The earthworm gut is a unique microzone in aerated soils that has been proposed to selectively stimulate ingested soil microorganisms by its in situ conditions, which include anoxia, high water content, a near-neutral pH, and high concentrations of organic compounds. The central objective of this study was to resolve potential links between in situ conditions and anaerobic microbial activities during the gut passage of Lumbricus terrestris. Both H 2 and N 2 O were emitted by living earthworms, and in situ microsensor analyses revealed both H 2 and N 2 O in the O 2 -free gut center. The highest H 2 concentrations occurred in foregut and midgut regions, whereas the highest N 2 O concentrations occurred in crop/gizzard and hindgut regions. Thus, H 2 -producing fermentations were more localized in the foregut and midgut, whereas denitrification was more localized in the crop/gizzard and hindgut. Moisture content, total carbon, and total nitrogen were highest in the foregut and decreased from the anterior to posterior end of the gut. Nitrite, ammonium, and iron(II) concentrations were highest in the crop/gizzard and decreased from the anterior to posterior end of the alimentary canal. Concentrations of soluble organic compounds were indicative of distinct fermentation processes along the alimentary canal, with maximal concentrations of organic acids (e.g., acetate and butyrate) occurring in the midgut. These findings suggest that earthworms (i) contribute to the terrestrial cycling of carbon and nitrogen via anaerobic microbial activities in the alimentary canal and (ii) constitute a mobile source of reductant (i.e., emitted H 2 ) for microbiota in aerated soils.Earthworms constitute the dominant soil macrofauna in many soils (15,34,36) and emit the greenhouse gas nitrous oxide (N 2 O) concomitantly with molecular nitrogen (N 2 ) via denitrifying bacteria in the gut (10,11,24,26,30,39). The in situ conditions of the earthworm gut, which include anoxia, high water content, a near-neutral pH, and high concentrations of organic compounds, may activate certain ingested soil bacteria, in particular bacteria capable of anaerobiosis (10, 11, 27, 28-30, 33, 62). In this regard, the numbers of cultured anaerobes and denitrifiers are two to three orders of magnitude greater in the earthworm gut than in soil (28-30), and the amount of bacterial long-chain fatty acids is higher in the gut than in soil (46).Gut contents of earthworms contain up to 80% intestinal mucus (i.e., up to 0.8 g mucus per g [dry weight] gut content) that consists of monosaccharides, low-molecular-weight amino acids, and glycoproteins (5,37,42,62). This large amount of degradable organic carbon in the anoxic gut suggests that fermentation is very active during gut passage (27). Indeed, fermentative bacteria are abundant in the gut of earthworms (25,28,30). Nitrate reducers also are abundant in the earthworm gut (28) and can produce fermentation products when nitrate is not available (58). Many fermentative or facultative microorganisms also can reduce nit...