Factors influencing nitrogen fixation and nitrogen release in biological soil crusts

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 (NH+4) 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 that lack 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 nonheterocystous 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). N fixation activity is generally measured as 15N2 incorporation or by the acetylene (C2H2) reduction assay (ARA). As 15N2 uptake is a direct measure of N fixation, it is the most reliable method. However, it is costly and requires highly controlled conditions; thus, most studies use ARA. In the presence of C2H2, the nitrogenase enzyme produces C2H4 (ethylene); thus, C2H4 production is a measure of nitrogenase enzyme activity (NA). Because NA is so widely used as a surrogate for nitrogen fixation, these terms will be used synonymously in this chapter. There are some problems associated with ARA. C2H2 can affect physiological functioning of the organism, especially when exposure lasts longer than 6h (David and Fay 1977). The biggest issue is that conversion of ARA data to absolute amounts of N2 fixed requires calibration by lSN2, which is seldom done. This is essential, as reported conversion rates in the literature for the C2H4:N2 ratio vary from 2 to 56, while conversion ratios for naturally occurring soil cyanobacteria generally range from 1.9 to 6.1 (Potts 1984). However, two recent studies indicate that conversion ratios can be much lower and also can be seasonally variable. S. Phillips and J. Belnap (unpubl. data) found that desert Nostoc commune sheets had an average conversion ratio of 0.31. Liengen (1999) reported conversion ratios varied with environmental conditions for both Nostoc sheets (0.11-0.4) and free-living soil cyanobacteria (0.02-0.07) in the Arctic. He also analyzed Anabaena in culture and obtained a ratio close to 4. These studies may explain why reported rates are so variable, as many studies have used cultured specimens or aquatic species, while actual rates in soils may be much lower. In addition, environmental conditions may lead to large variations in conversion rates. If these more recent results apply to other ecosystems or soil cyanobacterial species, N fixation rates may be currently underestimated by an order of magnitude. This matter clearly needs additional investigation