Crop rotation and soil amendments influence beneficial nematodes and suppress plant-parasitic nematodes in paddy rice fields in the Mekong Delta, Viet Nam

概要

In Vietnam, rice is the most important agriculture crop and planted in a predominant agriculture land. The annual cultivation system of three rice crops (triple rice system) has been adopted for more than 30 years ago. However long-term intensive cultivation of rice in clayey soils could lead to subsoil compaction and declined soil penetration resistance and yield loss. This system could provide suitable conditions for rice pests and other pathogens like plant-parasitic (PPN), and pathogens. PPN is an important pest in paddy rice, causing yield loss worldwide. Saltwater intrusion is a potential risk on degraded soil physicochemical properties, which may induce changes on nematode communities in paddy soils. However; little is known about their seasonal variation in extremely intensive rice cultivation in tropical regions and the effective agricultural techniques to control PPN in the rice field. Therefore, this study conducted to·(l)evaluate the impacts of saltwater on soil nematode communities, (2) find out seasonal distribution pattern of nematode communities in the rice field , and (3) assess the potential effects of crop rotation and soil amendments on nematode communities. Field experiments were conducted to evaluate the responses of the nematode community to a crop rotation (the first rice in a year was replaced with sesame and soybean, and the second and third crops were rice) and to organic amendment The experiment was based on a split-plots design with two factors that were crops (main factor , n = 3) and compost amendment (sub-factor, n = 3). The crops were rice, soybean, and sesame and the sub-factors were cow manure plus rice straw compost, sugarcane compost, and no amendment. Soil samples were taken at harvest of the first crops in topsoil(0-10 cm) and subsoil (10-20 cm) in 2017 and 2018. First study results showed that saltwater intrusion reduced the abundance of both free-living nematodes (FLN) and PPN (Hirschmanniella sp.) in soils. Also, FLN community was different among sites with different physicochemical properties. Saltwater intrusion reduced biological diversity Second study results showed that PPN were the largest group and accounted for 47 -80 % and 79 -92 % of total nematode abundance in the topsoil and subsoil, respectively. Hirschmanniella sp. was the most dominant taxon, whose abundance in topsoil varied from 275―739 in winter-spring to 1010 -1270 individuals 100 g-1 soil in spring -summer season. Bacterivore nematodes were the predominant group of FLN, with increasing relative abundance in the topsoil of spring -summer (average, 23.2%) < summer -autumn (average, 41.9%) < winter-spring (average, 57.3% of the total FLN). The present results suggest that the presence of Hirschmanniella sp. in soil may be involved in a reduction of rice yield in the triple rice system. Third study, results showed that the abundance of FLN, which were dominated by bacterivore and fungivore nematodes, was greater in upland crops, while that of PPN was greater in paddy rice in 2018. In particular, the density of the most predominant PPN genus, Hirschmanniella sp., was lower in upland crops than in paddy rice. Cultivation of sesame also reduced the number of the second predominant PPN genus, Bitylenchus, sp. resulted in an increase in the ratio of the free-livining to PPN. We suggest that crop rotation with sesame or soybean in a paddy field enhanced abundance and biodiversity of FLN and suppressed PPN and thus can be proposed for agricultural sustainability.