VARIATION IN N-15 ENRICHMENT OF CRIMSON CLOVER LABELED UNDER FIELD CONDITIONS

Plant material labeled with N-15 is often used to determine recovery of N from green manure crops by subsequent crops. In this study, N-15 enriched crimson clover (Trifolium incarnatum L.) was grown at a field site where it was to be utilized in a subsequent experiment. A foliar spray of (NH4)2SO4 (99 atom % excess N-15) was applied to a 1.2 m x 8.8 m plot of crimson clover at a rate of 10 kg N ha-1 in early March 1990, immediately prior to the period of rapid vegetative growth. Clover shoots harvested in April contained 1.72 atom % excess N-15. Total N concentration of enriched clover was similar to that in adjacent untreated clover. Clover shoots contained 20% of the applied N-15, and an additional 27% was recovered from the surface soil horizon (0 to 15 cm). A gradient was observed across the plot, with clover enrichment increasing from 1.3 to 2.2 atom % excess N-15. Recovery of applied N-15 in soil was highest in the subplots with lowest clover enrichment. Variability in N-15 enrichment was also observed among plant parts: leaves from the basal half of shoots had 2.2 atom % excess N-15; while leaves from the terminal half of shoots, terminal stems, and basal stems had between 1.1 and 1.4 atom % excess N-15.