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

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Abstract
Earthworms emit the greenhouse gas nitrous oxide (N2O), and ingested denitrifiers in the gut appear to be the main source of this N2O. The primary goal of this study was to determine if earthworms also emit dinitrogen (N2), the end product of complete denitrification. When [15N]nitrate was injected into the gut, the earthwormsAporrectodea caliginosaandLumbricus terrestrisemitted labeled N2(and also labeled N2O) under in vivo conditions; emission of N2by these two earthworms was relatively linear and approximated 1.2 and 6.6 nmol N2per h per g (fresh weight), respectively. Isolated gut contents also produced [15N]nitrate-derived N2and N2O under anoxic conditions. N2is formed by N2O reductase, and acetylene, an inhibitor of this enzyme, inhibited the emission of [15N]nitrate-derived N2by living earthworms. Standard gas chromatographic analysis demonstrated that the amount of N2O 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. caliginosaandL. terrestriswere 1.1 and 1.5 nmol N2per h per g (fresh weight), respectively; these emission rates approximated that of N2O. These collective observations indicate that (i) earthworms emit N2concomitant with the emission of N2O via the in situ activity of denitrifying bacteria in the gut and (ii) N2O is quantitatively an important denitrification-derived end product under in situ conditions.