PARENT SESSION
Poster Session # 10: Soil Ecology.
Tuesday, August 5 Presentation from 5:00 PM to 6:30 PM. SITCC Exhibit Hall B.
Effects of N deposition, increased summer rains and trampling on nitrogen fixing ability of biological crusts in the Mojave Desert.
Zimpfer, Jeff*,1, Wagner, Diane 2, Smith, Stanley1, 1 Department of Biological Sciences, Las Vegas, NV2 Biology and Wildlife Department, Fairbanks, AK
ABSTRACT- Biological crusts play an important role in the Mojave Desert by reducing soil erosion and fixing atmospheric nitrogen. Changing patterns of climate and land use are likely to affect the species composition and metabolic activity of crusts. In a 3-way factorial experiment at the Mojave Global Change Facility (MGCF), we increased rates of nitrogen deposition (0, 10, and 40 kg N ha-1 yr-1), summer rains (7.5 cm yr-1 irrigation), and physical disturbance (trampling) on 96 14 x 14 m plots. After two successive years of treatment, we measured effects on percent cover by the dominant N-fixing lichen (Collema), nitrogenase activity (via acetylene reduction, hereafter AR) and soil chemistry. Summer irrigation and trampling significantly reduced lichen cover, whereas N addition had no effect. Irrigation decreased soil organic matter and increased both soil and mineral N. Disturbance had no significant effect on soil chemistry, and N addition increased mineral N but had no effect on soil organic matter. Control plots had a mean AR rate of 13.2 n moles cm-2 h-1. Both 40 kg of added N ha-1 and trampling significantly decreased the rate of AR. Irrigation had no effect on AR rate, possibly because the positive effects of increased soil moisture on AR rates offset negative effects due to the irrigation-related decrease in areal coverage of N-fixing lichens. Our results suggest that (1) high levels of N deposition and increased physical disturbance will decrease rates of N fixation in the Mojave Desert and (2) cover by N-fixing lichens will decrease in response to increased summer precipitation.
Key words: nitrogen fixation, lichen, biological crust, cyanobacteria