How different or similar are nematode communities between a paddy and an upland rice fields across a flooding–drainage cycle?

The paddy field is being recognised as a biodiversity hotspot fostering a variety of organisms. However, there are few studies on the ecology of paddy field nematodes. We characterised nematode communities in rice paddy fields by comparing them with upland fields of rice or soybean. We examined nematode communities of the top (0–15 mm) and second (15–50 mm) soil layers before flooding (March or April), during flooding (June or July) and during the draining period (October) 2007–2009. We found that the nematode community in the paddy was different than that in the upland fields during all periods. Rhabdolaimus, Tobrilus, Mesodorylaims and Monhysteridae characterised the top of the paddy and Hirschmanniella characterised the second layer of the paddy. Total nematode density was generally lower in the paddy than in the upland field. However, the density in the paddy top layer increased with time from the flooding period to the draining period, during which time it was about the same as (or even greater than) the peak density in the upland fields. The density in the second layer of the paddy remained lower than that in the top layer of the paddy throughout the time course. Community diversity values were generally greater in the paddy top layer than in the paddy second layer across the six sampling periods, but periodic differences between the paddy and upland fields or between soil layers were not significant. During the flooding period, the F/(F + B) (13–37) and Enrichment Index (17–38) values were lower in the paddy than in the upland fields (32–47, 37–74, respectively) to reflect that bacteria dominate over fungi with slow decomposition due to anaerobic conditions in the flooded paddy field. In addition, particularly in the top layer, the Maturity (2.0–2.4) and Structure Index (23–72) values were greater in the paddy than in the upland fields (1.7–2.1, 9–15, respectively), indicating a well-developed ecosystem under water. These unique nematode communities persisted during the draining period, but there was a rapid increase in opportunistic bacterivores, which increased the EI values. We suggest that bactivorous nematodes in the families Cephalobidae and Chronogasteridae, herbivores in the genus Hirschmanniella, and fungivores in the genus Filenchus may be specific to paddy field soil rather than to pond and lake sediments. ► Nematode communities were different between paddy and upland fields in any period. ► Flooded paddy community reflected bacterial dom