A series of column experiments were carried out to elucidate the effects of microorganisms on reduction in the saturated hydraulic conductivity, Ks, of paddy field soil.
The K_s of surface 1 cm layer decreased 2 orders of magnitude during 118 day period of percolation of 50 ppm glucose solution. After the alternation of percolation to germicide (NaN3), K_s increased rapidly. Population of bacteria near the surface did not increase, but that of fungi near the surface increased hundred times. Therefore it was proved that the first cause of K_s reduction is clogging of soil pores by hyphae of fungi and microbial synthesized products.
The second cause of K_s reduction is occlusion of pore space by gas produced by anaerobic bacteria. The redox potential of all depths was below-150 mV at 15 day, which was sufficiently met for the CH₄ production. From calculation of production and resolving rate, it was explained that all CO₂ resolved in percolation, and that CH4 did not resolve perfectly. It is estimated that 14% of gas filled pore was occluded by CH₄ gas and the occlusion contributed to the K_s reduction.