THE LONG-TERM EFFECTS OF LAND-USE HISTORY ON NITROGEN CYCLING IN NORTHERN HARDWOOD FORESTS

Abstract

Nearly all northeastern U.S. forests have been disturbed by wind, logging, fire, or agriculture over the past several centuries. These disturbances may have long-term impacts on forest carbon and nitrogen cycling, affecting forests' vulnerability to N saturation and their future capacity to store C. We evaluated the long-term (80–110 yr) effects of logging and fire on aboveground biomass, foliar N (%), soil C and N pools, net N mineralization and nitrification, and NO3− leaching in northern hardwood forests in the White Mountain National Forest, New Hampshire. Historical land-use maps were used to identify five areas each containing previously logged, burned, and relatively undisturbed (old-growth) forests. Aboveground biomass averaged 192 Mg/ha on the historically disturbed sites and 261 Mg/ha on the old-growth sites, and species dominance shifted from early- successional and mid-successional species (Betula papyrifera and Acer rubrum) to late-successional species (Fagus grandifolia and particularly A. saccharum). Forest floors in the old-growth stands had less organic matter and lower C:N ratios than those in historically burned or logged sites. Estimated net N mineralization did not vary by land-use history (113 kg·ha−1·yr−1); mean (± 1 se) nitrification rates at old-growth sites (63 ± 4.3 kg·ha−1·yr−1) doubled those at burned (34 ± 4.4 kg·ha−1·yr−1) and logged (29 ± 4.7 kg·ha−1·yr−1) sites. Across all plots, nitrification increased as forest floor C:N ratio decreased, and NO3− concentrations in streamwater increased with nitrification. These results indicate that forest N cycling is affected by century-old disturbances. The increased nitrification at the old-growth sites may have resulted from excess N accumulation relative to C accumulation in forest soils, due in part to low productivity of old-aged forests and chronic N deposition.