Utilization of Fertilizer Nitrogen‐Nitrogen‐15 by Field‐grown Alfalfa

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 % 15 N, 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 15 N 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 15 N atom % in the alfalfa indicated a large effect of fertilizer on N uptake in the first two cuts only. The decrease in NH 4 ‐ 15 N 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.