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Algal nitrogen fixation on solid surfaces and temperate agricultural soilsThis thesis examines various factors controlling algal nitrogen fixation and nitrogen release in temperate agricultural soils. New apparatus for investigating nitrogenase activity on uniform soil grown algal crusts and for the in.situ estimation of nitrogen fixation in the field is described. The effects of oxygen concentration and temperature on acetylene reduction by soil grown cultures of Nostoc ellipsosporum in the light and in the dark suggest that ATP generated by oxidative phosphorylation contributes to fixation even in the light. The oxygen concentration giving optimum nitrogenase activity depends on the intensity of illumination, largely because of the continuing contribution from dark fixation. The oxygen dependent acetylene reduction continued throughout the night in the field at 20% of the midday rate. This proportion is dependent on daytime light intensity and day/night temperature drop. The release of nitrogenous compounds by soil and sand grown cultures of N .ellipsosporum was investigated in the laboratory. Algal cultures growing on sand released only 20/0. of the total nitrogen compared with 10% released by liquid cultures. Freezing to below -30C, drying to less than 4% moisture or pathogenic infection all cause extensive cellular lysis with a concomitant loss of nitrogenase activity, followed, after several days, by a surge in ammonia concentration. The recovery of nitrogenase activity after re-wetting was investigated in the field after a period of dry weather. All samples achieved a steady rate of nitrogenase activity after 400 minutes irrespective of the magnitude of recovery. Nitrogenase activity in the field was monitored at weekly intervals using an in situ technique over a two year period on Broadbank, one of the Rothamsted classic plots sown to winter wheat. The seasonal fixation rates varied from 1.4 to 28 Kg/ha depending on the fertilizer and herbicide treatment. The greatest seasonal fixation was given by non-herbicide plots receiving 48 Kg N/ha of applied nitrogen. The effect of various species of algal inoculum was investigated on 60 plots sown to winter wheat with and without irrigation and added nitrogen. A significant increase in fixation was produced and the most effective treatment was a liquid application of N. ellipsosporum. Both the N. punctiforme and Anabaena cylindrica were more effective when applied as dried sand cultures. The most important single factor limiting algal development in field experiments was soil moisture.