Patterns and Controls on Nitrogen Cycling of Biological Soil Crusts

In low-nutrient environments with few vascular plant symbiotic N fixers, biocrusts play an important role in ecosystem N cycling. A large number of studies across a wide range of biomes clearly confirm that not only the presence of biocrusts but biocrust community composition strongly influences N-fixation activity, with N fixation increasing with level of development (cyanobacterial-lichen biocrusts > dark cyanobacterial biocrust (e.g., Nostoc spp. and Collema spp.) > light Microcoleus-dominated biocrust). Nitrogen fixation by biocrusts results in N release to the soil in a variety of N forms (inorganic and organic N), thus elevating soil inorganic N pools in the top few millimeters of soil. The influence of N release on the bulk soil at greater soil depths is less clear, with biocrusts either elevating or having no influence on bulk soil inorganic N pools. The fate of N fixed and released by biocrusts, and whether this N is retained in the ecosystem in either soils or plants, determines ecosystem N balance over longer time scales, and results on the influence of biocrusts are mixed. Whereas we have multiple studies that examine a single compartment of N budgets, we lack studies that simultaneously address N inputs, losses, and soil and plant pools, thus precluding the construction of definitive N balances. One of the most consistent impact biocrusts have on ecosystem N is reducing N loss via wind and water erosion, with such losses consistently decreasing with increasing biocrust development.