NITROGEN-15 LABELING EFFECTIVENESS OF TWO TROPICAL LEGUMES

Authors: GLASENER, KARL - NORTH CAROLINA STATE UNIV; WAGGER, MICHAEL - NORTH CAROLINA STATE UNIV; MACKOWN, CHARLES; VOLK, RICHARD - NORTH CAROLINA STATE UNIV

Submitted to: Plant and Soil
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/1/1998
Publication Date: N/A
Citation: N/A

Interpretive Summary: Legume cover crops are often used to reduce erosion, control weeds, and provide a source of nitrogen (N) for subsequent crops. Most studies of the N contribution of legumes in crop rotations usually consider only the contribution of the aboveground legume biomass and often only indirectly estimate N derived from the decomposing shoots, which is masked by other benefits of the legume cover crop on soil derived N. This study describes a technique to label legumes with N-15 isotope in the field for making direct measurements without disturbing the natural soil system the N contribution of decomposing legume roots and shoots to a subsequent crop. About 82% of a foliar spray of urea enriched with N-15 was recovered from the shoots, roots, and soil of field grown legumes. Most of the N-15 recovered was present in the shoots (95%) while only 1.3% of the N-15 recovered was in the root. The soil organic N pool contained more of the recovered N-15 (3.1%) than the legume roots. The resulting N-15 enrichments of the plant parts were sufficient to allow tracing the fate of N release from the legumes and accumulation of the N by a subsequent crop. This technique will give researchers a direct approach to assess the N contribution of legume cover crops in agricultural systems. The information from this study will be useful to agronomists and soil scientists in devising studies to delineate the benefits and limitations of legume cover crops used as a N source in crop rotations.

Technical Abstract: A field experiment was conducted to determine the effectiveness of 15N-labe and the accumulation of 15N in various plant parts of two tropical legumes. Desmodium ovalifolium and Perrottet and Pueraria phoseoloides (Roxb.) Benth., grown in 0.5 m2 microplots, were labeled with foliar-applie a 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 greate 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.