Biological soil crusts (biocrusts) are a consortium of photosynthetic lichens, moss, and/or cyanobacteria living at the soil surface in arid and semiarid ecosystems worldwide. These diverse organisms play an important role in a wide range of ecosystem functions, including biological nitrogen fixation. Indeed, biocrusts can be the dominant input of new nitrogen to many dryland ecosystems, yet our understanding of the patterns, rates, and controls over biocrust nitrogen fixation remains notably limited. In this talk, we will synthesize a contemporary understanding of dryland biocrust nitrogen fixation, bringing new data to bear on questions of geographical variation and coupled biogeochemical controls and refining our understanding of how community dynamics regulate biocrust nitrogen fixation rates.
First, we will show how biocrust community dynamics affect nitrogen fixation rates using (1) a successional state framework (e.g., early successional biocrust communities vs. late successional biocrusts) and (2) population and community ecology perspectives to elucidate how community dynamics regulate the rates of this fundamental ecosystem process. We will show that biocrust community members interact to determine overall nitrogen fixation rates, and that nitrogen fixation can actually decline at the latest biocrust successional stages due to increases in lichen diversity. We will anchor these population- and community-scale perspectives within larger-scale considerations of climate and land use to show how biocrust composition interacts with abiotic controls to determine the amount of nitrogen fixed. Because the arid and semiarid ecosystems where biocrusts are common are both spatially extensive and nutrient-poor, an improved understanding of how climate and biocrust community dynamics interact to determine nitrogen inputs has global implications.