Barbara Cania, Gisle Vestergaard, Maike Krauss, Andreas Fliessbach, Michael Schloter og Stefanie Schulz
The authors gratefully acknowledge the funding provided by the German Federal Office for Agriculture and Food (BLE) and the financial support for the Frick trial provided by the Swiss Federal Office for Agriculture (FOAG), the Software AG-Stiftung, the Stiftung zur Pflege von Mensch, Mitwelt und Erde and the Edith Maryon Stiftung. Moreover, this study was associated to the project Fertility Building Management Measures in Organic Cropping Systems (FertilCrop), which received funding from the CORE Organic Plus Funding Bodies, being partners of the FP7 ERA-Net project CORE Organic Plus.
However, it has yet to be shown how tillage influences the bacterial potential to produce aggregate-stabilizing agents. Therefore, we sampled conventional and reduced tillage treatments at 0–10 cm, 10–20 cm and 20–50 cm from a long-term field trial in Frick, Switzerland. We compared the stable aggregate fraction of the soil and the bacterial potential to produce exopolysaccharides (EPS) and lipopolysaccharides (LPS) under different tillage regimes by employing a shotgun metagenomic approach. We established a method which combines hidden Markov model searches with blasts against sequences derived from the Kyoto Encyclopedia of Genes and Genomes database to analyze genes specific for the biosynthesis of these compounds.
Our data revealed that the stable aggregate fraction as well as the bacterial potential to produce EPS and LPS were comparable under both tillage regimes. The highest potential to produce these compounds was found in the upper soil layer, which was disturbed by tillage, but had higher content of organic carbon compared to the layer below the tillage horizon. Additionally, key players of EPS and LPS production differed at different sampling depths. Some families with high potential to produce EPS and LPS, such as Chitinophagaceae and Bradyrhizobiaceae, were more abundant in the upper soil layers, while others, e.g. Nitrospiraceae and Planctomycetaceae, preferred the lowest sampled soil depth. Each family had the potential to form a limited number of different aggregate-stabilizing agents.
Our results indicate that conventional tillage and reduced tillage equally promote the bacterial potential to produce EPS and LPS in the tillage horizon. However, as major bacterial groups triggering EPS and LPS formation were not the same, it is likely that gene expression pattern differ in the different treatments due to various pathways of gene induction and transcription in different bacterial species.
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