Plant carbon allocation under different nutrient availabilities and the role of mycorrhizal fungi therein
6 December 2019
Campus Drie Eiken, O.05 - Universiteitsplein 1 - 2610 Antwerpen-Wilrijk (route: UAntwerpen, Campus Drie Eiken
Organization / co-organization:
Departement of Biology
Sara Vicca & Ivan Janssens
PhD defence Arne Ven - Faculty of Science, Departement of Biology
Plants take up CO2 from the atmosphere and allocate this carbon to different plant functions, including growth and respiration. The fraction of carbon allocated to different processes, i.e., carbon partitioning (expressed as a fraction of gross primary production (GPP)), can vary substantially. Despite a central role in terrestrial carbon cycling, carbon partitioning and its relationship with nutrient availability remains poorly understood and seems to be very variable. Evidence is growing that nutrient availability plays a key role in this variation, with a variable carbon cost of plant-mycorrhizal symbiosis as the hypothesized underlying mechanism.
We performed two consecutive mesocosm nutrient manipulation experiments. The central aim of our experiments was to quantitatively determine the relationship between nutrient availability and carbon partitioning to all carbon pools and carbon-consuming processes, including the role of mycorrhizal fungi therein.
In the first experiment, the biomass production efficiency (BPE) increased with increasing nutrient availability. We found strong indications that this was linked to a reduced carbon partitioning to arbuscular mycorrhizal fungi (AMF). Our data indicate that autotrophic respiration relative to GPP (Raut:GPP) remained relatively constant under increasing nutrient availability. However, the second experiment showed an opposite response: BPE was relatively constant and even tended to decrease with increasing phosphorus availability. AMF abundance was much lower than in the first experiment; hence, when phosphorus conditions were low, plants likely invested in alternative pathways for phosphorus uptake, and this was associated with a surprising reduction of BPE with increasing phosphorus fertilization, while Raut:GPP increased.
Our experimental results indicated that the reason for these fundamental differences in carbon partitioning response to fertilization between both experiments was related to the abundance of AMF, which was substantially lower in the second than in the first experiment. Also the difference between pasteurized (no AMF) and AMF-inoculated treatments indicated AMF as one of the underlying mechanisms causing this variation in carbon partitioning. AMF were very important for plants, especially in low phosphorus conditions where symbiosis appeared critical for plant survival. They were essential for efficient phosphorus uptake by the plants, even when phosphorus was added.
To the best of our knowledge, we presented the first experiments where carbon balance closure was pursued and achieved, strengthening the robustness of the observed patterns. Our research emphasizes the need to take into account not only nutrient availability, but also mycorrhizal symbionts when studying and modelling carbon partitioning.