Investigations on the root nodule bacteria of leguminous plants: XX. Excretion of nitrogen in associated cultures of legumes and non-legumes

Abstract

With Plate X and Six Text-figures) ASSOCIATED cultures of legumes and non-legumes have been employed in ordinary farming practice since ancient times, as in such cultures the non-legumes have generally been found to develop better than when grown alone. However, the underlying reason for this beneficial effect has not been clearly understood. Lipman (1912) was probably the first worker to attempt an elucidation of this problem by direct experiments with pot cultures. His results, though in part conflicting, led him to the conclusion that inoculated legumes do somehow furnish nitrogenous food for the non-legumes. Lipman's findings were, however, not conclusive as proof for an excretion. Virtanen's (1929) first experiments with pot cultures showed that oats grew excellently in association with inoculated peas in N-free quartz sand. An excretion of nitrogen was therefore held to be probable. However, in ordinary pot cultures, where the action of various foreign micro-organisms is not excluded, the results may always be rendered valueless through the presence of free-living nitrogen-fixers which thrive particularly well in the rhizosphere. Beijerinck (1908) and others have shown long ago that bacterial life is indeed greatly enhanced in the immediate proximity of the roots of leguminous plants. Under such conditions the nitrogen taken up by the non-legumes is not necessarily derived from the legume nodules but might be fixed by the non-symbiotic organisms. The literature (up to 1934) on the derivation of the nitrogen in associated growth has been reviewed by Nicol (1934).The first conclusive proof for the excretion of considerable quantities of nitrogenous compounds from the nodules 1 of inoculated legumes was presented by Virtanen & v. Hausen (1931a). The work necessitated the use of "sterile" cultures containing only the desired nodule bacteria, 1 The excretion of the nitrogenous compounds has been definitely shown to occur from the nodules and not from the roots (Virtanen et al. 19366).

Transfer of fixed nitrogen from white clover to associated grasses in swards grazed by dairy cows, estimated using 15N methods

“…In grazed pastures, fixed N is transferred to grasses by an 'above ground' route via animal excreta and by 'below ground' transfer via either direct excretion of N compounds from legume roots (Ta et al, 1986;Virtanen et al, 1937) or decomposition of legume nodule, root and senescing root material (Butler et al, 1959).…”

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

“…In fact, the benefit of legumes to the growth of associated grass varies depending on both legume and grass species (Ta and Fails, 1987a). On the other hand, environmental variations, which are also known to affect NF (Feigenbaum and Mengel, 1979;Harding and Sheeley, 1980;Schweitzer and Harper, 1980;Wahua and Miller, 1978), N assimilation (Duke et al, 1979, Duke andDoehlert, 1981) and N excretion (Taet al, 1986;Virtanen et al, 1937) by legumes, would therefore also affect NT. Wyss and Wilson (1937) suggested that the relationship between photosynthesis and NF as affected by environmental conditions may be the dominant factor controlling N excretion.…”

Response of cereals to nitrogen in sole cropping and intercropping with different legumes

“…There has been some speculation, mostly based on laboratory experiments that legumes might benefit the associated cereals in intercropping by transfering part of the nitrogen fixed during the growing season (Rewari et al, 1972;Ruschel et al, 1979;Virtanen et al, 1937;Wilson and Wyss, 1937). However, very few field studies have actually demonstrated the direct benefit of legumes to cereals in intercropping.…”