Tree Patches Show Greater N Losses but Maintain Higher Soil N Availability than Grassland Patches in a Frequently Burned Oak Savanna |
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Authors: | Feike A Dijkstra Keith Wrage Sarah E Hobbie Peter B Reich |
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Institution: | (1) Department of Ecology, Evolution, and Behavior, University of Minnesota, St. Paul, Minnesota 55108, USA;(2) Department of Forest Resources, University of Minnesota, St. Paul, Minnesota 55108, USA;(3) Present address: Department of Environmental Studies, University of California, Santa Cruz, 1156 High Street, Santa Cruz, California 95064, USA |
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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. |
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Keywords: | cedar creek dissolved organic N (DON) grasses net N mineralization N leaching N volatilization oak savanna prescribed fires total soil N trees |
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