An investigation of the duration of phosphorous fertilization effects on phosphorous and nitrogen cycling patterns of Pinus elliottii plantations in the southern Cape
Master Thesis
1992
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University of Cape Town
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Abstract
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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Wienand, K. 1992. An investigation of the duration of phosphorous fertilization effects on phosphorous and nitrogen cycling patterns of Pinus elliottii plantations in the southern Cape. University of Cape Town.