Seasonally variable interactions between dissolved organic matter and mineral particles in an agricultural river

Streams and rivers form an important link in the global carbon cycle by transporting and transforming large amounts of carbon imported from terrestrial ecosystems to the oceans. Since streams in agricultural areas often experience increased concentrations of suspended mineral particles from soil erosion, they are important sites where dissolved organic carbon (DOC) may be adsorbed to particles and retained in the sediment. As the extent of adsorption varies with the molecular composition of dissolved organic matter (DOM), which is seasonally variable, we expect also the fraction of organic material that adsorbs to mineral particles to fluctuate over time. We sampled the agriculturally impacted River Fyrisan (Sweden) monthly during 1 year, and measured DOC concentration and DOM composition based on several optical properties. At each sampling occasion, we estimated the potential for adsorption by exposing the samples to a reference clay. The potential for adsorption was greatest when riverine DOM had the most terrestrial character, as this fraction of the DOM pool was selectively adsorbed to clay surfaces. The extent of adsorption was negatively related to the concentration of base cations, most notably calcium. We suggest that the observed relationships between the potential for adsorption, DOM composition and base cations are linked by discharge. A bioavailability test at one sampling occasion suggested that DOM remaining after exposure to clay particles was more biodegradable. This implies that adsorption may alter the degradation potential of DOM remaining in solution, which could have far reaching effects on the fate of organic carbon.

Automated summary

Streams and rivers play a crucial role in the global carbon cycle by transporting carbon from terrestrial ecosystems to the oceans. In agricultural areas, streams often experience increased concentrations of suspended mineral particles from soil erosion, leading to the adsorption of dissolved organic carbon (DOC) to particles and its retention in sediment. The extent of adsorption varies with the seasonal changes in the molecular composition of dissolved organic matter (DOM). This study conducted monthly sampling in a agriculturally impacted river in Sweden and found that the adsorption potential was highest when the riverine DOM had a terrestrial character. The adsorption extent was negatively related to base cation concentrations, particularly calcium.