Citation. ADijkstra, F. A.; Hobbie, S. E.; Reich, P. B.; Wrage, K.. 2005. Tree patches show greater N losses but maintain higher soil N availability than grassland patches in a frequently burned oak savanna. Ecosystems 9: 441–452.

Abstract. 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 C₄ 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.

Keywords.cedar creek - dissolved organic N (DON) - grasses - net N mineralization - N leaching - N volatilization - oak savanna - prescribed
fires - total soil N - trees