SOIL N AND PLANT-RESPONSES TO FIRE, TOPOGRAPHY, AND SUPPLEMENTAL N INTALLGRASS PRAIRIE

Tallgrass prairie in the Flint Hills region of Kansas is characterized by considerable topographic relief coupled with variation in soil pro perties. These topoedaphic gradients, together with variation in fire regimes, result in temporal and landscape-level variability in soil re source availability and plant responses. Nitrogen usually is considere d to be the nutrient most limiting to primary productivity in tallgras s prairie, but few studies have addressed how N availability varies se asonally, or across the landscape and with fire frequency. We measured soil inorganic N, in situ net N mineralization, aboveground net prima ry productivity (ANPP), and N mass on plots either fertilized with N i n 1993 or in 1994, or unfertilized, in uplands and lowlands of two ann ually burned and two long-term unburned sites during the 1994 growing season. In addition, our study was conducted in the year following rec ord rainfall, allowing us to assess the potential for high precipitati on amounts to affect subsequent N cycling and plant production. Both f ire 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 unfert ilized plots. Added N increased ANPP, but there were no differences be tween plots fertilized in 1994 and those fertilized in 1993. In genera l, patterns of ANPP on control plots were consistent with known produc tion responses to topography and burning (higher in annually burned si tes and in lowland sites) but were inversely related or unrelated to p atterns of N availability (higher in unburned sites and at upland topo graphic positions). Potential loss of N by volatilization during sprin g burning was greater than in years with normal rainfall amounts and r epresented a significant portion of aboveground plant N mass. Potentia l N losses did not appear to limit ANPP or N availability in the curre nt 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 avail ability in tallgrass prairie ecosystems.