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Biogeosciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/bg-2020-267
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2020-267
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  05 Aug 2020

05 Aug 2020

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This preprint is currently under review for the journal BG.

Soil organic carbon decomposition rates in river systems: effect of experimental conditions

Man Zhao, Liesbet Jacobs, Steven Bouillon, and Gerard Govers Man Zhao et al.
  • KU Leuven, Department of Earth and Environmental Sciences, 3001 Leuven, Belgium

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.

Man Zhao et al.

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Man Zhao et al.

Man Zhao et al.

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Latest update: 27 Sep 2020
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Short summary
We investigated the relative importance of two individual factors (mechanical disturbance and aquatic microbial organisms) that possibly control SOC decomposition rates in river systems. We found aquatic microbial organisms led to rapid SOC decomposition, while effect of mechanical disturbance is relative minor. We propose a simple conceptual model: mechanical disturbance is only important when soil aggregates are strong enough to withstand the disruptive forces imposed by immersion in water.
We investigated the relative importance of two individual factors (mechanical disturbance and...
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