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

Man Zhao,Steven Bouillon,Liesbet Jacobs,Gerard Govers

doi:10.5194/bg-18-1511-2021

Abstract

Abstract. Different erosion processes deliver large amounts of terrestrial soil organic carbon (SOC) to rivers. Mounting evidence indicates that a significant fraction of this SOC, which displays a wide range of ages, 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 metabolize 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 sensor (FireStingO2, PyroScience). Under both shaking and standing conditions, we found a significant difference in decomposition rate between SOC with aquatic microbial organisms added (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 the effect of hydrodynamic disturbance is relatively minor, under the temperature conditions, for the soil type, and for the suspended matter concentration range used in our experiments. We propose a simple conceptual model explaining these contrasting results.

Highlights

Rivers play an important role in the global carbon cycle by linking terrestrial and aquatic ecosystems
Understanding the mechanisms that contribute to this active mineralization process of soil organic carbon (SOC) is essential for understanding the role of rivers in the global carbon cycle and for assessing how the metabolism of rivers may respond to environmental perturbations such as an increase or decrease in terrestrial carbon delivery to the river system and/or changes in hydrology and climate
We investigated the relative importance of physical disturbance vs. exposure to a novel microbial community for SOC decomposition rates in aquatic environments

Summary

Introduction

Rivers play an important role in the global carbon cycle by linking terrestrial and aquatic ecosystems. Rivers receive and deliver large amounts of terrestrial organic carbon to the oceans (Raymond and Bauer, 2001; Ward et al, 2017). Many studies have demonstrated that the DOC transported by large rivers such as the Amazon and Mississippi is generally quite young (Mayorga et al, 2005; Rosenheim et al, 2013), while in other systems the POC is often associated with relatively old radiocarbon ages (in the range of 1000–5000 years BP) (Marwick et al, 2015; Raymond and Bauer, 2001; Dodds and Cole, 2007; McCallister and Del Giorgio, 2012). The amount of SOC imported into riverine ecosystems is often large compared to the autochthonous within-river primary

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