In situlabeling of legume residues with a foliar application of a15N‐enriched urea solution

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Abstract
Conventional methods for labeling of legume residues with 15N 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 15N for the study of N transfer to a succeeding crop. A single foliar application of 15N‐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 15N enrichment were determined for above‐ and below‐ground plant parts. Overall, recovery of the foliar applied 15N 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 % 15N 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 % 15N 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.