Elevated CO 2 increases cover crop sensitivity to imidazolinones but does not substantially alter post-cultivation soil dissipation

This study evaluated the effects of elevated (700 ± 50 µmol mol−1) and ambient (400 ± 50 µmol mol−1) atmospheric CO2 concentrations on soils treated with imazapyr, imazapic and imazethapyr using Italian ryegrass and Persian clover. Two open-top-chamber experiments assessed herbicide dissipation, soil microbial respiration (C–CO2) and plant biochemical responses, including acetolactate synthase (ALS) activity, free amino acids, proteins and soluble sugars. Soils amended with Persian clover residues showed greater herbicide dissipation (≈15–30%) than fallow soil after 40 days, regardless of CO2 level (except for imazethapyr under elevated CO2), whereas Italian ryegrass showed limited improvement, mainly for imazapic. Microbial respiration was highest in soils amended with Persian clover, while ryegrass resulted in the lowest respiration and reduced C–CO2 emission after imazapyr + imazapic application under elevated CO2. Elevated CO2 increased shoot biomass in both species but also reduced herbicide tolerance, decreasing GR50 values by 12–31% (from 64.5 to 44.6 g a.i. ha−1 in Italian ryegrass and from 29.8 to 26.1 g a.i. ha−1 in Persian clover), indicating greater plant susceptibility under future CO2 scenarios. At 28 days after emergence, herbicide application reduced ALS activity under both CO2 conditions, although ALS activity remained higher under elevated CO2. Persian clover showed greater biochemical resilience, maintaining higher amino acid contents under herbicide stress. Overall, cover-crop identity, rather than atmospheric CO2, was the primary determinant of herbicide dissipation in soil: Persian clover consistently enhanced microbial activity and imidazolinone degradation, whereas elevated CO2 increased plant sensitivity without improving post-cultivation soil remediation.