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Vertical distributions of sulfate-reducing bacteria and methane-producing archaea were investigated in the profundal sediment of a freshwater lake using membrane-immobilized small subunit rRNA hybridization with group- and genus-specific oligonucleotide probes. The annual average of the relative abundance of small subunit rRNA hybridized with all probes for sulfate-reducing bacteria to total small subunit rRNA was 2.3% at 0-2 cm and increased with depth up to 22.9% at 8-14 cm where sulfate concentration was less than 10 nmol ml(-1) in interstitial water, suggesting that these bacteria may survive on alternative metabolisms. The signal of probe Dsv687 (the family Desulfovibrionaceae and some Geobacteraceae) was the main factor in this increase. The relative abundance of methane-producing archaea to total small subunit rRNA was highest (7.8%) at 8-14 cm, dominated by the order Methanosarcinales. The metabolic rates measured in the sediments demonstrated that the peaks of sulfate reduction and methane production were separated vertically, and were not linked to their small subunit rRNA distributions. Our data indicate that sulfate-reducing bacteria can coexist with methane-producing archaea from 0 to 20 cm in the freshwater lake sediment.

[1] To clarify the mechanisms of N 2 O production and consumption in a humid temperate forest, we measured concentrations of gaseous N 2 O in unsaturated soil and dissolved N 2 O in groundwater in a forested headwater catchment. We especially focused on the hydrological controls on supplies of electron donors and acceptors for nitrification and denitrification, and their effects on N 2 O production and consumption. N 2 O concentrations were higher in the groundwater (0.6-148.0 times higher than water equilibrated with atmospheric N 2 O) than in the soil gas (1.0-5.4 times higher than atmospheric N 2 O), suggesting that N 2 O was mainly produced in the groundwater. Moreover, N 2 O production was greatest at the periphery of the groundwater body where both DOC and NO 3 À were readily available from shallower soils upstream. In contrast, N 2 O was reduced to N 2 in the deeper layer of the groundwater body where DO and NO 3 À were not readily available. N 2 O concentrations in the unsaturated soil layer were remarkably low despite high N 2 O concentrations in the groundwater. N 2 O flux from the spring out point was about 100 times higher than at any other observation site, and there was a decrease in dissolved N 2 O concentrations from springwater to streamwater. This indicates that N 2 O produced in the groundwater body did not readily diffuse into the unsaturated soil layer above the groundwater surface, but was transported by groundwater flow to the spring out point and then emitted into the atmosphere, implying the possible mechanism of N 2 O production, consumption and emission in forested catchments.

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