PhD Theses: Department of Department of Biological Sciences

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Browsing PhD Theses: Department of Department of Biological Sciences by Author "Nyaga, Justine M."

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    Empirical and model derived respiration responses to climate in different soils of an arid South African ecosystem
    (The University of the Western Cape, 2009) Nyaga, Justine M.
    This study examined the magnitude of soil CO2 efflux in an arid South African ecosystem, the flux responses as well as those of key limiting nutrients to soil temperature increases and moisture reductions consistent with a future climate change scenario, and compared measured soil respiration rates with those predicted with empirically and theoretically-based soil respiration models. Measurements of soil respiration rate, temperature, moisture, N and P contents were conducted monthly over a 12-month period in natural environments and those artificially manipulated with replicated open-top warming chambers (average 4.1oC increase) and precipitation exclusion chambers (average 30.1% decrease in rainfall, 26.2% decrease in fog and dewfall) distributed in five different soil-vegetation units.Measured soil respiration rates were over 3-fold less than those reported for temperate and tropical forest ecosystems with 61.5% of the total soil CO2 efflux contributed by root respiration (derived from the differences between moderately vegetated and sparsely vegetated areas) in moderately vegetated soils. Massive increases (up to 15 times) in soil CO2 efflux occurred during wet phases, but even these large CO2 pulses were only comparable in magnitude with soil CO2 effluxes reported for temperate semi-arid grasslands. There was considerable intra-annual and inter-site variability in the magnitude and direction of soil respiration and N and P responses to elevated temperatures and reduced precipitation levels with poor correspondence evident between soil CO2 efflux and soil organic matter content. Soil CO2 effluxes declined in response to precipitation exclusion by 7.1% over all sites and increased in response to warming by 42.1% over all sites. The large increase in response to warming was assisted by a 7.5% enhancement in soil moisture content due to precipitation interception by the chamber walls and its channelling to the soil surface.Relatively smaller respiration increases in response to warming occurred in moderately vegetated soils, these attributed to soil thermal insulation by the plant canopy cover. Soil P and N contents increased in response to warming by 11.3% and 13.3% respectively over all sites, with soil P declining in response to precipitation exclusion by 5.8% over all sites and soil N increasing in response to precipitation exclusion over all sites by 5.8%. Standard least squares regressions quantified the relationships between soil respiration rate and measured soil physical and chemical properties, and their interactions for each of the 5 soil-vegetation units. These relationships were incorporated in an empiricallybased soil respiration (EMR) model which was compared with a theoretically based generalized soil respiration model (GRESP). GRESP model functions included measured Q10 coefficients at soil moisture contents above field capacity, these assumed reduced by half for dry conditions, and maximum retentive and field capacities of soils. EMR modelled soil respiration rates displayed slightly better correspondence with measured soil respiration rates than GRESP modelled soil respiration rates. This apparent from the higher regression coefficients and lower sums of squared residuals, with EMR model residuals also more closely approximating normal distributions. However, despite the EMR model’s slight superiority, it was concluded that more precise laboratory-based measurements of soil retentive and field capacities and their Q10 coefficients at different soil moisture contents could improve the GRESP model’s accuracy thereby providing a more convenient and uncomplicated means of predicting respiration responses to current and future climates over a wide range of arid soil types
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    Nutritional contribution of atmospheric deposition to the Strandveld vegetation of West Coast South Africa
    (University of Cape Town, 2013-07) Nyaga, Justine M.
    Ecosystem nutrient availability depends on the balance between rates of nutrient inputs and losses. Nutrients may be lost through fire and displacement of ash, herbivory, leaching and volatilization. The main pathways through which nutrients may be acquired are weathering of rock and atmospheric deposition. Symbiotic and free-living diazotrophic bacteria and blue green algae also contribute N. In ecosystems with limited occurrence of N2-fixation and occurring on low-nutrient bedrock, atmospheric deposition is the most significant source of nutrients. Nutrients from atmospheric deposition may be of natural or anthropogenic origin, and can be “wet-deposited” dissolved in precipitation and “dry-deposited” when aerosols settle out of the atmosphere onto plant and soil surfaces. Studies on nutrient cycling around the world suggest that nutrient deposition can provide substantial amounts of nutrients to coastal ecosystems, although mineral weathering of rocks can also a significant source. Limited prior work on deposition in coastal areas of South Africa suggests that nutrient deposition could be an important component of nutrient budgets in the Cape Floristic Region. The west coast of South Africa borders a section of the Atlantic Ocean that is highly productive and characterized by strong seasonal winds, rough waters and strong wave action. This area is home to the Strandveld vegetation, which grows on marine-derived soils. Based on this, I hypothesized that marine aerosol deposition is a significant source of nutrients for the vegetation in west coast South Africa. To test this hypothesis, I examined the spatial and temporal characteristics of atmospheric deposition as well as the climatic and ecological characteristics of the area. I measured deposition rates and concentrations of essential plant nutrients (N, P, Na, Ca, Mg, and K) delivered in rain University of Cape Town v and horizontal precipitation. Horizontal precipitation was used to refer to all forms of precipitation deposited horizontally and included fog, windblown aerosols, and horizontal rainfall. I then estimated annual demand for these nutrients in 8 plant species growing in the area and compared them to the deposition rates measured in rain. I also compared nutrients deposited in rain water with those deposited in horizontal precipitation, measured the amounts of NO3 -, NH4 + and PO4 3- held in canopies of the 8 plant species during summer, and estimated the species’ capacity for foliar nutrient uptake. The Strandveld vegetation was found to have relatively high soil and plant nutrient concentrations compared to the rest of the CFR, despite its soils originating as nutrient-poor marine derived aeolian sands. Although N and P fluxes deposited in rain were lower than those measured in other pristine sites around the world, a large proportion of TN (84%) and TP (51%) was organic, pointing to a strong marine influence. The marine origin of N and P is supported by the high base cation fluxes compared to those reported globally. The high proportion of organic N and P, and the high base cation contents was also observed in horizontal precipitation. In this form of deposition, base cation concentrations were highest at the coast and contents declined with distance from the ocean, further supporting a possible marine source. This study also suggests that dust may be an important contributor to the deposition of some nutrients during the winter months, and both marine and terrestrial areas could therefore be important sources of nutrient deposition to this area. Based on leaf litter nutrient losses it was estimated that atmospheric deposition through rain alone could potentially supply 36% and 64% of N and P annual demand, respectively, and over 100% of the annual demand for K and Ca. This University of Cape Town vi suggests a strong marine influence in the supply of these nutrients to the Strandveld soils and vegetation. In addition, plants within the Strandveld vegetation intercepted substantial amounts of moisture and nutrients in their canopies. Species with small leaves intercepted significantly greater quantities of water and nutrients than those with larger leaves. It was also established that all the studied Strandveld plants could take up NO3 –, NH4 +, glycine (as a form of organic N) and Li (a proxy for K) through their leaves. Taken together, these results show that the Strandveld ecosystem of West Coast National Park receives substantial inputs of nutrients from marine aerosols, both in rain and horizontal precipitation. This deposition appears to be a critical source of nutrients in an ecosystem with limited bedrock nutrient supplies. Over the time scale of ecosystem development, atmospheric nutrient deposition combined with other ecological characteristics, such as strong moisture-laden winds, may help explain the unique biogeochemical and biogeographical characteristics of the Strandveld.