IMMOBILIZATION AND AVAILABILITY OF N AND P IN THE FOREST FLOORS OF FERTILIZED ROCKY-MOUNTAIN CONIFEROUS FORESTS

The rates of key processes related to the recycling of nutrients in the forest floor were monitored for four years following the application of ammonium phosphate sulphate to small plots in mature stands of lodgepole pine, white spruce, and Engelmann spruce - subalpine fir, and a small clearcut. Net mineralization of nitrogen and phosphorus were initially enhanced by fertilization, but microbial activity and biomass were unchanged. Nitrification was increased on fertilized plots on the clearcut immediately after fertilizer application, and in the mature forests the following year. No changes were detected in the mass or quality of the overstorey litter, but significantly larger amounts of N and P were returned in understorey litter. Decomposition and nutrient release from Epilobium angustifolium leaves were more rapid in leaves either harvested from, or decomposed on, fertilized plots. Four years after application, mineralizable N in the forest floor was higher on fertilized plots at the most N-rich sites only, highlighting the importance of the nutrient immobilization capacity of the site in determining the duration of enhanced nutrient availability following fertilization.