Controls on annual emissions of nitric oxide from soils of the Colorado shortgrass steppe

Estimates of NO_x (NO+NO₂) emissions from temperate grasslands range from 0.003 to 101 ng NO-N m(-2)s(-1) (average 4.17 ng NO-N m(-2)s(-1)). As a result of this uncertainty, the potential contribution of soil NO_x emissions to the global budget and to nitrogen loss for this ecosystem is unclear. The few existing measurements are short-term observations of growing-season fluxes. We measured soil NO_x emissions from six sites within the shortgrass steppe at the Central Plains Experimental Range in Colorado from June 1994 through October 1995. The soils at these sites provided a range of texture (from a sandy loam to a clay loam) and soil moisture. Mean NO_x emissions over the sampling period ranged from 2.6 to 5.7 ng NO-N m(-2)s(-1) from the four unfertilized sites. Temperature was the dominant control on seasonal variations in NO_x fluxes. Seasonal fluxes were highest in the summers (5.4 to 10.5 ng NO-N m(-2)s(-1)) and lowest in the winter (0.2 to 1.5 ng NO-N m(-2)s(-1)). The winter NO_x emissions contribute up to 25% to the mean annual flux. Water-filled pore space (WFPS) alone was a poor predictor of NO_x emissions; however, peak NO_x emissions were found near the field capacities for these soils (32-35% WFPS for coarse soils and 66% WFPS for fine-textured soils). Water additions produced large (22-51 ng NO-N m(-2)s(-1)) but short-lived (24 hour) pulses of NO_x emissions that were independent of both the amount of water added and the number of antecedent dry days. Short-term increases in NO_x flux stimulated by wetting are significant, and increase the summer estimate of NO_x emissions 8 times estimates calculated from periodic sampling. Nitrogen applied in previous studies, 5 to 12 years earlier, increased the average annual NO_x emissions approximately 1.5 times. Extrapolating our estimate to similar systems around the world, we estimate that grassland ecosystems, globally, emit 1.0 Tg N yr(-1) as NOx. A current estimate of NO_x emissions from grassland soils is 0.6 Tg N yr(-1) [Davidson, 1991]. [References: 39