Water dispersible (nano)colloids in riparian sediments have a significant influence on organic carbon fate and transport

Abstract:

Riparian sediments are an important link in the cycling of organic carbon (OC) at the interface of the terrestrial and aquatic environments, and mineral nano-colloids (MNCs) play a significant role in cycling. The global area of floodplain soils is estimated at 2´10⁶ - 8´10⁷ km², and contain 456–673 Pg OC (~10%–20% of the global soil carbon pool). The changing hydrodynamic conditions in watersheds stimulate formation and release of MNCs that are accompanied by increased transport of adsorbed OC. This is significant because a global estimate of river transported MNCs is 10³–10⁴ Tg yr^-1 and their surface reactivity facilitates sorption and transport. This presentation summarizes 3 riparian, capillary-fringe sediment studies. The first 2 simulated a shift in alpine temperatures as might occur via changing climate of Nisqually River sediments, Mt. Rainier, WA. These investigations involved a pretreatment where sediment with and without illite amendment was reacted for 3 months, to equilibrate at temperature (4˚ and 20˚C). Our initial, microbial investigation tested two moisture conditions (50 vs 100% water-filled pores) and found that inhibition of CO₂ emissions were from the combined effect of increased moisture content and illite addition. There were shifts in bacterial communities from a coupled moisture-mineral interaction, and these were temperature-dependent. Our second study showed that OC leaching rates at 20°C were 2 - 3 times greater than at 4°C, and our results suggest MNCs are responsible for moving increased OC. Additionally, a model developed for this study estimates a 1°C increase over a 3-month period increases the OC release rate from sediments by 79 %. The third investigation looked at the colloid fractionation of MNC- OC leached from contrasting rivers: Columbia River, WA and Tims Branch, SC. Here, sediments were leached aerobically and anaerobically. Samples were gravity separated to distinguish OC bound to MNCs from dissolved OC. From this study we found that MNC- OC was as high as 48% of OC for Tims Branch anaerobic extraction and as low as 0% for Columbia River aerobic extractions. Anaerobic leaching yielded higher MNC and OC release. The combined studies demonstrate that MNC facilitated release of OC in riparian sediments is significant and variable between locations and climatic conditions.