ORCHIDEE-MICT (revision 4126), a land surface model for the high-latitudes: model description and validation

Abstract. The high-latitude regions of the northern hemisphere are a nexus for the interaction between land surface physical properties and their exchange of carbon and energy with the atmosphere. At these latitudes, two carbon pools of planetary significance – those of the permanently frozen soils (permafrost), and of the great expanse of boreal forest – are vulnerable to destabilization in the face of currently observed climatic warming, the speed and intensity of which are expected to increase with time. Improved projections of future Arctic and boreal ecosystem transformation require improved land surface models that integrate processes specific to these cold biomes. To this end, this study lays out relevant new parameterizations in the ORCHIDEE-MICT land surface model. These describe the interactions between soil carbon, soil temperature and hydrology, and their resulting feedbacks on water and CO2 fluxes, in addition to a recently-developed fire module. Outputs from ORCHIDEE-MICT, when forced by two climate input data sets, are extensively evaluated against: (i) temperature gradients between the atmosphere and deep soils; (ii) the hydrological components comprising the water balance of the largest high-latitude basins, and (iii) CO2 flux and carbon stock observations. The model performance is good with respect to empirical data, despite a simulated excessive plant water stress and a positive land surface temperature bias. In addition, acute model sensitivity to the choice of input forcing data suggests that the calibration of model parameters is strongly forcing-dependent. Overall, we suggest that this new model design is at the forefront of current efforts to reliably estimate future perturbations to the high-latitude terrestrial environment. </jats:p

Different soil study tools to better understand the dynamics of carbon in soils at different spatial scales, from a single soil profile to the global scale

Soils are the major components ofthe terrestrial ecosystems and the largest organiccarbon reservoir on Earth, being very reactive tohuman disturbance and climate change. Despiteits importance within the carbon reservoirs, soilcarbon dynamics is an important source ofuncertainties for future climate predictions. Theaim of the thesis was to explore different aspectsof soil carbon studies (Experimentalmeasurements, modeling, and databaseevaluation) at different spatial scales (from thescale of a profile to the global scale). Wehighlighted that the estimation of the global soilcarbon stocks is still quite uncertain.Consequently, the role of soil carbon in theclimate dynamics becomes one of the majoruncertainties in the Earth system models (ESMs)used to predict future climate change. Thesecond part of thesis deals with the presentationof a new version of the IPSL-Land SurfaceModel called ORCHIDEE-SOM, incorporatingthe 14C dynamics in the soil. Several tests doneassume that model improvements should focusmore on a depth dependent parameterization,mainly for the diffusion, in order to improve therepresentation of the global carbon cycle inLand Surface Models, thus helping to constrainthe predictions of the future soil organic carbonresponse to global warming.Les sols sont la principale composantede l’écosystème terrestre et le plus grand réservoir de carbone organique sur Terre, étant très réactifs aux perturbations humaines et aux changements climatiques. Malgré leur importance dans les réservoirs de carbone, la dynamique du carbone des sols est une source importante d'incertitudes pour les prévisions climatiques futures. Le but de la thèse était d'explorer différents aspects d’études du carbone des sols (mesures expérimentales, modélisation et évaluation de bases de données) à différentes échelles spatiales (de l'échelle d'un profil à l'échelle globale). Nous avons souligné que l'estimation des stocks globaux de carbone du sol est encore assez incertaine.Par conséquent le rôle du carbone des sols dans la dynamique du climat devient l'une des principales incertitudes dans les modèles du système terrestre utilisés pour prédire les changements climatiques futurs. La deuxième partie de la thèse porte sur la présentation d'une nouvelle version du modèle IPSL-Land Surface appelé ORCHIDEE-SOM, intégrant la dynamique du 14C dans le sol. Plusieurs tests effectués supposent que les améliorations du modèle devraient se focaliser davantage sur une paramétrisation dépendante de la profondeur,principalement pour la diffusion, afin d'améliorer la représentation du cycle global du carbone dans les modèles de surface terrestre, contribuant ainsi à contraindre les prédictions futures du réchauffement climatique

Large Differences in Global and Regional Total Soil Carbon Stock Estimates Based on SoilGrids, HWSD, and NCSCD: Intercomparison and Evaluation Based on Field Data From USA, England, Wales, and France

International audienceSoils are the major component of the terrestrial ecosystem and the largest organic carbon reservoir on Earth. However, they are a nonrenewable natural resource and especially reactive to human disturbance and climate change. Despite its importance, soil carbon dynamics is an important source of uncertainty for future climate predictions and there is a growing need for more precise information to better understand the mechanisms controlling soil carbon dynamics and better constrain Earth system models. The aim of our work is to compare soil organic carbon stocks given by different global and regional databases that already exist. We calculated global and regional soil carbon stocks at 1 m depth given by three existing databases (SoilGrids, the Harmonized World Soil Database, and the Northern Circumpolar Soil Carbon Database). We observed that total stocks predicted by each product differ greatly: it is estimated to be around 3,400 Pg by SoilGrids and is about 2,500 Pg according to Harmonized World Soil Database. This difference is marked in particular for boreal regions where differences can be related to high disparities in soil organic carbon concentration. Differences in other regions are more limited and may be related to differences in bulk density estimates. Finally, evaluation of the three data sets versus ground truth data shows that (i) there is a significant difference in spatial patterns between ground truth data and compared data sets and that (ii) data sets underestimate by more than 40% the soil organic carbon stock compared to field data

How lysimetric facility can contribute to monitor Technosols dynamics.

International audienceThe dynamic of water in soils is mainly controlled by a set of hydraulic properties that are characteristic of each type of soil and that reflect the architecture – more generally defined as soil structure - of such a specific porous medium. Structural changes are induced by external factors (e.g. climate, biology, human action) and are the result of pedogenetic processes that modify the solid phase and redistribute ions and particles. Consequently, changes in the poral volume and in the size and the connectivity of soil pores are observed that significantly influence regulating ecosystem services that can be provided. The temporal and spatial dynamics of these properties is complex to highlight and poorly studied, especially as the soil processes in natural soils are slow at human timescales. To question this crucial issue, we chose to focus our study on the dynamics of Technosols porosity as a result of seasonal climatic variations, vegetation and early pedogenic evolution – which kinetic is known to be much faster - (Lin, 2011; Séré et al., 2012). Our purpose is then to develop an original approach to characterize, in a continuous way, the evolution of soil’s structure. To do so, a natural soil and SUITMAs - from a Luvisol to a Spolic Garbic Technosol (Histic) -, within an anthropization gradient, have been studied. They have been studied under two treatments (with or without vegetation) in monitored 2 m3 lysimetric columns over a 3 to 6 years’ time sequence. Water balances have been performed as well as the monitoring of water transfer at different depths. Experimental data have been compared to a modelling approach that relied on the use of Hydrus 1D (Simunek et al., 2008). The results exhibit contrasted hydraulic behaviors that are mainly correlated to the age of the soils and the level of human influence. Only cyclic variations – for example on the amount of water that is stored (Figure) - were visible on natural and slightly anthropogenic soils that were attributed to seasonal factors (e.g. climate and vegetation). In addition to that cyclic changes, more drastic acyclic evolutions were observed on the Technosols that demonstrated their significant settlement and an evolution of the porosity due to their early pedogenesis (Figure). An inverse modelling approach led to the estimation of hydraulic parameters that confirmed that findings by highlighting an evolution of poral architecture with time

Including stable carbon isotopes to evaluate the dynamics of soil carbon in the land-surface model ORCHIDEE

Soil organic carbon (SOC) is a crucial component of the terrestrial carbon cycle and its turnover time in models is a key source of uncertainty. Studies have highlighted the utility of δ¹³C measurements for benchmarking SOC turnover in global models. We used ¹³C as a tracer within a vertically discretized soil module of a land-surface model, Organising Carbon and Hydrology In Dynamic Ecosystems- Soil Organic Matter (ORCHIDEE-SOM). Our new module represents some of the processes that have been hypothesized to lead to a ¹³C enrichment with soil depth as follows: 1) the Suess effect and CO₂ fertilization, 2) the relative ¹³C enrichment of roots compared to leaves, and 3) ¹³C discrimination associated with microbial activity. We tested if the upgraded soil module was able to reproduce the vertical profile of δ¹³C within the soil column at two temperate sites and the short-term change in the isotopic signal of soil after a shift in C3/C4 vegetation. We ran the model over Europe to test its performance at larger scale. The model was able to simulate a shift in the isotopic signal due to short-term changes in vegetation cover from C3 to C4; however, it was not able to reproduce the overall vertical profile in soil δ¹³C, which arises as a combination of short and long-term processes. At the European scale, the model ably reproduced soil CO₂ fluxes and total SOC stock. These findings stress the importance of the long-term history of land cover for simulating vertical profiles of δ¹³C. This new soil module is an emerging tool for the diagnosis and improvement of global SOC model

Including Stable Carbon Isotopes to Evaluate the Dynamics of Soil Carbon in the Land‐Surface Model ORCHIDEE

International audienceSoil organic carbon (SOC) is a crucial component of the terrestrial carbon cycle and its turnover time in models is a key source of uncertainty. Studies have highlighted the utility of delta C-13 measurements for benchmarking SOC turnover in global models. We used C-13 as a tracer within a vertically discretized soil module of a land-surface model, Organising Carbon and Hydrology In Dynamic Ecosystems- Soil Organic Matter (ORCHIDEE-SOM). Our new module represents some of the processes that have been hypothesized to lead to a C-13 enrichment with soil depth as follows: 1) the Suess effect and CO2 fertilization, 2) the relative C-13 enrichment of roots compared to leaves, and 3) C-13 discrimination associated with microbial activity. We tested if the upgraded soil module was able to reproduce the vertical profile of delta C-13 within the soil column at two temperate sites and the short-term change in the isotopic signal of soil after a shift in C3/C4 vegetation. We ran the model over Europe to test its performance at larger scale. The model was able to simulate a shift in the isotopic signal due to short-term changes in vegetation cover from C3 to C4; however, it was not able to reproduce the overall vertical profile in soil delta C-13, which arises as a combination of short and long-term processes. At the European scale, the model ably reproduced soil CO2 fluxes and total SOC stock. These findings stress the importance of the long-term history of land cover for simulating vertical profiles of delta C-13. This new soil module is an emerging tool for the diagnosis and improvement of global SOC models