No indications for a priming effect on soil organic carbon mineralization in a temperate river system

Globally, rivers are vital conduits transporting and processing terrestrial carbon, and are generally considered to act as source of carbon dioxide (CO2) towards the atmosphere. A large amount of soil organic carbon (SOC) is transferred from the land surface to river systems each year, where it mixes and interacts with the autochthonous carbon pool (i.e., produced in-stream via photosynthesis). The latter has been suggested to be more labile and to potentially affect—positively or negatively—the mineralization rate of the more recalcitrant SOC, a mechanism referred to as the priming effect (PE). Here, we performed series of short-term (7 days) incubation experiments to investigate whether the addition of (13C-labelled) algal carbon (C) affected SOC mineralization in an aquatic environment, under different nutrient (nitrogen, N; phosphorus, P) conditions. Dissolved oxygen (DO) was continuously measured using a fiber-optic sensor. The concentrations and stable isotope composition of particulate organic carbon (POC), dissolved organic carbon (DOC), and dissolved inorganic carbon (DIC) were measured to help to distinguish the sources of the mineralized C. Overall, algal C mineralized much more rapidly compared to SOC, but its presence had only weak and insignificant effects on SOC mineralization. Overall, under our experimental conditions, our results do not indicate that phytoplankton-derived organic carbon substantially affects SOC mineralization rates in aquatic systems.