Effect of monospecific and mixed Mediterranean tree plantations on soil microbial community and biochemical functioning

Soil microbial communities (SMCs) are important drivers of forest ecosystem functions and services. To optimize afforestation practices, it is important to understand how pure and mixed plantations can affect SMCs and their functioning. Therefore, the objective of this work was to investigate the SMC structure and function of six Mediterranean woody species grown as monocultures, and of six selected 1: 1 combinations. SMC structure was assessed through the abundance estimation of different culturable heterotrophic microbial populations (i.e. bacteria, fungi, actinomycetes, Pseudomoas spp. and aerobic spore-forming bacteria) and the Biolog community level physiological profile, while soil biochemical functioning was estimated by quantifying soil dehydrogenase (DHG) activity. Species richness (SR) did not increase the abundance of culturable heterotrophic microbial populations, nor the number of carbon sources metabolized by the microbial communities (Biolog richness) or DHG activity. However, SR significantly decreased the potential metabolic activity (Biolog AWCD) of soil microbial populations. Leaf type significantly affected soil microbial and biochemical features, particularly for monocultures where conifer plots were characterized by a significantly higher fungal/bacterial ratio, reduced AWCD and richness as well as reduced DHG values. Significant interactions between tree species in mixed plots were noticed as well as significant correlations between plant community weighted mean (CWM) traits and soil microbial and biochemical features. Both SMC structure and function were clearly influenced by tree plantations and, importantly, microbial populations of monocultures and mixed plantations, and their functioning, were governed by different CWM traits.