Nitrogen cycling in desert biological soil crusts across biogeographic regions in the Southwestern United States

Biological soil crusts (BSCs) are thought to be important in the fertility of arid lands as gateways for carbon (C) and nitrogen (N). Studies on the Colorado Plateau have shown that an incomplete internal N cycle operates in BSCs that results in significant exports of dissolved organic N, ammonia and nitrate into the bulk soil through percolating water, thus mechanistically explaining their role as a N gateway. It is not known if this pattern is found in other arid regions. To examine this, we measured rates of major biogeochemical N-transformations in a variety of BSCs collected from the Colorado Plateau and the Mojave, Sonoran and Chihuahuan Deserts. Dinitrogen fixation and aerobic ammonia oxidation were prominent transformations at all sites. We found anaerobic ammonia oxidation (anammox) rates to be below the detection limit in all cases, and at least 50-fold smaller than rates of N-2-fixation, making it an irrelevant process for these BSCs. Heterotrophic denitrification was also of little consequence for the flow of N, with rates at least an order of magnitude smaller than those of N-2-fixation. Thus we could confirm that despite the demonstrable differences in microbial community composition and soil material, BSCs from major biogeographic regions in arid North America displayed a remarkably consistent pattern of internal N cycling. The implications for arid land fertility drawn from previous studies in the Colorado Plateau appear applicable to BSCs across other arid regions of the Southwestern United States.