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

Abstract

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.

Shifts in reclamation management strategies shape the role of exopolysaccharide and lipopolysaccharide‐producing bacteria during soil formation

“…This result indicates that the bacterial community grows in abundance, and eventually adapts and stabilizes with agricultural practices. This is in accordance with other studies showing a stable community under long‐term treatments (at least on the DNA level) (de Vries et al , ; Cania et al , ). Moreover, soil organic carbon concentrations peaked at RA6 and decreased at RA12 and RA24 (Pihlap et al , ), likely because organic fertilizers were applied only in the 4th and 7th year of the reclamation process.…”

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

“…Therefore, the positive effects of RT should be more emphasized in soils with lower clay content, where maintaining high aggregation is more challenging (Cooper et al, 2016). This is in agreement with studies using clayey soils that revealed no differences in aggregate stability when RT and CT were compared (Asgari, 2014;Cania et al, 2019a). However, soil aggregation is a complex process that is driven by both abiotic and biotic factors (Six et al, 2004), and still little is known about how tillage intensity influences soil biota and their capabilities to trigger aggregate formation.…”

“…According to Lehmann et al (2017), the bacterial production of adhesive polysaccharides is one of the most crucial biotic mechanisms of soil aggregation. We (Cania et al, 2019a) could show that while the relative abundance of bacteria capable to form EPSs and LPSs was comparable between CT and RT, the community composition of the potential producers of these compounds differed. As the aggregating efficiency of adhesive polysaccharides produced by different taxa varies greatly (Costa et al, 2018), tillage impact on the community composition of EPS and LPS producers could be critical for the stability of agricultural soils.…”

“…Although many researchers (Jacobs et al, 2009;Mikha et al, 2013;Bartlova et al, 2015;Sheehy et al, 2015;Singh et al, 2016) observed that soil aggregates are better preserved under RT compared with CT, others (Asgari, 2014;Cania et al, 2019a) reported no effect of tillage intensity on soil aggregation. According to Cooper et al (2016), the strength of the positive effects of RT strongly depends on soil texture.…”

“…However, while many researchers investigated the influence of tillage intensity on the aggregating capabilities of fungi, less attention was given to bacteria. In fact, only in a recent study (Cania et al, 2019a), we compared the effects of CT and RT on the bacterial potential for soil aggregation. Here, we investigated the bacterial potential for the production of exopolysaccharides (EPSs) and lipopolysaccharidesof (LPSs), which act as binding agents for soil particles (Six et al, 2004;Costa et al, 2018;Totsche et al, 2018).…”