Uptake of soil nitrogen by legumes in mixed swards

Abstract

The partitioning of uptake of soil nitrogen between legumes and grasses in mown, mixed swards was studied at two sites in south-eastern Queensland. The swards contained either Lotononis bainesii, Desmodium intortum or Trifolium repens with Digitaria decumbens at one site, and either L. bainesii, T. repens, Macroptilium atropurpureum, Vigna luteola or Stylosanthes guyanensis with Chloris gayana at the other site. (¹⁵NH₄)₂SO₄ equivalent to c. 0.3 kg nitrogen ha^-1 was added every 4 weeks in an attempt to label the soil mineral nitrogen, and the partitioning of ¹⁵N uptake between species used as an estimate of the partitioning of uptake of soil nitrogen.

Although two of the legumes (L. bainesii and T. repens) obtained 70–100% of the total ¹⁵N uptake at some of the spring harvests, when growth of the associated tropical grasses was limited by low temperatures, none of the legumes obtained more than c. 25% of the annual ¹⁵N uptake over a 2-year period. Proportional ¹⁵N uptake by the legumes was curvilinearly related to the proportion of legume dry matter yield in the mixtures. Annual ¹⁵N uptake by the grass-legume mixtures was up to 360% as high as by the grass control. This had only a small effect on estimates of the proportion of legume nitrogen derived from symbiotic fixation, but caused a large discrepancy in isotopic estimates of nitrogen transfer. The proportion of fixed nitrogen in the legumes averaged 94% at one site and 92% at the other, nearly always exceeded 80%, and was independent of legume yield.