Ph.D. Thesis Defense: Ms. Chen Chen: Earthworm interactions with denitrifying bacteria in riparian buffers: Signficance for nitrogen dynamics from the physiological to ecological scales
pre-defense will be at 13:45 (room MS2-022)
Denitrification is responsible for much of the gaseous nitrogen (N) loss from terrestrial ecosystems, particularly in riparian buffers where periodic flooding results in anoxic conditions that favor the activity of bacterial denitrifiers. Earthworms affect denitrification in controlled laboratory and field studies, indicating that earthworm-denitrifier interactions occur across temporal and spatial scales. However, the effects of earthworm on denitrification need to be evaluated at spatio-temporal scales that can be extrapolated to the ecosystem scale. The general objective of this thesis was to determine how earthworm-denitrifying bacteria interactions affect N dynamics at a physiological level (within the earthworm body) to the individual level (earthworm drilosphere), then finally determine whether these small-scale effects can be detected at the ecosystem scale (in riparian buffers). In the 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 L. terrstris had a significantly greater denitrification rate with the N-rich soybean mixture than with peat moss, but there was more variability in denitrification from A. tuberculata. These results suggest that earthworms are a source of gaseous N loss, such that ecosystems with abundant L. terrestris population and N-rich organic substrates would have greater N2O and dinitrogen (N2) fluxes due to earthworms. In a 69 day mesocosm study (drilosphere level), earthworm presence increased the cumulative N2O emissions by 50% in the dry soil treatment, but earthworms decreased cumulative N2O emissions by 34% in the wet soil treatment and reduced N2O emissions significantly by 82% in the treatment of 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 soil at wet soil and the WD treatment by earthworms was due to their stimulation of N2O consuming bacteria and a general alteration of the denitrifying bacterial community composition. Moreover, the results implied that earthworms would decrease the N2O emissions from saturated soils. In the ecological 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 (6 species). Earthworm population and biomass were largest in spring and autumn but declined in summer. Earthworm presence significantly decreased the DEA by 1.5 times in TR and 1.2 times in NR. Path analysis indicated that soil moisture was the most important factor influencing the DEA, and earthworms affected the DEA directly, probably due to earthworm bioturbation that disrupted the living habitats of denitrifiers, as well as indirectly by reducing the soil NO3- concentration. In conclusion, my results indicate that the effects measured at the small scale (within the earthworm body) cannot be extrapolated directly from the lab to the field because the indirect earthworm effects would overwhelm the soil N dynamics and N2O production. Nevertheless, the mesocosm scale work is more relevant to the earthworm influence on N2O in field. The studies from the mesocosm scale and the field scale suggest that the N2O output from riparian soils is the result of the moisture-earthworm-microbial interaction: soil moisture act as a crucial control on the final product of denitrification (N2O or N2) and earthworms reduced the soil denitrifier activity in the saturated soils. Moreover, earthworms influence the gaseous N losses from natural riparian buffers through both direct and indirect effects on denitrifiers and substrates required for the denitrification reaction.