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. Different erosion processes deliver large amounts of terrestrialsoil organic carbon (SOC) to rivers. Mounting evidence indicates that asignificant fraction of this SOC, which displays a wide range of ages, israpidly decomposed after entering the river system. The mechanismsexplaining this rapid decomposition of previously stable SOC still remainunclear. In this study, we investigated the relative importance of twomechanisms that possibly control SOC decomposition rates in aquatic systems:(i) in the river water SOC is exposed to the aquatic microbial communitywhich is able to metabolize SOC much more quickly than the soil microbialcommunity and (ii) SOC decomposition in rivers is facilitated due to thehydrodynamic disturbance of suspended sediment particles. We performeddifferent series of short-term (168 h) incubations quantifying the rates ofSOC decomposition in an aquatic system under controlled conditions. Organiccarbon decomposition was measured continuously through monitoring dissolvedO2 (DO) concentration using a fiber-optic sensor (FireStingO2,PyroScience). Under both shaking and standing conditions, we found asignificant difference in decomposition rate between SOC with aquaticmicrobial organisms added (SOC + AMO) and without aquatic microbialorganisms (SOC − AMO). The presence of an aquatic microbial community enhancedthe SOC decomposition process by 70 %–128 % depending on the soil type andshaking–standing conditions. While some recent studies suggested thataquatic respiration rates may have been substantially underestimated byperforming measurement under stationary conditions, our results indicatethat the effect of hydrodynamic disturbance is relatively minor, under thetemperature conditions, for the soil type, and for the suspended matterconcentration range used in our experiments. We propose a simple conceptualmodel explaining these contrasting results.

Rapid soil organic carbon decomposition in river systems: effects of the aquatic microbial community and hydrodynamical disturbance