A natural ¹⁵N approach to determine the biological fixation of atmospheric nitrogen by biological soil crusts of the Negev Desert

Abstract

Biological soil crusts are important cryptogamic communities covering the sand dunes of the north-western Negev. The biological crusts contain cyanobacteria and other free-living N(2)-fixing bacteria and are hence able to fix atmospheric nitrogen (N). This is why they are considered to be one of the main N input pathways into the desert ecosystem. However, up to now, in situ determinations of the N(2) fixation in the field are not known to have been carried out. We examined the natural (15)N method to determine the biological N(2) fixation by these soil crusts under field conditions. This novel natural (15)N method uses the lichen Squamarina with symbiotic green algae--which are unable to fix N(2)--as a reference in order to determine N(2) fixation. Depending on the sampling location and year, the relative biological fixation of atmospheric nitrogen was estimated at 84-91% of the total N content of the biological soil crust. The cyanobacteria-containing soil lichen Collema had a fixation rate of about 88%. These fixation rates were used to derive an absolute atmospheric N input of 10-41 kg N ha(-1) year(-1). These values are reasonable results for the fixation of atmospheric N(2) by the biological crusts and cyanolichens and are in agreement with other comparable lab investigations. As far as we are aware, the results presented are the first to have been obtained from in situ field measurements, albeit only one location of the Negev with a small number of samples was investigated.

Patterns and Controls on Nitrogen Cycling of Biological Soil Crusts

“…In general, these studies show that soils overlain with biocrusts have lower 15 N values than those without biocrusts, consistent with the notion that biocrusts do contribute fixed N to underlying soils and in agreement with global studies prior to 2001 (Evans and Lange 2003). Interestingly, Russow et al (2005) compared biocrusts dominated by a non-N-fixing lichen to those dominated by Collema, showing that Collema was responsible for 88 % of the N in biocrusts.…”

“…Several studies have used this method to assess if biocrusts are fixing atmospheric N (Aranibar et al 2003;Billings et al 2003;Russow et al 2005;Marsh et al 2006;Stewart et al 2011a). In general, these studies show that soils overlain with biocrusts have lower 15 N values than those without biocrusts, consistent with the notion that biocrusts do contribute fixed N to underlying soils and in agreement with global studies prior to 2001 (Evans and Lange 2003).…”

“…[reviewed by Evans and Lange (2003), Belnap (2002a, b), Russow et al (2005), Stewart et al (2011a, b, c), and Caputa et al (2013)]. Quantification of N fixation by desert biocrusts globally reported an average N-fixation rate of 6 kg N ha À1 a À1 (Elbert et al 2012).…”

Nitrogen input by cyanobacterial biofilms of an inselberg into a tropical rainforest in French Guiana

“…The amount of N released by cyanobacteria depends on the growth conditions and can reach 5-70% of the N fixed (Russow et al, 2005;Stewart, 1963). Especially in habitats exposed to frequent desiccation and rehydration like the tropical inselberg investigated here, a high proportion of N containing compounds is expected to be washed out.…”

Slowing of nitrogen cycling and increasing nitrogen use efficiency following afforestation of semi-arid shrubland

“…The N fixation rates by BSC were then estimated by applying the rates measured by Russow et al (2005) for a similar environment. Biological N fixation in the rhizosphere of pines was assumed to be null (Barkmann and Schwintzer 1998).…”