Topographic influences on nitrogen cycling within an upland pin oak ecosystem

Nitrogen mineralization and nitrification were studied within an upland pin oak (Quercus ellipsoidalis E.J. Hill) forest to determine if topography could be used to understand the spatial variability of N transformations within and among individual stands. Four upland pin oak stands were used for the study, and within each stand, transects were established on northeast (45° azimuth), southeast (135° azimuth), southwest (225° azimuth), and northwest (315° azimuth) facing aspects. Nitrogen mineralization and nitrification were measured at 5-wk intervals for 1 year at top, middle, and bottom slope position along each transect using an in situ soil incubation technique. Annual rates of mineralization among the four stands ranged from 3.0 to 6.7 g N m^-2 yr^-1, but were not significantly influenced by aspect or slope position. Although aspect had no effect on nitrification, bottom slope positions had significantly lower (0.3 g N m^-2 yr^-1) nitrification rates compared to middle (1.3 g N m^-2 yr^-1) and top (1.4 g N m^-2 yr^-1 slope positions. Annual N mineralization and nitrification increased with stand age. Slope position and aspect had little influence on N cycling rates within the subtle topography of east central Minnesota; time since disturbance seems to be the most important factor influencing the variability in N dynamics among upland pin oak stands.