Chitosan degradation and associated changes in bacterial community structures in two contrasting soils

Chitosan, a polymer of glucosamine, is one of the natural polysaccharides which are used as soil-amendment materials. We report here chitosan degradation and the corresponding effects on microbes in two distinctive types of soil. Having established a method to measure amounts of chitosan which was added to soil, the degradation of powdered chitosan in a sandy or a silty soil was investigated. In the chitosan-added sandy soil, chitosan amount and total carbon (C) content did not significantly change even after 30d. In contrast, chitosan added to the silty soil decreased to less than half of the initial amount (50mgg(-1) soil) after 10d incubation at 25 degrees C and seemed to be completely consumed after 30d. Total carbon and nitrogen (N) contents also declined according to the chitosan decrement, whereas the C/N ratio did not alter. Ammonium nitrogen was increasing over 0.7mgNg(-1) soil, during the 11d after the addition of chitosan. 16S rRNA gene-based polymerase chain reaction (PCR)-denaturant gradient gel electrophoresis (DGGE) analysis revealed that addition of chitosan caused drastic alteration of bacterial community structure in the silty soil, in contrast to fewer changes in the sandy soil. Nucleotide sequence analysis following DGGE revealed that actinobacteria belonging to genera Streptomyces and Kitasatospora strongly increased within 4d after the addition of chitosan to the silty soil. Seven out of eight chitosan-degrading bacteria, isolated from the silty soil 10d after the addition of chitosan, were identified to be Streptomycetes, based on the nucleotide sequence of the 16S rRNA gene. Among them, two strains, AMI7 and AMI10, exhibited identical nucleotide sequences with those detected in the PCR-DGGE analysis. The data strongly implied that those atcinobacterial genera, Streptomyces and probably Kitasatospora, are involved in the initial degradation process of chitosan in the silty soil. The presence and quantity of these genera may influence the chitosan degradation rate in soil.