Monday, November 5, 2007
96-6

Mechanisms for Seasonal Patterns in NH4 and NO3 Consumption and N2O Flux in Semiarid Cheatgrass and Sagebrush Soils.

Toby Hooker and John Stark. 5305 Old Main Hill, Utah State University, Utah State University, Biology Department, Logan, UT 84322-5305

Changes in the quantity and quality of plant detrital inputs to soil may be an important mechanism controlling soil C and N dynamics in sagebrush rangelands affected by annual grass invasion. We investigated the seasonal fate of soil NH4 and NO3 consumption processes in response to stimulated substrate pools in mesocosms taken from soils beneath cheatgrass stands and sagebrush canopies. Mesocosms were harvested 5 times over 17-months, and soil C and N cycling rates in intact cores were determined during 72-h laboratory incubations. Soil moisture ranged from 9 to 28% across sampling dates, and strongly affected rates of C mineralization, N cycling, and net N2O flux. Baseline N2O fluxes were low (<0.34 μg N/kg/d) in soils below field capacity, were correlated with gross nitrification, and increased by 1000% in moist spring soils. NH4 addition stimulated net NH4 consumption and net nitrification rates, and increased N2O flux by 50-500%. NO3 addition had no effect on N2O flux until moisture exceeded 25%, when both NH4 and NO3 stimulated N2O flux. High C mineralization rates, rapidly growing microbial biomass, and high Cmin:NO3 ratios in moist spring soils likely alleviated C-limitation to denitrification in anaerobic microsites. Cheatgrass soils typically had greater N2O fluxes than soils from beneath sagebrush-canopies, and larger N2O response to N additions. This is consistent with greater nitrification in response to increasing ammonification rates observed in cheatgrass compared to sagebrush soils. These results suggest that N2O flux in these soils is primarily due to trace gas loss during nitrification. Denitrification in anaerobic microsites may be C-limited during much of the year, but NO3 limited during the spring when plant C inputs are high.