Nitrogen-15 labeling effectiveness of two tropical legumes

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Nitrogen-15 foliar applications for the production of field-labeled plant tissues may achieve more effective labeling of plant shoot and root tissues and minimize directly labeling the soil N fraction as occurs when15 N is soil applied. Consequently, foliar-labeled plant tissues should be better suited for subsequent 15N mineralization studies. A field experiment was conducted to determine the effectiveness of 15N-labeling and the accumulation of 15N in various plant parts of two tropical legumes. Desmodium ovalifolium Guillemin and Perrottet and Pueraria phaseoloides (Roxb.) Benth., grown in 0.5 m2 microplots, were labeled with foliar-applied urea containing 99 atom% 15N. Plants in each microplot received a total of 0.1698 g 15N that was applied all at once or split equally into two, three or four applications. Legume shoots and roots and soil were destructively harvested and analyzed for total 15N content. Averaged over both legumes and foliar application rates, total plant (shoots, flowers, leaf litter, and roots) recovery was approximately 79% of the 15N applied. The soil contained 3% of the 15N applied, of which 2.5 and 0.5% were in the inorganic and organic fractions, respectively. Nitrogen-15 recovery in shoots (76%) was sixty-five fold greater than in roots (1%) and about nineteen fold greater than the sum of roots and soil (4.1%), a much greater percent recovery than observed in other foliar labeling studies. Averaged over all four foliar split-application rates, 15N recovery by Desmodium shoots was greater than Pueraria. Results demonstrate that 15N foliar application to legumes is an effective method for labeling, resulting in atom% excess 15N levels and 15N recoveries comparable to those reported with the more traditional soil-labeling approach. Another advantage of this method is a nondestructive, in situ labeling method that permits separation of shoot and root residual N contribution to subsequent crops in N tracer studies.