Rhizosphere effects of Helianthus annuus and Hordeum vulgare on allelochemical sorption and degradation in soil

• H. annuus and H. vulgare altered the fate of three allelochemicals in soil. • Root-influenced soil showed compositional and bacterial population changes. • Sorption of the allelochemicals was similar in rhizospheric and bulk soil. • Allelochemical persistence differed between rhizospheric and bulk soils. • Rhizospheric effects varied with plant species and allelochemical structure. Soil plays a crucial role in determining the activity of allelochemicals in natural and agricultural ecosystems. To identify potential differences in allelochemical availability and persistence between root-influenced soil (rhizosphere) and bulk soil, we designed experiments to compare the sorption and degradation patterns of three putative allelochemicals (S-abscisic acid, S-pulegone, and khellin) in soil near the roots of laboratory-grown Helianthus annuus and Hordeum vulgare with those obtained in unplanted soil controls. Soil compositional and bacterial population changes were monitored and linked to the observed allelochemical sorption and degradation patterns. Root-influenced soil underwent chemical and biological changes that affected its capacity to degrade the three allelochemicals, whereas the soiĺs inherent sorption capacity for the allelochemicals remained unaltered. Notably, rhizospheric soils showed a 36–70% increase in dissolved inorganic carbon, an 8–14% increase in dissolved organic carbon, a 0.20–0.51 unit decrease in pH, and distinct bacterial community structures compared to non-rhizospheric soil controls. The dissipation of the allelochemicals in root-influenced soil followed sigmoidal biodegradation patterns that depended on the plant species, the specific allelochemical, and the soil's proximity to the roots. We suggest that plant-specific changes in rhizospheric bacterial communities, likely influenced by root exudates, drove the observed differences in allelochemical dissipation between rhizospheric and non-rhizospheric soil. Our results indicate that evaluating the persistence of allelochemicals in soil near plant roots can help improve predictions of their environmental fate and activity in natural and agricultural ecosystems.