Legume Species and Proportion Effects on Symbiotic Dinitrogen Fixation in Legume‐Grass Mixtures

Abstract

AbstractOptimal utilization of the N2‐fixation capability of legumes improves forage productivity and quality and reduces the need for N fertilizer. This research assessed N2 fixation in binary mixtures of white clover (Trifolium repens L.) (WC), red clover (Trifolium pratense L.) (RC), or bitdsfoot trefoil (Lotus corniculatus L.) (BT) with tall fescue (Festuca arundinacea Schreb.) (TF) at four legume proportions. Two identical experiments were established in Uruguay, one in 1983 (Exp. 1) and one in 1984 (Exp. 2) that were evaluated for 2 yr. The soil of the area was a fine, montmorillonitic, mesic, Typic Argiudoll. Symbiotically fixed N was estimated by 15N isotope dilution by using TF pure stands as nonfixing reference. The proportion of legume N derived from air (%Ndfa) was larger in winter and spring harvests (82‐95%) than in other harvests, and differences among legumes were minimal. In first harvests after seeding, %Ndfa was 58% for BT and 74% for WC or RC, whereas in summer, WC showed the least %Ndfa. Total fixed‐N yield over 2 yr was greatest for RC‐TF (390 and 330 kg ha−1 for Exp. 1 and Exp. 2) because of both large %Ndfa and legume yield. As legume proportion increased %Ndfa decreased linearly, usually with similar slopes for all legumes. Fixed‐N yield increased linearly as legume proportion increased for RC‐TF and BT‐TF mixtures. For WC‐TF, there were optimal WC proportions between 50 and 70% in seeding years. We conclude that %Ndfa was negatively and linearly related to legume proportion for the three mixtures and that legume‐dominant swards were required to maximize fixed N yields for RC‐TF and BT‐TF but not for WC‐TF. Red clover fixed the greatest amount of N2 under the conditions of this study.