RésuméNatural ecosystems store large quantities of carbon in their soils, thereby
preventing it from ending up in the atmosphere and contribute to climate
change. A key question is whether climate change increases or decreases the
capacity of soils to sequester carbon, and hence whether ecosystems will buffer or accelerate climate change. However, experimentally in situ observed changes in soil carbon contents under climate change simulations are very variable and the underlying mechanisms are poorly understood. Heathlands are rare, seminatural ecosystems with soils dominated by fungi and relatively high carbon sequestration rates. These systems might thus play an important role in our understanding of the effects of climate change on soil carbon sequestration.
Therefore, in this PhD, we investigate how heathland soil fungi are affected by
climate change, as changes in soil fungal functioning to a large extent drive the
observed changes in heathland soil carbon sequestration.
In Chapter 1, the difficulty to parameterize a simple mechanistic food web
model that simulates the effect of climate change on soil carbon sequestration
indicated that we currently lack basic empirical data on species interactions and
stress tolerances. Therefore, we focused in the thesis on the stress ecology of
and interactions between fungi, as they are the most important group of
organisms with respect to carbon sequestration in heathland soils. But in order
to expose heathland soil fungi to abiotic stressors in laboratory experiments, we
had to isolate as many fungal species as possible. Therefore, in Chapter 2, we
tested four methods and seven growth media for their efficiency in isolating soil
fungi. All four tested isolation methods, that have largely varying methodologies,
showed high taxon specificity and complementarity. Contrary to expectations,
the nutrient composition of the growth medium did not affect cultivation.
However, long incubation times did prove to be useful for the isolation of
additional fungal taxa. Hence, by using various isolation methods combined with
long incubation times, we were able to cultivate a relatively diverse soil fungal
In Chapter 3, we in vitro quantified the tolerance to temperature and water
stress (drought) of the isolated fungal taxa by assessing their growth under
different treatments. Additionally, we measured several functional traits, such as
melanin content, that are considered to be important direct mechanistic drivers
of their tolerance to these abiotic stressors. We found a large variability in stress
sensitivities among taxa, whereby fungi were in general tolerant to the applied
mild temperature and water stress, but sensitive to high temperature stress.
These heathland soil fungi are thus relatively well-adapted to harsh abiotic
conditions. Contrary to expectations, the measured functional traits did not
explain the variation in abiotic stress tolerance among taxa, which is thus
probably driven by other traits than those that we quantified In Chapter 4, we
investigated how these abiotic stressors affect the capacity of fungi to grow in
presence of a more abundant competitor, which we defined as biotic stress
tolerance. We found that fungal growth rates were positively affected by biotic
stress under benign conditions, but that interactions between fungi become
negative under high warming stress, opposite to the stress gradient hypothesis
(SGH). Tolerance to biotic stress was not driven by tolerance to abiotic stress
nor intrinsic growth rate of the fungus, at any level of abiotic stress.
These results suggest that global change could potentially impact fungal
communities in unpredictable ways. Several perspectives would validate and
further complement the gathered knowledge, by addressing how the observed
changes in fungal growth rates under biotic and abiotic stress propagate into
more complex set-ups and more complex communities and eventually translate
into changes in soil carbon sequestration.
|la date de réponse||20 mars 2020|
|Superviseur||Frederik DE LAENDER (Promoteur), François Rineau (Promoteur), Jan V. Colpaert (Jury), Richard D. Bardgett (Jury), Erik Verbruggen (Jury), Sofie Thijs (Jury) & Tom Artois (Jury)|