Continuous effects of winter flooding on soil fauna, as revealed by community structure of soil nematodes in a paddy field in northern Japan

Beneficial effects of winter flooding in rice paddies have been proved for birds and other above-ground wildlife. However, there have been few studies that accounted for soil organisms. In this study the effects of winter flooding together with an organic amendment on soil fauna as represented by nematodes were examined. Soil samples were taken from experimental rice paddies in northern Japan with one of three treatments, i.e., winter flooding with organic farming (WFO), no winter flooding with organic farming (NFO), and no winter flooding with conventional farming (CVN, only chemical fertilizers and chemicals applied). During this study period of over one year and a half, nematodes were extracted and counted to estimate their density. Then to evaluate nematode diversity, a polymerase chain reaction-denaturing gradient gel electrophoresis analysis of the nematode 18S rRNA gene was conducted， which found 34 operational taxonomic units with Tobrilus spp. and Hirschmanniella sp. as the three most dominant taxa. Statistical analysis showed the following trends: 1) nematode density in the 0–5 cm soil layer was WFO > NFO > CNV, and 2) nematode diversity, H′, in the 5–10 cm layer was WFO < CNV < NFO. The results of our nematode and soil physico-chemical analyses suggest, 1) winter flooding and organic farming could increase nematode density at least in the upper soil layer with increased fertility, and 2) on the contrary, winter flooding could be harmful to maintain nematode diversity in the deeper part of soil, probably due to an increased soil reduction level.