1 Abstract
Biological soil crusts (BSC) are key components of ecosystem productivity in arid lands and they cover a substantial fraction of the terrestrial surface. In particular, BSC N2-fixation contributes significantly to the nitrogen (N) budget of arid land ecosystems. In mature crusts, N2-fixation is largely attributed to heterocystous cyanobacteria, however, early successional crusts also fix N2 but possess few N2-fixing cyanobacteria and this suggests that microorganisms other than cyanobacteria mediate N2-fixation during the critical early stages of BSC development. DNA stable isotope probing (DNA-SIP) with 15N2 revealed that Clostridiaceae and Proteobacteria are the most common microorganisms that assimilate 15N in early succes-sional crusts. The low abundance of these groups in BSC may explain why these heterotrophic diazotrophs have not previously been characterized. Diazotrophs play a critical role in BSC formation and characterization of these organisms represents a crucial step towards understand-ing how anthropogenic change will affect the formation and ecological function of BSC in arid ecosystems.
