Ungulate vs. landscape control of soil C and N processes in grasslands of Yellowstone National Park

Douglas A. Frank, Peter M. Groffman

Research output: Contribution to journalArticlepeer-review

287 Scopus citations


Within large grassland ecosystems, climatic and topographic gradients are considered the primary controls of soil processes. Ungulates also can influence soil dynamics; however the relative contribution of large herbivores to controlling grassland soil processes remains largely unknown. In this study, we compared the effects of native migratory ungulates and variable site ('landscape') conditions, caused by combined climatic and topographic variability, on grassland of the northern winter range of Yellowstone National Park by determining soil C and N dynamics inside and outside 33-37 yr exclosures at seven diverse sites. Sites included hilltop, slope, and slope bottom positions across a climatic gradient and represented among the driest and wettest grasslands on the northern winter range. We performed two experiments: (1) a 12-mo in situ net N mineralization study and (2) a long-term (62-wk) laboratory incubation to measure potential N mineralization and microbial respiration. Results from the in situ experiment indicated that average net N mineralization among grazed plots (3.8 g N · m-2 · yr-1) was double that of fenced, ungrazed plots (1.9 g N · m-2 · yr-1). Mean grazer enhancement of net N mineralization across sites (1.9 g N · m-2 · yr-1) approached the maximum difference in net N mineralization among fenced plots (2.2 g N · m-2 · yr-1), i.e., the greatest landscape effect observed. Furthermore, ungulates substantially increased between-site variation in mineralization; grazed grassland, 1 SD = 2.2 g N · m-2 · yr-1, fenced grassland, 1 SD = 0.85 g N · m-2 · yr-1. In the long-term incubation, potential microbial respiration and net N mineralization were positively related to total soil C and N content, respectively. There was greater variation in potential respiration and net N mineralization early in the incubation, when labile material was processed, compared to late in the incubation, when more recalcitrant substrate was processed, suggesting that between-site variation in labile organic matter was greater than that of recalcitrant material. Herbivores improved the organic matter quality of soil, increasing the labile fractions and reducing the recalcitrant fractions. Grazers reduced C respired/N mineralized ratios, an index of microbial N immobilization, by an average of 21%. However, the largest landscape influence on the immobilization index was 13-fold greater than the grazer effect. Given that the greatest landscape influence on in situ net mineralization (2.2 g N · m-2 · yr-1) was similar to the average grazer impact on that rate (1.9 g N · m-2 · yr-1), we hypothesize that the landscape effect on field N availability was primarily caused by variation in microbial immobilization, while the grazing effect was primarily due to stimulation of gross mineralization. These results indicate that the relative importance of ungulates in controlling soil N cycling may be more important than previously suspected for grasslands supporting large herds of migratory ungulates, and that the dominant mechanisms underlying the landscape and ungulate influences on soil mineral fluxes may differ.

Original languageEnglish (US)
Pages (from-to)2229-2241
Number of pages13
Issue number7
StatePublished - 1998


  • Carbon
  • Grassland
  • Grazing
  • Herbivory
  • Large herbivores
  • Nitrogen
  • Yellowstone National Park

ASJC Scopus subject areas

  • Ecology, Evolution, Behavior and Systematics


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