Nematode fauna of paddy field flooded all year round

The rice paddy fields in Asia are being recognized as biodiversity hotspots. We have reported the seasonal dynamics of nematode communities in a conventional type of paddy field (CPF) in Japan, which is flooded only in spring and summer. The CPF nematode fauna included the typical taxa of bacterial feeders (BAC) and algal feeders (ALG) that inhabit the sediment of freshwater ponds or lakes, as well as fungal feeder + facultative root feeder (FFR) and obligatory root feeders (ORF). Here, we report on the nematode fauna of paddy fields flooded all year round (PFF), which were developed to imitate" yatsu" paddy fields located at the bottom of a valley. We examined the fauna at soil depths of 0–50 mm in May, August, and November, 2008. One half or more of the total nematode abundance was concentrated into the 0–15 mm layer. This concentration was enhanced after the physical disturbances of puddling and rice transplanting in June. Nevertheless, the compositions of feeding groups and nematode taxa were stable across the study seasons, and generally identical across soil layers. PFF fauna comprised typical sediment taxa with two dominant BAC, Paraphanolaimus (Aphanolaimidae) and Paraplectonema (Leptolaimidae) (60–90% of total density), whereas the FFR and ORF taxa were scarce. Not surprisingly, the PFF fauna was quite different from those in the adjacent terrestrial habitat (ATH), which was composed mainly of FFR and ORF taxa. PFF fauna, however, also had some taxa in common with ATH; e.g., BAC of Monhysteridae, Aphanolaimus, and Chronogaster. Patches of wet peat moss in ATH might be important for the occurrence of these taxa.