Denitrification in a semi-arid grazing ecosystem

The effect of large herbivores on gaseous N loss from grasslands, particula rly via denitrification, is poorly understood. In this study, we examined t he influence of native migratory ungulates on denitrification in grasslands of Yellowstone National Park in two ways, by (1) examining the effect of a rtificial urine application on denitrification, and (?) comparing rates ins ide and outside long-term exclosures at topographically diverse locations. Artificial urine did not influence denitrification 3 and 12 days after appl ication at hilltop, mid-slope, and slope-bottom sites. Likewise, grazers ha d no effect on community-level denitrification at dry exclosure sites, wher e rates were low. At mesic sites, however, ungulates enhanced denitrificati on by as much as 3 kg N ha(-1) year(-1), which was double atmospheric N inp uts to this ecosystem. Denitrification enzyme activity (DEA, a measure of d enitrification potential) was positively associated with soil moisture at e xclosure sites, and herbivores stimulated DEA when accounting for the soil moisture effect. Glucose additons to soils increased denitrification and ni trate additions had no influence, suggesting that denitrification was limit ed by the amount of labile soil carbon, which previously has been shown to be enhanced by ungulates in Yellowstone. These results indicate that denitr ification can be an ecologically important flux in portions of semi-arid la ndscapes, and that there is a previously unsuspected regulation of this pro cess by herbivores.