PARENT SESSION
Oral Session #35: Carbon and Nitrogen Cycling.
Presiding: D. Rothstein
Tuesday, August 6. 8:00 AM to 11:30 AM. Palo Verde Room, Radisson.
The role of biological soil crusts in ecosystem N cycling on the Colorado Plateau.
Barger, Nichole*,1, Belnap, Jayne2, Ojima, Dennis1, 1 Colorado State University, Fort Collins, CO2 USGS-Biological Resources Division, Moab, UT
ABSTRACT- Biological soil crusts (also known as microbiotic, microphytic, or cryptobiotic crusts) are communities of fungi, lichens, cyanobacteria, and mosses that colonize soil surfaces in arid and semi-arid ecosystems. Associated soil cyanobacteria and bacteria species within biological soil crusts fix atmospheric N2 and are an important source of nitrogen [N] in desert ecosystems. On the Colorado Plateau, estimates of N fixation rates by biological soil crusts are high and range from 4-37 kg N/ha/yr depending on biological crust composition. However estimates of N accumulation within these ecosystems are generally <1 kg N/ha/yr, leading to considerable uncertainties regarding the fate of the fixed N by biological soil crusts and the dominant N loss pathways in desert ecosystems. To examine whether N gas losses increase with increasing N fixation potential of the biological crust community, we measured nitric oxide [NO] emissions and nitrous oxide [N2O] from three biological soil crust communities. NO fluxes from dark crusts were 4 fold higher relative to light crusts, and 2 fold higher relative to medium crusts in June. By July the pattern was not as strong but dark crusts still had 42% greater NO fluxes compared to light and medium crusts. We detected no N2O fluxes from soil cores in the field. Our hypothesis of increasing NO losses with increasing N fixation potential by biological soil crusts was supported by our data. Coupling N gas loss data to climate data we estimate that 0.01-0.37 kg N/ha/yr may be lost via nitrification and denitrification pathways during the summer months.
KEY WORDS: nitrogen fixation, biological soil crust, nitric oxide, nitrous oxide