NITROUS-OXIDE FLUX FROM A SHRUB-STEPPE ECOSYSTEM - SOURCES AND REGULATION

The semi-arid shrub-steppe is the largest grassland-type ecosystem of North America and may make significant contributions to the global atmospheric N2O budget. However, little information is available concerning sources and regulation of N2O flux in this ecosystem. Experiments were made to determine the relative importance of nitrification, denitrification and abiotic sources to total N2O flux and to investigate the factors regulating N2O flux rates from an undisturbed shrub-steppe ecosystem. The contributions to N2O flux by nitrification and denitrification were estimated using acetylene (10 Pa) to selectively inhibit N2O production by nitrifiers. Abiotic sources of N2O were evaluated using sterilized soil. Factors limiting N2O production were evaluated by monitoring N2O flux rates from soil-cores amended with combinations of NO-N, NH4+-N, soluble C and water. The effect of wet-dry cycles on N2O flux was determined by wetting field dry soil to field capacity and monitoring N2O flux rates, soil NH4+-N, NO3--N and water content throughout a drying period. Our results showed that nitrification accounts for 61-98% of the N2O produced from soil at water contents below saturation and that denitrification is the primary N2O source at saturated water contents. No detectable N2O was produced by abiotic sources. In intact soil cores N2O flux rates were found to be most limited by water and N availability. Wetting of dry soil resulted in a pulse of N2O flux due to increased N availability. It is likely that this ecosystem exhibits relatively low N2O flux rates for much of the year due to low soil moisture and inorganic N contents. Since soil moisture content is generally well below field capacity in this ecosystem, nitrification must be the dominant N2O source. These results suggest that conditions favorable for substantial N2O production in shrub-steppe ecosystems probably exist only at times following precipitation events.