Assessing the Age of Particulate Organic Matter Exported from a Temperate Rainforest Catchment of the North American Pacific Coast

Abstract:

Although temperate rainforests of the North American Pacific Coast contain a small proportion of the world’s forests, they contain some of the highest densities of biomass of any terrestrial system, and they store large quantities of carbon in soil. Understanding the residence time of organic carbon in these watersheds is of ecological significance. Given that rivers can mobilize sediment (and associated carbon) from across the catchment, carefully deciphering the organic signatures found within riverine particles can be a powerful tool to inform our understanding of carbon cycling catchment-wide. Here we examine the lignin phenol content (lignin is a biomarker unique to vascular plants) and the radiocarbon age (Δ¹⁴C) of fine particulate organic carbon (FPOC) exported by the Queets River of Washington State’s Olympic Peninsula over the course of one year, targeting winter storm events. This mountainous catchment is one of the largest and most pristine found on the Olympic Peninsula. The Δ¹⁴C of FPOC was quantified for each of the twelve sampling events, whereas the Δ¹⁴C of the individual lignin phenols was determined during a late-winter storm event. Sediments were enriched in lignin phenols at the end of the summer dry season and into the first storm of the fall, suggesting that surface soils were transported early on. The Δ¹⁴C of individual lignin phenols ranged from -161 to 26‰, with biomarkers for non-woody vegetation being most depleted. These results suggest that particulate lignin exported from temperate catchments is considerably aged, especially relative to the tropics. These findings are consistent with cool temperatures and abundant moisture limiting microbial decomposition, increasing the residence time of plant-derived organic carbon in temperate rainforests. We will compare the Δ¹⁴C content of lignin phenols to that of bulk organic matter to partition riverine FPOC amongst possible organic matter sources.