Soil organic carbon decomposition rates in river systems: effect of experimental conditions
Abstract. Rivers receive large amounts of terrestrial soil organic carbon (SOC) due to the action of different erosion processes. Mounting evidence indicates that a significant fraction of this SOC, which is often very old, is rapidly decomposed after entering the river system. The mechanisms explaining this rapid decomposition of previously stable SOC still remain unclear. In this study, we investigated the relative importance of two mechanisms that possibly control SOC decomposition rates in aquatic systems: (i) in the river water SOC is exposed to the aquatic microbial community which is able to metabolise SOC much more quickly than the soil microbial community and (ii) SOC decomposition in rivers is facilitated due to the hydrodynamic disturbance of suspended sediment particles. We performed different series of short-term (168 h) incubations quantifying the rates of SOC decomposition in an aquatic system under controlled conditions. Organic carbon decomposition was measured continuously through monitoring dissolved O2 (DO) concentration using a fiber-optic meter (FirestingO2, PyroScience). Under both shaking and standing conditions, we found a significant difference between SOC with aquatic microbial organisms (SOC + AMO) and without aquatic microbial organisms (SOC − AMO). The presence of an aquatic microbial community enhanced the SOC decomposition process by 70 %–128 % depending on the soil type and shaking/standing conditions. While some recent studies suggested that aquatic respiration rates may have been substantially underestimated by performing measurement under stationary conditions, our results indicate that this effect is relatively minor, at least under the temperature conditions, the soil type and for the suspended matter concentration range used in our experiments. We propose a simple conceptual model explaining these contrasting results.