Nitrification is the conversion of ammonia to nitrate via nitrite and is performed by ammonia oxidising archaea (AOA), complete ammonia-oxidiser (comammox) and ammonia and nitrite oxidising bacteria (AOB and NOB). The aim of this study is to examine the effect of ammonia concentration, temperature, drought and inhibitors on activity of AOA and AOB using soil microcosms and cultures. Ammonia concentration in soil increases during drought due to the reduced soil water content and, with desiccation stress or a combination of both factors, may result in reported greater inhibition of AOA than AOB during drought. The independent effects of both matric potential and initial ammonium concentration on AOA and AOB amoA abundances and nitrate production were studied in soil microcosms. AOA were more susceptible to increased desiccation stress than AOB, irrespective of initial soil ammonium concentration, and AOA cultures were more sensitive than AOB to osmotic stress induced by different concentrations of NaCl or sorbitol. This may represent an additional niche differentiating factor between AOA and AOB in soil. The effect of temperature and supply of high levels of inorganic ammonium on ammonia oxidation by AOA and AOB were also investigated in soil microcosms. Activity and growth of AOA and AOB were observed in soil amended with high ammonium concentration with increasing temperature, suggesting that AOA can contribute to nitrification in highly fertilised soil, particularly at 25 oC. Inhibition of AOA by simvastatin was investigated in culture and in soil. Simvastatin selectively inhibited AOA in both systems and soil microcosm studies provided evidence for oxidation of ammonia by AOB at low ammonium concentration. Generally, the results show the benefits of combining soil microcosm and culture-based approaches in soil microbiology. The findings advance our understanding of the influence of ammonium supply, temperature and osmotic stress on soil nitrification and its role in controlling the availability of ammonium-based fertilisers for plant uptake.

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N. europaea recovering from up to 7 days starvation showed no lag phase before nitrifying activity or growth could be measured. Cells starved for 1- 4 weeks had a lag phase of 20 - 40 min before activity. Cells starved for 0 - 7 d had no lag phase before growth. A lag phase before growth was initiated after 1 - 2 weeks starvation, which increased to 93 - 98 h after 6 weeks of starvation. Between 1 and 6 weeks starvation the nitrifying activity per ml of recovering cell suspension decreased significantly. N. europaea was also starved in continuous flow sand and soil columns. Cells recovering from starvation in sand columns resumed nitrifying growth and activity immediately at the same growth rate of cells initially inoculated into the column. Cells recovering from starvation in soil columns took longer to resume nitrifying activity than cells starved in sand. pH and nitrapyrin both had significant effects on nitrifying activity and the interaction between pH and nitrapyrin treatment was significant in the inhibition of both growth and activity. pH and PEX both significantly inhibited nitrifying activity but the interaction between pH and PEX was not significant. PEX completely inhibited growth at concentrations which only partially inhibited nitrifying activity. The optimum pH for nitrifying activity by N. europaea was between pH 8.5 and 9 and the optimum pH for growth was between 7.5 and 8.0. The optimum pH for activity increased as substrate concentration decreased. A previously published model quantifying the relationship between pH, substrate availability and ammonia oxidation (Quinlan, 1985) was tested. Data predicted using the model fitted experimental data when data from a limited pH range were used to calculate the coefficients (pH 7-9), but not when data from a wider range (6-9) were used.

Thesis (M. Phil.)--University of Hong Kong, 1989.

Our ability to analyse natural microbial populations has recently been revolutionised by the application of 16S rDNA-based techniques. These techniques have proven particularly appropriate for ammonia oxidising bacteria, which carry out the initial and rate-determining step in nitrification. The strong effect of pH on ammonia oxidisers has been well documented; growth in liquid batch culture rarely occurs at pH values below 6.5 but autotrophic nitrification can occur in acidic soils. This study aims to examine the effects of pH in ammonia oxidisers in soil and laboratory culture.  We employ both molecular techniques and traditional physiological investigations to examine community dynamics of <i>Nitrosospira </i>strains of the ammonia oxidising bacteria, and the links between species diversity and functional diversity.

Thesis (Ph. D.)--University of Michigan, 1916. / "Publication no. 156, Botanical department, University of Michigan. Cover title. Reprint from Soil science, vol. 3, no. 2, February, 1917. "Literature cited." p. 191-195.

Thesis (M.S.)--University of Wisconsin--Madison, 1969. / eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

Thesis (Ph. D.)--University of Wisconsin--Madison, 1971. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 168-173).

Thesis (M.S.)--University of Wisconsin--Madison, 1953. / Typescript. eContent provider-neutral record in process. Description based on print version record. Includes Bibliographical references (leaves 37-38).