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

Abstract

Optimal 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.

Dinitrogen fixation and nitrogen transfer among red clover cultivars

“…These f,rndings are similar to those reported by Boller and Nrisberger (1987) and Brophy et al (1987 Brophy et al (1987) and Mallarino et al (1990) …”

“…This agrees with findings by Mallarino et al (1990). They reported that legume-dominant swards were required to maximize fixed-N yields for red clover mix- (1991) that showed a decline in N transfer with time, with the decline being related to a reduced legume proportion in the sward, a loss of root-plus-crown dry matter accumulation, and reduced growth of legume and grass components of the swards.…”

“…They reported that legume-dominant swards were required to maximize fixed-N yields for red clover mix- (1991) that showed a decline in N transfer with time, with the decline being related to a reduced legume proportion in the sward, a loss of root-plus-crown dry matter accumulation, and reduced growth of legume and grass components of the swards. This is contradictory, however, to findings by Brophy et al (1987) and Mallarino et al (1990). Brophy et al (1987) XTOT/MEAN = mean seasonal %Ngdfa or total seasonal TNY.…”

Nitrogen transfers from Vicia sativa L. and Trifolium resupinatum L. to the companion grass and the following crop

“…The N-NO 3 -amounts in the soil of the grass/legume swards are only slightly reduced compared to legumes in monoculture when the catch crops are sown late or during the second research year, when growth conditions are sufficient until late autumn. Under conditions that enable growth of grass and legume, the N-NO 3 -uptake of the grass causes a temporary decrease of N-NO 3 -amounts in soil, which causes an increase of N 2 fixation (= g N/ plant) of the legume (Miller et al 1982, Mallarino et al 1990a) because nitrate affects the number and size of nodules and the fixation in active nodules (Giller 2001). This is the reason for estimated high amounts of fixed N 2 even by mixed swards.…”

“…However, the positive mixture effect is more decisive for the legume component and the estimated N transfer from the legume to the companion grass is generally on a low level. The benefit to grass plants by the neighbourhood of legumes is more distinct in perennial swards (Mallarino et al 1990a, Opitz v. Boberfeld and Biskupek 1995, Laser 1999. Apparently, the growth period is too short to enable an interspecific transfer of higher N amounts in catch crop swards, where it is more likely that surplus N from N 2 fixation is used by the following crop.…”