Effect of monospecific and mixed litters on bacterial communities' structure and functions under contrasting Mediterranean climate conditions

Abstract

Global climate change is expected to contribute to intense droughts and heatwaves in the Mediterranean basin, motivating ecological research to explore the responses of litter microbial communities to more drastic climatic conditions. Here, we investigated how the functioning of microbial decomposition systems may be affected by litter source and mixing in two contrasting Mediterranean climate contexts: a sub-humid climate in France vs a semi-arid climate in Algeria. Studies included in situ incubation of litter bags of monospecific litters (Pinus halepensis, Pistacia lentiscus, and Quercus ilex) and binary mixtures (Pinus/Pistacia; Pinus/Quercus) of litters in France and Algeria. Following a 12-month incubation, the remaining litter was chemically characterised using elemental analyses and solid-state 13C NMR spectroscopy. Microbial properties evaluated included basal respiration (BR), active microbial biomass (MB), community-level physiological profiling (CLPP), and molecular fingerprinting (T-RFLP). In both countries, negative correlations were observed between MB and ratios of C/N and (aromatics+phenols)/N. Basal respiration was also negatively correlated to both ratios in France; however, the opposite trend was found in Algeria. The effect of litter mixing on MB, BR, and microbial functional diversity (catabolic diversity index ECO H′) also differed between climates. The mixture of Pinus and Quercus litter led to limited heterotrophic CO2 release in the Algerian arid context i.e. a synergistic effect on MB associated with an antagonistic effect on BR. Higher bacterial genetic diversity (Terminal Restriction Fragment TRF richness and evenness) was found in Algeria whereas higher functional diversity (catabolic diversity index ECO H′) was found in France, suggesting that the higher genetic diversity potential in the arid climate context did not lead to functionally diverse bacterial communities. Our findings on the differing effect of litter chemical properties- (aromatics+phenols)/N and C/N- on microbial functioning (BR and MB) in sub-humid and semi-arid climates indicate that the impact of litter chemistry (litter source; litter admixtures) on C cycling is climate-dependent.