Transition from historic grasslands to woody plants in semiarid regions has led to questions about impacts in soil functioning, where microorganisms play a primary role. Understanding the relationship between microbes, plant diversity and soil functioning, is relevant to assess such impacts. We evaluate the effect that plant type change in semiarid ecosystems has for microbial diversity and composition, and how this is related to carbon mineralization (CMIN) as a proxy for soil functioning. We followed a mesocosms experiment during two years within the Biosphere 2 facility in Oracle, Arizona, USA. Two temperature regimes were established with two types of plants (grass or mesquite). Soil samples were analyzed for physicochemical and functional parameters, as well as microbial community composition using 16S rRNA amplicon metagenomics (MiSeq Illumina). Our results show the combined role of plant type and temperature regime on CMIN, where CMIN in grass has lower values in elevated temperatures compared with the opposite trend in mesquite. We also found a strong correlation of microbial composition with plant type but not with temperature regime. Overall, we provide evidence of the major effect of plant type in the specific composition of microbial communities as a potential result of the shrub encroachment.

Microbial communities respond to changes in environmental conditions; however, how compositional shifts affect ecosystem processes is still not well understood and it is often assumed that different microbial communities will function equally under similar environmental conditions. We evaluated this assumption of functional redundancy using biological soil crusts (BSCs) from two arid ecosystems in Mexico with contrasting climate conditions (hot and cold deserts) following an experimental approach both in the field (reciprocal transplants) and in laboratory conditions (common garden), focusing on the community's composition and potential for nitrogen fixation. Potential of nitrogen fixation was assessed through the acetylene reduction assay. Community composition and diversity was determined with T-RFLPs of nifH gene, high throughput sequencing of 16S rRNA gene amplicons and metagenomic libraries. BSCs tended to show higher potential nitrogen fixation rates when experiencing temperatures more similar to their native environment. Moreover, changes in potential nitrogen fixation, taxonomic and functional community composition and diversity often depended on an interactive effect of origin of the communities and the environment they experienced. We interpret our results as legacy effects that result from ecological specialization of the BSC communities to their native environment. Overall, we present evidence of non-functional redundancy of BSCs in terms of nitrogen fixation.