Browsing by Author "Sanga, Hilda Gerald"

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Localization of mineralizable organic carbon using X-Ray CT
(Ghent University Vrije Universiteit Brussel, 2010) Sanga, Hilda Gerald
The sensitivity of organic C (OC) to decomposition depends not only upon its chemical composition but also on its location within the soil matrix. However, the precise location of OC mineralization is not clearly known. This Master thesis research's objective was to investigate the role of soil pore structure on the organic matter (OM) decomposability in a model sandy loam soil. To achieve this, the influence of different artificial operations (compaction, artificial change in texture, acidification, and OM application) on soil pore size distribution, decomposer group community (assessed by the fungal: bacteria ratio) and C mineralization was studied. Two major experiments were included. In the first one, manipulated soil samples were incubated in larger tubes for five weeks. A second incubation experiment was set up to follow C mineralization of added C-sources (grinded sawdust or grass particles) in artificially manipulated soil with differing pore size distribution and decomposer community. Soil samples were incubated in small tubes (1cm diameter* 1cm height) for five weeks and the evolving CO2 gas was analyzed by a Gas chromatography method. Samples were then scanned after incubation by X-ray computed tomography (CT) and following image processing, the total pore volume and % volume of different pore size classes were calculated. Correlation analysis was used to investigate the relation between the % pore volume of five pore classes (10-200, 210-400, 410-600, 610-800 and >800 pm equivalent sphere diameter) and the cumulative C mineralization after 5 weeks. Artificial manipulation of the sandy loam soil, through compaction at BD 1.6 g cm -3 and artificial change in texture from coarse sand: fine sand: silt and clay (CS: FS: S&C) ratio of 10:40:50 to 15:50:35 and 20:60:20 was found to reduce the total soil porosity. Soil compaction at BD 1.6 g cm'3 reduced the proportion of macropores (pores with pore neck diameter 30-300 pm) while artificial change in soil texture found to affect mainly the distribution of micropores (pores with pore neck diameter 0.2-15pm) and to a lesser extent the macropores. Soil amendment with grass material found to increases the total soil porosity. Artificial change in soil texture, soil acidification to pH 4.3 and additions of sawdust material compared to grass changed the microbial community towards more fungi oriented. This suggests these artificial soil operations to be usable for manipulation of soil pore structure and the microbial community. In the second experiment, very pronounced interaction effects on C mineralization between substrate type and the artificial changes in soil pore structure were found. First an interaction effect between soil compaction at BD 1.6 g cm'3 compared to BD 1.3 g cm'3 and substrate type was present as the reduction on C mineralization was more pronounced in grass amended soil than in sawdust amended soil. Similarly, a very pronounced interaction effect between artificial change in texture and substrate type was also noted as addition of sawdust strongly reduced the net substrate derived C mineralization, while grass addition did not. C mineralization from native SOM positively correlated to % pore volume of the 200-600 pm, class implying the dominance of mineralization in intermediate sized pores. On the other hand, a positive correlation between C mineralization and % volume of 610-800 pm pores with both grass and sawdust addition demonstrates the importance of larger pores for substrate decomposition. An inhibiting effect of soil compaction on C mineralization was observed from pF and X-ray CT data and this was likely related to the effect of macropore reduction and probably aeration. The negative influence on C mineralization of artificial change in soil texture and the associated reduction in the microporosity only with addition of sawdust but not with grass seems to be related to observed differences in microbial community involved in decomposition of both substrates. Interestingly but complex interactions between soil pore structure and substrate type were demonstrated in this thesis. Pronounced negative or positive correlations between individual pore size class volumes and C mineralization could be established and this indicates their role in the OM decomposition process.
The use of coal ash from power plants as a soil conditioner
(University of Nottingham, 2015) Sanga, Hilda Gerald
The disposal of coal ash, produced in large quantities by power plants as a by-product of coal combustion, is a significant environmental concern. Coal ash can be used as an agricultural soil conditioner because of its liming potential and the presence of many essential plant nutrients. However, recommendations for the agricultural use of coal ash should be based on sound knowledge of the coal ash characteristics, particularly the concentrations of potentially toxic elements (PTEs) in the ash. Due to the uptake of PTEs by crop plants it may pose risks to human health following the consumption of food crops. The aim of this study was to evaluate the potential for the safe application of power station derived coal ash to soil as a beneficial disposal route. The specific objectives included; (i) testing the variability of fly ash obtained from different sources in the UK, Czech Republic and Tanzania, (ii) quantifying the short- and long-term changes in soil characteristics induced by applications of ash, (iii) determining the effects of coal ash on soil enzyme activities, (iv) quantifying the utility of coal ash as a fertilizer by evaluating its effect on growth and yield of wheat and (v) assessing risks of long-term use/multiple applications of coal ash to arable soils. Coal ash from the Czech Republic, the UK and Tanzania was characterized; the latter two were used in pot experiments to determine their effects on soil enzyme activities, wheat growth and PTE uptake when added to two contrasting soil types (woodland and arable sandy loams). Two incubation experiments were undertaken to quantify short- and long-term effects of the coal ashes on soil characteristics. Calculations were also performed to evaluate the probable risks of increased contamination of soil and plant material as well as human ingestion of PTEs following repeated applications of fly ash to arable soils. Coal ashes from each source contain varying quantities of essential nutrients and PTEs due to differences in coal ranks and the combustion conditions of the power plants producing each ash. Different batches of ash from the UK and from Tanzania had different characteristics, despite coming from the same industrial source within the respective countries. Application of the first batch of ash collected in the UK (UK1) to woodland soil increased the soil pH, soil respiration and nutritional status during a two-year incubation experiment. Soil amendment with high UK1 ash concentrations (8-16%) contaminated the soil with PTEs through the experiment. In a four-month incubation experiment, the effects of different coal ashes applied to acidic woodland soil varied depending on the characteristics of each individual ash and the amount of ash applied. In a pot experiment designed to evaluate the effect of coal ash on microbial activities, soil amendment with the UK1 ash increased the pH of woodland and arable soils, while application of the TZ1 ash reduced the pH of both soils. Application of low concentrations (0-4%) of UK1 ash to both soils increased dehydrogenase and urease activities and wheat growth while application of TZ1 ash at high concentrations (8-16%) inhibited the enzyme activities. In pot experiments to evaluate the effects of ash on wheat growth, application of 0- 32% of the UK1 ash to woodland and arable soils increased soil pH while application of the TZ1 ash at 0-32% decreased the pH of both soils. Soil amendment with 0-4% of either UK1 or TZ1 ash increased the concentrations and extractability of nutrients and wheat growth and yield, but application of 16-32% of both ashes to both soils contaminated the soils and wheat plants with PTEs. Despite PTE uptake by plants, grain PTE concentrations were within the FAO/WHO 'safe' limits for ingestion, except for As and Cd in grains from plants grown in woodland soil amended with the highest concentrations of UK1 and TZ1 ash respectively, which were both present in higher than acceptable concentrations. Soil and plant concentrations and human consumption of selected PTEs (As, Cd, Cr, Pb and Zn) were calculated following simulated annual applications of TZ1 ash to an arable soil for five consecutive years. This showed that, even when residual contamination over a 25-year period was considered, applications of 2% ash to the soil are unlikely to breach 'permissible' standards for soil, wheat grain contamination and human dietary intake of PTEs, which were far below 'permissible' limits. It would be possible to apply ash with similar characteristics to TZ1 more frequently or over more than five cropping cycles. In conclusion, coal ash can be used as an agricultural soil conditioner; however, low concentrations (0-4%) and the strategic agronomical use of ash, specifically targeting problematic soils, are highly recommended for future studies.