Tree patches show greater N losses but maintain higher soil N availability than grassland patches in a frequently burned oak savanna

Feike A. Dijkstra, Keith Wrage, Sarah E. Hobbie, Peter B. Reich

Research output: Contribution to journalArticlepeer-review

43 Scopus citations


Long-term prescribed fires have increased woody canopy openness and reduced nitrogen (N) cycling (that is, net N mineralization) in an oak savanna in Minnesota, USA. It is unclear how fire-induced shifts from oak-dominated to C 4 grass-dominated vegetation contribute to this decline in N cycling compared to direct effects of increasing fire frequency promoting greater N losses. We determined (1) the magnitude of decline in net N mineralization in oak versus grass-dominated patches with increasing fire frequency and (2) if differences in net N mineralization between oak and grass patches in frequently burned oak savanna (burned 8 out of 10 years on average during the last 40 years) could be attributed to differences in N losses through volatilization and leaching or to plant traits affecting decomposition and mineralization. In situ net N mineralization declined with increasing fire frequency overall, but this decline was less in oak-than in grass-dominated patches, with oak-dominated patches having more than two times higher net N mineralization than grass-dominated patches. Greater net N mineralization in oak-dominated patches occurred despite greater N losses through volatilization and leaching (on average 1.8 and 1.4 g m-2 y-1 for oak- and grass-dominated patches, respectively), likely because of higher plant litter N concentration in the oak-dominated patches. As total soil N pools in the first 15 cm did not differ between oak- and grass-dominated patches (on average 83 g N m-2), N inputs from atmospheric deposition and uptake from deep soil layers may offset higher N losses. Our results further show that net N mineralization rates decline within 5 years after tree death and subsequent colonization by C4 grasses to levels observed in grass-dominated patches. Although long-term prescribed fires often directly reduce N stocks and cycling because of increased N losses, this study has shown that fire-induced shifts in vegetation composition can strongly contribute to the declines in N cycling in systems that are frequently disturbed by fires with potential feedbacks to plant productivity.

Original languageEnglish (US)
Pages (from-to)441-452
Number of pages12
Issue number3
StatePublished - Apr 2006


  • Cedar creek
  • Dissolved organic N (DON)
  • Grasses
  • N leaching
  • N volatilization
  • Net N mineralization
  • Oak savanna
  • Prescribed fires
  • Total soil N
  • Trees


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