TURNOVER OF FLUVIAL PARTICULATE ORGANIC MATTER: THE ROLES OF SEDIMENT RESIDENCE TIME AND MICROBIOTA

Rivers are the primary conduits for organic carbon (OC) transfer from vegetation-rich uplands to long-term sinks, and thus are responsible for significant fluxes among different reservoirs of the carbon cycle. One important component of this system is the oxidation of particulate OC during floodplain storage and release of CO₂ to the atmosphere. Timescales and mechanisms of these processes are not yet constrained, but are important for modeling the global carbon cycle.

In this study we evaluate the timescales of sediment and organic carbon transport from source to sink. Our natural laboratory is the Rio Bermejo in northern Argentina, which transports material from the central Andes over 700 km across the foreland and out onto the craton without input of allochthonous material from tributaries. Rapid channel migration rates are responsible for exchanging sediment and OC between the river and floodplain. This material is then delivered to a large continent-scale river downstream.

We sampled suspended sediment, floodplain sediment, and soil from several locations along the length of the Rio Bermejo. We measured the radiocarbon content, ¹³C/¹²C ratio, concentration of the meteoric radionuclide ¹⁰Be of the material to evaluate the geomorphic processes that control the timescale of particulate OC transport in rivers. These data show that radiocarbon ages increases by ~500 years across the floodplain, while the clastic sediment has a longer overall residence time of ~6000 years, suggesting that a pool of old recalcitrant OC is preserved through river transit, while more labile OC is oxidized and replaced by modern vegetation. We combine these results with 16S rRNA-based microbial community analyses, which shows a high diversity along the whole river. The relative abundances of Streptomyces and Sphingomonadacae in river sediment and floodplain soil, which are known to be chemoorganotrophs, can be linked to the turnover of OC and preferential enrichment of ¹³C vs. ¹²C in river sediment over the timescale of 10²-10⁴ years. With these data, we can begin to explain the timescales and mechanisms of OC oxidation during fluvial transit, enabling carbon cycle models to account for these losses.