Background and aims <p>Irrigation retirement is expected to increase in semiarid regions due to groundwater shortage, with potential consequences for soil organic carbon (SOC) storage. While reductions in carbon (C) inputs are anticipated, the response of C outputs due to heterotrophic respiration (Rh) remains poorly understood. We investigated how irrigation retirement influences C inputs, soil respiration (Rs) components, and SOC formation in contrasting cropping systems.</p> Methods <p>We installed a three-year transition experiment to compare irrigated and dryland (retired) treatments under continuous maize and continuous wheat. We measured C inputs, different components of Rs, and SOC isotopic composition and distribution in particulate and mineral associated fractions.</p> Results <p>In maize, C inputs were reduced more than twofold after irrigation retirement, but effects on Rs were smaller and mostly related to decreases in autotrophic respiration. Rh decreased by 20% only after two seasons of accumulated input reduction. In wheat, irrigation retirement had little effect on either C inputs or outputs. Evidence indicated that Rh changes were driven by substrate limitation rather than soil moisture: (i) Rs from long-term fallow subplots was not impacted by irrigation, (ii) Rh in maize was not correlated with soil moisture, and (iii) isotopic analyses indicated that irrigation retirement affected the formation of new SOC rather than old SOC decomposition.</p> Conclusion <p>SOC responses to irrigation retirement are mediated by changes in crop productivity and residue return. Cropping systems that minimize biomass production declines have the potential to mitigate SOC losses during the transition from irrigated to dryland.</p>