<p>Carbon sequestration in agricultural soils is an economically viable pathway to achieve climate and food co-benefits, but it remains unclear whether the positive soil organic carbon (SOC)–yield relationship observed in field trials applies to real-world farms. Here we analysed maize yield responses to SOC on 1,246 field trials and 192 farms in Northeast China using multiple causal inference approaches. We found that an increase in SOC consistently enhanced maize yield in field trials but had no detectable effect in farms. Field trials showed significant yield increases of 8–13% up to a threshold of 30–40 g SOC kg⁻¹ soil. On farms, however, yields rose by only up to 5% with increasing SOC to a threshold of 10.0 g SOC kg⁻¹ soil and then decreased to around the initial yield. The absence of a positive yield–SOC response in real-world farms was mainly attributable to excessive nitrogen fertilizer applications, along with limited farmer knowledge and inadequate infrastructure hindering effective adoption of soil conservation practices. Our findings underscore the need to integrate fertilization, soil conservation strategies and farmer support to fully realize the potential of SOC on yield.</p>