LANDSCAPE PATTERNS OF INTRAECOSYSTEM NITROGEN CYCLING

Nitrogen mineralization and nitrification were studied in three upland forests differing in landscape position, species composition and structure to gain an understanding of the spatial dynamics of intraecosystem N cycling. The ecosystems studied were an oak ecosystem associated with glaciofluvial features and two sugar maple ecosystems that occurred on morainal features but differed in ground flora composition. Stands were spatially replicated across a two county area in northwestern Lower Michigan.; Net N mineralization and nitrification potentials were determined by an eight-week aerobic laboratory incubation. Litter was collected in each ecosystem during autumn; components were separated by species and analyzed for total N. Litter production, N returned to the forest floor, N mineralization and nitrification in both sugar maple ecosystems were two times greater than in the oak ecosystem. Nitrification potentials were minimal in the oak forest. Nitrification was four times greater in the species-rich sugar maple ecosystem compared to the species-poor sugar maple ecosystem. High nitrification potentials were consistently related to the distribution of spring ephemeral communities.; Seasonal fluxes of N mineralization and nitrification were estimated using a buried polyethylene bag technique; surface soil samples were incubated at monthly intervals for one year. Aboveground woody biomass and its allocation to annual increment were determined using species-specific allometric biomass equations. Distinct patterns of overstory production, mineralization and nitrification corresponded to the spatial distribution of forest ecosystems. Nitrogen mineralization was 86.1 kg ha('-1) yr('-1) in the oak ecosystem; significantly less than annual mineralization in the sugar maple forests (106 kg ha('-1) yr('-1)). Nitrification was greatest in the species-rich sugar maple forest; 82% of all mineral N was oxidized to NO(,3)('-) (88.1 kg ha('-1) yr('-1)). Nitrification in the oak forest was 5% of mineral N production (4.5 kg ha('-1) yr('-1)). The spatial distribution of forest ecosystems could be used to predict landscape patterns of N mineralization and nitrification.; Nitrate reductase (NR) activity was measured in Allium tricoccum and Asarum canadense to investigate the mechanism of N retention by spring ephemeral communities. NR activity was low in both species, indirectly suggesting NH(,4)('+) assimilation and plant-nitrifier competition as a possible mechanism of N retention.