Dissolved organic matter (DOM) quality in agricultural streams and its impact on carbon dioxide concentrations in stream water
Abstract: Inland waters have lately been recognized to be an important component in the carbon cycle, having a significant role in carbon sequestration and carbon dioxide (CO2) emissions. Attention has also been drawn to the impact of the quality (i.e. composition and source) of dissolved organic matter (DOM) on CO2 production. The objective of this study was to identify geographical and hydrochemical controls on DOM quality and quantity in agricultural streams, and to investigate if DOM quality has an impact on CO2 concentrations in stream water. Water samples were collected in July-November from ten streams in agricultural catchments in Uppsala, in which partial pressure of CO2 (pCO2) had been measured in a related M.Sc. thesis project. Fluorescence measurements and Parallel Factor Analysis (PARAFAC) were carried out to analyse DOM quality. Fluorescence Index (FI), Freshness Index (β/α), Humification Index (HIX) and a five-component PARAFAC model were derived and used to parametrize DOM quality. Dissolved organic carbon (DOC) concentration was used as a measure of DOM quantity. The fraction of arable land in the catchment was found to be positively correlated with FI, indicating a shift towards more microbially derived DOM with more arable land in the catchment. Two PARAFAC components associated with terrestrial and highly decomposed DOM were found to correlate positively with pCO2 at one site. Specific discharge and electrical conductivity were correlated with DOM quality and quantity at several sites. The correlations indicated that both the discharge magnitude as well as the flow paths affected the quality and the quantity of DOM.
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