Soil microbial activity in a Mediterranean garrigue responds more to changing shrub community than to reduced rainfall

Abstract

The functioning of soil microbial communities is co-determined by plant community composition and environmental factors. Decreased precipitation predicted in the Mediterranean area will affect both determinants, yet their interplay on soil microbial functioning is poorly understood. Here we assessed the interaction of plant community diversity and reduced precipitation on microbial metabolic activity and diversity in the topsoil of a Mediterranean shrubland in Southern France. With a large field experiment using 92 plots that differed in the diversity of the four dominant shrub species (Quercus coccifera, Cistus albidus, Ulex parviflorus, and Rosmarinus officinalis) we manipulated the average precipitation (a mean reduction of 12%) over three years and analyzed the community level physiological profile (MicroResp™) after 7 and 31 months of partial rain exclusion. Partial rain exclusion had only subtle effects on soil microbial parameters. Soil microbial global metabolic activity and diversity increased with total shrub cover but tended to decrease with shrub diversity under control conditions, relationships that were absent with partial rain exclusion. We showed strong shrub composition control over the soil microbial parameters, with a particularly strong effect of Q. coccifera. Our results suggest that climate change may have greater impact on soil microbial functioning via shifts in plant community composition rather than through direct effects of reduced precipitation, yet this may depend on how precipitation will change.