This thesis reports a series of experiments performed to investigate the importance of dissolved organic nitrogen (DON) within UK agro-ecosystems, with specific reference to sample pre treatment and characterisation.
A UK wide survey was conducted, in order to quantify the concentration of DON within 119 sites on 7 different land use categories (Chapter 3). Further examination was conducted to identify links between DON concentrations and the chemical properties of plant residues. This survey highlighted the importance of sample treatment and extraction prior to analysis, while also indicating that DON constitutes a large proportion of the extractable N pool within UK soils.
Land uses with poor litter quality and prevailing environmental conditions were found to contain greater concentrations of DON than more intensive land categories, however there were very few relationships between soil DON and
plant residue chemical properties.
A year long monthly monitoring study was conducted in order to investigate seasonal dynamics, possible relationships between DON and other soil properties and to examine the effects of different soil extraction methods on DON concentrations (Chapter 4 ). The 4 extraction methods employed were, quartz suction cups, centrifuge extractions, water extractions and 2 M potassium chloride extractions. Seasonal relationships were identified at 2 sites, with peaks
in DON concentration occurring in the summer and winter. The soil with the greatest DON concentrations also contained the greatest concentration of phenols, which was thought to be the result of greater inputs of low quality plant litter. This site also had a the greatest concentration of amino acids which may result from a lower microbial population and therefore a lower turnover of amino acids. Of all the extraction methods 2 M potassium chloride consistently extracted more DON than any other method (Chapters 3-6). This was thought to result from disturbed chemical equlibria at DON sorption sites, although this was not tested here. Quartz suction cups extracted the least DON of all because it only accesses the DON free in solution.
In order to further characterise DON and understand the effects of soil extraction method on DON compounds an ultra filtration method was investigated (Chapter 5). This has been used previously, within a range of environments in order to
identify the "molecular weight signatures" of organic compounds within rivers and streams and soil solutions. This method highlighted the importance of solution pH and ionic strength to the behaviour, shape and size of organic compounds.
A study was then conducted in order to examine the decomposition of plant residues through different DON extractable pools using a sequential extraction method (Chapter 6). This was devised in order to examine each extractable DON pool associated with a specific method. Each extraction method was therefore conducted on the same soil sample one after the other. It was found that the DON flush, as a result of decomposing plant residue, was first identified within the potassium chloride extraction. However, it was highlighted that the turnover time of labile DON, (thought to be predominantly extracted by centrifuge), can be as short as a few hours and subsequently any recognisable peak of DON within this extract may not have been identified within this study.
The final part of this thesis concentrates on the transformation and transport of DON from the terrestrial environment into the aquatic system (Chapter 7). This study examines the data collected from 102 streams and 16 lakes within
catchments of differing complexity situated in Wales. This study confirmed that DON constitutes an important component of the N cycle in freshwater systems. Seasonal relationships were also assessed and it was identified that relationships between DOC and DON exist within the summer months but not in the winter, this was thought to be the result of differing water flow paths, although it was not possible to test this here.
This thesis shows how DON can be an important part of the N cycle within both terrestrial and aquatic systems and examines the behaviour of DON during extraction from the soil. Areas where further work is required are identified and a
general discussion concludes that more compound specific research is needed in order to fully understand the importance and functioning of DON within soils.