Plant and Soil 128: 293-297, 1990. t~) 1990 KluwerAcademic Publishers. Printedin the Netherlands. PLSO 8084 Nitrogen transfer from arrowleaf clover to ryegrass in field plantings D.R. MORRIS ~, R.W. WEAVER 2, G.R. SMITH 3 and F.M. ROUQUETrE 3 aSoutheast Research Station, Louisiana State University, Agricultural Center, P.O. Drawer 567, Franklinton, LA 70438, USA, 2Soil and Crop Science Department, Texas A & M University, College Station, TX77843, USA, 3Texas A & M University, Agricultural Research and Extension Center, Drawer E, Overton, TX 75684, USA Received 27 December 1988. Revised May 1990. Key words: arrowleaf clover, labelled N, Loliurn multiflorum, ~5N depleted ammonium nitrate, N transfer, ryegrass, Trifolium vesiculosum Abstract Arrowleaf clover (Trifolium vesiculosum Savi) and annual ryegrass Lolium multiflorum Lam.) common- ly are overseeded in dormant bermudagrass (Cynodon dactylon L. Pers.) sod on coastal plain soils in the southeastern United States. Two field experiments were conducted in consecutive years at different sites to estimate the amount of N transferred from the clover to the annual grass. Nitrogen treatments included 50 kgNha -1 as 15N depleted ammonium nitrate applied in either February or April, and a check (no N applied). Three clippings were made during the cool-season from March to June. In both experiments, less than 5 kg N ha- were transferred from the clover to the grass. Ryegrass yields of dry matter and total N were not increased by growing with clover. Clover growth was typical for the region; average dry matter yield in pure stand was 2,615 kg ha -1 over the two-year period. Clover in mixed stand fixed between 20 and 60 kg N/ha. Less than 13% of N contained in ryegrass was transferred from arrowleaf clover to ryegrass at any clipping while clover was actively growing. The quantity of N transferred over the entire season was not statistically significant. Introduction Overseeding winter annual legumes and grasses in perennial sod is an important management practice in the southeastern United States espe- cially on coastal plain soils. These soils are rela- tively infertile and occupy a large percentage of the area in the southeastern United States. Fer- tilizer N inputs can be reduced by growing clover/grass mixtures because clover receives its N from biological N fixation and may transfer a portion of the fixed N to the companion grass. The transfer of N would enhance the yield of the grass. The preferred method used for determining N transfer under field conditions is by isotope dilu- tion. In isotope dilution the mineral N in soil is enriched or depleted with 15N by adding fertilizer N. Because the nitrogen fixed by the legume is close to natural abundance values of 15N, trans- fer of this N to a companion grass makes the 15N content of the grass closer to the natural abund- ance value. Isotope dilution is more sensitive for detecting N transfer than total N difference be- cause it is yield independent (Labandera et al., 1988). Also, N transfer can be measured in the presence of fertilizer N applications (Boiler and Nosberger, 1988). A complete description of the theory may be obtained by reading the publica- tions of Broadbent et al. (1982) and Hauck and Bremner (1976). When white clover (Trifolium repens L.) and red clover (Trifolium pratense L.) were grown with perennial and annual ryegrass (Lolium