Factors Influencing Nitrogen Fixation and Nitrogen Release in Biological Soil Crusts

Abstract

Nitrogen (N) occurs in the atmosphere as N2, a form that is not useable by vascular plants. N2 must first be ``fixed'', or reduced, to ammonia (NH4+) by prokaryotic organisms such as eubacteria and cyanobacteria. Thus, an important feature of the cyanobacteria and cyanolichens in soil crusts is their ability to fix atmospheric N. As this fixation is an anaerobic process, most cyanobacterial fixation takes place in heterocysts, which are specialized, thick-walled cells with enhanced respiration and no oxygen-producing photosystem II (Paerl 1990). Heterocystic genera commonly occurring in soil crusts include Anabaena, Calothrix, Cylindrospermum, Dicothrix, Hapalosiphon, Nodularia, Nostoc, Plectonema, Schizothrix, and Scytonema (Harper and Marble 1988). Nitrogen fixation has also been demonstrated in non- heterocystous soil genera such as Lyngbya, Microcoleus, Oscillatoria, Phormidium, and Tolypothrix (Rogers and Gallon 1988; Belnap 1996), although this may be a result of associated bacteria (Steppe et al. 1996). Nonheterocystic species can exclude oxygen in several ways: (1) behaviorally by clumping; (2) spatially or chemically within a cell; (3) temporally, by fixing at night when no oxygen is being evolved by photosynthesis; or (4) through a combination of these (Paerl 1978; Rogers and Gallon 1988; Paerl 1990). Bacteria associated with cyanobacteria may also contribute to N inputs by scavenging oxygen (thus creating anaerobic microzones for the cyanobacteria) or by fixing N themselves. This has been demonstrated for Microcoleus vaginatus isolated from soil crusts (Steppe et al. 1996). Soil lichens with cyanobacterial photobionts also fix N. Common N-fixing soil lichens include Nostoc-containing Collema spp. and Peltigera spp. and Scytonema-containing Heppia spp. Cyanobacteria also live as epiphytes on soil mosses and phycolichens; thus, this consortium of organisms can show fixation activity (Peters et al. 1986).