Epsilon Open Archive

Abstract

Denitrifying bacteria comprise a functional guild that under anaerobic conditions is able to use nitrogenous compound for respiration. Agricultural soils are often fertilized with nitrogen, and the reduction of nitrate and nitrite to nitric oxide, nitrous oxide and dinitrogen via the denitrification pathway, leads to gaseous emissions from the nitrogenous compounds, a subject of great environmental and economic concern. This thesis examined denitrifiers, nitrate reducers, ammonia oxidizers and total bacteria, in terms of community structure, abundance and function in different arable soils. Denitrifying bacteria were described using the signature genes that encode the analytic subunits of denitrification enzymes. Appropriate targets and methods for this were evaluated. An assessment was carried out of how these biological factors are affected by different fertilizer and management practices. The spatial distribution of the denitrifying bacteria, and how this was correlated to soil properties, was also explored. Long-term application of different organic and mineral fertilizers altered both the community structure and function of the different bacterial groups. However, fertilization practices producing the most different community structure were not correlated to the treatments with the most different activities. Thus activities were not coupled to community composition. However, a study with different carbon additions, with and without nitrogen additions, showed that nitrogen addition increased the microbial activity and biomass, and in this case community composition was correlated with potential activity. In long-term fertilization, pH, C:N ratio and organic carbon content were important drivers shaping the community composition of the denitrifiers and total bacteria. Determination of the structure and size of denitrifying bacteria at field scale revealed significant relationships between denitrifying community composition, size and activity, and soil factors. These results are important in identifying the resource-based niches for denitrifiers at scales relevant for developing sustainable land management and agricultural practices.