3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Slash burning and the effects on nutrient dynamics and soil faunal compostition in an Eucalyptus grandis plantation in South Africa(2006-11-14T11:20:00Z) Nadel, Ryan LeslieSustainability of commercial plantations is dependent on the conservation of soil nutrients, especially on the ancient, leached soils of the tropics. This is particularly important when limited fertilization is practiced. To assess the sustainability of commercial plantations, various sustainability indicators have been suggested. One such indicator is that of biodiversity. The measurement of soil faunal diversity is important as soil fauna play an active role in the cycling of nutrients through the decomposition of plant residues and organic matter. The diversity and different feeding activities of soil fauna influence the rate at which nutrient cycling occurs either through microbial grazing, faecal deposition, the mixing of litter with the mineral soil as well as through the spreading of microbial inoculum. Management practices may, however, negatively influence nutrient availability through losses associated with the burning and removal of slash residues, thus changing the soil faunal community composition. In Eucalyptus grandis plantations in South Africa, the site is usually burned following the harvest and prior to replanting, which may markedly disrupt soil faunal function and nutrient availability. Soil biological processes, nutrient loss and soil faunal composition were the focus of this study. Nutrient losses associated with fire and slash management practices were determined by comparing the quantity of nutrients lost from the slash that was either spread or piled following burning. The in situ nitrogen and phosphorus mineralization rates and soil faunal composition were measured in six randomly located plots, three burned and three unburned, prior to and after a low intensity fire. Results indicate that 78 – 99% of phosphorus, nitrogen and carbon were lost from the slash as a result of burning, with a greater percentage loss occurring from the slash that was spread. It was further shown that within the burned plots the nitrogen availability was enhanced four fold within the first month following the fire. Phosphorus availability was also enhanced ten fold as a result of the burning, however, these effects on nitrogen and phosphorus availability were short lived. Soil faunal diversity was low both prior to and after burning. Ants are the dominant soil fauna. Soil faunal composition was more markedly influenced by season than by fire, with millipedes occurring in all plots in spring, prior to the fire and ants dominating in summer, post fire.Item The nutrient status of grasslands and adjacent Pinus patula and Eucalyptus grandis plantations on the eastern escarpment of South Africa(2006-03-13) Kathawaroo, DeshikaThe replacement of indigenous grasslands by commercial forestry plantations on the mountainous eastern escarpment of South Africa, has inevitably led to alteration of nutrient cycling. The overall objective of this study was to determine the nutrient status (soil, litter and foliar nutrient concentrations) of Pinus patula and Eucalyptus grandis plantations and adjacent grasslands. The 2, 13 and 30 year old P. patula plantations and adjacent Driekop grassland were located relatively high up on the escarpment (altitudes ranging between 950–1610 m) and were underlain by dolomite while the 2, 8 and 13 year old E. grandis plantations and adjacent Brooklands grassland were located lower down (altitudes ranging between 900–1100 m) and were underlain by granite. It was assumed that the nutrient status of the soil under the plantations and the adjacent grasslands were similar prior to afforestation. Samples were collected once in winter (August 2002) and once in summer (January 2003). Soil samples were collected to a depth of 20 cm and litter material, to a maximum depth of 20cm. Lower canopy foliar samples were collected within the lowermost metre of the canopies and upper canopy foliar samples were collected within the uppermost metre of the canopies. The data were analysed in the laboratory for various elements and soil physical parameters. The soil pH was lower under the P. patula (pH 4.8-5.2) and E. grandis plantations (pH 4.6-5.3) when compared with the soil under the adjacent grasslands (pH 5.3-5.6). The soil under the P. patula plantations had lower exchangeable base cation concentrations (204-300 mg kg-1) compared with the adjacent Driekop grassland (452-645 mg kg-1), while there were no clear trends when comparing the exchangeable basic cation concentrations in the E. grandis plantations and the adjacent Brooklands grassland. The soil aluminium saturation was similar between the plantations, ranging between 39.7 and 63.3% in the P. patula plantations and 27.4 and 75.6% in the E. grandis plantations. The grasslands had lower soil aluminium saturations, ranging between 17.7 and 35.7% in the Driekop grassland and 17.5 and 39.1% in the Brooklands grassland. Exudates from mycorrhizal associations chelate iv acidic cations, rendering plantations more tolerant of acidic soils. Soil total nitrogen, phosphorus and carbon in the plantations were similar to the adjacent grasslands. Higher nitrogen mineralisation rates in the P. patula plantations corresponded with higher litter and foliar total N concentrations relative to the adjacent Driekop grassland, while there were no clear trends when comparing rates of N mineralisation in the E. grandis plantations and the Brooklands grassland. The concentrations of foliar amino acids and protein were significantly higher (p<0.01) in the plantation foliage, when compared the grassland foliage. The amino acid arginine, occurred in higher proportions relative to other amino acids in plantation foliage, when compared with the relative proportions found in the grasslands. Relative to nitrogen, foliar ratios indicated that phosphorus and potassium were the most limiting nutrients for the plantations, which possibly resulted in N storage in the form of protein and amino acids. The soil and foliage were the more sensitive indicators of the nutrients status than litter. Differences in nutrients concentrations between the lower and upper canopies in both plantations were minor and no major trends were found. Therefore the sub division of lower and upper canopies is not recommended for further foliar nutrient analyses. Age related trends were unclear as a result of higher soil clay contents found in the 13 year old P. patula plantation and the 8 year old E. grandis plantation as well as the various silvicultural practices implemented. In terms of the relative impacts of P. patula and E. grandis plantations, the results of this study imply that the upper 20cm of soil was more impacted under the P. patula plantations than under the E. grandis plantations, in terms of reduced base cation and increased acid cation concentrations. The P. patula plantations are shallow rooters and thus the base cation status of the upper soil horizons may affect the sustainability of the P. patula plantations.