Evidence for involvement of gut-associated denitrifying bacteria in emission of nitrous oxide (N2O) by earthworms obtained from garden and forest soils

Earthworms (Aporrectodea caliginosa, Lumbricus rubellus, and Octolasion lac teum) obtained from nitrous oxide (N2O)-emitting garden soils emitted 0.14 to 0.87 nmol of N2O h(-1) g (fresh weight)(-1) under in vivo conditions. L. rubellus obtained from N2O-emitting forest soil also emitted N2O, which co nfirmed previous observations (G. R. Karsten and H. L. Drake, Appl, Environ , Microbiol, 63:1878-1882, 1997), In contrast, commercially obtained Lumbri cus terrestris did not emit N2O; however, such worms emitted N2O when they were fed (i.e., preincubated in) garden soils.A. caliginosa, L. rubellus, a nd O. lacteum substantially increased the rates of N2O emission of garden s oil columns and microcosms. Extrapolation of the data to in situ conditions indicated that N2O emission by earthworms accounted for approximately 33% of the N2O emitted by garden soils. In vivo emission of N2O by earthworms o btained from both garden and forest soils was greatly stimulated when worms were moistened with sterile solutions of nitrate or nitrite; in contrast, ammonium did not stimulate in vivo emission of N2O, In the presence of nitr ate, acetylene increased the N2O emission rates of earthworms; in contrast, in the presence of nitrite, acetylene had little or no effect on emission of N2O. In vivo emission of N2O decreased by 80% when earthworms were prein cubated in soil supplemented with streptomycin and tetracycline, On a fresh weight basis, the rates of N2O emission of dissected earthworm gut section s were substantially higher than the rates of N2O emission of dissected wor ms lacking gut sections, indicating that N2O production occurred in the gut rather than on the worm surface. In contrast to living earthworms and gut sections that produced N2O under oxic conditions (i.e,, in the presence of air), fresh casts (feces) from N2O-emitting earthworms produced N2O only un der anoxic conditions. Collectively, these results indicate that gut-associ ated denitrifying bacteria are responsible for the in vivo emission of N2O by earthworms and contribute to the N2O that is emitted from certain terres trial ecosystems.