Ungulate stimulation of nitrogen cycling and retention in Yellowstone Parkgrasslands

We studied how ungulates and a large variation in site conditions influence d grassland nitrogen (N) dynamics in Yellowstone National Park. In contrast to most grassland N studies that have examined one or two soil N processes , we investigated four rates, net N mineralization, nitrification. denitrif ication. and inorganic N leaching, at seven paired sites inside and outside longterm (33+ year) exclosures. Our focus was how N fluxes were related to one another among highly variable grasslands and how grazers influenced th ose relationships. In addition, we examined variation in soil delta(15)N am ong grasslands and the relationships between soil N-15 abundance and N proc esses. Previously, ungulates were reported to facilitate net N mineralizati on across variable Yellowstone grasslands and denitrification at mesic site s. In this study, we found that herbivores also promoted nitrification amon g diverse grasslands. Furthermore, net N mineralization, nitrification, and denitrification (kg N ha(-1) year(-1), each variable) were postively and l inearly related to one another among all grasslands (grazed and fenced), an d grazers reduced the nitrification/net N mineralization and denitrificatio n/net N mineralization ratios, indicating that ungulates inhibited the prop ortion of available NH,I that was nitrified and denitrified. There was no r elationship between net N mineralization or nitrification with leaching (in dexed by inorganic N adsorbed to resin buried at the bottom of rooting zone s) and leaching was unaffected by grazers. Soil delta(15)N was positively a nd linearly related to in situ net N mineralization and nitrification in un grazed grasslands; however, there was no relationship between isotopic comp osition of N and those rates among grazed grasslands. The results suggested that grazers simultaneously increased N availability (stimulated net N min eralization and nitrification per unit area) and N conservation (reduced N loss from the soil per unit net N mineralization) in Yellowstone grasslands . Grazers promoted N retention by stimulating microbial productivity, proba bly caused by herbivores promoting labile soil C. Process-level evidence fo r N retention by grazers was supported by soil delta(15)N data. Grazed gras sland with high rates of N cycling had substantially lower soil delta(15)N relative to values expected for ungrazed grassland with comparable net N mi neralization and nitrification rates. These soil N-15 results suggest that ungulates inhibited N loss at those sites. Such documented evidence for con sumer control of N availability to plants, microbial productivity, and N re tention in Yellowstone Park is further testimony for the widespread regulat ion of grassland processes by large herbivores.