Carbon dynamics and their link to dissolved organic matter quality across contrasting stream ecosystems.

Streams represent active components of the carbon cycle as emitters of carbon dioxide (CO2) and methane to the atmosphere at a global scale. However, the mechanisms and governing factors of these emissions are still largely unknown, especially concerning the effect of land use. We compared dissolved and gaseous carbon dynamics in streams bordered by contrasting types of land use, specifically agriculture and forest. Carbon dioxide and methane partial pressures (pCO2 and pCH4, respectively) in the water body and carbon emissions via both gases were studied for 24h during four field expeditions. pCH4 did not differ between the two system types. pCO2 was constantly oversaturated in all streams and significantly higher in agricultural streams (annual mean 4282 ppm) compared to forest streams (annual mean 2189 ppm) during all seasons. However, emissions of CO2 were not significantly different between the stream types due to significantly higher gas transfer velocity in forest compared to agricultural streams. pCO2 was significantly positively correlated to the concentrations of dissolved organic carbon, dissolved nitrogen and soluble reactive phosphorus in the water. Furthermore, pCO2 was correlated to optical parameters of dissolved organic matter (DOM) quality, e.g., it increased with indicators of molecular size and an allochthonous fluorescent component identified by Parallel Factor Analysis (PARAFAC). This study demonstrates that different forms of land use may trigger a cascade of effects on the carbon production and emission of streams linked to changes in DOM quality.