Water Table Dynamics Control Carbon Losses from the Destabilization of Soil Organic Matter in a Small, Lowland Agricultural Catchment

Laurent Jeanneau,Valérie Viaud,Pauline Buysse,Gérard Gruau,Anne Jaffrézic,Chris Flechard,Marie Denis,Patrice Petitjean

doi:10.3390/soilsystems4010002

Abstract

The biogeochemistry of soil organic matter (SOM) is driven by a combination of stabilization and destabilization mechanisms. Among the various ways in which SOM is lost, soil moisture controls the leaching of dissolved organic and inorganic carbon (DOC and DIC) and CO2 fluxes (FCO2). The aim of this study was to investigate the impact of naturally occurring water table dynamics on the couplings between these three types of C losses. The DIC and DOC concentrations in the soil solutions and the FCO2 values at the soil surface were collected fortnightly over a nine-month period at four sampling points located along two topographic transects characterized by different water table dynamics. The water table depth, soil temperature and water-filled pore space (WFPS) were monitored at each site. Linear and nonlinear regressions were used to explore the couplings between C losses, WFPS and soil temperature. The dynamics of the water table seem to drive DOC solubilization, diffusion, and export mechanisms in addition to microbial processes and the equilibrium between DIC and CO2. The main descriptors of this water table dynamic were the residence time, return time and number of oscillations of the water table. Considering both transects, FCO2 was positively correlated with DOC, which highlights the importance of substrate accessibility for SOM mineralization. This paper emphasizes the importance of the water table dynamic for the coupling between SOM carbon losses.

Highlights

Soil organic matter (SOM) is the source of energy for terrestrial heterotrophic organisms
In the scientific report of this workshop, soil organic matter (SOM) vulnerability is used as a synonym of destabilization and is assumed to increase “when intrinsic soil properties, environmental conditions, and perturbations reduce the amounts of stable aggregates, soil microbes, particulate C, and mineral-associated and sorbed C, leading to greater proportions of leached C and greenhouse gas emissions” [5]
This is in accordance with the results of Creed et al [41] that described, at the catchment scale, a positive correlation using an exponential function between the soil CO2 emissions and the DOC concentrations

Summary

Introduction

Soil organic matter (SOM) is the source of energy for terrestrial heterotrophic organisms. In the scientific report of this workshop, SOM vulnerability is used as a synonym of destabilization and is assumed to increase “when intrinsic soil properties, environmental conditions, and perturbations reduce the amounts of stable aggregates, soil microbes, particulate C, and mineral-associated and sorbed C, leading to greater proportions of leached C and greenhouse gas emissions (carbon dioxide and methane)” [5] Based on these definitions, SOM stored in a specific volume of soil can be conceptualized as a reactor receiving natural inputs, such as plant-derived organic matter coming from the fixation of atmospheric CO2 and anthropogenic inputs such as organic waste products. Primarily CO2 , and methane and volatile organic compounds, have been estimated to represent an annual flux of

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