Effect of potato continuous cropping on genetic diversity of soil microorganisms

Because of its high nutritive value and extensive use, potato has become a critical crop in the southern mountain areas of Ningxia, China. However, the expansion of cultivated land areas under potato has made not only crop rotation difficult, but as has also worsened the obstacles of potato continuous cropping in recent years. In order to alleviate and eventually resolve the obstacles of potato continuous cropping, rhizosphere soils were sampled under the following potato treatments after 2 years, 7 years and 11 years of continuous cropping in the southern mountain areas of Ningxia, China. Terminal restriction fragment length polymorphism (T-RFLP) was used to study the genetic diversity of soil microorganisms, to explore the dynamics of main soil microbial groups in rhizosphere soils and to find the possible reasons for the obstacles of potato continuous cropping in the region. The results showed that some specific T-RFs fragments changed or disappeared in rhizosphere soil samples under different periods of continuous cropping, though higher polymorphism of T-RFLP of bacteria and fungi in rhizosphere soils of continuously cropped potato was observed. While the Shannon-Wienen, Simpson and Sorenson indexes of bacterial dropped, those of fungus increased with increasing years of continuous cropping. Analysis of soil microorganism biota revealed that Firmicute, Bacilli and Clostridia were the highest proportion of total bacteria communities in potato rhizosphere soils. Continuous cropping significantly changed structures of soil bacteria and fungi communities. While the proportions of Bacilli and Sphingobacteria decreased or even disappeared with increasing cropping years, β-proteobacteria and Deinococci proportions increased. The proportion of Dothideomycete and Capnodiales of fungi community in rhizosphere soils dropped whereas that of Sordariomycete and Hypocreales grew. The results suggested that potato continuous cropping limited the population of beneficial microbes such as Bacillus, while boosting that of pathogenic bacteria such as Ralstonia in the rhizosphere soils. In conclusion, soil microbial diversity in rhizosphere soils under different periods of continuous cropping was obviously influenced. The fall of the diversity of bacteria community gave way to the rise of fungi community under continuous cropping, causing imbalance in microbial community and soil micro-ecological/environmental deterioration. The obstacle of continuous cropping possibly resulted from complex interaction of multiple soil factors in rhizosphere soils. There was therefore the need for continuous field research to help comprehensively determine the mechanism of continuous cropping obstacle.