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Second only to water among limiting factors, nitrogen controls the fertility of most arid regions. Where dry and wet depositions are weak, as in the western US deserts, N inputs rely heavily on biological N-2 fixation. Topsoil cyanobacterial communities known as biological soil crusts (BSCs) are major N-2 fixation hot spots in arid lands, but the fate of their fixed N remains controversial. Using a combination of microscale and mesoscale process rate determinations, we found that, in spite of theoretically optimal conditions, denitrification rates in BSCs were paradoxically immaterial for nitrogen cycling. Denitrifier populations within BSCs were extremely low. Because of this absence of denitrification, and because of the limitation of respiration and ammonia oxidation by diffusive O-2 supply, we could demonstrate that BSCs function as net exporters of ammonium, nitrate and organic N to the soils they cover, in approximately stoichiometrically equal proportions. Overall export rates during periods of biological activity are in the range of tens to hundreds of mu mol-N m(-2) h(-1), commensurate with those of N-2 fixation. These results explain the long-term dependence of BSCs on N-2 fixation, confirm their role in landscape fertility, and provide a robust argument for conservation of these endangered communities.