Frank and Groffman examined the influence of native migratory ungulates on denitrification in grasslands of Yellowstone National Park in two ways, by (1) examining the effect of artificial urine application on denitrification, and (2) comparing rates inside and outside long-term exclosures at topographically diverse locations. Artificial urine did not influence denitrification 3 and 12 days after application at any sites. Grazers had no effect on community-level denitrification at dry exclosure sites, where rates were low. At mesic sites, however, ungulates enhanced denitrification by as much as 4 kg N ha-1 year-1. Irrigation increased denitrification, as did glucose addition. Frank and Groffman hypothesize that ungulates increase labile carbon in soils, and subsequently denitrification, provided that sufficient moisture is present.
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