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Analytical methods evolved in this study included (1) use of ferrous sulphate for determining nitrate in the microdiffusion method, (2) use of boiling 6N-HC1 for determining total major and trace elements in plant materials and organic residues and (3) use of atomic absorption spectroscopy for determining major and trace elements in extracts used for fractionating these elements in soils and sewage sludge. The first part of the study included work on the metabolism of nitrogen during incubation of soil in relation to moisture content, temperature, pH and addition of organic materials. The effect of levels of applied ammonium sulphate on loss of ammonia by volatilisation and the rate of nitrification were studied in soils of different pH. The comparative utilisation of mineral-N, applied as ammonium or nitrate, by decomposing organic materials was also studied. The main part of the study was concerned with the effects of addition of trace elements (copper and zinc in particular and also chromium and manganese) on ammonification and nitrification during incubation of soils. The trace elements were added as pure salts and also in the form of natural sewage sludge containing these elements as contaminants. In addition studies were also made on mineralisation of nitrogen in samples taken from field soils which had received varying levels of digested sewage sludge over three years. Fractionation of copper and zinc in sewage sludge and peat before and after treatment with copper and zinc salts was studied by the use of various chemical extractants.

A study was made with three soil extractants with a view to finding one which would be suitable for indicating changes in the levels of ammonia, nitrate, iron, manganese, aluminium and phosphorus induced by different moisture treatments during incubation of soils. It was shown that iron could be determined directly in soil extracts by atomic absorption spectroscopy. Vacuum drying over silica gel was found to be a very suitable method for decreasing soil moisture contents without affecting microbiological activity when the soils were again wetted and incubated. The effects or moisture content during incubation of soils on metabolism of ammonia, nitrate, iron, manganese, aluminium and phosphorus were studied (a) at varying static moisture contents ranging from field capacity up to waterlogging and (b) under alternate aerobic and anaerobic conditions induced by changing moisture contents during incubation from high to low levels and vice versa. The effects of moisture contents was also studied in short term incubation and pot test. The metabolism of nutrients under different moisture treatments was also studied in relation to soil pH, texture and the addition of organic materials.

Microbial degradation represents the major removal process of hydrocarbon contaminants from soil. Understanding the factors which impact the behaviour and biodegradation of hydrocarbons in soil is fundamental for facilitating a successful bioremediation strategy. Little is known about the impact other contaminants have on hydrocarbon biodegradation, which is of particular relevance as contaminated soils often contain complex organic chemical mixtures. This thesis investigated the biodegradation of aromatic and aliphatic hydrocarbons in soils; with emphasis directed towards the impact of other contaminants (polycyclic aromatic hydrocarbons, carbon nanomaterials, cable insulating oil and weathered hydrocarbons) and enhancing biodegradation through biostimulation and bioaugmentation. The catabolic activity of indigenous soil micro flora was further found to be dependent on prior exposure to hydrocarbons, with biodegradation also found to be strongly affected by (a) hydrocarbon type and concentration microflora are exposed to and (b) exposure time, form and frequency. SOM was found to restrict the development of phenyldodecane degradative activity in soil, and indicated to have more of an inhibitory influence upon the biodegradation and bioaccessibility of aliphatic hydrocarbons. The effectiveness of biostimulation and bioaugmentation techniques to optimise biodegradation was influenced by many parameters, particularly hydrocarbon type and concentration; the presence and activity of indigenous microbes; and treatment type and application to soil. As such, the suitability of these techniques to facilitate contaminant biodegradation should be assessed on a site by site basis. Continued research in to the biodegradation of hydrocarbons is crucial for an environment as complex and heterogeneous as soil, and the ongoing development of risk assessment and bioremediation practices.

Dryland ecosystems contain small amounts of Carbon (C), yet very little information is available about C dynamics in dryland soils. Cyanobacterial soil crusts are considered a major source of organic C in drylands through photosynthesis. Through photosynthesis, cyanobacteria produce extracellular polysaccharides (EPS) increasing soil organic C (SOC) pool as carbohydrates.