Fate of fertilizer nitrogen in a subarctic agricultural soil

Abstract

A nitrogen balance approach was taken to determine the fate of fertilizer nitrogen in a subarctic agricultural soil. Urea and calcium nitrate fertilizers were compared in a three-year spring barley recrop field study. Methods of N application included incorporating the N fertilizer into the soil during spring tillage versus broadcasting it on the soil surface after planting. $\sp{15}$N labeled urea was applied on one-meter square subplots within the main fertilizer plots. Nitrogen transformations and movement were monitored with ammonia volatilization traps, suction cup lysimeters, deep soil cores, plant tissue samples, and grain samples. Environmental data including precipitation, soil temperatures and soil moisture tensions were collected. Fertilizer N loss by ammonia volatilization was negligible, amounting to only a few grams N/ha/day. Rate of urea hydrolysis was rapid in the cool soil and was not considered to be a limiting factor affecting N availability to the crop. There appeared to be a little nitrate leaching during the growing season, but some may have occurred between cropping seasons. Only 16 percent of the fertilizer N could not be detected when the crop was physiologically mature, and that loss was accredited mostly to denitrification. Fertilizer N use efficiency, determined by the Difference Method, was 73 and 60 percent for calcium nitrate and urea, respectively. When the crop was physiologically mature, average fertilizer N recovery rates determined by the Isotope Dilution Method were: 40 percent in the plants, 43 percent immobilized in the soil, 1 percent available in the soil, and 16 percent unrecovered. Barley yields were not significantly affected by N source, but plants took up more N where nitrate had been applied. Position of N placement had little effect on either N loss or barley yield, but the surface application of N resulted in delayed barley maturity when spring rains were deficient.