No description available.

No description available.

No description available.

No description available.

No description available.

No description available.

No description available.

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.

No description available.

Low recovery rates of fertiliser N in tree biomass are frequently reported due to the inefficiency of N fertilisers in afforested ecosystems. At Culloden (North East Scotland), only 13&'37 of 15N-urea fertiliser applied to Sitka spruce could be recovered in the above-ground tree biomass two years after fertilisation. Fertiliser N not taken up by trees was largely 'locked-up' in stable organic forms of N within the LFH layers of the soil profile. 15N-labelled litter was used in both field and microcosm experiments, the release and fate of litter-derived-N (LDN) being traced over the course of two growing seasons. In both experiments, the microbial biomass acted as a major sink for LDN. Measurement of soil microbial biomass was calibrated for Culloden soil samples by determination of a kEN-factor. Tree uptake of LDN, in the field, occurred within one month of labelled-litter application, with the foliage being the largest sink for LDN. Approximately 30&'37 of the N within the labelled-litter layer was taken up by the trees over the course of two growing seasons and was equivalent to 5.4 kg LDN ha-1 y-1. There was considerable mixing of the LFH and peat layers in Sitka spruce microcosm soil profiles. This was probably due to elevated soil animal population densities. After 18 months, approximately 83&'37 of LDN had been redistributed to other N pools in the microcosm. Uptake of LDN by seedlings accounted for 15.7&'37 of LDN after 12 months, the largest sink being the foliage, equivalent to 6.16 kg LDN ha-1 y-1. Again, the microbial biomass was a major sink for LDN. Measurement of availability (NH4+) N in Culloden soil samples incubated at different matric potentials and temperatures, appeared not to reflect N mineralisation rates. There was a strong interaction between temperature and soil matric potential, seedling uptake of N being greatest at 15oC and -16.0 kPa. The rate of turnover of the microbial biomass pool was identified as the key determinant of the rate of processing of LDN in forest soils.