Organic carbon decomposition rates controlled by water retention time across inland waters

The loss of organic carbon during passage through the continuum of inland waters from soils to the sea is a critical component of the global carbon cycle(1-3). Yet, the amount of organic carbon mineralized and released to the atmosphere during its transport remains an open question(2,4-6), hampered by the absence of a common predictor of organic carbon decay rates(1,7). Here we analyse a compilation of existing field and laboratory measurements of organic carbon decay rates and water residence times across a wide range of aquatic ecosystems and climates. We find a negative relationship between the rate of organic carbon decay and water retention time across systems, entailing a decrease in organic carbon reactivity along the continuum of inland waters. We find that the half-life of organic carbon is short in inland waters (2.5 +/- 4.7 yr) compared to terrestrial soils and marine ecosystems, highlighting that freshwaters are hotspots of organic carbon degradation. Finally, we evaluate the response of organic carbon decay rates to projected changes in runoff(8). We calculate that regions projected to become drier or wetter as the global climate warms will experience changes in organic carbon decay rates of up to about 10%, which illustrates the influence of hydrological variability on the inland waters carbon cycle.