| Forest harvesting has potentially long-term effects on N cycling. This research investigated N cycling pools and processes and the major external and internal controls on N cycling at different stages of forest development in order to better understand mechanisms of ecosystem recovery from major disturbances. Net N mineralization and N uptake showed a trend toward long-term declines after harvest. Soil microbial N content varied with stand age but showed not long-term effects of harvesting. Fine root decomposition and net N mineralization were higher in younger forest stands. N mineralization, N uptake, and microbial N were higher in the A than the B soil horizon. Significant seasonal fluctuations occurred for all measured pools and processes. Estimates of actual evapotranspiration, an integrated measure of moisture and temperature availability and plant activity, was the most influential environmental variable correlated to N cycling pools and processes. Net N mineralization, net nitrification, and N uptake were all highly correlated with each other. These data were placed into N balance models at each stage of forest development. N mineralization, N uptake, and litter N return were similar for the regenerating forest stands (1--33 years post-harvest) and averaged approximately 100 kg N ha -1 yr-1. N mineralization and N uptake in the mature forest stand (∼145 kg N ha-1 yr -1) were higher than N return in forest floor and fine root litter (∼100 kg N ha-1 yr-1 ). The results from this study can be used to validate or modify predictive N cycling models and guide long-term soil productivity research currently being carried out by the U.S. Forest Service. |
