Influence of carbon source and availability on denitrification, nitrous oxide emissions and denitrifier community abundance
Bacterial denitrification plays an important role in the global N cycle and is a principal contributor of nitrous oxide (N2O) to the atmosphere. The objective of this study was to quantify the effect of carbon (C) source and availability on the amount of denitrification and N2O molar ratio of gaseous denitrification losses, and abundance of soil total bacterial and denitrifier communities. Both simple (glucose) and complex (plant residues and liquid manures) C sources were examined. Total bacterial and denitrifier communities were quantified using real-time PCR targeting the 16S rRNA, nitric oxide (NO) reductase ('cnorB') and N2O reductase ('nosZ') genes. The relationship between the supply of, and demand for, terminal electron acceptors, as determined by the relative availability of C and NO3-, influenced the amount of denitrification and the N2O molar ratio (i.e., N2O:(N2+N 2O)), regardless of C source. Denitrification activity and the abundance of denitrifiers were not homogenously distributed across different aggregate size fractions from contrasting management systems, however, the activity and distribution of denitrifiers were uncoupled across aggregate fractions. In bulk soil, both the quality and quantity of C amendments affected the abundance of the total bacterial community and components of the soil denitrifier community, however, the response varied with the community studied. Significant positive correlations were measured between cumulative CO2 emissions and denitrifier community densities, whereas no significant correlations were found between the abundance of denitrifier communities and total denitrification, N2O emissions or the N2O molar ratio, suggesting that differences or changes in the abundance of the denitrifiers were a result of differences in C substrate availability rather than being coupled to activity changes (i.e., denitrification). This study demonstrated the value of studying a combination of both broad and narrow groups of microbial communities: narrow primer sets revealed measurable changes of more specific denitrifier communities over time while a broader primer set reflected the response of a larger component of the denitrifier community. This study illustrated that using quantitative PCR to measure the abundance of soil microbial communities can be an effective tool in studies aimed at understanding the affect of C amendments on denitrification and the N2O molar ratio.