PARENT SESSION
Contributed Oral Session 125: Soil Ecology: Carbon - Nitrogen Cycling
Thursday, August 11, 8:00 AM - 11:30 AM, Meeting Room 524 B, Level 5, Palais des congrès de Montréal

Earthworm gut-associated processes stimulate denitrification.

Sampedro, Luis^{1, 2}, Whalen, Joann^*,1, ¹ Dpt Natural Resource Sciences, McGill University, Ste-Anne-Bellevue, Quebec, CANADA² Centro de Investigacions Forestais e Ambientais Lourizan, Pontevedra, Galicia, SPAIN

ABSTRACT- Earthworms are recognized as key regulators of nitrogen cycling, particularly nitrogen mineralization, but little is known about how earthworms influence denitrification. In this paper we studied how denitrification from an organic substrate was affected by nitrate availability, earthworm density and CO₂ concentration in the headspace of 500 ml microcosms. The organic substrate used in this study was washed cow manure (primarily lignocellulose), and the treatments consisted of 4 levels of nitrate and 4 earthworm densities. We analyzed the fluxes of CO₂ and N₂O, denitrification potential and ancillary soil properties. The net flux of N₂O from the organic substrate increased markedly with increasing nitrate availability and earthworm numbers, while carbon mineralization was not so strongly affected. The natural N₂O emission rate was sensitive to the CO₂ concentration in the headspace of the microcosms with earthworms, but no relationship between N₂O emission and CO₂ concentration was observed in the controls. N₂O natural emission after 21 days was detected only in microcosms with earthworms, and the denitrification potential was 25-fold to 50-fold greater than natural N₂O emission. Denitrification was low when earthworms and casts (fecal material) were removed from the organic substrate, although denitrification was greater when earthworms were removed but fresh casts were present in the organic substrate. Denitrification activity from the earthworm′s body decreased significantly when the animals were not fed for 24 h, compared to when they had a full gut. Earthworms were identified as key regulators of denitrification in the organic substrate, and the gut of epigeic earthworms was revealed as the functionally active microsite for denitrifying bacteria. Denitrification by the earthworm gut microbiota is a Gut-Associated Process (GAP) with proxy effects at the earthworm scale, but may have important consequences at the ecosystem-level that remain to be determined.

Key words: Denitrification, earthworms, Gut-Associated Processes