Soil organic carbon across space and Time: Where chronosequences Still matter

Soil organic carbon (SOC) is a major reservoir of the terrestrial carbon cycle and a key regulator of soil fertility, ecosystem functioning, and climate. However, its long-term dynamics remain difficult to observe directly because they unfold over timescales far exceeding typical experimental records. Chronosequences address this limitation by substituting space for time, allowing temporal trajectories of SOC to be inferred from spatial gradients under comparable environmental conditions. For more than a century, this approach has supported research on ecological succession, pedogenesis, and SOC recovery following disturbance or land-use change. Its explanatory power, however, depends on key assumptions, including site comparability and the stability of system trajectories, which are rarely fully met in natural systems. Here, we review the conceptual foundations of chronosequences, propose a structured classification, and critically evaluate their methodological strengths and limitations. We then explicitly link chronosequence evidence to contemporary frameworks of SOC formation and stabilization, including microbial and mineral-mediated processes. Across diverse geomorphic and bioclimatic settings, chronosequences consistently reveal shared patterns of SOC accumulation, vertical redistribution, and, in some cases, long-term retrogression. Despite their imperfections, chronosequences remain indispensable tools for constraining long-term SOC dynamics and for connecting process-based models with the deep temporal dimension of soils. These inferences ultimately rely on comparisons between alternative system trajectories, whose validity depends on the plausibility of underlying assumptions.