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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 ORCHIDEESOM, 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. [ABSTRACT FROM AUTHOR]

Current global models of the carbon (C) cycle consider only vertical gas exchanges between terrestrial or oceanic reservoirs and the atmosphere, thus not considering the lateral transport of carbon from the continents to the oceans. Therefore, those models implicitly consider all of the C which is not respired to the atmosphere to be stored on land and hence overestimate the land C sink capability. A model that represents the whole continuum from atmosphere to land and into the ocean would provide a better understanding of the Earth's C cycle and hence more reliable historical or future projections. A first and critical step in that direction is to include processes representing the production and export of dissolved organic carbon in soils. Here we present an original representation of dissolved organic C (DOC) processes in the Joint UK Land Environment Simulator (JULES-DOCM) that integrates a representation of DOC production in terrestrial ecosystems based on the incomplete decomposition of organic matter, DOC decomposition within the soil column, and DOC export to the river network via leaching. The model performance is evaluated in five specific sites for which observations of soil DOC concentration are available. Results show that the model is able to reproduce the DOC concentration and controlling processes, including leaching to the riverine system, which is fundamental for integrating terrestrial and aquatic ecosystems. Future work should include the fate of exported DOC in the river system as well as DIC and POC export from soil. [ABSTRACT FROM AUTHOR]

Abstract: Reduced tillage and cover cropping are often considered as measures to increase carbon sequestration in cropland soils. We hypothesized that these management practices could result in an increase in carbon leaching. To examine this possibility we assessed carbon leaching from an Irish arable soil under spring barley, either with conventional management or non-inversion tillage plus cover cropping. Concentrations of biogenic dissolved inorganic carbon (DIC) were considerably higher under reduced tillage probably due to a higher supersaturation of soil solution with respect to partial pressures of CO2 in soil air resulting from a reduced abundance of tillage-induced macropores. Leaching losses of DOC equalled 3±0.3gm−2 yr−1 for conventional management as well as for non-inversion tillage plus cover cropping. Losses of biogenic DIC were 14.5±4.4gm−2 yr−1 for conventional tillage and 34.0±4.7gm−2 yr−1 for non-inversion tillage plus cover crop. Higher leaching losses from the non-inversion tillage plus cover crop plot thereby reduced potential soil carbon gains by 20gm−2 yr−1 compared to the conventionally managed treatment highlighting the need to consider leaching losses in estimates of carbon sequestration, especially if these are deduced from balancing carbon inputs and outputs. [Copyright &y& Elsevier]