TRANSFORMATIONS OF FALL-APPLIED NITROGEN-15-LABELED FERTILIZERS

Losses occurred during the fall (before the soil was frozen) and in the following spring (after soil thaw) and were primarily due to denitrification. Application of nitrapyrin in conjunction with urea delayed nitrification as well as downward movement of fertilizer N in the soil profile and was effective in decreasing the denitrification losses. Immobilization of fertilizer N was minimum under fallow conditions (7%) but ranged from 15 to 21% and 24 to 26% of the applied. Nitrapyrin did not affect the immobilization of fertilizer N or the amount of fertilizer N found in the microbial-biomass.