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|Title:||Nitrogen fixation by three tropical forage legumes and the utilization of legume-fixed nitrogen by their associated grasses|
|Authors:||Whitney, A. S (Arthur Sheldon), 1933|
|Keywords:||Nitrogen -- Fixation|
|Abstract:||Three tropical legumes, Desmodium intortum, Desmodium canum and Centrosema pubescens, were grown alone and in combination with napier grass (Pennisetum purpureum) and pangola grass (Digitaria decumbens) in fresh volcanic cinders under continuously moist climate on the Island of Hawaii. D. intortum gave high yields of both dry matter (ca. 17,000 pounds per acre) and nitrogen (ca. 300 pounds per acre) in a 12-month period, and transferred small but significant amounts of nitrogen to its associated grasses. D. canum yields were low under these conditions, and the nitrogen yields of grasses associated with this legume were depressed. f. pubescens in pure stand was intermediate in yield of dry matter, but equalled D. intortum in nitrogen yield. However, when combined with grasses, the dry matter and N yields of this legume were reduced by one-half. Transfer of nitrogen to the grasses by C. pubescens was noted only when a 6-month growing period was allowed. The total fixation of nitrogen from the atmosphere during the test period averaged 340 pounds per acre for D. intortum, 82 pounds per acre for D. canum, and 156 pounds per acre for C. pubescens. Of the total nitrogen fixed by D. intortum, 5% or less was transferred to the associated grasses; but with C. pubescens, transfer amounted to 11% of the nitrogen fixed in one instance. Transfer due to the release of nitrogen from roots of these legumes was evaluated by circulating nutrient solution through the root systems of plants growing in cinders in the glasshouse. The roots equilibrated with only trace amounts of solution nitrogen, but marked increases in the levels of ammonium and amino nitrogen occurred immediately after defoliation. When the root systems of nitrogen-starved pangola plants were included in the perfusion systems, significant transfer of nitrogen occurred from the more vigorous legume plants', especially following defoliation. Of the nitrogen mobilized in the legume roots in the 3week period after defoliation, the proportions transferred ranged from slightly over 1% for D. canum to 9% for the more vigorous D. intortum plant. Transfer of nitrogen through the leaching of nitrogen from legume leaves was studied by shaking intact leaves of varying ages in distilled water. The amounts extracted were small, between 0.4% and 0.7% of the total leaf nitrogen. Extractable amino nitrogen tended to be relatively high in rapidly expanding leaves, yellowing leaves, and shaded leaves. Leaf fall accounted for significant nitrogen losses from D. intortum and C. pubescens in situations where leaf senescence equaled the rate of production of new leaves. Under these conditions, the dead leaves from these legumes supplied nitrogen equivalent to over 1.2 pounds per acre per week. This pathway could thus account for appreciable transfer if long growing periods were allowed. The combined action of these three pathways provides an adequate explanation for the nitrogen transfer observed in the field. A number of ways in which transfer by these means would be affected by management and by soil and weather conditions are discussed.|
Thesis (Ph. D.)--University of Hawaii, 1966.
Bibliography: leaves 109-117.
viii, 117 l mounted illus. (part col.), tables
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|Appears in Collections:||Ph.D. - Soil Science|
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