Vertical profiles of CH concentrations, dissolved substrates and processes involved in CH production in a flooded Italian rice field

Vertical profiles were measured in soil cores taken from flooded rice fields in the Po valley during July and August 1990. Methane concentrations generally increased with depth and reached maximum values of 150-500 μM in 5-13 cm depth. However, the shape of the profiles was very different when studying different soil cores. The CH content of gas bubbles showed a similar variability which apparently was due to spatial rather than temporal inhomogeneities. Similar inhomogeneities were observed in the vertical profiles of acetate, propionate, lactate, and formate which showed maximum values of 1500, 66, 135, and 153, μM, respectively. However, maxima and minima of the vertical profiles of the different substates usually coincided in one particular soil core. Large inhomogeneities in the vertical profiles were also observed for the rates of total CH production, however, the percentage contribution of H/CO to CH production was relatively homogeneous at 24 ± 7% (SD). Similarly, the H content of gas bubbles was relatively constant at 93.3 ± 9.6 ppmv when randomly sampled in the rice field at different times of the day. A small contribution (6%) of H/CO to acetate production was also observed. Vertical profiles of the respiratory index (RI) for [2-C] acetate showed that acetate was predominantly degraded by methanogenesis in 5-11 cm depth, but by respiration in the surface soil (3 cm depth) and in soil layers below 13-16 cm depth which coincided with a transition of the colour (grey to reddish) and the physical characteristics (porosity, density) of the soil. The observations indicate that the microbial community which degrades organic matter to CH is in itself relatively homogenous, but operates at highly variable rates within the soil structure. © 1993 Kluwer Academic Publishers.