Nitrogen dynamics following clear-cut harvest of northern hardwood ecosystems in Michigan.

Northern hardwood forests are susceptible to nitrogen (N) loss through nitrate (NO$\sb3\sp-)$ leaching and denitrification following harvesting, resulting from elevated rates of net N mineralization and nitrification in the absence of plant uptake. Predicting harvesting effects on N loss is contingent on understanding processes which control the cycling of N between soil microorganisms and soil organic matter. Lake States northern hardwood forests vary in nitrification rates, but it is not known whether they also differ in NO$\sb3\sp-$ loss following harvesting or whether microbial N immobilization reduces N loss from these ecosystems. My objective was to examine the soil microbial processes regulating N loss following clear-cutting in two northern hardwood ecosystems of northern Lower Michigan that differ floristically, edaphically, and in rates of nitrification. Paired intact and clear-cut plots were established in two northern hardwood ecosystems. For one year following clear-cut harvest, I measured NO$\sb3\sp-$ leaching and denitrification, gross and net N transformation rates, and microbial biomass. Clear-cut harvest led to significant loss of N through NO$\sb3\sp-$ leaching in both ecosystems, averaging 4.9 g-N m$\rm\sp{-2}\ yr\sp{-1}$ in clear-cut and 0.2 g-N m$\rm\sp{-2}\ yr\sp{-1}$ in intact plots. Denitrification was minimal in both ecosystems and did not increase significantly following clear-cutting. Averaged across ecosystems, annual net N mineralization increased by 100% in clear-cut plots (14.2 g-N $\rm m\sp{-2}\ yr\sp{-1})$ relative to intact plots (7.3 g-N m$\rm\sp{-2}\ yr\sp{-1}).$ Annual net nitrification rates similarly increased after harvest (11.4 g-N $\rm m\sp{-2}\ yr\sp{-1}$ clear-cut vs. 5.5 g-N $\rm m\sp{-2}\ yr\sp{-1}$ intact). Gross rates of N mineralization and nitrification displayed a response similar to that of net rates. Gross rates of N immobilization increased following clear-cutting, but microbial biomass did not. Thus, increased turnover of N through microbial biomass produced the observed increase in net N mineralization rates. Consequently, N immobilization was not important in reducing N loss. Nitrogen loss following clear-cut was primarily regulated by ammonium (NH$\sb4\sp+)$ availability to nitrifying bacteria. Both ecosystems lost equivalent amounts of NO$\sb3\sp-,$ regardless of differences in net nitrification rates prior to harvest. Therefore, N loss following clear-cutting in these ecosystems cannot be predicted by rates of nitrification prior to harvest. Rather, N loss depends on the combined effects of changes in microclimatic conditions and heterotrophic activity on NH$\sb4\sp+$-availability to nitrifiers.PhDBiogeochemistryBiological SciencesEarth SciencesEcologyForestryUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/130254/2/9721995.pd