Denitrification from nitrogen-fixing biologically crusted soils in a cool desert environment, southeast Utah, USA

Abstract

Introduction: Nitrogen fixation by microorganisms within biological soil crust ("biocrust") communities provides an important pathway for N inputs in cool desert environments where soil nutrients are low and symbiotic N-fixing plants may be rare. Estimates of N fixation in biocrusts often greatly exceed that of N accretion rates leading to uncertainty regarding N loss pathways. Methods: In this study we examined nitrogen fixation and denitrification rates in biocrust communities that differed in N fixation potential (low N fixation = light cyanobacterial biocrust, high N fixation = dark cyanolichen crust) at four temperature levels (10, 20, 30, 40°C) and four simulated rainfall levels (0.05, 0.2, 0.6, 1 cm rain events) under controlled laboratory conditions. Results: Acetylene reduction rates (AR, an index of N fixation activity) were over six-fold higher in dark crusts relative to light crusts. Dark biocrusts also exhibited eight-fold higher denitrification rates. There was no consistent effect of temperature on denitrification rates, but there was an interactive effect of water addition and crust type. In light crusts, denitrification rates increased with increasing water addition, whereas the highest denitrification rates in dark crusts were observed at the lowest level of water addition. Conclusions: These results suggest that there are no clear and consistent environmental controls on short-term denitrification rates in these biologically crusted soils. Taken together, estimates of denitrification from light and dark biocrusts constituted 3 and 4% of N fixation rates, respectively suggesting that losses as denitrification are not significant relative to N inputs via fixation. This estimate is based on a previously published conversion ratio of ethylene produced to N fixed that is low (0.295), resulting in high estimates of N fixation. If future N fixation studies in biologically crusted soils show that these ratios are closer to the theoretical 3:1 ratio, denitrification may constitute a more significant loss pathway relative to N fixed.

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

“…Barger et al (2013) report a relationship between nitrogen fixation and N 2 O release in biological soil crusts, which include lichens and bryophytes, as well as soil bacteria and algae. For this approach, however, reliable nitrogen fixation data are sparse.…”

“…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).…”

Emission of nitrous acid from soil and biological soil crusts represents an important source of HONO in the remote atmosphere in Cyprus

“…Low correlations were found between HONO or NO emissions and NH + 4 contents (R 2 = 0.165 and 0.232; p = 0.05). Thus, in the present study it seems that reactive nitrogen emissions predominantly depend on NO − 2 and NO − 3 contents and not on surface cover types, although biocrusts (especially with cyanobacteria and cyanolichens) are able to fix atmospheric nitrogen (Belnap, 2002;Elbert et al, 2012;Barger et al, 2013;Patova et al, 2016). The results of a two-factorial ANOVA showed that HONO or NO emissions were not significantly related to soil cover type but rather to nitrite content, i.e., its direct aqueous precursor.…”

“…Globally it has been estimated that 100-290 Tg (N) yr −1 is fixed biologically (Cleveland et al, 1999), of which 49 Tg yr −1 (17-49 %) is fixed by cryptogamic covers, which comprise biocrusts, but also other microbially dominated biomes, like lichen and bryophyte communities occurring on soil, rocks and plants in boreal and tropical regions (Elbert et al, 2012). Studies have suggested that nitrogen cycling in soil (N 2 fixation, nitrification, denitrification) and hence reactive nitrogen emission (NO, N 2 O, HONO) is often enhanced by well-established biocrusts, especially by dark cyanobacteria (Cleveland et al, 1999;Elbert et al, 2012;Belnap, 2002;Barger et al, 2013;Johnson et al, 2005;Abed et al, 2013;Strauss et al, 2012;Weber et al, 2015). However, much of the molecular biology and chemistry that is important for atmosphere-land interactions likely occurs just below the crust (that is visible at the surface).…”

Biological soil crusts accelerate the nitrogen cycle through large NO and HONO emissions in drylands

“…But what are the consequences of nitrogen fixation by biocrusts? Incubation studies have shown that biocrusts can release nitrogen oxides (20)(21)(22), while field and model studies suggested large emissions of nitrogen oxides from drylands, raising questions about their origin (9,11,23,24).…”