Abstract
Lateral carbon transfer along the land-ocean continuum is a key component of global carbon cycle, yet its response to global change is poorly quantified. Here, we use a land-surface model to ...simulate vertical (soil-plant-atmosphere) and lateral (land-river-ocean) carbon exchanges in Europe between 1901–2014 and investigate the effect of atmospheric carbon dioxide, climate and land use changes on lateral carbon transfer. We find that global change during 1901–2014 led to a significant increase in the total terrestrial carbon delivery to European rivers (33% increase) and to the sea (20% increase). Carbon delivery increased in the dissolved phase and decreased in the particulate phase. Climate change, increased atmospheric carbon dioxide, and land-use change explain 62%, 36% and 2% of the temporal change in European lateral carbon transfer during the study period, respectively. Our findings suggest that redistribution of soil carbon due to lateral carbon transfer induced a 5% reduction in the net land carbon sink in Europe.
Contaminated soils are widespread and contamination is known to impact several biotic soil processes. But it is still not clear to what extent soil contamination affects soil carbon efflux (CO
2
) ...occurring through soil microfauna respiration. Regarding the large stocks of organic carbon (Corga) stored in soils, even limited changes in the outputs fluxes may modify atmospheric CO
2
concentration with important feedbacks on climate. In this study, we aimed at assessing and quantifying how soil respiration is affected by contamination. For that, we performed a quantitative review of literature focusing on 1) soil heterotrophic respiration measurements thus excluding autotrophic respiration from plants, 2) soil copper contamination, and 3) the influence of pedo-climatic parameters such as pH, clay content or the type of climate. Using a dataset of 389 data analyzed with RandomForest and linear mixed statistical models, we showed a decrease in soil CO
2
emission with an increase in soil copper contamination. Specific data from
ex-situ
spiking experiments could be easily differentiated from the ones originated from
in-situ
contamination due to their sharper decrease in soil Corga mineralization. Interestingly,
ex-situ
spikes data provided a threshold in soil Cu contents for CO
2
emissions: CO
2
emission increased for inputs below 265 mgCu.kg
−1
soil and decreased above this concentration. Data from long-term
in-situ
contaminations due to anthropogenic activities (industrialization, agriculture, … ) also displayed an impact on soil carbon mineralization, much particularly for industrial contaminations (smelter, sewage sludge, … ) with decreased in CO
2
emissions when Cu contamination increased. Soil pH was identified as a significant driver of the effect of Cu on CO
2
emissions, as soil C mineralization was found to be more sensitive to Cu contamination in acidic soils than in neutral or alkaline soils. Conversely the clay content and the type of climate did not significantly explain the responses in soil C mineralization. Finally, the collected data were used to propose an empirical equation quantifying how soil respiration can be affected by a Cu contamination. The decrease in soil CO
2
emissions cannot be related, however, in a role of C sink as it comes together with a decrease in soil microbial biomass.
Land-use change affects both the quality and quantity of soil organic carbon (SOC) and leads to changes in ecosystem functions such as productivity and environmental regulation. Future changes in SOC ...are, however, highly uncertain owing to its heterogeneity and complexity. In this study, we analyzed the outputs of simulations of SOC stock by Earth system models (ESMs), most of which are participants in the Land-Use Model Intercomparison Project. Using a common protocol and the same forcing data, the ESMs simulated SOC distribution patterns and their changes during historical (1850-2014) and future (2015-2100) periods. Total SOC stock increased in many simulations over the historical period (30 ± 67 Pg C) and under future climate and land-use conditions (48 ± 32 Pg C for ssp126 and 49 ± 58 Pg C for ssp370). Land-use experiments indicated that changes in SOC attributable to land-use scenarios were modest at the global scale, in comparison with climatic and rising CO2 impacts, but they were notable in several regions. Future net soil carbon sequestration rates estimated by the ESMs were roughly 0.4‰ yr−1 (0.6 Pg C yr−1). Although there were considerable inter-model differences, the rates are still remarkable in terms of their potential for mitigation of global warming. The disparate results among ESMs imply that key parameters that control processes such as SOC residence time need to be better constrained and that more comprehensive representation of land management impacts on soils remain critical for understanding the long-term potential of soils to sequester carbon.
Lateral transfer of carbon (C) from terrestrial ecosystems into the inland water network is an important component of the global C cycle, which sustains a large aquatic CO2 evasion flux fuelled by ...the decomposition of allochthonous C inputs. Globally, estimates of the total C exports through the terrestrial–aquatic interface range from 1.5 to 2.7 Pg C yr−1 (Cole et al., 2007; Battin et al., 2009; Tranvik et al., 2009), i.e. of the order of 2–5 % of the terrestrial NPP. Earth system models (ESMs) of the climate system ignore these lateral transfers of C, and thus likely overestimate the terrestrial C sink. In this study, we present the implementation of fluvial transport of dissolved organic carbon (DOC) and CO2 into ORCHIDEE (Organising Carbon and Hydrology in Dynamic Ecosystems), the land surface scheme of the Institut Pierre-Simon Laplace ESM. This new model branch, called ORCHILEAK, represents DOC production from canopy and soils, DOC and CO2 leaching from soils to streams, DOC decomposition, and CO2 evasion to the atmosphere during its lateral transport in rivers, as well as exchange with the soil carbon and litter stocks on floodplains and in swamps. We parameterized and validated ORCHILEAK for the Amazon basin, the world's largest river system with regard to discharge and one of the most productive ecosystems in the world. With ORCHILEAK, we are able to reproduce observed terrestrial and aquatic fluxes of DOC and CO2 in the Amazon basin, both in terms of mean values and seasonality. In addition, we are able to resolve the spatio-temporal variability in C fluxes along the canopy–soil–water continuum at high resolution (1°, daily) and to quantify the different terrestrial contributions to the aquatic C fluxes. We simulate that more than two-thirds of the Amazon's fluvial DOC export are contributed by the decomposition of submerged litter. Throughfall DOC fluxes from canopy to ground are about as high as the total DOC inputs to inland waters. The latter, however, are mainly sustained by litter decomposition. Decomposition of DOC and submerged plant litter contributes slightly more than half of the CO2 evasion from the water surface, while the remainder is contributed by soil respiration. Total CO2 evasion from the water surface equals about 5 % of the terrestrial NPP. Our results highlight that ORCHILEAK is well suited to simulate carbon transfers along the terrestrial–aquatic continuum of tropical forests. It also opens the perspective that provided parameterization, calibration and validation is performed for other biomes, the new model branch could improve the quantification of the global terrestrial C sink and help better constrain carbon cycle–climate feedbacks in future projections.
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 models.
Key Points
Soil 13C was incorporated as a tracer in the land surface model ORCHIDEE‐SOM
The model was able to reproduce short‐term changes such as those observed after a shift in C3/C4 vegetation
Long‐term changes in δ13C were more difficult to represent, highlighting the importance of some missing mechanisms
Global water erosion strongly affects the terrestrial carbon balance. However, this process is currently ignored by most global land surface models (LSMs) that are used to project the responses of ...terrestrial carbon storage to climate and land use changes. One of the main obstacles to implement erosion processes in LSMs is the high spatial resolution needed to accurately represent the effect of topography on soil erosion and sediment delivery to rivers. In this study, we present an upscaling scheme for including erosion‐induced lateral soil organic carbon (SOC) movements into the ORCHIDEE LSM. This upscaling scheme integrates information from high‐resolution (3″) topographic and soil erodibility data into a LSM forcing file at 0.5° spatial resolution. Evaluation of our model for the Rhine catchment indicates that it reproduces well the observed spatial and temporal (both seasonal and interannual) variations in river runoff and the sediment delivery from uplands to the river network. Although the average annual lateral SOC flux from uplands to the Rhine River network only amounts to 0.5% of the annual net primary production and 0.01% of the total SOC stock in the whole catchment, SOC loss caused by soil erosion over a long period (e.g., thousands of years) has the potential to cause a 12% reduction in the simulated equilibrium SOC stocks. Overall, this study presents a promising approach for including the erosion‐induced lateral carbon flux from the land to aquatic systems into LSMs and highlights the important role of erosion processes in the terrestrial carbon balance.
Plain Language Summary
Global land surface models (LSMs) are the main tools used to simulate the terrestrial carbon (C) cycle and to predict its response to climate and land cover changes. Currently, the processes of vertical C fluxes between soils, plants, and the atmosphere (e.g., photosynthesis, plant growth, and litter and soil organic matter decomposition) has been well represented in many LSMs; however, the lateral soil C delivery through the river network caused by water erosion is still missing in most LSMs. This study introduces a LSM approach which is suitable to simulate the large‐scale soil C delivery from upland soils to inland waters at high temporal resolution (daily) and accounting for the small‐scale (~90 m) spatial variability in topography and soil properties. Evaluation of our model in the Rhine catchment demonstrates a good performance with regard to reproducing the spatial and temporal (daily, seasonal, and interannual) variability of soil C delivery rate from uplands to river networks at large spatial scale and for exploring the impacts of changes in vegetation cover (land use change) and climate (e.g., changes in rainfall amounts and regimes) on the regional soil C balance.
Key Points
We presented an upscaling scheme for including erosion‐induced lateral soil and carbon transfers into a global land surface model
Our model is a useful tool to estimate the impacts of climate and land cover changes on erosion‐induced soil carbon loss at large scale
Model application for the Rhine basin demonstrates that erosion‐induced soil carbon losses substantially reduce soil carbon stocks
Abstract
Numerical models are crucial to understand and/or predict past and future soil organic carbon dynamics. For those models aiming at prediction, validation is a critical step to gain ...confidence in projections. With a comprehensive review of ~250 models, we assess how models are validated depending on their objectives and features, discuss how validation of predictive models can be improved. We find a critical lack of independent validation using observed time series. Conducting such validations should be a priority to improve the model reliability. Approximately 60% of the models we analysed are not designed for predictions, but rather for conceptual understanding of soil processes. These models provide important insights by identifying key processes and alternative formalisms that can be relevant for predictive models. We argue that combining independent validation based on observed time series and improved information flow between predictive and conceptual models will increase reliability in predictions.
Soils 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.
Key Points
Estimates of the total soil organic carbon stock for global land mask are still quite diverse. Uncertainties associated with soil bulk density are higher and apply to all regions of the globe, whereas those of the carbon concentration are mostly marked at high latitudes
Whatever the region considered, the SoilGrids‐, HWSD‐, and NCSCD‐derived carbon stocks are not in agreement with the field data: Using field data gathered over USA, databases underestimate the stocks by 40% for SoilGrids and by 80–90% for HWSD. Using regional inventories over France and England and Wales, SoilGrids overestimates the carbon stock by 30% in the first case and by more than 150% in the second one
It is possible that previous estimates of the total organic carbon stock have seriously underestimated organic carbon stock in northern latitudes, peatlands, and wetlands
Soil contamination by trace elements like copper (Cu) can affect soil functioning. Environmental policies with guidelines and soil survey measurements still refer to the total content of Cu in soils. ...However, Cu content in soil solution or free Cu content have been shown to be better proxies of risks of Cu mobility or (bio-)availability for soil organisms. Several empirical equations have been defined at the local scale to predict the amount of Cu in soil solution based on both total soil Cu content and main soil parameters involved in the soil/solution partitioning. Nevertheless, despite the relevance for risk assessment, these equations are not applied at a large spatial scale due to difficulties to perform changes from local to regional. To progress in this challenge, we collected several empirical equations from literature and selected those allowing estimation of the amount of Cu in solution, used as a proxy of available Cu, from the knowledge of both total soil Cu content and soil parameters. We did the same for the estimation of free Cu in solution, used as a proxy of bio-available Cu. These equations were used to provide European maps of (bio-)available Cu based on the one of total soil Cu over Europe. Results allowed comparing the maps of available and bio-available Cu at the European scale. This was done with respective median values of each form of Cu to identify specific areas of risks linked to these two proxies. Higher discrepancies were highlighted between the map of bio-available Cu and the map of soil total Cu compared to the Cu available map. Such results can be used to assess environmental-related issues for land use planning.
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