Increased rainfall variability promotes cyanobacterial diversity in biological soil crusts

Arid land topsoils are commonly colonized by biological soil crusts (biocrusts), photosynthesis-based communities of microbes that provide a variety of services to desert ecosystems, from soil fertilization to stabilization against erosion. To investigate potential effects on biocrust cyanobacteria of the more frequent, but smaller, rainfall events predicted under global climate change scenarios for North American deserts, we monitored long-term changes in their abundance, composition and diversity after treatment with altered precipitation regimes, which included addition of rainfall events during the summer monsoon season totaling 60 mm, either as 12 small 5 mm events, or as 3 larger, 20-mm events. To assess interannual variability, we quantified parameters after 10 and 12 years of recurrent treatment. Smaller rainfall events treatment promoted an increase in cyanobacterial dversity, with a significant increase in the abundance of the nitrogen-fixing cyanobacterium Scytonema spp. Bundle-forming pioneer cyanobacteria species dominance also shifted from Microcoleus vaginatus to members of the M. steenstrupii complex with this increase in water availability caused by our rainfall addition treatment. Our study suggests that smaller more frequent rainfall events will increase both abundance and diversity of cyanobacteria in biocrusts, thus increasing their beneficial services to desert ecosystems.