Forest soils store the bulk of terrestrial carbon, but the factors that determine how much carbon a forest soil can store and for how long are not fully understood. A recently published study from the University of Helsinki demonstrates that soil organic carbon turnover slows as forests age, which increases long-term carbon storage. Soil organic matter originates from decaying plant material that consists of a variety of plant compounds with different chemical properties. For a long time, scientists have understood that organic matter containing the more slowly decomposable plant compounds accumulates over time as forest soils age.