Utilization of Fertilizer Nitrogen-Nitrogen-15 by Field-grown Alfalfa
1980
Feigenbaum, Sala | Hadas, Aviva
Although legumes are usually not fertilized with N, response of irrigated alfalfa (Medicago sativa L.) to N fertilization has been observed frequently. More precise information is required, however, on the time and extent of the fertilizer-N utilization by alfalfa and its distribution in the soil. Uptake of ammonium fertilizer by alfalfa, its rate of nitrification and its recovery in a clay and sandy soil, were studied under field conditions using labeled ammonium sulfate as a fertilizer source. The fertilizer, enriched to 10.28 atom % ¹⁵N, was applied to microplots, with and without plants, in a commercial alfalfa field at a rate of 100 kgN/ha. Plant tops were analyzed for total N and soil samples for mineral N. Excess ¹⁵N content in plants and soil was also determined. Alfalfa recovered 40 to 45% of the applied fertilizer during the whole growth period on both soils, but in the first cut the recovery was higher in the sandy than in the clay soil. The ¹⁵N atom % in the alfalfa indicated a large effect of fertilizer on N uptake in the first two cuts only. The decrease in NH₄-¹⁵N recovery in the soil with time during the first 20 days after fertilization was exponential, indicating first-order kinetics of the nitrification process. The rate constant was 0.13 day⁻¹ and the half-life of ammonium in the field was 5 days, which is considerably faster than reported in the literature for laboratory incubation experiments at similar temperatures.
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