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Consecutive monoculture cropping of tobacco has led to a decline in tobacco yield and quality. In this study, quicklime (900 kg/hm2), calcium cyanamide (375 kg/hm2) and microbial agents (210 kg/hm2) were applied to soil where tobacco was continuously planted for 6 years. The potential of these soil conditioners to eliminate problems associated with consecutive tobacco monoculture was evaluated using agronomic traits, tobacco yield and quality, microbial diversity of tobacco rhizospheric soil and soil enzymatic activity as indicators. The quicklime, calcium cyanamide and microbial agent treatments significantly improved tobacco yield and quality compared with the control (CK). The quicklime treatment yielded the best results. Traditional culture methods, which were used to examine tobacco rhizospheric microecology, showed that the quicklime treatment resulted in the most abundant microbial population followed by the calcium cyanamide treatment. Further analysis using the BIOLOG method indicated that the greatest tobacco rhizospheric microbial functional diversity was measured in the quicklime treatment, i.e., the diversity index was higher compared with the other treatments. Soil enzyme measurements indicated that the highest enzymatic activity of tobacco rhizospheric soil occurred in response to the quicklime treatment. The application of quicklime was conducive to improving and restoring the rhizospheric microecological environment and this treatment promoted tobacco growth, improved tobacco yield and quality and alleviated the problems associated with consecutive tobacco monoculture. © 2015 Friends Science Publishers [ABSTRACT FROM AUTHOR]

Pseudostellariae heterophylla, an important medicinal plant, has been shown to suffer from serious replanting disease that causes significant declines in both yield and quality. The objective of this study was to isolate the soil-born pathogen and antagonistic microorganism for investigation of the relationship between these microorganism and consecutive monoculture problems of P heterophylla. In this study, we isolated the bacteria with different morphology from P heterophylla rhizosphere soil, detected the pathogenicity of these bacteria to P heterophylla plantlet and antagonistic ability to Fusarium of P heterophylla biotype, identified their species and analyzed their abundance with qRT-PCR in rhizosphere soil of different planting years. We isolated Enterobacter sp1 and Burkholderia sp1 from P heterophylla rhizosphere soil, and found Enterobacter sp1 possessed the pathogenicity to P heterophylla plantlet and Burkholderia sp1 was antagonistic for Fusarium of P heterophylla biotype. However, Burkholderia sp1 was not the antagonistic microorganism to Enterobacter sp1. The growth of Enterobacter sp1 was stimulated by phenolic acids at a certain concentration nearly closed to that in P heterophylla rhizosphere soil, which increased as planting years of P heterophylla increased. On the contrary, the phenolic acids had no promoting effect on the growth of Burkholderia sp1. And with the increase of planting years, the abundance of Burkholderia sp1 decreased and abundance of Enterobacter sp1 increased in the P heterophylla rhizosphere soil. The results indicated that harmful microorganisms increase and beneficial microorganisms decrease, which were mediated by the accumulated autotoxins in rhizosphere soil of consecutively monocultured P heterophylla, and hence leads to the imbalance of microbial community structure and the degradation of soil ecological function. So several abatement methods, such as crop rotation, specific microbial fertilizer and organic matter, could be used to improve the structure and functional diversity of microbial community, and overcome the obstacles of continuous cropping. © 2015 Friends Science Publishers [ABSTRACT FROM AUTHOR]