A BIOASSAY OF THE AVAILABILITY OF RESIDUAL N-15 FERTILIZER 8 YEARS AFTER APPLICATION TO A FOREST SOIL IN INTERIOR BRITISH-COLUMBIA

Although a high proportion of fertilizer N may be immobilized in organ ic forms in the soil, no studies have examined the long-term availabil ity of residual fertilizer N-15 in forestry situations. We investigate d this by growing lodgepole pine (Pinus contorta) seedlings in surface (0-10 cm) soil sampled eight years after application of N-15-urea, (N H4NO3)-N-15 and (NH4NO3)-N-15, to lodgepole pine in interior British C olumbia. After nine months of growth in the greenhouse, seedlings took up an average of 8.5% of the N-15 and 4.6% of the native N per pot. M ost of the mineral N in the pots without seedlings was in the form of nitrate, while pots with seedlings had very low levels of mineral N. I n contrast to the greenhouse study, there was no significant uptake of N-15 by trees in the field study after the first growing season, alth ough half of the soil organic N-15 was lost between one and eight year s after fertilization. This indicates the need to understand the mecha nisms which limit the uptake of mineral N by trees in the field, and t he possible mismatch of tree demand and mineral N availability.