Microbiological characterization and nitrate reduction in subsurface soils

Summary Two borings (20 m depth) were performed in a sandy-clayey soil over a limestone bed and in a sandy soil with lumps of clay in some depths. Bacteria were found in the deeper soil layers of both profiles. The methods used to detect bacteria were those normally used for topsoil layers, plate counts of bacteria, ATP content, and direct microscopy. Measurements of CO₂ evolution showed that the potential for bacterial activity was present in all depths of the two profiles. However, the activity was strongly dependent on the presence of easily available organic C. An indication of the denitrification potential was obtained by measuring the N₂O evolution. Under aerobic incubation without the addition of glucose, N₂O was detected only in the topsoil. When glucose was added to the soil samples, N₂O was found at a low level in the deeper soil layers. Under anaerobic incubation, N₂O was detected in all deeper layers, and increased markedly when glucose was added to the soil samples.