity experiment. In the classical biodiversity experiment, we focus particularly on how plant community identity and diversity affects the aboveground and belowground interactions that occur on phytometer plants (tansy ragwort: Senecio jacobaea) that have been planted into these plots, to test for associative resistance and susceptibility, and to test to what extent plant-antagonist interactions are driven by host plant quality and by the surrounding community.

Preparation of the field site with Olga (and Andre, not on the photo)

Field site during early summer of 2010

Chronosequence of old fields:

• We use a series of old fields that differ in time since cessation of agriculture (1 to 40 years) to study how plant, soil, and aboveground communities develop and how ecosystem stability changes over time during secondary succession. We study how nature restoration can be improved, but also study population dynamics of the plant tansy ragwort in this chronosequence. After an initial period of five to ten years during which this plant species dominates the vegatation, its abundance starts to declline. We examine whether this decline is due to a negative plant soil feedback that develops in these old fields or due to increased competition with other plant species or changes in soil chemical properties.

Luis, Estela and Joop collect soil and biomass samples at Reijerskamp (early successional site)

Dennekamp (later successional site)

 Aboveground belowground interactions:

• Plants grow aboveground and in the soil, but ecologists frequently study aboveground or belowground processes in isolation. We are interested in question by which mechanisms belowground herbivores, pathogens or decomposers can influence the interactions between plants, herbivores and parasitoids aboveground, and vice versa. We introduce aboveground and belowground organisms on plants in microcosm studies in the greenhouse and study how these organisms interact via induced plant defense responses and whether feeding in one compartment can affect the behaviour of organisms in the other compartment. We also study these interactions in the field. For example, we study ature restoration on ex arable fields and how soil diversity can influence the diversity of plants and herbivores aboveground.

Studying aboveground belowground interactions

Microcosm experiment with aboveground and belowground herbivores

Plant-soil feedback:

• We study how plants via changes in soil abiotic or biotic properties can affect the performance of plants of the same or other species that subsequently grow in the same soil. This is called plant soil feedback. We examine how important plant soil feedback effects are in the field and how important the identity is of the plant individual that conditions the soil. We also study plant-soil feedback responses in plant communities and a lot of our plant soil feedback work is related to the biodiversity and chronosequence work described above.
   

A plant soil feed back study in the greenhouse

Effects of host plant quality and neighbouring plant community composition on aboveground and belowground food webs:

• In the field, we study how resource quality and the identity of the neighbouring plant community affect above and belowground food webs on individual plants. Belowground, we construct entire foodwebs underneath individual plants belonging to different species (including Plantago lanceolata and Lotus corniculatus) that grow in the plots of the successional biodiversity epxeriment. Although the food webs are entirely open, and plants grow internmingled with other plants of the same and different species, we are detecting considerable differences in the composition of the soil food webs. Surprisingly, these differences occur in particular in the decomposer part of the foodweb rather than in the root associated organisms. Aboveground we have reared out all herbivores and parasitoids from seed pods of Lotus corniculatus. So far the food web consists of two herbivore species and 20 parasitoid and hyper parasitoid species. We analyze the food webs for complexity, stability, link density etc, and have data on the body size of the emerging adults. We also have detailed information about the size of the pods, the number of seeds, and the origin of the pods (from which plant community were they colleted and from which plant within the community). For these aboveground and belowground food webs we examine the hypothesis that aboveground food webs are influenced most stronly by resource quality while belowground food webs are determined most strongly by neighbouring and legacy effects.

Indidivual Lotus and Plantago plants cooccur in the same community but have entirely different soil food webs 

Linking plant soil feedback and aboveground belowground interactions

• Plants and soil microbes are interdependent. Via their influence on the composition of the soil microbial community in the rhizosphere, plants can influence the growth of other plants that grow later in the soil. In greenhouse experiments, common garden experiments and in the field, We examine how these plant soil feedback effects influence the composition of plant defense compounds and the emission of volatiles of the later growing plants, and how this, in turn, influences the response of aboveground insect herbivores and their natural enemies.

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Soil transplantation for ecosystem restoration

• Agricultural intensification has resulted in a strong reduction of species-rich grasslands in The Netherlands and great efforts are made to restore diverse grasslands on former arable land. For long, the standing view has been that the diversity and functioning of ecosystems is largely determined by abiotic conditions such as soil fertility and the focus in restoration in the Netherlands has been traditionally on reducing soil fertility, e.g. by removal of the entire topsoil. With topsoil removal the "agricultural"soil microbial community that was present in the soil is also remvoed. Soil microbes o play an important role in plant community development and hence in restoration: a target plant community requires a target soil community. We study how inoculation of areas where top soil has been removed with soil communities collected from well developed nature areas can improve establishment of later-successional (target) plant species. In the “NWO-ALW Biodiversity Works” programme, together with the organization Natuurmonumenten we are evaluating a large soil transplantation trial carried out in the Netherlands in a restoration area where the topsoil has been removed. (See also links to broadcasts on Dutch radio and TV) 

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Biodiversity and functioning of green roofs at the NIOO complex

• At the green roofs of the main building of the NIOO since 2012 we monitor the diversity of plant and soil organisms and study the development of the vegetation and soil community. The aim is to study how biodiversity on green roofs can be improved and how green roofs can be used as "steppingstones" in urban environments. While the roof was planted in 2012 with a standard sedum vegetation it has developed rapidly in a species rich vegetation and more than 100 plant species have been recorded in the past three years, including rare species such as orchids. In an experimental area at the roof temperature and water regulation is measured continuously in 45 plots of 2x2 m each which enables us to examine the relationship between plant diversity and compostion, and the functioning of green roofs.

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Ecology of the biobased economy: Biochar amendment in a natural ecosystem:

• Together with colleagues from Wageningen University (Jan Willem van Groenigen and Liesje Mommer) and two appointed post docs (Tess van de Voorde and Simon Jeffery) recently started a four year project in which we examine possibilities for  biofuel production from biomass of old fields. We will study the effects of biochar amendment in a nature restoration area on plant communities and soil food webs and ecosystem functions. Biochar (similar properties as active coal), is a left over product from oil production after pyrolisation of biomass. We also organized two workshops on biofuels and biochar. More information on: www.base-project.org

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