A missing carbon sink from land to the ocean: Process and assessment
A missing process in forest carbon-sink assessment
Forests are among the most visible and widely recognized terrestrial carbon sinks, and afforestation and reforestation are often regarded as feasible nature-based options for climate mitigation. Forest carbon sinks are usually assessed by measuring carbon retained within forest boundaries, including aboveground biomass, roots, litter, dead wood, and soil organic carbon. This plot-based view is essential for inventories, restoration assessments, and mitigation projects. Yet forests are open biogeochemical systems with a less visible dissolved carbon outlet: forest-derived dissolved organic carbon (DOC) that is leached from soil and transported by water.
The critical knowledge gap concerns the fate of exported dissolved carbon after it enters aquatic systems. Humified forest-soil organic matter is mobilized by runoff, groundwater flow, and erosion and transported through inland waters, estuaries, and shelves, where microbial, photochemical, mineral, and hydrological processes transform it. After partial degradation, part may enter refractory DOC (RDOC) reservoirs that persist over centennial-to-millennial timescales.1 This lateral DOC export and downstream RDOC formation are generally omitted from project-level forest carbon accounting, although they remain part of the broader land-ocean carbon cycle.
Terrestrial humification provides an intuitive starting point: dead wood, litter, roots, and humus-rich soils are visible and inventory-ready carbon pools, whereas the persistence of mobilized DOC depends on its transformation history and downstream processing. Brown or tea-colored waters draining forested catchments provide a visible expression of humic DOC mobilization, yet the transition from visible terrestrial carbon pools to less visible, long-lived dissolved reservoirs remains poorly represented in forest carbon-sink assessment (Figure 1).
We refer to this connection as the land-ocean humic carbon pathway. It captures a simple but overlooked possibility: forests, especially planted forests and shelterbelts, may alter production, leaching, and downstream transformation of humic-like DOC and RDOC precursors. Only the fraction that survives successive aquatic processing and persists in marine waters contributes to RDOC preservation. This pathway is most relevant where large-scale planting is hydrologically connected to river networks, deltas, continental shelves, and active marginal seas. The land-ocean continuum also transports particulate organic carbon, whose preservation is governed mainly by erosion, sediment transport, mineral association, and sedimentary burial.
The missing component in conventional forest carbon-sink assessment is the downstream fate of DOC exported beyond the forest boundary. Terrestrial DOC export and aquatic RDOC formation are established processes. By linking them within a forest source-to-sink framework, the land-ocean humic carbon pathway extends forest assessment beyond within-boundary stocks to the downstream fate and persistence of exported DOC. Its carbon-sink significance lies in the fraction that survives aquatic processing and persists as RDOC. Because forest-associated DOC and RDOC may contain both contemporary forest carbon and legacy soil organic carbon, attribution to recent afforestation, reforestation, or forest management requires an appropriate baseline that separates the incremental contribution from the pre-existing catchment signal.
