Biogeochemical cycling and N dynamics of biological soil crusts in a semi-arid ecosystem

Abstract

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.