Accumulation of recalcitrant dissolved organic matter in aerobic aquatic systems

Abstract

An oxygenated atmosphere led to many changes to life on Earth but it also provided a negative feedback to organic matter accumulation over billions of years by increasing decomposition rates. Nonetheless, dissolved organic carbon (DOC) is a huge carbon pool (>750 Pg) and it can accumulate to high concentrations (20-100 mg C L-1) in some freshwater aquatic systems, yet it is not clear why. Here, we examine DOC in several arctic lakes with varying concentrations and identify processes that alter its composition to make it recalcitrant to further degradation processes. Aging of DOC (from radiocarbon Δ14C ratios) corresponded with changes in its concentration, degradation rates, δ13C-DOC isotope ratios and optical quality, all suggesting that photochemical and microbial degradation processes contributed to decreased DOC reactivity over time. The degradation of young DOC was strongly stimulated by inorganic phosphorus, but older DOC was not, suggesting an important role for nutrients in regulating organic carbon degradation rates and pool sizes. Photochemical processing coupled with decreased habitat and microbial diversity in hydrologically isolated systems may enable recalcitrant DOC to accumulate with important implications for the Earth's carbon and oxygen cycles.