In Vivo Emission of Dinitrogen by Earthworms via Denitrifying Bacteria in the Gut

Earthworms emit the greenhouse gas nitrous oxide (N2 O), and ingested denitrifiers in the gut appear to be the main source of this N2 O. The primary goal of this study was to determine if earthworms also emit dinitrogen (N2 ), the end product of complete denitrification. When [15 N]nitrate was injected into the gut, the earthwormsAporrectodea caliginosa andLumbricus terrestris emitted labeled N2 (and also labeled N2 O) under in vivo conditions; emission of N2 by these two earthworms was relatively linear and approximated 1.2 and 6.6 nmol N2 per h per g (fresh weight), respectively. Isolated gut contents also produced [15 N]nitrate-derived N2 and N2 O under anoxic conditions. N2 is formed by N2 O reductase, and acetylene, an inhibitor of this enzyme, inhibited the emission of [15 N]nitrate-derived N2 by living earthworms. Standard gas chromatographic analysis demonstrated that the amount of N2 O emitted was relatively linear during initial incubation periods and increased in response to acetylene. The calculated rates for the native emissions of N2 (i.e., without added nitrate) byA. caliginosa andL. terrestris were 1.1 and 1.5 nmol N2 per h per g (fresh weight), respectively; these emission rates approximated that of N2 O. These collective observations indicate that (i) earthworms emit N2 concomitant with the emission of N2 O via the in situ activity of denitrifying bacteria in the gut and (ii) N2 O is quantitatively an important denitrification-derived end product under in situ conditions.