Effects of various continuous cropping times on soil nematode structure in cotton fields of Xinjiang, China

Long-term continuous cropping of cotton had ledto substantial agricultural losses. However, continuous cottoncropping could maintain high crop yields for many years in someareas. The composition and structure of soil nematode communitieswere investigated to explore the effect of continuous croppingand soil depths on these communities. Soil samples werecollected at two soil depths (0-20 cm and 20-40 cm) from cottonfields with a history of 5, 10, 15 or 20 years of continuouscotton cropping in the Karamay region. The results showed that36 genera were found. Significant differences in the numbers oftotal nematodes and trophic groups were observed among continuouscropping times. The highest number of total nematodeswas observed at 20-40 cm depth under continuous 10-croppingyears, and the lowest number at 20-40 cm depth under continuous5-cropping years. There were significant soil depth effects on thedensity of fungivores and plant parasites. Continuous croppingeffects on soil nematode communities could be reflected by valuesof ecological indexes. Continuous cropping times, soil depths andtheir interaction significantly influenced H´, λ, WI, PPI (index ofplant parasites), MI (maturation index excluding plant parasites),PPI/MI, F/B and NCR. The Shannon index (H´) was higher inthe 10-year cotton field than in the other-year fields. The Simpsonindex (λ) was the lowest in the 10-year cotton field. The lowestvalue of WI (index of Wasilewska) appeared at the 20-year cottonfield, which had the majority of plant parasites. Increases of cottoncropping times determined a decreased trend of NCR [=B/(B + F)], and an increased trend of F/B [F and B represent theconsumer abundance of fungi (F) and bacteria (B)]. The bacterialdecomposition pathway was more important in the 5-year situations,and the fungal decomposition pathway was more importantin the 20-year situations. Nematode analysis showed that changes of soil nematode communities and trophic groups could indicatechanges in the soil environment and nematode community structurewith changes of continuous cropping times.