Environmental changes interactively alter communities of biological soil crusts with large impacts of nitrogen addition.

Multiple environmental changes are projected to alter the structure and function of ecological communities during the coming century, but relatively few studies have investigated the interactive effects of multiple stressors such as nitrogen deposition, altered precipitation, and climate warming, on soil microbial communities. In dryland ecosystems, biological soil crusts (biocrusts) feature strongly in feedbacks between soil microbes and ecosystem functions, but the responsiveness of biocrust organisms to interacting environmental changes is relatively understudied. We used long-term experiments to investigate whether interactions among nitrogen deposition, intra-annual variability in rainfall, and nighttime warming disrupted biocrust community structure or function (photosynthetic capacity) in the northern Chihuahuan Desert, New Mexico, USA. Among the environmental changes, nitrogen deposition had the largest net impact, reducing cyanobacteria abundance (52-81%), suppressing the function of photoautotrophic capacity (67-200%), and increasing total bacteria diversity (3-10%). However, the influence of nitrogen addition depended on additional axes of environmental change: nitrogen effects were amplified by altered intra-annual rainfall and for some responses, by nighttime warming. These findings reveal surprisingly large biocrust declines under nitrogen eutrophication and reveal how multiple stressors interactively affect soil microbial structure and function in drylands.