A small lysimeter system to investigate oxygen and carbon dioxide profiles in soils with water tables
Schoonwinkel, Benjamin Christiaan
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In South Africa a huge amount of time and energy has been spent on evapotranspiration research over the past 30 years, mainly to predict the amount of plant available water needed to prevent crop stress. In the quest to conserve water losses due to transpiration, researchers tended to neglect the importance of soil-air concentrations in relation with soil water. Rising water tables caused by recharged groundwater through irrigation is one of the most important factors that change soil-air concentrations. For measurements, researchers found lysimeters more convenient due to the fact that they can simulate transient or constant water table conditions, which is otherwise very difficult to study in agricultural fields. The dissertation focuses mainly on the development of a monolith-lysimeter to measure soil water and soil-air regimes under rising water table conditions for different soils. The research was conducted on five soils (sandy Hutton, loamy-sand Hutton, Bainsvlei, Sepane and Valsrivier) sampled in small (200 kg) lysimeters. A disturbed and undisturbed Bainsvlei soil was sampled at the experimental farm of the Department of Soil, Crop and Climate Science (University of the Free State) at Kenilworth in the Bloemfontein district while the remaining four undisturbed soils were sampled at the Orange-Riet River Irrigation Scheme. A total of 6 lysimeters was arranged in the glasshouse of the University of the Free State located on the main campus in Bloemfontein, South Africa. The aim was firstly to develop and test a small weighing-lysimeter system for measuring soil temperature, soil water and soil air (oxygen and carbon dioxide) responses under water table conditions in a disturbed and undisturbed Bainsvlei soil monolith. These monolith lysimeters were used to characterize the influence of the lysimeter compared to in situ data that determined the accuracy of the method. After saturation of the soils with de-aired water from the bottom, drainage curves were determined by measuring weight-loss with both a weighing bridge and a capacitance DFM probe. Results showed that the shapes of the drainage curves for both the disturbed and undisturbed soils were similar due to the similarity of the easily drainable pores. However, the water retention was significantly lower in the undisturbed soil compared to the disturbed soil. Furthermore, a water-table height control system was used for both raising, and to keep the water table steady at three heights while soil air measurements took place. According to the results it was found that an undisturbed soil is better to use for studying O2 and CO2 concentrations in soils. This conclusion is supported by the results which showed the sampling method with disturbed soil induced significantly higher O2 and lower CO2 concentrations, respectively compared to that of the undisturbed soil. Overall, the results indicated that the proposed small weighing-lysimeter system contribute towards the understanding of a very important subject, namely soil aeration. Secondly, the monolith-lysimeter technique developed was used to evaluate five undisturbed soils in their O2 and CO2 response to a rising water-table over a period of 6 days. The water table was set at each height for six consecutive days for measurements where after it was raised to the next height. It was found that the O2 and CO2 concentration profiles were significantly influenced by the rise in water-table heights for the five soils under investigation. However, there were some distinct differences in the gas profiles observed between the sandy soils (sandy Hutton, loamy-sand Hutton and Bainsvlei) compared to the clay soils (Sepane and Valsrivier) due to differences in physical composition. The results further showed that time had significantly influenced O2 and CO2 concentrations over the 6-day period. As O2 concentrations gradually decreased, CO2 concentration gradually increased for all five soils. The only difference between the two soil groups was the intensity of respiration that resulted in lower O2 and higher CO2 concentrations for the clay soil group than for the sandy soil group over the 6 day period.