TASK 2 : Forest PSMs production as drivers of biodiversity and decomposition process AND DECOMPOSITION PROCESSES
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Task leader : V. Baldy -Participants : J. Balesdent, S. Boiry, A. Bousquet-Mélou, S. Dupouyet, Z. Fekiacova, C. Fernandez, T. Gauquelin, P. Mirleau, M. Péan, I. Reiter, A. Saatkamp, M. Santonja, P. Signoret - Subcontracting : C. Gers (CR, EcoLab).
Objectives and hypothesis : The goal of task 2 is to measure the impact of intensified drought on PSMs and on nutrients of leaf litter and the resulting consequences on litter decomposition through decomposer biodiversity and activity. Moreover, the effect of litter allelopathic compounds on fitness of adjacent plants (e.g. inhibition of seed germination) will be evaluated. We hypothesize a significant impact of plant species, including possible allelopathic impact of PSMs (e.g., phenolic compounds), on decomposer community structure and activity.
Sub-Task 2-1 : Climate dependency of litter-mixing effects on leaf litter decomposition
The objective of this sub-task is to evaluate if the effect of litter mixture on litter decomposition is affected by drought. On the one hand, we expect a direct negative effect of drought on decomposers and thus on the rate of litter decomposition. On the other hand, an indirect effect, negative or positive, could occur due to changes in litter biochemical composition since i) the potential decrease in some mineral elements may change the stoichiometry of green leaf nutrients and then reduce the litter degradation by decomposers due to lower litter pal-atability, and ii) the potential increase in the quantity and diversity of secondary metabolites in leaves and so litter (link to the task 1) may either cause a decrease in the rate of decomposition if refractory compounds (phenolics) dominate, or an increase of decomposer diversity as a result of high terpenoid chemical diversity and thus carbon resources.
Litter presents a high chemical diversity in terms of PSMs, which increases with plant species diversity. To account for the effect of different litter chemical signatures (PSMs and nutrients) on litter decomposition, senescent leaves of different species will be mixed : Q. pubescens, A. monspessulanum and C. coggygria, all three naturally present at the experimental site and Pinus halepensis which is close to the experimental site and could co-occur more fre-quently with Q. pubescens under climate change. Q. pubescens and A. monspessulanum mainly produce phenolics, C. coggygria is a terpenoid producer and P. halepensis produces both PSMs in large amount and diversity.
We use two methodological approaches to tackle in situ leaf litter decomposition, leaf litter decomposition of the 4 selected species and
leaf litter decomposition of 13C-labelled Q. pubescens litter in mixture with P. halepensis non labelled litter.
Sub-Task 2-2 : Allelopathy x drought : sources of plant, microbial, mesofauna biodiversity ?
PSMs leached into the soil via the litter decomposition can cause chemical interactions be-tween plants, microorganisms or between plants and microorganisms.
Objectives are i) to assess the impact of litter PSMs under different decomposition stages on plant, microbial and mesofauna biodiversity, and ii) to test if intensified drought conditions influence the production of PSMs (allelochemicals) in a given plant (donor plant) and, as a result, the sensitivity of the neighbouring plants (target plant) to these allelochemicals is accentuated.
Two separate experiments will be performed to test these two specific objectives : i) quantitative and qualitative changes of litter allelochemicals at different decomposition stages and litter mixtures and ii) sensitivity of the target species (Q. pubescens) to allelochemicals of four species having grown under intensified drought.