Export of nitrogenous compounds due to incomplete cycling within biological soil crusts of arid lands

Abstract

Second only to water among limiting factors, nitrogen controls the fertility of most arid regions. Where dry and wet depositions are weak, as in the western US deserts, N inputs rely heavily on biological N(2) fixation. Topsoil cyanobacterial communities known as biological soil crusts (BSCs) are major N(2) fixation hot spots in arid lands, but the fate of their fixed N remains controversial. Using a combination of microscale and mesoscale process rate determinations, we found that, in spite of theoretically optimal conditions, denitrification rates in BSCs were paradoxically immaterial for nitrogen cycling. Denitrifier populations within BSCs were extremely low. Because of this absence of denitrification, and because of the limitation of respiration and ammonia oxidation by diffusive O(2) supply, we could demonstrate that BSCs function as net exporters of ammonium, nitrate and organic N to the soils they cover, in approximately stoichiometrically equal proportions. Overall export rates during periods of biological activity are in the range of tens to hundreds of mumol-N m(-2) h(-1), commensurate with those of N(2) fixation. These results explain the long-term dependence of BSCs on N(2) fixation, confirm their role in landscape fertility, and provide a robust argument for conservation of these endangered communities.

Estimating global nitrous oxide emissions by lichens and bryophytes with a process-based productivity model

“…Measurements of N 2 O emissions by lichens and bryophytes, too, show considerable variation. Regarding biological soil crusts, several studies analysed denitrification rates to be negligible (Johnson et al, 2007;Strauss et al, 2012), and N 2 O production was calculated to constitute only 3-4 % of the N fixation rate (Barger et al, 2013). Other studies, however, described high denitrification rates that either increased (Brankatschk et al, 2013) or decreased with advancing crust development (Abed et al, 2013).…”

Species identity of biocrust-forming lichens drives the response of soil nitrogen cycle to altered precipitation frequency and nitrogen amendment

“…Biocrusts are important to the stability and productivity of dryland ecosystems where plants are typically sparse (Eldridge and Greene, 1994;Belnap et al, 2008;Lindo and Gonzalez, 2010;Zelikova et al, 2012). Previous studies have demonstrated that biocrusts play crucial roles in mediating key processes of N turnover, such as N fixation, nitrification and denitrification (Belnap, 2003;Johnson et al, 2007;Strauss et al, 2012;Abed et al, 2013;DelgadoBaquerizo et al, 2013c;Kidron et al, 2015). Much less, however, is known about the importance of biocrusts on the diversity and abundance of particular microbial communities such as those related to N processes.…”

The role of microorganisms at different stages of ecosystem development for soil formation

“…However due to increased microbial activities and leaching of N to deeper soil layers (Johnson et al, 2007), nitrogen is still one of the limiting factors in BSCs. Brankatschk et al (2011) demonstrated that deposited N and C are important drivers for the ecosystem development at initial sites; the ability of Cyanobacteria to trap nutrient-rich deposits via their exopolysaccharide sheath even facilitates that effect (Reynolds et al, 2001).…”