Nitrifier and Denitrifier Response to Swine Effluent Application Methods.

Soil microbial communities have been proposed as indicators of soil quality due to their importance as drivers of global biogeochemical cycles and their sensitivity to management and climatic conditions. It is only recently that it has been possible, through application of sophisticated molecular microbiological methods, to sensitively and specifically target these important microbial populations at the field-scale and in differentially managed soil systems. In general, much is unknown regarding the effect of management fluctuations on important functional groups in soils systems (i.e., methanogens, nitrifiers and denitrifiers).  In this study, quantitative, real-time PCR (qPCR) was used to quantify changes in nitrifier (amoA), denitrifiers (narG) and methanogens (mcrA) in agricultural soils with three different swine effluent application methods including surface application, direct injection, and application in combination with soil aeration. Two groups showed significant response to effluent application; nitrifying bacteria (AOB) and nitrate reducers (narG). Nitrifying bacteria averaged 4.6 ± 1.9 X 10⁵ cells and 1.4 - 3.3 X 10⁷ cells per gram of soil for control and treatment samples, respectively. Interestingly, nitrifying archaeal populations did not change in response to swine effluent application 2.4 ± 0.5 X 10⁷ cells and 1.5 ± 0.4 X 10⁷ cells per gram of soil for control and treatment samples, respectively. Methanogens and methane oxidizers also increased more than seven orders of magnitude but were likely brought in with the swine effluent. Results suggest that effluent application significantly affects some microbial groups, but not others. Manipulation of these important functional communities may aid in reducing GHG production in soils with applied manures.