Phosphorus status of paddy soils from the central region of the Mekong River in Laos

Microorganisms colonizing the leaf sheaths of rice plants placed in a waterlogged paddy field were studied using a scanning electron microscope. Decomposition was faster on the adaxial side than on the abaxial side of the leaf sheaths. The veins and trichomes were highly resistant to microbial decomposition throughout the waterlogged period of rice growth. The decomposition and disintegration of leaf sheaths markedly progressed from the third month (from July) after the placement in the paddy field. Microbial colonization was gradual in the first month after the placement of leaf sheaths in the field. One of the common first colonizers had a terminal endospore (Clostridium dissolvens-like microorganisms). Microbial colonization proceeded actively in the second month, and microbiota became diversified. Microbial colonization on the adaxial surface was more than that on the abaxial surface. Respective microorganisms colonized the surfaces separately from each others in this period. They grew mainly on the thin membrane that covered the epidermal layer. Microbial colonies spread all over the abaxial and adaxial surfaces, and there was no significant difference in the degree of colonization between the abaxial surface and the adaxial surface in the third month (in July). Although separate colonization of the respective microorganisms was common, the co-existence of different microorganisms within a colony was also observed. The thin membrane that covered the epidermal layer and the epidermis seemed to be the main substrate during this period. Co-existence of different microorganisms was a common occurrence in colony formation, and organic debris and remaining tissues of leaf sheaths were assumed to be the major substrates in August. These findings on microbial colonization were well correlated with the findings on the degree of decomposition.