Warmer and drier conditions associated with climate change could reduce vegetation productivity and cover in Mediterranean-type drylands, thus increasing the risk of land degradation and desertification. We conducted a 4-year manipulative experiment in a semiarid shrubland in Southeastern Spain in which we simulated the warmer and drier climate conditions forecasted for the Mediterranean Region. We evaluated the effects of experimental climate change on the performance of a native plant community by exposing six coexisting shrub species to a ~2.5ºC temperature increase using open top chambers (Warming, W), ~30% rainfall reduction using rainout shelters (Rainfall Reduction, RR), and their combination (W+RR). We measured leaf gas exchange, plant stoichiometry and nutrient status, foliar carbon isotopic composition, shoot biomass production and plant survival, as well as total microbial biomass in rhizosphere soil and enzymatic activities related to the C (β-glucosidase and dehydrogenase), N (urease and glycine-aminopeptidase), and P (alkaline phosphatase) cycles. Finally, microbial exudation efficiency of key enzymes was calculated as the quotient between enzymatic activity and microbial biomass.
Warming (W and W+RR treatments) consistently decreased net photosynthesis rate and water use efficiency across species throughout the study. Shoot dry biomass production was strongly decreased by the three climate manipulation treatments in all the target species. Leaf nutrient (N, P, K, Fe, Zn, Cu) concentrations and pool sizes in foliage were decreased by warming across species, indicating reduced plant nutrient uptake and status. Plant survival rate at the end of the 4 yr. study period was also drastically decreased by the combination of warming plus rainfall reduction (W+RR). In contrast to the strong detrimental effects of warming on plant performance, microbial biomass in rhizosphere soil increased in response to warming. However, despite increased soil microbial biomass, the activity and/or exudation efficiency of key microbial extracellular enzymes for soil nutrient cycling (phosphatase, urease, glycine-aminopeptidase) were significantly decreased by warming, suggesting slowed N and P mobilization and cycling rates and increased microbial immobilization, especially in the W+RR treatment. Overall, the data indicate that a warmer and drier climate could shift the competitive balance between plants and soil microbes, thereby exacerbating nutrient limitation of photosynthesis, with detrimental feedback effects on vegetation productivity, water use efficiency and cover in this primarily water-limited ecosystem.