THE FATE OF APPLIED NITROGEN IN A FRASER VALLEY SOIL USING 15N IN FIELD MICROPLOTS

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Abstract
A tracer (15N) study using fallowed field microplots was conducted at Agassiz Research Station to examine the fate of applied N over an entire year. The tracer confirmed nontracer (difference between fertilized and control treatments) observations that applied N does not leach beyond the rooting zone (45 cm) during the growing season, despite the considerably more than average precipitation that occurred in July, but that all residual NO3-N is leached over the winter. The tracer did, however, show that net immobilization of applied N occurred late in the fall resulting in 17% of the N recovered in the 75-cm profile 1 yr after application even though the nontracer method showed that none of the applied N remained. There was significant net mineralization of soil N over the summer (100 kg N ha-1 from early May to late August) and nitrification of the applied NH4+-N (120 kg ha-1) was essentially complete within 14 d of application. Tracer analyses suggested that 36% of the applied NH4+-N was immediately fixed by the clays but after 14 d in the field in decreased to less than 1%. The fixed NH4+-N remained at this level throughout the rest of the year. The apparent large decrease in fixed NH4+-N within the first 14 d may have been an analytical artifact which resulted when the initial soil was air dried. Negligible denitrification was observed during the growing season despite the soil remaining quite moist throughout most of the year. Delta 15N measurements of total N, fixed NH4+-N and extractable inorganic N fractions showed only enrichment of total N. The delta 15N results support the observation that denitrification tends to be low under Agassiz soil and weather conditions. Comparisons and contrasts to previously reported similar tracer studies in Ottawa were made.
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