Landscape drivers of biodegradable dissolved organic carbon across a gradient of human impact

Abstract:

Intensive agriculturalization and urbanization are reshaping the structure and functioning of aquatic ecosystems worldwide. These activities alter the composition and reactivity of dissolved organic matter (DOM) in receiving waters, with uncertain implications for nutrients and carbon (C) processing. Here, a nested watershed sampling approach was applied across seasons to link land use and biodegradable dissolved organic carbon (BDOC) processing in a model continental watershed (from mountain headwaters to agricultural landscapes in the Oldman River Basin, Alberta, Canada). Using standardized 28-day incubations, BDOC patterns ranged widely across sites (5 to 45% of initial DOC pool). The greatest fractions of DOC loss were found in regions of intense agricultural and urban land use, while less impacted headwater sites had proportions of DOC loss consistent with global averages (approximately 12%). At the most impacted site receiving both slaughterhouse and municipal effluents, rates of BDOC were dampened during the first seven days of the incubation period before extreme DOC removal in subsequent weeks. We observed varying responses of DOM absorbance and fluorescence parameters, with general increases in DOM aromaticity throughout the incubation period in both impacted and headwater sites. These findings highlight the importance of the often-overlooked organic pool when considering agricultural impacts on aquatic nutrient and C cycles. They also highlight the potential for agricultural and urban effluents to supply labile DOM that may render these sites hotspots in the river network for microbial greenhouse gas production.