Nitrogen Transformations and Transport Over 17 Months in Field Fallow Microplots Using ¹⁵n

A field experiment with an Ottawa area clay loam soil utilizing open-ended microplots and 15 N-labelled fertilizer showed the relative importance of seasons on transformation and transport of nitrogen. Denitrification appeared to be appreciable during the growing season; about 39% of the fertilizer N was denitrified in 86 days (May–Sept.) and 65% was lost after 511 days but leaching losses were included in the latter period. Nitrification of fertilizer N was very rapid with extractable NH 4 + -N approximating background level within the first 43 days. Immobilization of fertilizer N was negligible in the first 159 days and only a small amount was immobilized during the remainder of the experiment. Mineralization of soil N averaged 0.77 and 1.10 kg N/ha/day in the first two sampling periods. Clay fixation of NH 4 + -N was significant in this soil with 59% of the 152 kg N/ha applied being immediately fixed. Over one-half (66%) of this recently fixed NH 4 + -N was released in the first 86 days of the experiment with the remainder held tightly through the sampling period. Movement of fertilizer N was greatest in the late fall and early spring, i.e. periods of high precipitation and low evaporation.