Long-term intensive rice monoculture, triple rice cultivation (rice crops are cultivated three times per year) inthe Mekong Delta, Vietnam, causes a reduction in topsoil quality. Rotating rice with an upland crop in a tropicalpaddyfield may improve the soil physicochemical properties but effects on the soil nematode community areunknown. This study was conducted in a triple ricefield to evaluate the responses of the nematode community toa crop rotation (thefirst rice in a year was replaced with sesame and soybean, and the second and third cropswere rice) and to organic amendment. The experiment was based on a split-plot design with two factors thatwere crops (main factor, n = 3) and compost amendment (sub-factor, n = 3). The crops were rice, soybean, andsesame and the sub-factors were cow manure plus rice straw compost, sugarcane compost, and no amendment.Soil samples were taken at harvest of thefirst crops in topsoil (0–10 cm) and subsoil (10–20 cm) in 2017 and2018. In 2018, the abundance of free-living nematodes (FLN), which were dominated by bacterivores andfungivores, was greater in upland crops, while that of plant-parasitic nematodes (PPN) was greater in paddy rice.In particular, the density of the most predominant PPN genus,Hirschmanniella, was lower in upland crops than inpaddy rice. Cultivation of sesame also reduced the number of the second predominant PPN genus,Bitylenchus,resulted in an increase in the ratio of the free-living to plant-parasitic nematodes. The diversity evenness index(J), Shannon-Wiener (H′) and Hill's indices increased in upland crops compared to those in ricefields in 2018.Crop rotation with sesame or soybean in a paddyfield enhanced abundance and biodiversity of FLN and sup-pressed PPN and thus can be proposed for agricultural sustainability.1. IntroductionIn Vietnam, rice is the most important agriculture crop, which isplanted in 82% of the total agricultural land (Vu et al., 2018). It ac-counts for 22% and 54% of the nation's Gross Domestic Product (GDP)and labour force, respectively (General Statistics Office, 2017). TheVietnamese Mekong Delta (VMD) is located in the South and is called“arice bowl”of the country, as rice is the largest agricultural product ofthe delta, comprising 45% of the total rice area and producing 57% ofthe total rice output of Vietnam (Tong, 2017). In VMD, the annualcultivation system of three rice crops (triple rice system) has beenadopted since 1980 (Khoa, 2002), reflecting changes in hydrology toimprove rice production (Minh and Kawaguchi, 2002). In this system,soil is irrigated andflooded during the rice crop, and this process isrepeated for each crop. To maintain productivity, this cultivationsystem relies greatly on chemical fertilisers and pesticides (Tran et al.,2018). Overuse of inorganic fertilisers and pesticides causes seriousenvironmental problems, such as water pollution (Stone andHornberger, 2016), emission of greenhouse gases (Hoa et al., 2018),and increase in pests and diseases (Stuart et al., 2014). In addition, soilquality may decline (Cassman and Harwood, 1995). Long-term in-tensive cultivation of rice in clayey soils could lead to subsoil com-paction and declined soil penetration resistance (Linh et al., 2016).Therefore, triple rice cultivation system could not be a sustainablehttps://doi.org/10.1016/j.apsoil.2020.103683Received 7 October 2019; Received in revised form 27 May 2020; Accepted 28 May 2020⁎Corresponding author.E-mail address:kokit@cc.tuat.ac.jp(K. Toyota).Applied Soil Ecology 156 (2020) 103683Available online 16 June 20200929-1393/ © 2020 Published by Elsevier B.V.T