| Biological soil crusts are communities of fungi, lichens, cyanobacteria, and mosses that colonize soil surfaces in arid and semi-arid ecosystems. Biological soil crusts fix atmospheric N2 and are an important source of nitrogen [N] in many aridland ecosystems. Since N accretion in these ecosystems is low, I hypothesized that N inputs via fixation must nearly balance N losses.; I measured NO loss from biological soil crusts with three levels of N fixation potential. The upper limit of annual NO loss from dark, cyanolichen and light, cyanobacterial crusts was 0.13 and 0.07 kg N/ha/yr respectively. Overall, it appears that annual inputs via N fixation greatly exceed estimates of annual N gas losses in dark crusts, whereas N gas loss nearly equals N inputs in light, cyanobacterial crusts.; I also examined the effect of trampling disturbance and biological soil crust composition (dark, cyanolichen vs. light, cyanobacterial crust) on C and N fluxes in surface runoff. Trampling disturbance resulted in higher C and N losses as compared to scraped (crust removed but soil structure intact) and intact biological soil crusts. Biological soil crust composition also impacted C and N losses in runoff, where C and N export was higher in light, cyanobacterial crusts relative to dark, cyanolichen crusts.; I determined acetylene reduction (AR) to N fixation conversion ratios for the soil cyanobacterium Nostoc commune collected from three sites (New Mexico, Texas, and Inner Mongolia). Conversion ratios ranged from 4.3–6.2 for the New Mexico sites and 5.7–11.4 for the Texas site. 15N2 incorporation was not detectable in samples from the Inner Mongolia site.; In the final chapter, I explored the impacts of sheep grazing on plant composition and soil nutrients in a nine-year grazing study. My results suggest that as grazing intensity increases, litter quantity decreases resulting in C limitation of soil microbial communities. Even though C limitation of microbes results in lower N immobilization and higher plant available N pools, there is also greater potential for N loss from these pools in leaching and gaseous loss pathways. |
