caa a22 4500 46788563X CHVBK 20180406152733.0 cr unu---uuuuu 170328e20060401xx s 000 0 eng 10.1007/s10021-006-0004-6 doi (NATIONALLICENCE)springer-10.1007/s10021-006-0004-6 Tree Patches Show Greater N Losses but Maintain Higher Soil N Availability than Grassland Patches in a Frequently Burned Oak Savanna [Elektronische Daten] [Feike Dijkstra, Keith Wrage, Sarah Hobbie, Peter Reich] 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. Springer Science+Business Media, Inc., 2006 cedar creek nationallicence dissolved organic N (DON) nationallicence grasses nationallicence net N mineralization nationallicence N leaching nationallicence N volatilization nationallicence oak savanna nationallicence prescribed fires nationallicence total soil N nationallicence trees nationallicence Dijkstra Feike Department of Ecology, Evolution, and Behavior, University of Minnesota, 55108, St. Paul, Minnesota, USA aut Wrage Keith Department of Forest Resources, University of Minnesota, 55108, St. Paul, Minnesota, USA aut Hobbie Sarah Department of Ecology, Evolution, and Behavior, University of Minnesota, 55108, St. Paul, Minnesota, USA aut Reich Peter Department of Forest Resources, University of Minnesota, 55108, St. Paul, Minnesota, USA aut Ecosystems Springer-Verlag; www.springer-ny.com 9/3(2006-04-01), 441-452 1432-9840 9:3<441 2006 9 10021 https://doi.org/10.1007/s10021-006-0004-6 text/html Onlinezugriff via DOI 1 research-article jats NATIONALLICENCE 856 40 https://doi.org/10.1007/s10021-006-0004-6 text/html Onlinezugriff via DOI NATIONALLICENCE 700 1- Dijkstra Feike Department of Ecology, Evolution, and Behavior, University of Minnesota, 55108, St. Paul, Minnesota, USA aut NATIONALLICENCE 700 1- Wrage Keith Department of Forest Resources, University of Minnesota, 55108, St. Paul, Minnesota, USA aut NATIONALLICENCE 700 1- Hobbie Sarah Department of Ecology, Evolution, and Behavior, University of Minnesota, 55108, St. Paul, Minnesota, USA aut NATIONALLICENCE 700 1- Reich Peter Department of Forest Resources, University of Minnesota, 55108, St. Paul, Minnesota, USA aut NATIONALLICENCE 773 0- Ecosystems Springer-Verlag; www.springer-ny.com 9/3(2006-04-01), 441-452 1432-9840 9:3<441 2006 9 10021 Metadata rights reserved Springer special CC-BY-NC licence nationallicence BK010053 XK010053 XK010000 NATIONALLICENCE NATIONALLICENCE NL-springer