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.

Melilotus officinalis (yellow sweetclover) causes large changes in community and ecosystem processes in both the presence and absence of a cover crop

“…This was somewhat surprising, since legumes harbor N-fixing bacteria. Melilotus officinalis likely competed most strongly for nitrogen in the early stages of its development when legumes primarily take up unfixed soil nitrogen (Vallis et al 1977) and before root nodules have developed (Wolf and Rohrs 2001). Therefore, this non-native legume competed for all the resources that we measured, even nitrogen.…”

Effect of high rates of nitrogen fertiliser on white clover growth, morphology, and nitrogen fixation activity in grazed dairy pasture in northern New Zealand

“…However, in a mixed pasture the grass was usually more efficient at taking up fertiliser N from the soil than legumes. For example, Vallis et al (1977) showed that white clover in mixed pastures with a selection of tropical grass species rarely obtained more than 25% of the available soil N whereas Murphy & Ball ( 1985) found ryegrass accumulated more than twice the amount of nitrate as clover even when both were grown in monoculture. The more efficient use of fertiliser N by ryegrass compared with clover leads to increased ryegrass growth.…”

Symbiotically fixed nitrogen from field- grown white and red clover mixed with ryegrasses at low levels of15N-fertilization

“…The validity of this procedure requires that a direct N transfer does not occur during the test (McAuliffe et al, 1958). Available data from a similar experiment (Broadbent et al, 1982) suggest that, in a field situation, direct N transfer is of little importance as compared to indirect transfer, which is of no consequence (Vallis et al, 1977).…”