Elevated CO2 increases cover crop sensitivity to imidazolinones but does not substantially alter post-cultivation soil dissipation.

Abstract

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.