Nitrogen fixation of blue-green algae associated with Avicennia marina (Forssk.) Vierh. pneumatophores and wet and dry surface sediments were investigated in the Mgeni Estuary mangrove swamp by means of the acetylene reduction technique. Optimum partial pressures of acetylene ranged from 0,15 to 0,2 atm. for the different habitats. A lag phase of 3 h was observed in all habitats, followed by a period of linear ethylene production of 42 h for the pneumatophores and 72 h for the wet and dry mat areas. An assay period of 24 h was employed in all habitats. Laboratory studies revealed percentage moisture and temperature to be the prime factors influencing ARA (acetylene reduction activity) in all habitats and rates were highest under submerged conditions and at 22˚C. Short-term variations in ARA and salinity in the wet and dry mat areas, measured at 6 day intervals, were also related to percentage moisture. High concentrations of inorganic nitrogen (between 1 and 5 mg 1-1) significantly depressed ARA in all habitats. Increases in ARA occurred with increase in light intensity up to 40 μE m -2 s-1, with negligible dark rates being recorded in the wet and dry mat areas. Significant dark rates of ARA and stimulation of ARA by sucrose in association with the pneumatophores indicated that bacteria may also be contributing to ARA in this habitat. No organic carbon stimulation was noted in the other sites. Salinity had little effect on ARA over the range generally experienced in each habitat. Field studies revealed a marked seasonal variation in ARA, with summer maxima of 78, 678 and 341 nmol C2H4 cm-2 24 h-1 associated with the pneumatophore, wet and dry mat areas respectively. This coincided with maximum nitrogen-fixing blue-green algal numbers, temperature, light intensity and day length. No seasonal variations in organic carbon, inorganic nitrogen, salinity, percentage moisture or bacterial numbers were apparent. Rates of bacterial ARA associated with decomposing litter of A. marina were highest under exposed conditions and reached a maximum of 25 935 nmol C2H4 g dry wt -1 24 h -1 after 3 weeks. Maximum rates of ARA under submerged conditions of 5394 nmol C2H4 g dry wt -1 24 h -1were reached after 4 weeks of decomposition. An increase in percentage nitrogen occurred during decomposition and was greatest under submerged conditions. Rates of decomposition were highest under exposed conditions. It was estimated that nitrogen fixation by blue-green algal communities supplies 23,8% of the annual nitrogen requirements of the mangrove swamp. / Thesis (M.Sc.)-University of Durban-Westville, 1987.

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LD2668 .T4 1949 P61 / Master of Science

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Bibliography: leaves 170-191.

Bibliography: leaves 151-177.

Abstract Few studies have assessed the levels of symbiotic N nutrition in legumes grown by fanners in Africa. In this study, the shoots of cowpea plants were sampled from 63 farms in 12 villages within 5 districts of the Upper West Region of Ghana, and assessed for growth and symbiotic N nutrition. The data revealed considerable differences in cowpea plant density per m2 , plant growth, 15N natural abundance (OI5N), %Ndfa, and N-fixed among different farms under one village, and between villages under the same district, and between districts in the Upper West Region. In farms where there were fewer cowpea plants per m2 , plant growth was better and dry matter yield per plant significantly greater, leading to strong variations in O!5N values. Except for four farms at Bamahu which had cowpea shoot Ndfa values of 12.1%,30.0%,36.5% and 46.6%, one farm at Babile with Ndfa value of 58.1%, and three farms at Silbelle with Ndfa values of56.8%, 57.9% and 68.7%, the remaining 55 out of the 63 farms studied showed high shoot Ndfa values, ranging from 70.6% to 99.7%, which clearly indicates that cowpea cultivated by farmers in the Upper West Region of Ghana meet a large proportion of their N requirements from symbiotic fixation. At the district level, isotopic analysis showed that, on average, the 15N natural abundance values (%0) of cowpea shoots were -0.496±0.04 for Jirapa, -O.083±0.06 for Nadowli, 0.368±O.08 for Lawra, l.333±0.29 for Wa and 0.365±0.09 for Sissala district. Estimates of the legume's N derived from fixation were 66.3% for Wa district, 89.9% for Nadowli, 79.4% for Lawra, 78.9% for Sissala and 80.9% for Jirapa district. The amount ofN-fixed ranged from 402.3 mg.plant' for Nadowli, 176.5 mg.plant" for Wa, 235.4 mg.plant' for Sissala, 179.0 rng.plant' for Lawra to 249.2 mg.plane! for the Jirapa district. Expressed on per-hectare basis using cowpea density per m2 , the total amount ofN-fixed was around 16.6 kg ha-! in the Nadowli district, 19.1 kg ha-! in Wa, 23.0 kg ha' in Sissala, 21.1 kg ha-I in Lawra and 17.6 kg ha-1in the Jirapa district. Averaged across all 5 districts, N-fixed by cowpea was about 19.5 kg ha-! in the Upper West Region of Ghana. These data suggest that, increasing N2 fixation in fanners' fields in Ghana would require optimization of cowpea plant density rather than biological manipulation of the symbiotic process (as %Ndfa values were generally very high).

Abstract Few studies have assessed the levels of symbiotic N nutrition in legumes grown by fanners in Africa. In this study, the shoots of cowpea plants were sampled from 63 farms in 12 villages within 5 districts of the Upper West Region of Ghana, and assessed for growth and symbiotic N nutrition. The data revealed considerable differences in cowpea plant density per m2 , plant growth, 15N natural abundance (OI5N), %Ndfa, and N-fixed among different farms under one village, and between villages under the same district, and between districts in the Upper West Region. In farms where there were fewer cowpea plants per m2 , plant growth was better and dry matter yield per plant significantly greater, leading to strong variations in O!5N values. Except for four farms at Bamahu which had cowpea shoot Ndfa values of 12.1%,30.0%,36.5% and 46.6%, one farm at Babile with Ndfa value of 58.1%, and three farms at Silbelle with Ndfa values of56.8%, 57.9% and 68.7%, the remaining 55 out of the 63 farms studied showed high shoot Ndfa values, ranging from 70.6% to 99.7%, which clearly indicates that cowpea cultivated by farmers in the Upper West Region of Ghana meet a large proportion of their N requirements from symbiotic fixation. At the district level, isotopic analysis showed that, on average, the 15N natural abundance values (%0) of cowpea shoots were -0.496±0.04 for Jirapa, -O.083±0.06 for Nadowli, 0.368±O.08 for Lawra, l.333±0.29 for Wa and 0.365±0.09 for Sissala district. Estimates of the legume's N derived from fixation were 66.3% for Wa district, 89.9% for Nadowli, 79.4% for Lawra, 78.9% for Sissala and 80.9% for Jirapa district. The amount ofN-fixed ranged from 402.3 mg.plant' for Nadowli, 176.5 mg.plant" for Wa, 235.4 mg.plant' for Sissala, 179.0 rng.plant' for Lawra to 249.2 mg.plane! for the Jirapa district. Expressed on per-hectare basis using cowpea density per m2 , the total amount ofN-fixed was around 16.6 kg ha-! in the Nadowli district, 19.1 kg ha-! in Wa, 23.0 kg ha' in Sissala, 21.1 kg ha-I in Lawra and 17.6 kg ha-1in the Jirapa district. Averaged across all 5 districts, N-fixed by cowpea was about 19.5 kg ha-! in the Upper West Region of Ghana. These data suggest that, increasing N2 fixation in fanners' fields in Ghana would require optimization of cowpea plant density rather than biological manipulation of the symbiotic process (as %Ndfa values were generally very high).