EFFECT OF VARIOUS PERIODS OF SEED-DOWN TO ALFALFA AND BROMEGRASS ON SOIL NITROGEN

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Abstract
Total soil nitrogen decreased substantially during the 12-year period (1954 to 1965) in a coarse-textured Chernozemic soil located at Brandon, Manitoba. The amount of nitrogen loss was related to cropping practices. The loss was 24% when the soils were continuously summerfallowed and 16% when the soil was producing alfalfa, during eight of the 12 years. Other perennial crops (bromegrass, and a mixture of bromegrass and alfalfa) had similar effects on total nitrogen to those of alfalfa. The loss of soil nitrogen was inversely proportional to the intensity of cropping during the 12-year period. The availability of soil nitrogen, as assessed for wheat production during the four-year period 1966 to 1969, was directly proportional to the intensity of cropping during 1954 to 1965, and was increased by alfalfa production. The total nitrogen absorbed by three crops of wheat was closely related to the amount of nitrate nitrogen in the soil to a depth of 122 cm at the beginning of the three years of production. It was not related to total soil nitrogen. The protein content of the grain was related to intensity of cropping and alfalfa production. The change in the amount of nitrogen in the amino acid, hexosamine, ammonia, and unidentified fractions of an acid hydrolysate of these soils was proportional to the change in total soil nitrogen. It did not appear to be related to changes in available nitrogen as measured by soil nitrate, plant nitrogen absorption or nitrate released on aerobic incubation.