Effects of continuous soybean monoculture on soil nematode community

【Objectives】Soybean yield decrease and soil sinckness in the northeast China have been attributed to continuous monoculture. Continuous soybean monoculture may provide preferential food resources for specific plant-parasitic nematodes namely cyst nematode (Heterodera glycines), resulting in the increase in competitive advantage of disease-causing plant-parasitic nematodes over beneficial free-living forms, with resulting economic losses. Thus, understanding dynamics of nematode communities in continuous monoculture is an important aspect of soil sickness of soybean. We used terminal restriction fragment length polymorphism (T-RFLP) and Quantitative PCR (qPCR) assays to estimate soil nematode community in soils taken from a long-term field experiment with continuous soybean monoculture up to 13 years. The objective of this study was to investigate whether the dynamic processes in free-living nematodes and plant-parasitic nematodes with increasing durations of continuous soybean monoculture are linked to soil sickness.【Methods】 T-RFLP assays were used to estimate structure of soil nematode community in rhizosphere soils. According to T-RFLP analysis, the species richnessand relative abundances of different sizes of T-RF were determined. In order to assign phylogenetic affiliation to specific T-RF, a clone library was constructed using the pMD 19-T Vector. Phylogenetic tree was constructed using the MEGA software with the Kimura two-parameter method for distance matrix calculations and the neighbor-joining method for the tree design. Finally, quantitative PCR was used to assess nematode community sizes in rhizosphere soils.【Results】 The species richness of nematode communities is decreased rapidly to the lowest value in the ninth year and then remains a constant from the ninth year to the thirteenth year. The T-RFLP analysis of nematode gene shows that the nematode communities in soybean rhizosphere soil of the northeast China are dominated by Acrobeloides. The relative abundances of plant-parasitic nematode in the second year and third year of monoculture are higher than those from the sixth year to 13th year. The free-living nematodes are more abundance in the first year and less abundance in the second year and third year, and are more abundance from the sixth year onwards. No omnivores-predators are identified. The nonmetric multi-dimensional scaling (NMDS) indicates that the nematode communities in rhizosphere soil collected from the 9th, 11th, and 13th years closely cluster together, outside of the first three years. This result suggests that the differences of nematode communities measured by pairwise Bray-Curtis dissimilarities have less change in the first three years. However, the nematode communities are highly differed between subsequent years and initial years. Furthermore, the nematode community composition is affected by available P, soil organic matter (SOM), pH, bacterial and fungal abundance. The abundance of nematode community is increased from the first year to sixth year, and then decreased from the nineth year to 13th year. In addition, the abundance of nematode community is positively correlated to NH+4 and genisten, and is negatively correlated to NO-3 and bacterial abundance.【Conclusions】The species richness and relative abundance of nematode is decreased significantly in the second and the third year of soybean monoculture, and revived form the 6th to the 9th year to some extent, but not completely. In the first year of soybean culture, the abundance of Acrebeloides is the highest, representing healthy and normal soil function. In the following 2 and 3 years, the abundance of plant-parasitic nematode is increased, indicating the high risk of disease infection.The lower ratio of free-living nematodes to plant-parasitic nematode in the second year and third year could respond to the potential severity of the disease.