Effects of environmental conditions on the fixation and transfer of nitrogen from alfalfa to associated timothy

Abstract

Nitrogen fixation (NF) by alfalfa and nitrogen transfer (NT) from alfalfa to associated timothy was studied under different environmental conditions in controlled growth chambers, using the ~N dilution technique. Evidence was obtained of NT from alfalfa to the associated timothy. Conditions that favored NF by alfalfa resulted in an increase in its NT. Of 3 different temperature regimes (25/20, 16/14, and 12/9~ day/night), 16-25/14-20~ was the best range for NF by alfalfa and resulted in the greatest NT. High light intensity (550 uE.m -2.sec -1) and long days (16-20 h) also caused increased NF by alfalfa and benefitting timothy more than in a regime of low light intensity (by shading 50% or 75%) or short days (12/12 or 16/8 h day/night). When the inoculated (Rhizobium melilotO root systems of plants were kept free from other microorganisms (axenic condition) to minimize possible decomposition of dead tissues, lower NT from alfalfa was observed, especially at later cuts, compared to non-axenic plants. This suggests that both direct excretion and decomposition of dead alfalfa tissues are sources of N benefit from alfalfa to associated timothy.

Temperature effects on root nodule activity and nitrogen release in some sub-tropical and temperate legumes

“…This observation supports our previous finding which showed that treatments such as defoliation, pod removal and stem girdling which caused a decline in dinitrogen fixation in soybean did not change the amount of nitrogenous compounds exuded by the root systems except for stem girdling. As a result we could not confirm an earlier report (Ta and Faris 1988) indicating the presence of a direct relationship between the conditions which favor N fixation and N transfer in alfalfa. Further studies should be carried out to examine the relationship between dinitrogen fixation and the release of nitrogenous compounds from the root system of legumes.…”

15N estimates of nitrogen fixation by white clover (Trifolium repens L.) growing in a mixture with ryegrass (Lolium perenne L.)

“…Since the 15N content of the grass in the mixture is lower than that of the grass monoculture, it seems likely that the additional source of N is at, or near, natural abundance (0.366at.% and is derived from N fixed by the legume. This could involve direct transfer of N (Eaglesham, 1981;Haystead and Marriott, 1978;Ledgard et al, 1985;Ta and Faris, 1988), or indirect transfer via mineralisation of senescing clover roots and nodules (Haynes, 1986;Henzell, 1972;Walker, 1956). There is also evidence from 15N studies that N is released from the root-nodule system (Ruschel et al, 1979).…”

Growth characteristics and total N content of a Leucaena/Sorghum agroforestry system

“…This response period occurred in both experiments, even though Experiment 1 took place earlier in the growing season than Experiment 2. The increased accumulation of N in sorghum grown in mixed culture during this period may be associated with Leucaena exudation of fixed N and/or root and nodule decomposition as suggested by Ta and Faris (1988) for N transfer from alfalfa (Medicago sativa) to timothy (Phleum pratense).…”