Nature and accessibility of organic matter in lacustrine sediment

Abstract

Organic matter (OM) in reservoir sediment consists of a range of biomolecules, but their individual contribution to the biogeochemical cycling of carbon and nitrogen nutrients is not documented. This work proposes to investigate whether the nature of the OM determines its accessibility in lacustrine sediment matrix. We adapted an OM chemical sequential method developed for soils (particle size ≤ 20 μm) for use on sediments collected from four reservoirs (particle size ≤ 2 mm), coupled with a carbon and nitrogen elementary analysis and colorimetric quantification. This method allowed for the extraction of more than 70% of carbon and 61% of nitrogen. This OM includes exchangeable, extractable, free particulate, and residual OM, whose carbon content represented < 2%, 64 to 86%, 4 to 16%, and 9 to 24%, respectively. The sum of chemically extracted biochemical molecules that recover the extracted elementary carbon and nitrogen represents the chemically extracted OM. Phenols are the main carbon contributor (55 to 60%), followed by carbohydrates and proteins (14 to 18% and 21 to 29%, respectively). Nitrogen is mainly composed of proteins, amino acids, and ammonium (46 to 56%, 20 to 34%, and 8 to 28%, respectively). Among the four reservoir sediment samples, this same trend applies to the exchangeable and extractable phases: phenols, carbohydrates, and proteins are similarly distributed along the extracted phases, whereas nitrogen, forming as ammonium, amino acids, and nitrogen oxides, exhibits specific distributions. In reservoir sediments, the nature of the carbon does not impact its physicochemical accessibility; nitrogen material presents more varied profiles depending on its accessibility.