Shifts in soil bacterial communities associated with the potato rhizosphere in response to aromatic sulfonate amendments

Desulfonation is an important process in the sulfur cycle, through which organic sulfur compounds are mineralized, releasing S. Desulfonating bacteria are known to respond to inorganic S soil amendments. However, the extent to which these communities shift as a response to the addition of organic sulfur in the soil remains elusive. Here, we investigated how amendments of soil with inorganic or organic sulfur compounds influence the bacterial communities associated with potato, in a microcosm experiment. The soil was amended with two doses of linear alkylbenzene sulfonate (LAS), here used as a model aromatic sulfonate compound, or with sulfate. Degradation of LAS was observed already at the young plant stage, as in all treatments 10- to 50-fold reductions of the initial (background) LAS concentrations were noted. Quantitative PCR analyses showed no significant effects of treatment on the bacterial abundances, which tended to increase from the young plant to the flowering stages of plant development. The bacterial community structures, determined via PCR-DGGE, were strongly affected by the presence of plants. This rhizosphere effect became more apparent at the flowering stages. Both the bacterial and β-proteobacterial community structures were affected by the presence of LAS, but dose-related effects were not observed. LAS also caused significant changes in the community structures, as compared to those in inorganic sulfate amended soil. Sulfate did not influence the bacterial community structures and only affected the β-proteobacterial ones at the flowering stage. Surprisingly, the presence of LAS did not exert any significant effect on the abundance of the Variovorax asfA gene, although clone libraries revealed a dominance of Variovorax types in the rhizosphere, especially in the high-level LAS treatment. Our results suggest that rhizosphere communities are key players in LAS degradation in soils, and that desulfonator Variovorax spp. plays a minor role in the mineralization of aromatic sulfonates in soil cropped with potato.