Einfluss von Bewirtschaftungsmaßnahmen auf die Struktur und Funktion der Bodenmikroflora

Im ökologischen Landbau gilt es unter umweltschonender Bewirtschaftung trotzdem hohe Erträge zu erzielen. Bisher wurde dies durch konventionelle Pflügung realisiert, diese Maßnahme zerstört aber die Bodenstruktur und damit den Lebensraum wichtiger Organismengruppen. Daher ist es wünschenswert die im konventionellen Landbau schon übliche reduzierte Bodenbearbeitung auch im ökologischen Landbau zu etablieren. Im Rahmen dieses Projektes wurde daher untersucht wie sich die reduzierte Bodenbearbeitung auf das Mikrobiom auswirkt. Da Pflügen insbesondere die Bodenstruktur zerstört, lag der besondere Fokus auf dem bakteriellen Potential strukturbildende Substanzen wie Exo-(EPS) und Lipopolysaccharide (LPS) zu produzieren. Dazu wurden vier Standorte mit unterschiedlicher Bodentextur untersucht. Das Potential zur Bildung von EPS/LPS war generell im Pflughorizont am größten. Als Indikatorgene wurden wza für die EPS-Synthese und lptG und lptF für den LPS Transport identifiziert. Während die Abundanz der Gene nicht durch die Bodenbearbeitung beeinflusst wurde, hat sich die Zusammensetzung der Schlüsselorganismen je nach Standort, Tiefe und Bodenbearbeitung unterschieden. Da die strukturbildenden Eigenschaften der Polysaccharide bei jedem Organismus anders sind, können kleine Unterschiede in der mikrobiellen Zusammensetzung zu großen Unterschieden in der Aggregatstabilität führen. So hat sich gezeigt, dass trotz vergleichbarer relativer Genabundanzen schluffiger Boden am sensibelsten auf die Bodenbearbeitung reagiert, mit höheren Werten unter reduzierter Bodenbearbeitung. Die sowieso schon schlechte Aggregierung in sandigen Böden, konnte nicht verbessert werden. Alternativ könnte man hier aber durch reduzierte Bodenbearbeitung die Entwicklung von BBK unterstützen, die wie sich gezeigt hat ein großes Potential für die Speicherung von Nährstoffen haben, als auch die Bildung von EPS/LPS. Interessanterweise haben die Netzwerkanalysen ergeben, dass insbesondere Bakterien, die das Pflanzenwachstum unterstützen wie Micromonapsora und Actinobacteria durch die Bodenbearbeitung beeinflusst werden. Sie akkumulieren im Oberboden bei reduzierter Bearbeitung und unter dem Pflughorizont bei den Pflugvarianten und folgen somit der Wurzelpenetrationstiefe

Hydrocarbon degradation potential and plant growth-promoting activity of culturable endophytic bacteria of Lotus corniculatus and Oenothera biennis from a long-term polluted site

Many endophytic bacteria exert beneficial effects on their host, but still little is known about the bacteria associated with plants growing in areas heavily polluted by hydrocarbons. The aim of the study was characterization of culturable hydrocarbon-degrading endophytic bacteria associated with Lotus corniculatus L. and Oenothera biennis L. collected in long-term petroleum hydrocarbon-polluted site using culture-dependent and molecular approaches. A total of 26 hydrocarbon-degrading endophytes from these plants were isolated. Phylogenetic analyses classified the isolates into the phyla Proteobacteria and Actinobacteria. The majority of strains belonged to the genera Rhizobium, Pseudomonas, Stenotrophomonas, and Rhodococcus. More than 90% of the isolates could grow on medium with diesel oil, approximately 20% could use n-hexadecane as a sole carbon and energy source. PCR analysis revealed that 40% of the isolates possessed the P450 gene encoding for cytochrome P450-type alkane hydroxylase (CYP153). In in vitro tests, all endophytic strains demonstrated a wide range of plant growth-promoting traits such as production of indole-3-acetic acid, hydrogen cyanide, siderophores, and phosphate solubilization. More than 40% of the bacteria carried the gene encoding for the 1-aminocyclopropane-1-carboxylic acid deaminase (acdS). Our study shows that the diversity of endophytic bacterial communities in tested plants was different. The results revealed also that the investigated plants were colonized by endophytic bacteria possessing plant growth-promoting features and a clear potential to degrade hydrocarbons. The properties of isolated endophytes indicate that they have the high potential to improve phytoremediation of petroleum hydrocarbon-polluted soils

Site-Specific Conditions Change the Response of Bacterial Producers of Soil Structure-Stabilizing Agents Such as Exopolysaccarides and Lipopolysaccarides to Tillage Intensity

Agro-ecosystems experience huge losses of land every year due to soil erosion induced by poor agricultural practices such as intensive tillage. Erosion can be minimized by the presence of stable soil aggregates, the formation of which can be promoted by bacteria. Some of these microorganisms have the ability to produce exopolysaccharides and lipopolysaccharides that "glue" soil particles together. However, little is known about the influence of tillage intensity on the bacterial potential to produce these polysaccharides, even though more stable soil aggregates are usually observed under less intense tillage. As the effects of tillage intensity on soil aggregate stability may vary between sites, we hypothesized that the response of polysaccharide-producing bacteria to tillage intensity is also determined by site-specific conditions. To investigate this, we performed a high-throughput shotgun sequencing of DNA extracted from conventionally and reduced tilled soils from three tillage system field trials characterized by different soil parameters. While we confirmed that the impact of tillage intensity on soil aggregates is site-specific, we could connect improved aggregate stability with increased absolute abundance of genes involved in the production of exopolysaccharides and lipopolysaccharides. The potential to produce polysaccharides was generally promoted under reduced tillage due to the increased microbial biomass. We also found that the response of most potential producers of polysaccharides to tillage was site-specific, e.g., Oxalobacteraceae had higher potential to produce polysaccharides under reduced tillage at one site, and showed the opposite response at another site. However, the response of some potential producers of polysaccharides to tillage did not depend on site characteristics, but rather on their taxonomic affiliation, i.e., all members of Actinobacteria that responded to tillage intensity had higher potential for exopolysaccharide and lipopolysaccharide production specifically under reduced tillage. This could be especially crucial for aggregate stability, as polysaccharides produced by different taxa have different "gluing" efficiency. Overall, our data indicate that tillage intensity could affect aggregate stability by both influencing the absolute abundance of genes involved in the production of exopolysaccharides and lipopolysaccharides, as well as by inducing shifts in the community of potential polysaccharide producers. The effects of tillage intensity depend mostly on site-specific conditions

A long-term field experiment demonstrates the influence of tillage on the bacterial potential to produce soil structure-stabilizing agents such as exopolysaccharides and lipopolysaccharides

Background: Stable soil aggregates are essential for optimal crop growth and preventing soil erosion. However, tillage is often used in agriculture to loosen the soil, which disrupts the integrity of these aggregates. Soil aggregation can be enhanced by bacteria through their ability to produce exopolysaccharides and lipopolysaccharides. These compounds stabilize soil aggregates by “gluing” soil particles together. 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. Results: 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. Conclusions: 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