Priming of terrestrially-derived dissolved organic matter: Implications for coastal carbon cycling

English Abstract:

<span'',serif;">Although inland waters comprise a small fraction of Earth’s surface, they play a critical role in the global C cycle. Global estimates of riverine flux of dissolved organic carbon (DOC) to the oceans range from about 250 to 360 Tg y^-1. Interestingly, only a small fraction of the roughly 2900 Tg C yr^-1 transported through inland waters globally ever reaches the ocean. Recent observations of immense CO₂ evasion from streams and rivers thus suggest that terrestrially-derived DOC (TDOC) is not as recalcitrant as previously thought. In a lab-based experiment pine-litter leachate was converted to CO₂ at roughly the same rate in the trehalose and diatom leachate treatments, less than 1% of the diatom leachate was converted to CO₂, as opposed to ~50% of the trehalose over a few days. It is likely that a large fraction of the diatom leachate was assimilated by bacteria, whereas the trehalose was mostly respired to CO₂. Pilot field experiments in the Amazon River, with the incubation system performed during two cruises show that δ¹³CO₂ rapidly increased after adding labeled lignin; roughly 50% of the added lignin was converted to CO₂ in 10 hours. This is equivalent to a remineralization rate of ~0.021 mg C L^-1d^-1, or 2x the rate of FACE-only remineralization measured in the lab experiments.