The recovery of 15N in the aerial biomass of alfalfa–bahiagrass mixtures

Abstract

AbstractAlfalfa (Medicago sativa L.) overseeded into bahiagrass (Paspalum notatum Flueggé) has the potential to increase nutritive value and dry matter (DM) yield as well as extend the growing season in the southeastern United States. To maximize N2 fixation by alfalfa plants, alfalfa/grass mixtures are typically not fertilized with N. As a result, the grass component can lack in available N for uptake which can be detrimental to grass production. On the other hand, when N fertilizer is applied, the degree of fertilizer recovery by both species is not well understood. The objective of this study was to quantify competitive fertilizer 15N recovery between aerial biomass portions of alfalfa and bahiagrass when grown in a mixture. This was accomplished by using two rates of 15N labeled fertilizer (25 and 50 lb N acre−1) and two application timings (after second and third harvest). Fertilizer recovery was greatest for bahiagrass when application was delayed until after the third harvest (21%). Alfalfa recovered up to 66% of the applied N, when fertilizer was applied after the second harvest of the season while grass averaged <12%. Delaying fertilizer application limited alfalfa fertilizer recovery but did not necessarily increase grass recovery as expected. The percent 15N fertilizer fraction within the grass biomass was similar to that found in alfalfa biomass suggesting a greater reliance on applied fertilizer.