Soils are one of the largest terrestrial sinks for organic carbon, and therefore present a promising opportunity to mitigate climate change. Over the past decade, many global initiatives have been launched to enhance soils’ capacity to sequester and store organic carbon. A noteworthy example is the ‘4 per mille’ scheme, launched at the Paris Climate Change Conference in 2015. This initiative proposed that annual CO2 emissions from fossil fuel burning could be offset if the global stock of soil organic carbon was increased annually at the rate of 4 parts per 1000. Eight years have elapsed since this initiative was launched, and a debate ensues about the extent to which soils have the capacity to endlessly increase their carbon storage. In this session, we will showcase research that interrogates both arguments of the ‘carbon saturation threshold’ debate. Is there a threshold above which a soil profile can no longer increase its carbon storage? If so, what is this threshold, and what are the implications for both land management and our Net Zero Carbon targets? What are the mechanisms determining differences between soils’ soil carbon saturation thresholds, and over what timescales may saturation limit the capacity of soils to mitigate climate change? We welcome empirical work, model-based efforts, or desk-based reviews. Early career researchers are strongly encouraged to apply.