Seasonal variations in vertical distribution of methanogenic activity and Fe(II) content and relationship between them in wetland rice field soil

Methanogenic activity and Fe(II) content in soil samples collected at diffe rent depths of the plow layer in a wetland rice field were determined over a period of 1 y. The methanogenic activity increased at first in the top la yer of soil (0-1 cm in depth) during the continuous flooding period of the field and the activity decreased after the period of midseason drainage. In contrast, in the deeper soil layers, the increase in the activity persiste d during the period of intermittent irrigation after midseason drainage and the period of the highest methanogenic activity at each depth in the year varied for about 1 month depending on the depth. The Fe(II) content in soil did not change appreciably during the continuous flooding period regardles s of the soil depth. However, after midseason drainage, the content signifi cantly varied with depth. Thus, until the midseason drainage, a sharp incre ase in the methanogenic activity preceded the increase in the Fe(II) conten t in the surface (0-1 cm) soil layer, while the opposite was observed in th e deeper soil layers. Based on these data, the relationship between the met hanogenic activity and Fe(II) content in soil was investigated by calculati ng the correlation coefficients between them and comparing the slopes and c onstants of regression lines obtained. It was found that although during th e cropping season of rice the methanogenic activity depended mainly on the Fe(II) content in soil as a whole, the relationship between the two paramet ers was different before and after the period of midseason drainage. The re lationship between them before the midseason drainage was distinctly differ ent depending on the depth, while after the midseason drainage the relation ship was almost the same irrespective of the depth. During the non-cropping season, no correlation was obtained between the two parameters. Most of th e correlation coefficients between the methanogenic activity and the soil, temperature were low for all the data sets examined except for the deepest soil layer (13-15 cm) during the cropping season. Correlation coefficients between the methanogenic activity and water content in soil were also low.