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Soil N and Plant Responses to Fire, Topography, and Supplemental N in Tallgrass Prairie

Clarence L. Turner, John M. Blair, Rita J. Schartz and Jeffrey C. Neel
Ecology
Vol. 78, No. 6 (Sep., 1997), pp. 1832-1843
Published by: Wiley
DOI: 10.2307/2266105
Stable URL: http://www.jstor.org/stable/2266105
Page Count: 12
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Soil N and Plant Responses to Fire, Topography, and Supplemental N in Tallgrass Prairie
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Abstract

Tallgrass prairie in the Flint Hills region of Kansas is characterized by considerable topographic relief coupled with variation in soil properties. These topoedaphic gradients, together with variation in fire regimes, result in temporal and landscape-level variability in soil resource availability and plant responses. Nitrogen usually is considered to be the nutrient most limiting to primary productivity in tallgrass prairie, but few studies have addressed how N availability varies seasonally, or across the landscape and with fire frequency. We measured soil inorganic N, in situ net N mineralization, aboveground net primary productivity (ANPP), and N mass on plots either fertilized with N in 1993 or in 1994, or unfertilized, in uplands and lowlands of two annually burned and two long-term unburned sites during the 1994 growing season. In addition, our study was conducted in the year following record rainfall, allowing us to assess the potential for high precipitation amounts to affect subsequent N cycling and plant production. Both fire treatment and topography affected soil N availability. In general, N mineralization was greater on unburned than on burned sites and was up to five times greater on uplands than lowlands. Total extractable soil N was highest early in the season and least at midseason, and it also tended to be higher in unburned sites than burned sites on unfertilized plots. Added N increased ANPP, but there were no differences between plots fertilized in 1994 and those fertilized in 1993. In general, patterns of ANPP on control plots were consistent with known production responses to topography and burning (higher in annually burned sites and in lowland sites) but were inversely related or unrelated to patterns of N availability (higher in unburned sites and at upland topographic positions). Potential loss of N by volatilization during spring burning was greater than in years with normal rainfall amounts and represented a significant portion of aboveground plant N mass. Potential N losses did not appear to limit ANPP or N availability in the current growing season. Our results suggest that different factors control soil N mineralization and plant productivity, which explains, in part, why patterns of ANPP are not well correlated with patterns of N availability in tallgrass prairie ecosystems.

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