Agricultural matrix affects differently the alpha and beta structural and functional diversity of soil microbial communities in a fragmented Mediterranean holm oak forest

Given the increase in habitat fragmentation in the Mediterranean forests, understanding its impacts over the ecology of soil microbial communities, responsible for many ecosystem functions, and their capacity to metabolize different substrates from soil organic matter, is of upmost importance. We evaluated how the influence of the agricultural matrix, as one of the main consequences of forest fragmentation, may affect both the composition and the functioning of soil microbial communities in Mediterranean holm oak forests. We determined structural and functional alpha and beta-diversity of microbial communities, as well as microbial assemblages and metabolic profiles, by using a commonly used fingerprinting technique (Denaturing Gel Gradient Electrophoresis) and a community level physiological profiles (CLPP) technique (EcoPlate). Key drivers of soil microbial structure and metabolism were evaluated by using structural equation models (SEM) and multivariate ordination (envfit) approaches. Our results pointed out that forest fragmentation affects microbial community structure and functioning through a complex cascade of causal-effect interactions with the plant-soil system, which ultimately affects the nutrient cycling and functioning of forest soils. We also found a strong scale-dependency effect of forest fragmentation over the ecology of microbial communities: fragmentation increases the local (alpha) diversity, but affected negatively microbial diversity at the landscape scale (beta diversity). This homogenization of the microbial communities and their metabolism at landscape scale resulting from habitat fragmentation may have unknown potential consequences on the capacity of these communities, and hence these ecosystems, to respond to the climate change. Finally, we found a consistent relation between the structure and functional diversity of bacterial community, which further showed the important role that the assemblage of microbial communities might have over their functioning.