Season and management related changes in the diversity of nitrifying and denitrifying bacteria over winter and spring

This study assessed the effects that season and tillage practices have on the diversity of nitrous oxide producing bacteria (nitrifiers and denitrifiers) and to relate this to measured N2O fluxes at our field site. Large-scale held plots (1.5 ha) were established in Elora, Ontario in 2000, and managed using conventional tillage (CT) or no-tillage (NT). Each field plot was instrumented with micrometeorological equipment to determine N2O fluxes on a field scale. Soil samples were taken at four time points between the fail of 2005 and the spring of 2006. The diversity of the nitrifier and denitrifier communities was assessed by PCR-denaturing gradient gel electrophoresis (DGGE) using primer pairs targeting the amoA, nirS and nirK gene. Seasonal variation (a combination of soil temperature, available soil moisture, nutrient levels and other potential factors) had the largest influence on the diversity of nitrifier and denitrifier populations: while tillage practice also influenced the diversity of the microbial community at certain time periods. Tillage significantly affected all communities in March and affected denitrifiers on all other dates except for the nirS community in February. Further statistical analysis revealed that diversity of the nitrifying and denitrifying populations was the lowest in February, in frozen soils, and rapidly increased in March, corresponding with spring thaw N2O emissions. Long-term soil nutrient, temperature and N2O data taken at this site added additional information on the dynamics of the nitrogen cycle. (C) 2009 Elsevier B.V. All rights reserved.