Abstract
IntroductionNitrogen 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.MethodsIn 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.ResultsAcetylene 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.ConclusionsThese 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.
Highlights
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
We examined nitrogen fixation and denitrification rates in two biocrust communities that differed in their N fixation potential 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
Chla content, which is an index of cyanobacterial biomass and N fixation potential of the biocrusts, was nearly two-fold higher in the dark crusts relative to the light crusts (Table 1)
Summary
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. Biological soil crusts (“biocrusts”) are diverse communities of cyanobacteria, algae, lichens, mosses, fungi, and other bacteria, which exist in open soil areas not favorable for the growth of higher autotrophs They comprise up to 70% of the living cover at many sites in arid and semi-arid regions of the western United States (Belnap et al 2001), contributing to a broad range of ecological. Estimates of N fixation in biocrusts often greatly exceed that of N accretion rates (Peterjohn and Schlesinger 1990), leading to uncertainty regarding the fate of the fixed N by biologically crusted soils and the potentially important N loss pathways. Soils in dryland ecosystems are characterized by pulse precipitation events resulting in wet and drying cycles These pulsed dynamics in soils may result in N “leakage” by biological soil crust organisms to the surrounding soil environment in such forms as NH4+ and other soluble organic nitrogen compounds (Mayland and McIntosh 1966; Johnson et al 2007). Nitrogen leakage from biocrusts may enhance soil nutrient availability to support plant growth (Mayland and McIntosh 1966; Belnap and Harper 1995 but may be lost from the system via gaseous N loss in transformations related to nitrification and denitrification processes (Zaady 2005; Barger et al 2005; Johnson et al 2007; Strauss et al 2012; Brankatschk et al 2013)
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