Do ungulates accelerate or decelerate nitrogen cycling? Francis J. Singer * , Kathryn A. Schoenecker US Geological Survey, Natural Resources Ecology Lab, Fort Collins Science Center, Colorado State University, Fort Collins, CO 80523, USA Received 1 November 2001; received in revised form 22 July 2002; accepted 17 September 2002 Abstract Nitrogen (N) is an essential nutrient for plants and animals, and N may be limiting in many western US grassland and shrubland ungulate winter ranges. Ungulates may influence N pools and they may alter N inputs and outputs (losses) to the ecosystem in a number of ways. In this paper we compare the ecosystem effects of ungulate herbivory in two western national parks, Rocky Mountain National Park (RMNP), Colorado, and Yellowstone National Park (YNP), Wyoming. We compare ungulate herbivory effects on N pools, N fluxes, N yields, and plant productivity in the context of the accelerating and decelerating nutrient cycling scenarios [Ecology 79 (1998) 165]. We concluded that the YNP grasslands fit the accelerating nutrient cycling scenario for ungulate herbivory: in response to grazing, grassland plant species abundance was largely unaltered, net annual aboveground primary productivity (NAPP) was stimulated (except during drought), consumption of key N- rich forages by ungulates was moderate and their abundance was sustained, soil N mineralization rates doubled, N pools increased, aboveground N yield increased, and N concentrations increased in most grassland plant species. Grazing in grasslands in RMNP resulted in no consistent detectable acceleration or deceleration of nutrient cycling. Grazing effects in short willow and aspen vegetation types in RMNP fit the decelerating nutrient cycling scenario of Ritchie et al. [Ecology 79 (1998) 165]. Key N- rich forages declined due to herbivory (willows, aspen, herbaceous vegetation). Aboveground production declined, soil N mineralization rates declined, N pools declined (NO 3  pools were 30% that of ungrazed controls), and aboveground N yield declined. We believe that the higher ungulate densities and rates of plant consumption in RMNP, large declines in N-rich forage plants, and possibly a tendency of ungulates to move N from willow and aspen vegetation types to other types in RMNP, contributed to deceleration of nutrient cycling in two vegetation types in RMNP compared to acceleration in grasslands in YNP. # 2003 Published by Elsevier Science B.V. Keywords: Bison bison; Cervus elaphus; Elk; Mineralization; Nitrogen; Rocky Mountain National Park; Ungulate herbivory; Yellowstone National Park 1. Introduction Grassland and shrubland ecosystems located within the conifer forest ecoregion in the western US are limited in productivity by nitrogen (N). Nitrogen abundance is closely tied to both higher soil fertility and higher soil water retention rates, since N and soil organic matters are closely related. Nitrogen abun- dances may influence not only rates of primary plant production but also plant species abundances, since many plants have specific N needs. An increase, or alternatively, a decrease in N abundance may alter plant species composition (Ritchie et al., 1998). Any depletion of N may have serious long-term Forest Ecology and Management 181 (2003) 189–204 * Corresponding author. Tel.: þ1-970-491-7056; fax: þ1-970-491-1965. E-mail address: francis@nrel.colostate.edu (F.J. Singer). 0378-1127/$ – see front matter # 2003 Published by Elsevier Science B.V. doi:10.1016/S0378-1127(03)00133-6