Abstract
Abstract. Current land surface models (LSMs) typically represent soils in a very simplistic way, assuming soil organic carbon (SOC) as a bulk, and thus impeding a correct representation of deep soil carbon dynamics. Moreover, LSMs generally neglect the production and export of dissolved organic carbon (DOC) from soils to rivers, leading to overestimations of the potential carbon sequestration on land. This common oversimplified processing of SOC in LSMs is partly responsible for the large uncertainty in the predictions of the soil carbon response to climate change. In this study, we present a new soil carbon module called ORCHIDEE-SOM, embedded within the land surface model ORCHIDEE, which is able to reproduce the DOC and SOC dynamics in a vertically discretized soil to 2 m. The model includes processes of biological production and consumption of SOC and DOC, DOC adsorption on and desorption from soil minerals, diffusion of SOC and DOC, and DOC transport with water through and out of the soils to rivers. We evaluated ORCHIDEE-SOM against observations of DOC concentrations and SOC stocks from four European sites with different vegetation covers: a coniferous forest, a deciduous forest, a grassland, and a cropland. The model was able to reproduce the SOC stocks along their vertical profiles at the four sites and the DOC concentrations within the range of measurements, with the exception of the DOC concentrations in the upper soil horizon at the coniferous forest. However, the model was not able to fully capture the temporal dynamics of DOC concentrations. Further model improvements should focus on a plant- and depth-dependent parameterization of the new input model parameters, such as the turnover times of DOC and the microbial carbon use efficiency. We suggest that this new soil module, when parameterized for global simulations, will improve the representation of the global carbon cycle in LSMs, thus helping to constrain the predictions of the future SOC response to global warming.
Highlights
The soil is the largest terrestrial carbon pool and its response to global warming is crucial for the global carbon (C) cycle and its feedback to climate change (Jobbagy and Jackson, 2000; Todd-Brown et al, 2014)
ORCHIDEE-SOM is a new vertically resolved soil module embedded in the land surface model ORCHIDEE that represents litter, soil organic carbon (SOC) and dissolved organic carbon (DOC) dynamics, and transport in and out of the soil
Key model improvements compared to the trunk version of ORCHIDEE are that ORCHIDEE-SOM can simulate deep SOC dynamics and loss of organic carbon through leaching
Summary
The soil is the largest terrestrial carbon pool and its response to global warming is crucial for the global carbon (C) cycle and its feedback to climate change (Jobbagy and Jackson, 2000; Todd-Brown et al, 2014). These models differ in the definitions of the soil C pools (from turnover times to chemically differentiated fractions), the level of detail in the process formulation (e.g., from simple first-order kinetics to nonlinear relationships, including or not including sorption to soil minerals), and the spatial and temporal resolution (from site to global and from hourly to annual or longer timescales) While these models have been successfully tested and are able to reasonably simulate DOC dynamics, at present only two models exist that can predict DOC export from soil on the global scale (Langerwisch et al, 2016; McGuire et al, 2010) and there is no global LSM embedded within an Earth system model that represents a vertically resolved module of SOC and DOC production, consumption, sorption, and transport. If the model structure is valid, ORCHIDEE-SOM should be able to reproduce the values of DOC and SOC concentrations within the range of the observations, and the internal soil processes that drive the site-specific differences in SOC stocks following differences in soil texture, vegetation, and climate and the decrease in SOC and DOC down the soil profile
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