PARENT SESSION
Poster Session # 13: Biogeochemistry, Photosynthesis, and Respiration.
Tuesday, August 5 Presentation from 5:00 PM to 6:30 PM. SITCC Exhibit Hall B.
Soil nutrient dynamics across a fire frequency gradient in oak savanna.
Norris, Mark1, Reich, Peter 1, Hobbie, Sarah2, Wedin, David3, 1 Department of Forest Resources, St. Paul, MN, USA2 Department of Ecology, Evolution and Behavior, St. Paul, MN, USA3 School of Natural Resource Sciences, Lincoln, NE, USA
ABSTRACT- Savanna ecosystems provide an opportunity to examine the dynamic competitive balance between two life forms, an herbaceous understory and woody overstory, and how this balance is influenced by global change. This research evaluates species effects on ecosystem functions and the mechanisms controlling observed patterns of nitrogen cycling in a midwestern oak savanna. The fire frequency gradient (0-0.8 fires per year) at Cedar Creek LTER, Minnesota, drives a gradient in oak canopy dominance which in turn leads to substantial differences in N cycling rates. Available soil N pools and cycling rates from multiple years are negatively correlated with fire frequency as annual net N mineralization rates in savanna plots of low fire frequency (high oak dominance) are 3 times that of high fire frequency (high grass dominance) plots (60 vs. 20 kg N/ha/yr). A short-term lab incubation that standardized microclimatic conditions found that this pattern of increased N mineralization is likely due in large part to increased microbial biomass and activity as fire frequency decreases. An ongoing reciprocal transplant decomposition experiment (oak vs. grass litter in oak vs. grass dominant plots) suggests that decay rates of aboveground material do not vary greatly after two seasons with respect to fire frequency, however belowground patterns indicate that litter decomposes quicker in its native habitat. Results thus far indicate that short-term patterns of N cycling are closely linked to microbial activity irrespective of plant community composition and fire effects.
Key words: savanna, fire, Quercus, nitrogen