SOIL MICROBIAL CONTROL OF NITROGEN LOSS FOLLOWING CLEAR‐CUT HARVEST IN NORTHERN HARDWOOD ECOSYSTEMS

Establishing the relationship among the spatial distribution of forest ecosystems, N cycling processes, and N loss following harvesting could enable land managers to anticipate and predict the potential for N loss at the scale of local and regional landscapes. In the Great Lakes region, northern hardwood forests with distinct floristic, edaphic, and physiographic characteristics vary predictably across the landscape in N cycling processes, especially in rates of nitrification. Although their landscape distribution and patterns of N cycling are well established, it is uncertain whether this type of information could be used to predict landscape-level patterns of N loss following overstory harvest or other types of disturbance. We studied the microbial processes in soil that control the retention and loss of N following clear-cutting in two northern hardwood ecosystems that are widely distributed in the upper Great Lakes states and differ floristically, edaphically, and in N cycling processes. The overstory of one hardwood ecosystem is dominated by Acer saccharum and Quercus rubra, and the other is dominated by A. saccharum and Tilia americana. These ecosystems differ in annual rates of net nitrification (5 vs. 15 g NO3−-N·m−2·yr−1), and we studied paired intact and clear-cut plots within them to understand whether spatial patterns of nitrification could be used to predict N loss following harvest. We measured microbial biomass, NO3− leaching, denitrification, and microbial N transformations for one year following a clear-cut harvest. Clear-cut harvest led to significant loss of N through NO3− leaching in both ecosystems, averaging 4.9 g N·m−2·yr−1 in clear-cut plots and 0.2 g N·m−2·yr−1 in intact plots. Denitrification was low in both ecosystems (0.08–0.42 g N·m−2·yr−1) and did not increase significantly following clear-cutting (0.16–0.29 g N·m−2·yr−1). Averaged across ecosystems, annual net N mineralization increased by a factor of 2 in clear-cut plots (14.2 g N·m−2·yr−1) relative to intact plots (7.3 g N·m−2·yr−1); net nitrification similarly increased after harvest (11.4 g N·m−2·yr−1 clear-cut vs. 5.5 g N·m−2·yr−1 intact). Gross rates of N mineralization and nitrification displayed a response similar to that of net rates. Although gross rates of N immobilization increased following clear-cutting, microbial biomass did not change. Thus, increased turnover of N through microbial biomass ([biomass N]/[gross N immobilization]) resulted in the observed increase in net N mineralization rates. Our results indicate that microbial immobilization of N was not an important process of N retention following clear-cut harvest in sugar maple-dominated northern hardwood forests. Rather, increases in net N mineralization following harvest increased substrate availability to nitrifying bacteria, which eventually resulted in substantial losses of N through leaching. Regardless of initial differences in net nitrification, harvesting led to similar rates of rates NO3− leaching from both northern hardwood ecosystems. We conclude that N loss following clear-cutting in these forests cannot be predicted by rates of nitrification prior to harvest. Rather, N loss depends on high initial rates of net N mineralization and on the effects of changes in microclimatic conditions and heterotrophic activity on NH4+ availability to nitrifying bacteria following overstory removal.