Nitrogen-15 labeling effectiveness of two tropical legumes

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 when N-15 is soil applied. Consequently, foliar-labeled plant tissues should be better suited for subsequent N-15 mineralization studies. A field experiment was conducted to determine the effectiveness of N-15-labeling and the accumulation of N-15 in various plant parts of two tropical legumes. Desmodium ovalifolium Guillemin and Perrottet and Pueraria phaseoloides (Roxb.) Benth., grown in 0.5 m(2) microplots, were labeled with foliar-applied urea containing 99 atom% N-15. Plants in each microplot received a total of 0.1698 g N-15 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 N-15 content. Averaged over both legumes and foliar application rates, total plant (shoots, flowers, leaf litter, and roots) recovery was approximately 79% of the N-15 applied. The soil contained 3% of the N-15 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, N-15 recovery by Desmodium shoots was greater than Pueraria. Results demonstrate chat N-15 foliar application to legumes is an effective method for labeling, resulting in atom% excess N-15 levels and N-15 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.