The normal operating range of soil functioning. Understanding the natural fluctuations of nitrogen cycling communities

PhD ceremony: Ms. M. de Cássia Pereira e Silva, 14.30 uur, Academiegebouw, Broerstraat 5, Groningen

Dissertation: The normal operating range of soil functioning. Understanding the natural fluctuations of nitrogen cycling communities

Promotor(s): prof. J.D. van Elsas

Faculty: Mathematics and Natural Sciences

The most important results of the research of Michele de Cássia Pereira e Silva include the fact that the archaeal communities in agricultural soils are surprisingly dynamic, when compared to bacterial and fungal communities, and also highly sensitive to management regimen. Also from her research it was observed that two very important groups of soil microorganism, the ammonia oxidizers, respond to stress in different ways, although they perform the same function, nitrification. This has consequences for the selection of good bioindicators of soil stress.

When the structure and dynamic changes in the composition of nitrogen-fixing microorganisms were investigated De Cássia Pereira e Silva observed that different soil types could be discriminated based on this community. Moreover, soil characteristics such as ammonium content, pH and texture strongly correlated with the variations observed in this community, suggesting that nitrogen-fixing microorganisms are sensitive to abiotic parameters and could represent potential indicators of soil disturbances. Soil represents a highly heterogeneous and dynamic environment for its microbiota, a mixture of different constituents. Such fluctuations may affect the dynamics and activities of soil organisms and the interactions between them, and taken together, will define the normal operating range of soils, with normality including the expected fluctuations in function in response to all conditions that occur in a particular soil system, either naturally or due to common anthropogenic influences. The dynamics of ammonia oxidizers were also studied and the effects of soil abiotic parameters on the abundance, structure and function of these communities were addressed. The results indicated that soil pH and soil type were also main factors that influenced the size and structure of the archaeal (AOA) and bacterial (AOB) ammonia oxidizers, as well as their function. From a microcosm experiment the influence of soil texture and soil pH on the abundance and function of nitrogen fixers and ammonia oxidizers. The results suggested that AOA might drive nitrification in soils that have been recently disturbed, whereas AOB might be more relevant in pristine ecosystems. Finally, a mathematical method is presented which describes a quantitative approach to assess the NOR of soil functioning. Such approach will certainly facilitate and greatly influence soil quality assessments.