Seasonal patterns of nitrogen fixation in biological soil crusts from British Columbia’s Chilcotin grasslands

Abstract

This study provides estimates of N2 fixation in biological soil crusts (BSCs) from shrub–steppe grasslands in British Columbia’s Chilcotin plateau. We identify key seasonal periods and environmental conditions during episodes of BSC nitrogen-fixation activity. Predicted nitrogen-fixation activity showed two peak periods: one during late winter and early spring snowmelt events, and the second during summer precipitation events. BSC nitrogen fixation, especially summer activity, was quite variable from year to year. The magnitude of predicted nitrogen fixation at the landscape level was dependent upon the ratio used to convert rates of acetylene reduction to nitrogen fixed. Using a theoretical ratio of 3:1, estimates of average annual BSC nitrogen fixation were 1.0 kg N·ha−1. However, using a conversion ratio of 0.06, obtained from side-by-side measurements of acetylene reduction and 15N uptake, annual estimates are 52 kg N·ha−1. These estimates are based on assessments of the aerial extent of dark BSC communities, which at the Farwell Canyon study site exceeded 50%. The fate of fixed nitrogen, especially during the late winter period, when underlying soils are frozen, remains unclear.

Patterns and Controls on Nitrogen Cycling of Biological Soil Crusts

“…14.3). Barger (2003), Barger et al (2013), Belnap (2002a, b), Billings et al (2003), Caputa et al (2013), Dickson (2000), Housman et al (2006), Issa et al (2001), Liu et al (2009Liu et al ( , 2012, Stewart et al (2011aStewart et al ( , b, c, 2012, Strauss et al (2012), Su et al (2011 b), Wu et al (2009), Zhao et al (2010, and Zielke et al (2002) The high variability in ARA rates reported in the literature may at least partly reflect methodological limitations of the assay. ARA is based on the fact that the enzyme nitrogenase, which is normally responsible for catalyzing the reduction of N 2 to NH 3 , can also reduce acetylene to ethylene, the latter reaction being an indirect measure of nitrogenase activity.…”

“…In a series of early experiments, Hardy et al (1968) confirmed that this theoretical ratio ranged between 3 and 4 for bacterial cultures and legumes under laboratory and field conditions. Since this time, however, actual results from field and laboratory studies of biocrusts report that conversion ratios can range from 0.022 to 3.49 (Liengen 1999a;Stewart et al 2011a;Caputa et al 2013). Deviation (usually downwards) from the 3-4:1 ratio can result from several factors and may include the following:…”

“…15 N 2 Method We could only find three studies since 2001 (Holst et al 2009;Stewart et al 2011a;Caputa et al 2013) and two studies prior to that time (Mayland and McIntosh 1966;Liengen 1999b) that used 15 N 2 to measure N fixation in biocrusts. These studies were not focused on how environmental factors influenced N-fixation rates, as analysis was done under standard temperatures, moisture, and light and with materials collected in the same season.…”

“…[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).…”

“…In a recent study, biocrusts collected from the pristine lower Sonoran desert and then transferred to the high-N atmosphere of the city of Tempe, Arizona, resulted in a complete loss of N-fixation capacity within weeks (Vaugh and Garcia-Pichel, unpublished). The choice of ARA to N-fixation conversion ratios in estimates of N fixed is also an issue, as this may result in estimates of annual N inputs differing by an order of magnitude (Liengen 1999b;Caputa et al 2013). Notably, however, upper estimates of annual N fixation are of similar magnitude as those of annual N fertilization inputs to agricultural systems [reviewed in Evans and Lange (2003) and Caputa et al 2013).…”