Photochemical and microbial processing of stream and soilwater dissolved organic matter in a boreal forested catchment in northern Sweden

• Natural organic matter (NOM) from stream and soil water in a humic-rich headwater catchment in northern Sweden (initial total organic carbon (TOC) concentrations 10-40 mg C L-1) was rapidly degraded by light and microbial activity in an incubation experiment. Concentration losses were 33-50% after 12 days of exposure to 69 W m(-2) artificial PAR and 16 W m(-2) UV radiation. Natural, unshaded mid-day solar radiation in the region (68degreesN 18degreesE) during the month of june is 159 W m(-2) for PAR. In contrast to microbial organic carbon removal, TOC exponentially decreased upon radiation, which suggests that TOC is more rapidly oxidized by light than by ambient microbes. Further, rapid decline in TOC concentration implies the presence of a dominant pool of photo-labile compounds (p > 95%). A measured mass balance for carbon identified 50-75% of the degraded TOC as carbon dioxide after 12 days of exposure to light. The observed conversion of organic to inorganic carbon was accompanied by increases in pH and alkalinity, suggesting that photo-degradation of NOM potentially contributes to in-stream buffering capacity. The remaining refractory TOC changed in chemical character, including an altered molecular weight distribution with decreased average weight and a change in the proportions of humics as evidenced by absorbance ratios (A(254)/A(420)). Extrapolation of the experiment to natural headwater conditions show that photo-degradation is an important in-stream process that should be considered in calculations of carbon turnover in surface waters because of its influence on both TOC amount and character.

Nyckelord

photo-degradation

natural organic matter (NOM)

headwaters

carbon dioxide

dissolved organic matter (DOM)

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