Fractionation of lignin during leaching and sorption and implications for organic matter “freshness”

Much of what we know about the cycling of terrigenous organic matter (OM) in freshwater and marine systems can be attributed to evidence derived from biomarkers such as lignin. Here we report the fractionation of lignin phenols both during solubilization from plant litters and again during sorption of resulting leachates to soils. Source parameters in both leached and sorbed lignin varied from the parent litters by as much as ten‐fold, while elevated lignin oxidation parameters in riverine dissolved OM (DOM) can be completely explained by dissolution and sorption. Carbon‐normalized dissolved lignin endmembers indicate a substantial fraction of riverine DOM may not be vascular plant‐derived, and thus derived from algal, microbial or non‐vascular plant sources such as mosses. These results demonstrate that riverine DOM is less degraded and likely more labile than previously thought, and helps resolve the paradox of diagenetically‐altered riverine DOM degrading rapidly in marine systems.