Effects of intensified corn stalk degradation by biocontrol agents on soil microbial community in continuous cucumber cropping

Stunted crop growth, yield decline and disease susceptibility have been associated with monoculture cropping. Soil borne diseases have emerged as the main obstacle to continuous cropping, which current biological techniques have failed to overcome. In recent years, however, intensified corn stalk degradation by biocontrol agents has been a widely used technique in winter greenhouses in North China. This technique combining application of crop stalk and biocontrol agents, which can improve soil quality, reduce soil diseases, increase crop productivity, etc. Thus this study aimed to investigate what effects this technique has on soil microbial community in continuous cucumber cropping. The dynamics of fungi, bacteria, actinomycetes, biocontrol agents and pathogens in cucumber fields were determined using both the plate culture counting and terminal-restriction fragment length polymorphism (T-RFLP) methods. The results showed that intensified corn stalk degradation by biocontrol agents significantly influenced soil microbial community in cucumber fields. After treatment, the number of fungi in soils of the second crop (6.33×10⁴ CFU·g^-1) was lower than that in the first crop (1.81×10⁵ CFU·g^-1) while bacteria and actinomycetes showed the reverse trend. Bacteria to fungi (B/F) ratio increased from 222.30 at the end of the first crop to 667.16 at the end of the second crop. The contents of biocontrol agents, Purpureocillium lilacinum and Bacillus subtilis, remained stable in soils under continuously cropped cucumber. Although insignificantly, the amount of pathogen Fusarium oxysporum f. sp. cucumerinum decreased. On the other hand, soil bacteria diversity significantly increased. This somehow promoted beneficial bacteria TRF139 propagation while inhibiting harmful bacteria TRF341 and TRF501. Moreover, the technique improved cucumber yield by 25.9% under continuous cropping and reduced root-knot nema-tode disease index by 71.4% at the end of the third crop. Thus the combination of corn stalk and biocontrol agents positively influenced soil microbes and their community structures, promoted biocontrol agent colonization within a few crops, inhibited the growth of pathogens and subsequently improved the health of soil microenvironment.