A balanced nitrogen (N) cycle is paramount for the ecology and biogeochemistry of planet Earth. Human activities are now causing an imbalance in the N cycle, with several negative effects on the marine environment. However, our knowledge of the marine N cycle remains incomplete, especially with regards to the role of dissolved organic N (DON). Therefore, there is need to study the role of DON more extensively to aid in restoring balance to Earth's ecosystems and biogeochemical cycles. This project investigated DON dynamics and influence on phytoplankton in coastal and shelf seas (CSS) to the west of Britain where DON was understudied. First, selected methods for the measurement of N uptake rates, urea and dissolved free amino acid (DFAA) concentrations were reviewed and tested. Then, they were used to study the spatial and temporal DON dynamics in the shelf region to the west of Britain (Irish Sea and adjacent shelf), the in-situ seasonal DON dynamics and N uptake in relation to phytoplankton community composition and abundance in Loch Creran, and the influence of DON on the phytoplankton community composition and abundance in controlled nutrient uptake kinetics and growth experiments and ecosystem modelling. The main findings were: DON was important in the N dynamics of the Irish Sea and adjacent shelf waters; DON was correlated with changes in phytoplankton community composition in Loch Creran; its seasonal cycle suggested its importance as an N source for the development of the spring bloom; these ideas were further supported by controlled laboratory experiments and ecosystem modelling.

Anaerobic ammonium oxidation (anammox) is a pathway that has been known for almost 2 decades, but few studies have investigated its importance in natural groundwaters. This thesis investigated the presence of anammox cells and the groundwater geochemistry of 2 sites (Elmira and Putnam) in southwestern Ontario where groundwaters are contaminated with high levels of nitrate and ammonium. Fluorescence in situ hybridization (FISH) was used to quantify the relative abundance of anammox cells in these waters. Our results showed that anammox cells could be detected in many wells at both sites and that their relative abundance varied between 0.45 and 4.81 % at the Putnam site, whereas it ranged between 0.8 to 8.4 % at the Elmira site. These values are within the same range as those obtained for marine and freshwater environments where anammox cells have been detected. In addition, indirect observations point to the fact that N cycling at the 2 sites might be linked to Fe and Mn reduction, but additional experiments are needed. In summary, our results corroborate the findings of N-labeled microcosm experiments which demonstrated that anammox was an important pathway of N cycling in those groundwaters and molecular analyses that detected important anammox organisms at the same sites.

No description available.

Thesis (M. S.)--Earth and Atmospheric Sciences, Georgia Institute of Technology, 2009. / Committee Chair: Ellery Ingall; Committee Member: Andrew Stack; Committee Member: Greg Huey; Committee Member: Joseph Montoya; Committee Member: Judith Curry.

Thesis (Ph. D.)--University of Washington, 1996. / Vita. Includes bibliographical references (leaves [147]-165).

No description available.

This study set out to investigate the effects of level of nitrogen supply on the root dynamics, carbon (C) and nitrogen (N) return to soil from root turnover, decomposition, and stability of improved grasslands. Field studies involved a two-year field experiment, where plant species composition, above ground biomass production, root production, litter decomposition and short term soil organic matter (SOM) dynamics, as a response to varying levels of inorganic fertiliser nitrogen supply were studied. The results showed that root dynamics are strongly affected by N fertilisation, with an increase in root production and death with increasing N supply. Reduced water availability lead to a greater root disappearance rate and the production of roots with a shorter lifespan. Root decomposition was strongly influenced by the age of the grass ley, which also exerted a strong influence on the structure of the soil microbial communities (SMC). Rate of litter decomposition and SMC structure were also influenced by the level of N supply. Level of mineral N supply and age of the grass ley also influenced the formation and stability of water stable aggregates, the microbial community structure and microbial community function within the aggregates. Mineral N applications are likely to influence SOM and soil nutrient dynamics. The finding that climatic conditions, and in particular water availability, had the strongest impact on both above and below ground productivity, and strongly influenced the amount of C, N and root/shoot biomass returned to soils, is of great importance in helping to make more accurate predictions of the response of plant communities to projected changes in the global climate.

Nitrification is central to the global cycling of nitrogen and is important for primary production and other processes in marine systems, where nitrogen is generally the limiting nutrient. The majority of studies on nitrification, however, have focused on terrestrial systems, where this process can cause significant losses of N-based fertilisers from agricultural land. Comparatively little is known of nitrification in marine systems and the organisms involved. In addition, technical difficulties in isolation of pure cultures of ammonia-oxidising bacteria have severely restricted studies of species diversity and community structure of these organisms. In this study, molecular technology, based on the 16S ribosomal RNA (rRNA) molecule was applied to the characterisation of marine communities of ammonia-oxidising bacteria. PCR primers were designed for specific amplification of the rRNA gene (rDNA) from ammonia-oxidisers belonging to the beta-subdivision of the Proteobacteria. These primers were used to characterise both enrichment cultures and communities within polluted and unpolluted sediment samples. These studies indicated considerable diversity of beta-subgroup ammonia-oxidisers within marine systems which has not previously been detected. It was also shown that enrichment and isolation techniques select for strains belonging to the genus Nitrosomonas while the majority of sequences obtained by direct analysis of rDNA amplified from total genomic extracts belonged to the genus Nitrosospira. In addition, novel isolation methods were developed which considerably reduced the level of heterotrophic contamination and greatly facilitated isolation of pure cultures. In situ probing, using fluorescently labelled rRNA oligonucleotide probes, indicated that CCD microscopy is less sensitive than UV microscopy alone due to quenching of the signal.

Nitrogen (N) cycling involves the gains, losses, and transformations of N from sources such as soil organic matter, crop residues, and fertilizers. These sources are the primary N supplies potentially available to non-leguminous crops. Through the use of a stable N isotope tracer (¹⁵N), transformations among various soil N pools can be studied. We conducted three separate studies using ¹⁵N. Two studies dealt with methodologies of ¹⁵N use and analysis, while the third study investigated mineralization of ¹⁵N labeled crop residues under field conditions. The first study evaluated a new apparatus for applying ¹⁵N by fertigation to subplots under buried drip irrigation. We determined that this method was an effective means of uniformly applying tracers using buried drip irrigation. The second study evaluated a new method for fine-grinding soils based on particle size distribution and variability of organic N and ¹⁵N analyses. Soils of varying texture were rapidly ground to achieve acceptable analytical precision for N and ¹⁵N analysis. The objectives of the third experiment were to: (i) evaluate mineralization of inorganic N from ¹⁵N -labeled crop residues with different C/N ratios and at different loading rates and (ii) evaluate the influence of residue loading rate and type on the percent net mineralization from ¹⁵N-labeled crop residues in a basin irrigated wheat cropping system in Southern Arizona. Mineralization of crop residues in this hyperthermic soil was rapid and was often followed by periods of re-immobilization. Net end-of-season mineralization of residue N was 30-50% for lettuce, and 30-40% for wheat.

Biofilm communities were cultivated in rotating annular bioreactors using water from the South Saskatchewan River. The impacts of seasonal variations, nutrients, pollutants and dissolved oxygen on the activity and composition of the biofilms were assessed by using a combination of microcosm assays and molecular biology techniques. / The seasonal pattern in nitrification, denitrification and hexadecane mineralization, and in the occurrence of nirK in the South Saskatchewan River biofilms was: fall greater than winter, which was equivalent to spring. Hexadecane mineralization was higher in fall 1999 than in fall 2001, denitrification was similar in these two years, and no seasonal pattern of nitrification was observed. / The addition of combined nutrients (C, N, and P) resulted in significant increases in the measured bacterial activities and in the predominance of alkB, nirS and nirK in all seasons and years. The addition of individual nutrients did not stimulate hexadecane mineralization, denitrification, and the PCR amplification of nirS and nirK. In fall 1999, CNP and, to a lesser extent P, stimulated nitrification, whereas in fall 2001, no pattern was observed. The results showed that nutrients, especially P, were limiting for bacterial activities, and that the biofilm activities and composition varied with nutrient availability and time of year. / At the concentration assessed (1 ppb), hexadecane partially inhibited denitrification to similar extents in both years, had a negative impact on nitrification and hexadecane mineralization in fall 1999, and a positive impact on these two latter activities in fall 2001. Nickel (0.5 mg liter-1 ) negatively affected denitrification but had no effect on hexadecane mineralization. The alkB and nirS genes were less predominant and absent, respectively, in biofilms grown in the presence of nickel. DGGE analyses indicated that nickel reduced the biofilm bacterial diversity. / The results presented herein provide much needed information on the microbial ecology of river biofilms, and on the impact and interactive effects of pollutant and nutrient inputs on these biofilms. These results and the techniques used in this project can be applied to monitor environmental effects of anthropogenic activities on aquatic biofilms, and can contribute to establish or revise environmental regulations.