Solid Olive‐Mill Waste and Biochar Effects on the Bioavailability and Bioactivity of Abscisic Acid in Soil

ABSTRACT Natural plant growth regulators (PGRs) are attracting increasing interest in agriculture for their potential to support plant development, increase resistance to environmental stresses, and reduce reliance on synthetic agrochemicals. Although several innovative applications of natural PGRs involve the soil as the medium for their action, the fate and bioactivity of natural PGRs in soil environments remain insufficiently explored. In this study, we evaluated the plant growth inhibitory activity of the phytohormone S‐abscisic acid (ABA) in a sandy clay loam Mediterranean soil and investigated how it was influenced by two organic amendments—solid olive‐mill waste (SOMW) and biochar (BC)—commonly used to improve soil fertility in Mediterranean regions. Both amendments delayed the biodegradation of ABA in the soil, but through different mechanisms that led to contrasting effects on its activity. SOMW prolonged ABA persistence without altering its negligible sorption by the soil, likely by producing major shifts in the soil bacterial community and/or reducing microbial preference for metabolising ABA. This effect maintained high levels of bioavailable ABA in solution for a longer period, thereby increasing its activity compared to unamended soil. In contrast, BC increased ABA persistence by enhancing its sorption, which reduced its bioavailability and consequently its inhibitory activity. The irreversibility of the germination‐inhibitory effects of high concentrations of ABA was also differentially affected by SOMW and BC. These findings highlight that agronomic choices such as the use of organic amendments can substantially influence the bioactivity of natural PGRs in soil, with the type of amendment and underlying mechanisms affecting PGR bioavailability critically determining the final effects.