PATHWAYS OF NITROGEN TRANSFER IN SOME TROPICAL LEGUMEGRASS ASSOCIATIONS

Three ways by which legumes release N for possible transfer to an associated grass were evaluated in greenhouse experiments utilizing Desmodium intortum (Mill.) Urb., D. canum (Gmel.) Schintz and Thellung, and Centrosema pubescens Benth. Legume root systems, growing in cinders and perfused with N-free solutions, normally released only trace amounts of N to the solutions. However, immediatedly after the plants were cut back marked increases in the release of ammonium and amino nitrogen were observed. When the root systems of N-starved grass plants (Digitaria decumbens Stent) were perfused in series with the legume roots, significant nitrogen transfer occurred from the more vigorous legume plants, especially after cutting back the legumes. Release of soluble N from legume leaves due to leaching was studied by shaking excised leaves of varying ages in distilled water. The amounts of soluble N extracted were always small, but rapidly expanding leaves, yellowing leaves, and shaded leaves contained much higher levels of extractable amino N than did other classes of leaves. Leaf fall accounted for significant N losses from D. intortum and C. pubescens plants allowed to shed mature leaves over a period of time, thus providing a 3rd source of transferable N. The combined effects of root losses, leaching of leaves, and leaf fall provide an adequate explanation for the N transfer observed in an earlier small plot experiment.