The role of earthworms in nitrous oxide emissions from forage agroecosystems.

摘要

Nitrous oxide (N2O) emission from agricultural soils contributes to the greenhouse effect, depletion of the ozone layer, and acid rain formation. In laboratory studies, soil in contact with earthworms had significantly higher N2O production than soil devoid of earthworm activity. The elevated N2O production occurs via activation or priming of nitrifying and/or denitrifying microorganisms. Yet, field scale experiments have been inconclusive with respect to N2O fluxes attributed to earthworms. The objectives of this thesis were in the field to (1) determine the effect of elevated anecic and endogeic earthworm populations on N2O fluxes from legume and non-legume enclosures, and in the laboratory to (2) qualify the denitrification community and (3) quantify the denitrification rate of earthworm-related structures including the gut, casts, burrows and middens of the anecic Lumbricus terrestris. In the field study, elevated earthworm populations increased N2O fluxes significantly (P0.05) in legume but not in non-legume enclosures. Anecic species appeared to have a greater effect on N2O fluxes. The linear regression model of the flux data and number of earthworms added was significant (P0.05); on average earthworms emitted 335 ng N2O m-2 h -1 from the legume enclosures when soil moisture exceeded 60% water-filled pore space. In the laboratory study, a 454 pyrosequencing analysis on the diversity and phylogeny of bacterial nosZ gene fragments (encoding N2O reductase) revealed a set of 39 unique OTUs, of which 14 were found only in the earthworm gut. The earthworm gut appears to have a unique set of endemic denitrifying bacteria. An acetylene block assay revealed that the earthworm itself had the greatest denitrification rate of 2.67 mug N2O-N g soil-1 h-1, which was significantly (P0.05) greater than bulk soil and other earthworm structures except fresh casts. Fresh casts had a denitrification rate of 0.94 mug N 2O-N g soil-1 h-1 which was significantly (P0.05) greater than the rate of aged cast and bulk soil. The set of unique gut denitrifiers is highly active due to the favourable conditions in the gut, including anoxia and quality substrate. When fresh, anecic earthworm related structures are important "hot spots" favouring the activity of soil denitrifiers. This thesis demonstrates the importance of earthworm-denitrifier interactions for N2O emissions, particularly in legume-based agroecosystems, which should be considered when developing predictive models of greenhouse gas emissions from agricultural soils.