Is it feasible to increase soil carbon stocks by 4 per 1000 per year?

Extreme heatwaves, wildfires, floods, droughts… The effects of climate change are no longer predictions: they are happening right now. And, according to the experts of the Intergovernmental Panel on Climate Change (IPCC), they will continue to happen and become more and more intense in the future.

So, what can we do to stop them?

Most importantly, we need to reduce the emissions of greenhouse gases (GHG), such as carbon dioxide (CO2) from fossil fuel burning and deforestation, and nitrous oxide (N2O) and methane (CH4) from agriculture. But this is still not enough. In order to achieve carbon neutrality (that is, a net zero difference between what we emit and what we put back in the soil) we will need additional efforts to sequester CO2 from the atmosphere and store it in the soil.

This can be done, for example, by avoiding deforestation and improving the management of agricultural soils.

Soil

Soils play a crucial role for C sequestration:

  • They store the largest amount of C compared to all other terrestrial ecosystems.
  • They also store twice to three times more C than the atmosphere and – because the soil and the atmosphere interchange C – this means that even small changes in the soil C stocks can have a huge impact on the concentration of CO2 in the atmosphere.
  • C improves the fertility and other functions of the soils, so having soils rich in C is beneficial for food production.

Basically, storing more C in the soil can help to mitigate climate change and feed the world!

How can we restore carbon in the soil?

Carbon is naturally brought to the soil through plant photosynthesis and organic matter deposition. It leaves the soil once microbes and other organisms decompose it and respire it back to the atmosphere. Hence, storing additional soil organic C (SOC) can be done in two ways:

  • By increasing the amount of C entering the soil (e.g., the amount of CO2 fixed by the plants, or the amount of organic material added to the soil)
  • By decreasing the C output from the soil (e.g., decreasing the rate of microbial decomposition)

Researchers have found that the most efficient way to store C is to increase the C input to the soil.

Figure 1 Global carbon cycle between the land and the atmosphere. The values were taken from Ciais et al. (2013). Uncertainty in the atmospheric CO2 growth rate is ± 0.02 Gt C yr-1. Figure adapted from Le Quéré et al. (2018). Icons from www.flaticon.com.

How much should we increase the C input?

Assuming that we aim to increase the SOC stocks by a certain fixed target, we estimated the additional C input required to reach this target, and assessed whether this amount is realistic with current land use management practices. Following the 4 per 1000 initiative, we set the target to an annual 4‰ increase of the SOC stocks.

In order to address these questions, we used mathematical models that simulate the processes that influence the accumulation of C in the soil and estimated the additional C input required to reach a 4‰ objective in Europe.

The methods used

We used data from 16 long-term agricultural experiments where agricultural practices with organic matter addition were carried out for several years under controlled conditions. At these experiments, pedo-climatic conditions were monitored over time in order to see the effect of organic matter addition on the system.

Mathematical models are largely used by soil scientists and policy makers to predict the evolution of SOC stocks with time, following changes in land management practices and climate. They help to better understand the behavior of the system, and to predict its future responses to external changes. However, they are still highly uncertain. This is mainly due to the uncertainty of:

  • The processes described in the models, which are still largely unknown;
  • The data used to run them, such as climate and soil variables;
  • The parameters that are included in the model equations, which are usually considered constant although they actually vary in space and time, and which values are not always related to measurable physical processes.

In order to estimate some of this uncertainty, instead of running one single model we ran a multi-model ensemble, which allowed to consider different ways of representing soil processes.

We also calibrated model parameters in order to correctly reproduce the evolution of measured SOC stocks. This means that we adjusted the values of model parameters to reproduce the conditions of the sites that we studied.

Feasibility of the 4 per 100

On average across the models and across the sites, we found that the C input had to increase by 119% compared to the initial conditions. That is, an additional 1.5 ± 1.2 Mg C ha-1 would need to be annually input to the soil in order to increase the SOC stocks by 4‰ each year.

To give an order of magnitude, we estimated from Zhang et al. (2017) that the annual C input applied to European croplands, derived only from livestock manure, is around 0.3 to 0.9 Mg C ha-1. But this is already applied! Meaning that, if we wanted to provide additional 1.5 ± 1.2 Mg C ha-1, that would need to come from different sources of organic material. Doubling the C input where mineral and organic fertilizers are already applied is unlikely without the implementation of other agricultural practices, such as agroforestry systems, cover cropping, improved crop rotations, and crops with a high belowground biomass. This is the case for Europe, for example, where croplands are usually minerally fertilized and where organic fertilizers are already widely applied.

Figure 2 Annual SOC stock increase (%) for different levels of additional C input in agricultural experiments (black spots) and additional C input required to reach the 4‰ SOC increase according to the 1) non-calibrated multi-model median (MMM) (blue cross) and the 2) calibrated MMM (orange cross). Errors are shown as confidence intervals (CI) The regression line between additional C input and SOC stock increase in the EOM treatments is indicated in the figure (y=m (±〖SD〗_m)∙ x + b ± 〖(SD〗_b)).

Model uncertainties

Concerning the model simulations, we found large uncertainties across models, even when they were calibrated to reproduce the observed SOC stocks of the experiments. In particular, we found that the uncertainty across models was mainly due to the way the models represented (or not) water related variables, such as precipitations and potential evapotranspiration. This indicates that the choice of the hydrological processes included in the models and their representation affect model predictions, and it suggests that major efforts should be made to better represent these processes and reduce their associated uncertainties.

Figure 3 Required additional C input (± standard deviation, SD) relative to the unfertilized control, to reach a mean annual 4‰ SOC stock increase for 30 years across the 16 sites. The bars represent the different models and multi-model median (MMM). The non-calibrated and calibrated configurations are in blue and orange, respectively. For the MMM, the SD bar represents the median SD across models.
Figure 4 Heatmap of the simulated additional C input to reach the 4‰, for each calibrated model and each site. Darker cells show lower C input and lighter cells represent higher C input. Dendrograms above the heatmap represent the relationship of similarity among groups of models, calculated as the minimal correlation distance.

Conclusions and perspectives

Despite uncertainties in model simulations, there is compelling evidence that a radical change in agricultural management will be required to cope with climate change and food security in the near future. In a recent report from the Mission Board for Soil Health and Food, the European Commission was suggested to set ambitious targets to increase SOC stocks and improve the health of European soils. Yet, we are far from being optimistic. The last fifty years of international agreements on the response of world nations to climate change have proven that, no matter how compelling evidence the scientific community provides, indicators of adverse change are still on a rise (Glavovic et al., 2021). Governments need to take action before it is too late.

Diversity in science

In the last decades, thousands of works have been published on the effects of land management, land-use change and climate change on SOC. However, studies are narrowed to a selected number of specific drivers and geographical regions. In fact, studies on agricultural management practices have mostly focused on mineral fertilization, organic amendments, and tillage, while drivers of SOC changes have only occasionally been studied in North and Central Africa, and in the Middle East and Central Asia (Beillouin et al., 2022).

Future research should focus on more local and diversified knowledge on how to preserve and restore SOC stocks, while covering understudied geographical regions.

Besides, increased knowledge on the effects of diversified practices on SOC stock changes, under different pedo-climatic conditions, will help to improve model simulations and provide reliable SOC stock projections under future climate change.

Read more about this in the open access paper published in the European Journal of Soil Science: Multi-modelling predictions show high uncertainty of required carbon input changes to reach a 4‰ target

References and related articles

Beillouin, Damien, Rémi Cardinael, David Berre, Annie Boyer, Marc Corbeels, Abigail Fallot, Frédéric Feder, and Julien Demenois. “A Global Overview of Studies about Land Management, Land‐use Change, and Climate Change Effects on Soil Organic Carbon.” Global Change Biology 28, no. 4 (February 2022): 1690–1702. https://doi.org/10.1111/gcb.15998.

Bruni, Elisa, Bertrand Guenet, Yuanyuan Huang, Hugues Clivot, Iñigo Virto, Roberta Farina, Thomas Kätterer, Philippe Ciais, Manuel Martin, and Claire Chenu. “Additional Carbon Inputs to Reach a 4 per 1000 Objective in Europe: Feasibility and Projected Impacts of Climate Change Based on Century Simulations of Long-Term Arable Experiments.” Biogeosciences 18, no. 13 (July 2, 2021): 3981–4004. https://doi.org/10.5194/bg-18-3981-2021.

Bruni, Elisa, Bertrand Guenet, Hugues Clivot, Thomas Kätterer, Manuel Martin, Iñigo Virto, and Claire Chenu. “Defining Quantitative Targets for Topsoil Organic Carbon Stock Increase in European Croplands: Case Studies With Exogenous Organic Matter Inputs.” Frontiers in Environmental Science 10 (February 14, 2022): 824724. https://doi.org/10.3389/fenvs.2022.824724.

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Glavovic, Bruce C., Timothy F. Smith, and Iain White. “The Tragedy of Climate Change Science.” Climate and Development, December 24, 2021, 1–5. https://doi.org/10.1080/17565529.2021.2008855.

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Riggers, Catharina, Christopher Poeplau, Axel Don, Cathleen Frühauf, and René Dechow. “How Much Carbon Input Is Required to Preserve or Increase Projected Soil Organic Carbon Stocks in German Croplands under Climate Change?” Plant and Soil 460, no. 1–2 (March 2021): 417–33. https://doi.org/10.1007/s11104-020-04806-8.

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