Gut-Associated Denitrification and In Vivo Emission of Nitrous Oxide by the Earthworm Families Megascolecidae and Lumbricidae in New Zealand

Abstract

Previous studies have documented the capacity of European earthworms belonging to the family Lumbricidae to emit the greenhouse gas nitrous oxide (N(2)O), an activity attributed primarily to the activation of ingested soil denitrifiers. To extend the information base to earthworms in the Southern Hemisphere, four species of earthworms in New Zealand were examined for gut-associated denitrification. Lumbricus rubellus and Aporrectodea rosea (introduced species of Lumbricidae) emitted N(2)O, whereas emission of N(2)O by Octolasion cyaneum (an introduced species of Lumbricidae) and emission of N(2)O by Octochaetus multiporus (a native species of Megascolecidae) were variable and negligible, respectively. Exposing earthworms to nitrite or nitrate and acetylene significantly increased the amount of N(2)O emitted, implicating denitrification as the primary source of N(2)O and indicating that earthworms emitted dinitrogen (N(2)) in addition to N(2)O. The alimentary canal displayed a high capacity to produce N(2)O when it was supplemented with nitrite, and alimentary canal contents contained large amounts of carbohydrates and organic acids indicative of fermentation (e.g., succinate, acetate, and formate) that could serve as sources of reductant for denitrification. nosZ encodes a portion of the terminal oxidoreductase used in denitrification. The nosZ sequences detected in the alimentary canals of L. rubellus and O. multiporus were similar to those retrieved from soil and were distantly related to sequences of uncultured soil bacteria and genera common in soils (i.e., Bradyrhizobium, Azospirillum, Rhodopseudomonas, Rhodospirillum, Pseudomonas, Oligotropha, and Sinorhizobium). These findings (i) suggest that the capacity to emit N(2)O and N(2) is a general trait of earthworms and not geographically restricted, (ii) indicate that species belonging to different earthworm families (i.e., Megascolecidae and Lumbricidae) may not have equal capacities to emit N(2)O, and (iii) also corroborate previous findings that link this capacity to denitrification in the alimentary canal.