Effects of long -term tillage and rotational treatments on the rhizosphere microbial community and development of soybean

Management practices such as tillage and crop rotation can have significant effects upon the growth, development, and yield of crops. Most studies fail to consider the ecosystem as a whole, but rather evaluate above and below ground effects separately. Here, it is hypothesized that long-term agronomic management practices will affect the rhizosphere microbial community and plant development of soybean under specific treatments. The objectives of this work were to compare the rhizosphere microbial (Bacteria, Eukarya and fungi) community structure and soybean plant development over time and under different agronomic treatments, and to evaluate the presence of Fusarium solani f. sp. glycines Roy (FSG) - (=F. virguliforme O’Donnell) within soybean rhizosphere over time under the same treatments. Treatments included: plowed-monoculture soybean, plowed soybean rotated with corn, no-till monoculture soybean, and no-till soybean rotated with corn. Soybean plants were collected at specific growth stages over two seasons. Microbial community structure was determined by denaturing-gradient-gel electrophoresis (DGGE) of 16S rRNA (Bacteria) and 18S rRNA (Eukarya) gene fragments amplified by PCR from rhizosphere soil DNA extracts, and ribosomal interspacer analysis (RISA) of ITS1 – 5.8S – ITS2 (fungal) amplicons from the same extracts. Presence-absence screening with species specific PCR primers was used to detect FSG within rhizosphere DNA extracts. Principal components analysis (PCA) showed that the microbial community differed with plant growth and treatment. The bacterial community under the monoculture treatments differed most often from other treatments. In contrast, the eukaryotic community under the rotated no-till treatment differed most often. No fungal community differences were detected. Analysis of plant growth factors and grain yield indicated that in general, these values were consistently significantly higher in plants from the rotated plow treatment than the from the monoculture no-till treatment. Sequence analysis of statistically significant members of the bacterial and eukaryal communities indicated similarity to organisms including Pseudomonas sp. nematodes, and potential oomycete pathogens. FSG screening indicated increased presence under the monoculture plow treatment. Results from this study suggest that agronomic management practice has a clear effect on both rhizosphere microbial community and plant development, and that in some growth stages, these effects are similar. Future research is needed to further establish mechanisms for these changes.