Soil Ammonia-Oxidizing Bacterial Communities in Paddy Rice Fields as Affected by Upland Conversion History

Rotation of paddy rice and upland crop cultivation is a major type of land use in eastern Asia. Temporary upland conversion of paddy field also occurs frequently in this region. Little is known about the affects of these agricultural practices on soil microbial communities. The purpose of this study was to investigate the effect of former upland conversion (upland rice [Oryza sativa L.] cultivation or soybean [Glycine max (L.) Merill]-wheat [Triticum aestivum L.] cultivation) on sod ammonia-oxidizing bacteria (AOB) in a paddy rice field. The AOB population size and community structures were determined by real-time polymerase chain reaction (PCR) and PCR-denaturing gradient gel electrophoresis (DGGE), respectively. Soil AOB communities were more diverse and had larger population size in the plots with upland conversion history than in consecutive paddy rice plots. Former soybean-wheat cultivation had a much larger effect on sod AOB communities than former upland rice cultivation. Phylogenetic analysis showed that DGGE bands derived from former upland conversion belonged to Nitrosospira amoA cluster 1. Nitrosospira amoA Cluster 1 became dominant over Nitrosospira amoA Cluster 3 in the plots wit former soybean-wheat cultivation. Our results demonstrate that upland conversion history exhibit a significant effect on sod AOB communities in paddy rice field, and the effect depends strongly on the type of upland crop.