AbstractsEarth & Environmental Science

Denitrification is responsible for gaseous nitrogen (N) loss from in riparian buffers. Earthworms affect denitrification in controlled laboratory and field studies; however, the small-scale effects of earthworm on denitrification need to be extrapolated to the field scale. The general objective of this thesis was to determine how earthworm-denitrifying bacteria interactions could affect N dynamics at a physiological level (within the earthworm body), the individual level (earthworm drilosphere), then finally determine whether these small-scale effects could be detected at the field scale (in riparian buffers). In a microcosm study (physiological level), earthworms were fed with organic substrates with different C:N ratio, but earthworm maintained a constant C:N ratio of 3.37 to 5.25 in their muscular tissue, regardless of the food N content. Adult Lumbricus terrestris had a significantly greater denitrification rate with the N-rich soybean mixture than with peat moss. These results suggest that adult L. terrestris consuming N-rich organic substrates may contribute to N2O and N2 fluxes from soil. In a mesocosm study (drilosphere level), earthworm presence increased the cumulative N2O emissions by 50% in the dry soil treatment, but earthworms reduced the cumulative N2O emissions by 34% in the wet soil treatment and reduced N2O emissions significantly by 82% in soil with rewetting-drying cycles (WD). Denitrification enzyme activity (DEA) increased significantly when earthworms were present and the abundance of 16S rRNA, nirS, and nosZ genes was affected significantly by the earthworm × soil moisture interaction. These results suggested that the decrease in cumulative N2O emissions from wet soil and the WD treatment by earthworms was due to a general alteration of the denitrifying bacterial community composition. Moreover, the results implied that earthworms would decrease the N2O emissions from saturated soils. At the field scale, earthworm demographics were investigated in temporary flooded riparian region (TR) and non-flooded riparian region (NR) in Quebec, Canada, from spring to autumn, 2012. The TR had more earthworm diversity (9 species) and larger population and biomass than NR. Earthworm population and biomass were largest in spring and autumn but declined in summer. Path analysis indicated that soil moisture, NH4+ and soil C:N ratio, but not earthworm biomass, directly affected the DEA. This observation suggests that earthworm-denitrifier interactions in riparian buffers were the result of soil moisture content and available substrate concentrations. In conclusion, my results indicate that physiological scale effects cannot be extrapolated directly from the lab to the field. Studies at the mesocosm and field scales suggest that the N2O output from riparian soils is the result of the moisture-earthworm-microbial interaction: soil moisture act as a crucial controller on the final product of denitrification (N2O or N2), while earthworms influence the gaseous N losses from natural riparian buffers through both…