1,720,991 research outputs found
The effects of elevated atmospheric CO2 on below ground processes in fynbos and C4 ecosystems
No full textIncludes bibliography.This thesis investigated ways in which elevated atmospheric carbon dioxide (CO2) concentrations influence the plant: soil interface and soil biological processes. The studies focused on microbial populations associated with the plant rhizosphere, and how these responses could feed into changes in the soil carbon pool
The ecology and nitrogen-fixing ability of selected Aspalathus spp. in fynbos ecosystems
No full textBibliography: pages 128-140.Aspects of the ecology of selected species of Aspalathus spp. (Fabaceae) were investigated to elucidate the role that this genus may play in the nitrogen economy and ecology of fynbos ecosystems. From the literature it is argued that fire plays a pivotal role in these ecosystems as decomposition is very slow. Decomposition returns nutrients trapped in the vegetation back to the soil. However, nitrogen is lost via volatilisation during fires and as deposition of nitrogen in precipitation is low, symbiotic and non-symbiotic nitrogen-fixation may be of importance in replacing lost nitrogen. The influence of soil total nitrogen and phosphorus on the occurrence of dense stands of Aspalathus plants in vegetation of different ages was investigated. Soil of young vegetation containing stands of Aspalathus plants had the same or higher concentration of phosphorus as nearby older vegetation which lacked stands of Aspalathus. Total nitrogen however showed no trends. Nodulation of 15 Aspalathus spp. at 11 study sites showed varying amounts of nodulation although all these species nodulated well under pot culture. Nodulation of three Aspalathus spp. at three seasonal study sites was followed over the course of one year. A. carnosa showed peak nodulation in the late winter months but A. retroflexa and A. abietina displayed no similar trends. Across the three seasonal sites, which had very similar soil, nodulation was positively correlated with soil total phosphorus and negatively correlated with soil total nitrogen in a multiple regression analysis. The ratio of soil total nitrogen to phosphorus was negatively correlated with nodulation. The opposite correlations for soil total nitrogen and phosphorus were found across all 11 study sites. Differences in soil types and organic content may be responsible for this apparent contradiction. Other soil factors such as pH, temperature and moisture were not correlated with nodulation. Total organic content, however, was negatively correlated with nodulation across the geographic survey study sites
The long-term effect of elevated CO₂ on grassland biomass production
Get PDFThis study investigates the influence of elevated CO₂ on grassland biomass production at a naturally elevated CO₂ spring situated on the Bongwan gas fault in Natal. The effect of elevated CO₂ on monocotyledenous (C₄) and dicotyledenous (C₃) above ground plant biomass production and their dominance patterns along a CO₂ gradient were studied. Three 7x7m plots were located 18m, 39m and 73m away from the elevated CO₂ spring. The 18m site was the experimental site, while the other two sites were the controls. The primary focus of the study was to determine the biomass production of monocotyledenous and dicotyledenous plants at the above-mentioned distance from the spring. However, to ascertain possible factors that could influence the increase in biomass production with distance from the CO₂ source, plant nutrient analyses (N and P), soil moisture contents (which could have an effect on plant wateruse efficiency) and carbon isotope discrimination values were determined at the three sites. The results show that elevated CO₂ had a significant effect on the monocotyledenous dry matter production, but had no significant effect on any of the other plants or soil. It was also shown that elevated CO₂ increased the soil water retention capacity as one moves toward the spring, however this result is not confirmed
An investigation of the duration of phosphorous fertilization effects on phosphorous and nitrogen cycling patterns of Pinus elliottii plantations in the southern Cape
No full textIncludes bibliographical references.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
Response of Cynodon dactylon (L.) Pers. to ammonium and nitrate nutrition
No full textPlants that come early in succession predominantly prefer_ ammonium form of nitrogen than nitrate. The hypothesis that Cynodon dactylon (L.) Pers., a grass in the primary succession stage would respond well when supplied with ammonium than nitrate fertiliser was tested. Seedlings of Cynodon dactylon were grown in culture solution and fertilised with either ammonium sulphate or potassium nitrate at ~ continuous concentrations of 200 ppm, 400 ppm and 600 ppm in the growth chamber. In support of my hypothesis, growth of Cynodon dactylon was greater from ammonium than nitrate nutrition in terms of aboveground and total plants yields at 200 ppm and 400 ppm and this was associated with efficient utilisation of this form of nitrogen by this species. Production from the nitrate treatments was restricted and showed no change with increase in external nitrogen supply. Decreased dry weights from the ammonium nutrition at 600 ppm were associated with toxicity of ammonium ions in plant tissues. High levels of nitrogen measured from the nitrate treatments were associated with the soluble nitrogen that was not assimilated for yield increase by this grass, Uptake of ammonium ion was shown to result· in high of uptake phosphorus. This study shows that Cynodon dactylon has the capacity for increased growth under ammonium nutrition while this was limited in the nitrate nutrition. However, response is restricted when excessively fertilised with ammonium nitrogen
Atmospheric deposition of combined nitrogen in the Cape Metropolitan Area: a threat to a species rich ecosystem?
No full textIncludes abstract.Includes bibliographical references (leaves 161-173).In this project. two broad approaches were used in order to gauge levels of atmospheric N deposition to the natural vegetation of the Cape Metropolitan Area (CMA). In the first approach, bulk collectors were constructed and set up at five sites around the CMA in order to trap atmospheric deposition for chemical analysis. In the second approach, moss samples collected from different parts of the CMA during various historical periods, were analysed for N concentration, C: N ratio and 15N/14N isotope ratio in order to investigate the increase in N deposition over the past 100 years and the primary source of this increase
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Why is Chasmanthe spp. absent from the archaeological record of the south-western Cape?
Get PDFIn the archaeological record of the south-western Cape one finds corm residues in deposits mostly in the form of the netting which surrounds the corm. These plant residues seem to be a widespread feature in the archaeological record of Later Stone Age sites in Southern Africa. Corm residues nave been identified as mainly representative of the Iridaceae family. The most common geophyte corms identified are those of Watsonia, Babiana, Hexaglottis, Moreae and Gladiolus. Interestingly, Chasmanthe spp. commonly found growing on the west coast, have not been found in archaeological deposits of this area. The carbohydrate-rich corms follow seasonal growth patterns and mainly flowering in spring and early summer and growing during the winter months. Hunter-gatherers must have been familiar with their growth patterns and their palatability so that they could exploit these plants when corms were at their optimum and harvest them before the stored carbohydrates were used up by the plant. Utility plant indices for varying plant-resource components and mineral content analysis for N, P and total non-structural carbohydrates of the corms were calculated. From the results it appears that the reason for Chasmanthe spp. not appearing in the archaeological record is due to choices made by foragers regarding field processing of low utility plant parts (i.e. plant waste), rather than its relative importance or more precisely lack thereof, in the diets of early foragers
Phylogenetic and functional growth from diversification in the Cape grass genus Ehrharta Thunb
No full textIncludes bibliography.This thesis uses phylogenetic and comparative data to test an hypothesis of adaptive radiation in the Cape grass genus Ehrharta Thunb. sensu stricto. Morphological data and sequence data from two noncoding regions of DNA (lTS1 and trnL-F) are used to produce a phylogenetic hypothesis for the tribe Ehrharteae. Combined analysis of these data sets resolves four principal clades that approximate the genera Ehrharta s. s., Micro/aena, Tetrarrhena and Zotovia and this result thus supports a four-genus classification. Poor resolution and a reduction in branch length at the base of a clade nested within Ehrharta s. s. suggests past radiation. Parsimony-based reconstruction of ancestral habitats and growth form attributes indicates that such radiation is associated with a historical transition to seasonallydrier but more fertile habitats, and the coincident or subsequent evolution of several growth form novelties (e.g. buried and swollen culm bases and annualness). These traits are interpreted to reflect divergent strategies for surviving seasonal drought (Le. via seed or storage). Much higher transpiration rates in summer-deciduous leaves than in perennating culms of two species suggest that the evolution of summer-deciduous foliage was important in the occupation of seasonally-arid habitats. Controlled growth experiments are used to test the hypothesis that divergence in persistence traits is associated with differences in seedling biomass allocation and relative growth rate (RGR). Ehrharta s. s. shows wide variation in seedling RGR and regressions based on phylogenetically independent contrasts suggest that differences are better explained by early biomass allocation than leaf area indices. Species with a high allocation to leaves grow faster and flower sooner, so these traits are typical of seeding species
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