Browsing by Author "Wienand, Karen Tania"

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    An investigation of the duration of phosphorous fertilization effects on phosphorous and nitrogen cycling patterns of Pinus elliottii plantations in the southern Cape
    (1992) Wienand, Karen Tania; Stock, WD
    The effects of phosphorus fertilization and its duration of impact on nutrient cycling patterns in Pinus elliottii plantations were investigated by examining soil phosphorus and nitrogen turnover and litterfall in a 8, 20 and 25 year old plantation age sequence. Each stand contained plots fertilized with between 30-60 kg ha-1 superphosphate at establishment and an equal number of control (unfertilized) plots. At the oldest stand there were additional variables, namely timing of fertilizer application (at establishment versus 10 years after establishment) and fertilization frequency (double application, at establishment and 10 years later). Phosphate fertilization produced a significant increase in soil phosphorus availability at the 8 and 20 year old stands. At the 25 year old stand, increased phosphorus availability was only significant in the plots fertilized twice. It was only at these latter plots that a significant increase in soil annual net phosphorus turnover with fertilization was evident. Thus, only with a double application of phosphate will increased phosphorus availability and turnover be maintained up to 25 years. Phosphate fertilization significantly reduced soil nitrogen availability and soil annual nitrogen turnover at all the stands. This was ascribed to inorganic nitrogen being immobilized by the large microbial population supported by the increased phosphorus availability. This contention is supported by the investigation of the factors influencing nutrient mineralization - while fertilization was the most significant factor determining phosphorus mineralization, nitrogen mineralization was shown to be strongly controlled by environmental factors, indicating the role of micro-organisms in this process. Thus phosphorus release is mainly a physiochemical phenomenon while nitrogen turnover is biologically controlled. The reduction of nitrogen turnover rates with phosphate fertilization intensified with increasing age, presumably as the nitrogen became bound in biomass which decomposes slowly.
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    A preliminary study of the impacts of alien Acacia infestation (A saligna) on the relative rates of nitrogen and phosphorus cycling in Lowland Fynbos, southwestern Cape, South Africa
    (1988) Wienand, Karen Tania; Stock, William D
    This study was carried out on Sand Plain Lowland fynbos at Pella from April to September 1988. Environmental factors, nitrogen and phosphorus pool sizes and mineralization processes were investigated in the surface soils (1-10cm) of 7-8 year old fynbos vegetation and an adjacent Acacia saligna (Labill.) Wendl. infestation. While there was no significant difference in soil temperature between fynbos and acacia sites, soil moisture and organic matter content was significantly higher in the acacia soils. This favoured decomposition so that soil nutrient analyses showed enrichment of the soils (higher N and P) by the acacia infestation. Soil N and P mineralization was assayed using in situ incubations. Due to their higher soil total N concentrations, acacia soils showed greater inorganic N concentrations. In both acacia and fynbos soils ammonium was the dominant N form. This was ascribed to the high soil moisture content, while the low temperatures appeared to be the factor most strongly influencing ammonium accumulation. The low nitrate accumulations even in the field incubations indicated that the nitrification process was inhibited, probably by the high soil moisture content and low temperatures. The variable patterns of inorganic P accumulation were ascribed mainly to fluctuations between microbial mineralization and immobilization. Contrary to the hypothesis that the higher soil organic matter and greater concentrations of total N and Pin acacia soils would result in higher mineralization rates, there was no significant difference in the rates of N and P mineralization between fynbos and acacia soils. Thus, it was concluded that the higher decomposition rates in acacia soils was not associated with greater mineralization rates during the wet season (period of study).
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