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

Alfalfa ( 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 N 2 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 15 N recovery between aerial biomass portions of alfalfa and bahiagrass when grown in a mixture. This was accomplished by using two rates of 15 N 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 15 N fertilizer fraction within the grass biomass was similar to that found in alfalfa biomass suggesting a greater reliance on applied fertilizer.