Linking N2O concentrations in different soil depths to denitrification genes abundances under the influence of an elevated atmospheric CO2 concentration

The production of nitrous oxide (N2O) from agricultural soils by denitrification forms a major part of the global emission of this important greenhouse gas. Due to a lack of the nosZ gene encoding the nitrous oxide reductase, some denitrifying bacteria are not able to reduce N2O to N2. Until now, factors influencing the proportion of denitrifying bacteria having this truncated pathway are not well understood. Under a changing climate, namely increasing concentrations of CO2 in the atmosphere, plant growth and exudation as well as soil moisture will be influenced which probably also affects conditions for denitrifying bacteria in soil. We investigated in the present study N2O production rates as well as densities of functional genes (narG, napA, nirK, nirS and nosZ) encoding reductases involved in the denitrification process in different soil depths of an oilseed rape field under both, ambient and elevated CO2 concentrations (Mini-FACE Experiment) during the growing season 2007. Gas samples were collected in soil air sampling probes installed at three different soil depths (5, 15 and 30 cm). Soil samples were collected at three depths (0-10, 10-20 and 20-30 cm) on sixdates during the season 2007. Soil temperature, precipitation and soil moisture content were monitored in the field. N2O concentrations in soil were highest in deeper soil layers under moist and high nitrate conditions. Total amounts of DNA as well as functional gene densities of denitrifying bacteria changed during the vegetation period with lower values when soil was dry. Effects of soil depth dominated over those of elevated atmospheric CO2 concentrations. However, during some periods throughout oilseed rape growth altered ratios of nosZ to napA indicate temporal changes in the soil bacterial denitrifier community.