The use of radiocarbon &amp;lt;sup&amp;gt;14&amp;lt;/sup&amp;gt;C to constrain carbon dynamics in the soil module of the land surface model ORCHIDEE (SVN r5165)

Marwa Tifafi,Sebastián Barbaro,Christine Hatté,Sophie Cornu,Bertrand Guenet,Lucas Moretti,Hector Morras,Marta Camino-Serrano

doi:10.5194/gmd-11-4711-2018

Abstract

Abstract. Despite the importance of soil as a large component of the terrestrial ecosystem, the soil compartments are not well represented in land surface models (LSMs). Indeed, soils in current LSMs are generally represented based on a very simplified schema that can induce a misrepresentation of the deep dynamics of soil carbon. Here, we present a new version of the Institut Pierre Simon Laplace (IPSL) LSM called ORCHIDEE-SOM (ORganizing Carbon and Hydrology in Dynamic EcosystEms-Soil Organic Matter), incorporating the 14C dynamics into the soil. ORCHIDEE-SOM first simulates soil carbon dynamics for different layers, down to 2 m depth. Second, concentration of dissolved organic carbon and its transport are modelled. Finally, soil organic carbon decomposition is considered taking into account the priming effect. After implementing 14C in the soil module of the model, we evaluated model outputs against observations of soil organic carbon and modern 14C fraction (F14C) for different sites with different characteristics. The model managed to reproduce the soil organic carbon stocks and the F14C along the vertical profiles for the sites examined. However, an overestimation of the total carbon stock was noted, primarily on the surface layer. Due to 14C, it is possible to probe carbon age in the soil, which was found to be underestimated. Thereafter, two different tests on this new version have been established. The first was to increase carbon residence time of the passive pool and decrease the flux from the slow pool to the passive pool. The second was to establish an equation of diffusion, initially constant throughout the profile, making it vary exponentially as a function of depth. The first modifications did not improve the capacity of the model to reproduce observations, whereas the second test improved both estimation of surface soil carbon stock as well as soil carbon age. This demonstrates that we should focus more on vertical variation in soil parameters as a function of depth, in order to upgrade the representation of the global carbon cycle in LSMs, thereby helping to improve predictions of the of soil organic carbon to environmental changes.

Highlights

The complexity of the mechanisms involved in controlling soil activity (Jastrow et al, 2007) and the carbon flux from the soil to the atmosphere makes predicting the response of these systems to climate change extremely complex
Results from the initial version of ORCHIDEE-SOM-14C show that in all the studied sites, the model succeeds in reproducing the trend of the total carbon profiles, with more carbon at the surface which decreases according to depth (Fig. 3)
Total soil carbon stock simulated down to 2 m depth is in accordance with data in the case of Misiones and Feucherolles where the major difference mainly lies on the surface

Summary

Introduction

The complexity of the mechanisms involved in controlling soil activity (Jastrow et al, 2007) and the carbon flux from the soil to the atmosphere makes predicting the response of these systems to climate change extremely complex. Tifafi et al.: The use of radiocarbon 14C to constrain carbon dynamics count within the Intergovernmental Panel on Climate Change (IPCC; Taylor et al, 2012) for assessment of the impacts of climate change and design of mitigation strategies Their predictions need to be as accurate as possible. These models represent the physical, chemical and biological processes within and between the atmosphere, ocean and terrestrial biosphere. They allow us to follow and understand both the effect of the climate on carbon storage and vice versa. This component primarily manages the carbon cycle, energy and water on land and simulates the carbon exchange between the land surface and the atmosphere, namely the gross primary production (GPP) and the autotrophic and heterotrophic respiration

Methods

Results

Conclusion