Litter microbial responses to climate change: How do inland or coastal context and litter type matter across the Mediterranean?

Warmer and drier climates are expected in the Mediterranean basin which is considered as a climate-change hotspot. Here we used climate contrasts across the Mediterranean to mimic climate change effects on litter microbial communities. Litterbags of monospecific (Pinus halepensis and Pistacia lentiscus) and of binary mixtures of litter (Pinus/Pistacia, 50/50) were transferred across contrasted climates between France and Algeria (from a sub-humid to a semi-arid climate). We tested the effect of litter type (species identity, litter-mixing) and of environmental context (coast/inland) on microbial responses to more constraining climate conditions. After 12-months of field incubation, litter chemical properties (C/N and solid-state 13C NMR) and microbial markers (active microbial biomass (MB), basal respiration (BR), bacterial catabolic and genetic structures (BIOLOG and T-RFLP) were characterized. After transfer, catabolic and genetic structures were different from those of both French and Algerian control and responses depended on the context and on litter type: stronger modifications were observed in inland context, except for microbial communities of Pistacia lentiscus litter (weak variations). However, MB and BR decreased in response to transfer for this litter type. On the other hand, and for either inland or coastal context, MB of Pinus halepensis litter did not vary in response to transfer and BR decreased reflecting microbial performance in sustaining biomass and limiting C losses through respiration. Here, litter mixture did not mitigate the effect of climate stress on microbial communities neither in inland nor in coastal context: lower MB and BR were detected after transfer in inland context whereas lower MB was accompanied by a higher BR in coastal context, reflecting an increase in CO2 release for a weaker amount of microbial biomass. This study shows that microbial responses to climate change vary with litter type and also with the environmental context.