The growing issue of climate change has placed high significance on the carbon cycle, its interactions, sources, and sinks. Within the greater cycle, rivers are important bridges between terrestrial sources and oceanic and atmospheric sinks, however their role has only been recently better understood and quantified. Recent research suggests that watershed geology and land use may control the amount and age of organic carbon (OC) within rivers, however their roles are not well understood.
Determining the quantity and age of labile (reactive) OC is important in understanding how geology and land use influence river carbon cycles. To address this question, we sampled six sites within the Hudson and Mohawk River watersheds in Upstate New York. These sites, with different geologies (shale-limestone, mixed, crystalline-metasedimentary) and different land usage (i.e. forest, agriculture, developed), allow us to explore the relationship between carbon cycling, geology, and land use. In addition to general geochemical characterization of the stream water, we conducted incubation experiments on the water samples collected. Incubation experiments were performed to determine the age and proportion of labile dissolved organic carbon (DOC) in each river. Our incubation experiments were coupled with geologic information and analyses to assess the influence of land use and watershed geology on DOC chemical characteristics and age.
Our incubation experiments indicate that the proportion of labile DOC is affected by site geology. Shale-limestone geology was found to have the highest proportion of labile DOC, while crystalline-metasedimentary had the lowest. Furthermore, we find that DOC 14C ages are dependent on site geology, with shale-limestone geology having the oldest 14C ages. From these ages, we were also able to calculate the age of labile DOC. We find that while crystalline-metasedimentary geologies have younger bulk DOC ages, the age of their labile DOC is significantly older (on the order of thousands of years), meaning once aged OC is mobilized, it becomes labile and is actively cycled. Our findings suggest that watershed disturbances, which mobilize aged OC from soils and rocks could result in this previously buried carbon being actively cycled and ultimately contributing to the present-day carbon cycle. Our findings help to further develop the understanding of how river carbon cycling responds to both natural and anthropogenic disturbances.