INSITU LABELING OF LEGUME RESIDUES WITH A FOLIAR APPLICATION OF A N-15-ENRICHED UREA SOLUTION

Conventional methods for labeling of legume residues with N-15 for the measurement of the transfer of nitrogen (N) to a succeeding crop are inadequate for zero-tillage management where the residue is chemically desiccated. The study tests the feasibility of in situ labeling of legumes with N-15 for the study of N transfer to a succeeding crop. A single foliar application of N-15-labeled urea was misted onto the stems and leaves of second cut red clover and alfalfa in the field in 0.75 x 0.75 m microplots. Nitrogen content and N-15 enrichment were determined for above- and below-ground plant parts. Overall, recovery of the foliar applied N-15 in the forage was 30% and 57% for the alfalfa and red clover, respectively. Of the nitrogen recovered, approximately 70% and 30% were recovered in the above- and below-ground plant parts, respectively, for both forage species. Atom % N-15 was similar for the red clover and alfalfa for each of the above-ground plant parts and was slightly lower in the crown and roots for the red clover than for the alfalfa. Values of atom % N-15 ranged from 0.5 for fine roots to 1.5 for leaves above 0.20 m from the ground. Labeling of the plant material was not uniform between plant parts but was adequate for many studies of N transfer to a succeeding crop.