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Competition for Nitrogen-15 Depleted Ammonium Nitrate and Nitrogen Fixation in Arrowleaf Clover-Gulf Ryegrass Mixtures¹

ABSTRACT

‘Yuchi’ arrowleaf clover (Trifolium vesiculosum Savi) and ‘gulf’ ryegrass (Lolium multiflorum Lam) are commonly grown together but their competitive relationship for fertilizer N has not been determined. Two greenhouse experiments were conducted to examine this relationship and to determine the effects on biological N₂ fixation. Seeds were sown in a Norwood soil (fine-silty, mixed [calcareus], thermic Typic Utifluvent) that was mixed with vermiculite (Exp. I) or sand (Exp. II). Nitrogen was applied at different rates during the seedling stage to plants sown in pure and mixed stands. In Exp. I, plants were grown for 8 weeks. In Exp. II, plants were grown for 20 weeks and three clippings were taken. The N source was 99% ¹⁵N depleted NH₄NO₃. Ryegrass in mixed stand accumulated several times more fertilizer N than its clover component. But clover in pure stand accumulated similar quantities of fertilizer N as the ryegrass in pure stand. The percentage of total N₂ fixed in the clover was usually decreased with increasing fertilizer N rates due to increased fertilizer N uptake. Total N in the clover from N₂ fixation was often not significantly altered with increasing fertilizer N rates which indicates that fixed N was complimenting the N taken up from the fertilizer.

¹ Contribution from the Soil and Crop Sciences Dep., Texas A&M Univ., College Station, TX 77843. This research was supported in part from Hatch funds (H-3701) and a Tom Slick Fellowship provided by Texas A&M Univ.

² Graduate Research Associate and Professor at Texas A&M Univ. Senior author is currently Assistant Professor, Southeast Res. Stn., Franklinton, LA 70438.

Received for publication December 19, 1985.