Soil modification with organic amendments and organo-clays: Effects on sorption, degradation, and bioactivity of the allelochemical scopoletin

Abstract

We assessed the effect of three organic amendments and two organo-clays on sorption, persistence, and phytotoxicity of scopoletin, an allelochemical compound with potential as bioherbicide, in a Mediterranean alkaline soil. The aim was to elucidate whether the phytotoxicity of scopoletin could be expressed better in amended than unamended soil. The three organic amendments were fresh solid olive-mill waste (OMW), composted solid olive-mill waste (OMWc), and biochar (BC) prepared from OMWc. The two organo-clays were a commercial organo-montmorillonite (Cloi10) and lab-synthesized oleate-modified hydrotalcite (HT-OLE). The amendments enhanced sorption of scopoletin by the soil consistently with their individual affinities for the allelochemical: Cloi10 ≫ OMW > BC > OMWc > HT-OLE. The soil persistence of scopoletin increased significantly because of the addition of Cloi10, OMW, and BC. This increase was attributed to a combination of sorption, which protected the allelochemical from rapid biodegradation, and microbial activity changes. Although the inhibitory effect produced by the amendments themselves obscured the phytotoxicity of scopoletin to Lactuca sativa L. in soil treated with OMW and Cloi10, applying scopoletin to BC-amended soil led to a marked reduction in root length and aerial biomass of the emerged seedlings even though BC alone did not negatively affect these parameters. This inhibitory effect of scopoletin in BC-amended soil was in contrast to the negligible effect exerted by the allelochemical when applied to unamended soil. The results show that soils treated with suitable amendments, such as BC, might provide a scenario in which the herbicidal properties of 7-hydroxycoumarins could be better expressed.