Regional variability in nitrogen mineralization, nitrification, and overstory biomass in northern Lower Michigan

Potential net N mineralization, potential net nitrification, and overstory (boles and branches) biomass were measured in 9 forest ecosystems commonly found within the well-drained uplands of northern Lower Michigan. The ecosystem types ranges from oak-dominated forests on coarse-textured outwash sands to mesic northern hardwood forests on sandy glacial till. Overstory biomass ranged from 92 t.ha ^-1 in a xeric oak ecosystem to 243 t.ha^-1 in a northern hardwood ecosystem; annual biomass production ranged from 1.3-3.5 t.ha^-1.yr^-1 respectively. Potential net N mineralization was lowest in the xeric oak ecosystem (52.0 μg N.g^-1) and greatest in the mesic northern hardwood ecosystem (127.8 μg N.g^-1). Potential net nitrification was 45.5 μg NO₃ ^--N.g^-1 in the northern hardwood ecosystem; 10-230 times greater than in other ecosystems. Maximum overstory biomass and biomass increment predicted were 247 t.ha^-1 and 3.7 t.ha^-1, respectively. The exponential function used to predict biomass increment from N mineralization suggests that the productivity of some northern hardwood forests in northern Lower Michigan is not limited by N availability. -from Authors