Huizinga, K. M. 2006. The diversity of dissimilatory nitrate reducers in an agroecosystem. Ph.D. Dissertation, Michigan State University, East Lansing, Michigan, USA.

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Microbial communities are essential to nutrient cycles in agricultural soils, since their activity helps regulate the availability of nutrients to crops. Despite this important role, factors affecting microbial community structure are just beginning to be uncovered. As well as identifying factors affecting microbial communities, it is important to determine the links between particular communities and their ecosystem functions. Therefore, the focus of the work presented is on microbial communities known to be involved in soil nitrogen cycling (denitrifiers) or with the potential for involvement ( Planctomycetales ).

Statistical modeling determined how much of the variability in N 2 O and two other greenhouse gas fluxes (CO 2 and CH 4 ) could be explained from treatments varying in history of agriculture. The amount of explained variation was generally low, with CO 2 flux having the greatest amount of explainable variation. It was concluded that the high amount of explainable variability in CO 2 flux was due to the fact that it is a byproduct of heterotrophic metabolism in soil, making its production responsive to any factors affecting microbial metabolism. Nitrous oxide and CH 4 , conversely, are produced or consumed by microorganisms with specialized metabolisms, causing environmental variables alone to be insufficient for explaining amounts of flux from these gases.

Denitrifier community composition and diversity was assessed in three soil treatments varying in average annual N 2 O flux and history of agriculture. Comparisons of nir K sequence libraries indicated that community composition of successional sites abandoned from agriculture 15 years ago still showed impacts from agriculture. Productivity did not have a significant relationship with measures of denitrifier diversity, but there was a weak, positive relationship between relative disturbance and diversity. In addition, a significant, positive relationship between ribosomal RNA (rrn) operon copy number and growth rate of dissimilatory nitrate reducers was found, indicating that denitrifiers with low and high rrn operon copy numbers differ in ecological strategies. There was a significant effect of soil treatment on evenness, and there are nir K OTU’s in the never tilled and historically tilled treatments that may play an important role in minimizing the amount of N 2 O flux from sites currently not used for agriculture.

Since members of the order Planctomycetales may play important roles in nitrogen cycling, a phylogenetic survey of 16S rRNA gene sequences was undertaken to assess soil planctomycete diversity in soils differing in history of agriculture. Sequences clustered with the four-recognized genera of planctomycetes as well as in clusters outside those genera. Three sequences were similar to 16S rDNA sequences from organisms capable of anaerobic ammonia oxidation. Comparative analyses indicated that two soil treatments that have been used for agriculture are more similar in community composition and diversity to each other than to a treatment never used for agriculture. However, there were no significant differences between the three communities, suggesting that Planctomycetales are marginally affected by agricultural practices.

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