What is Soil carbon 4 per mille initiative?
The 4 per mille (or 4 per 1000) initiative was launched during the COP21 meeting in Paris by The French Minister of Agriculture, Stéphane Le Foll. This initiative aspires to increase the global soil organic carbon stocks by 0.4 percent per year as a compensation for the global emissions of greenhouse gases by anthropogenic sources. It was supported by almost 150 signatories from representatives of countries, regions, international agencies, private sectors and NGOs. The official title is “4 per 1000 – Soils for food security and climate”.
What is the significance of the number 4 per mille or 4 per 1000?
Well, you can interpret it several ways. The official 4 per 1000 website (http://4p1000.org/understand) said: “A 4‰ annual growth rate of the soil organic carbon stock would make it possible to stop the present increase in atmospheric CO2”. Based on the Global Carbon Budget, then the objective would be to mitigate the annual increase in CO2 in the atmosphere, around 4.3 Gt C. Taking a global SOC stock of 1500 Gt C (to 1 m depth) and multiplying it by 0.4%, we get 6 Gt C, which is larger than the annual CO2 increase.
But according to a publication by Ademe (2015): “An annual 4 per 1000 (0.4%) increase in organic matter in soil would be enough to compensate the global emissions of greenhouse gases”. If we take the global SOC stocks of 2400 G t C (to 2m depth) and only consider CO2 emitted by fossil fuel combustion which is estimated at 8.9 Gt C, then the ratio 8.9/2400 is 0.0037, or around 4 per mille figure. Actually, it should be 9.6/2400 = 0.4%.
Amazingly, Balesdent and Arrouays (1999) who proposed 4 per mille for the first time in the literature, wrote (in French) ‘A relative increase of total SOC stocks by 0.4% would mitigate the global fossil fuel emissions’. We used an estimate of global SOC stock of 1600 Gt C (to 1 m) which, if multiplied by 0.4% gives 6.4 Gt C, close to the annual fossil fuel emissions during the 1990s. Since then, the estimates of SOC stocks have been refined to 2400 Gt down to 2 m, and the emissions have increased in a similar proportion, and thus the 0.4% figure does not change.
Additionally, if we quote Soussana et al. (2015) “Over a meaningful depth for carbon sequestration, i.e. 0-40 cm, the 4‰ target would result in a carbon sequestration that could peak at 3.5 billion tons C per year (Gt C yr-1) when considering soils from all biomes. Agricultural soils have a technical carbon sequestration potential between 0.7 and 1.2 Gt C yr-1, while the potential from all other land uses (including forests and integrated systems like agroforestry) could reach 2.5 Gt C yr-1.” Indeed, if we sum 2.5 Gt C and the average of the range 0.7-1.2 Gt C, we come to 3.45 billion tons C per year, which fits the target especially when combined with halting deforestation.
Can you elaborate a bit more on the Balesdent and Arrouays paper.
The Balesdent and Arrouays paper (1999) was written in the framework of a French research programme on agriculture and greenhouse gases emission. In that paper, we produced a first rough estimate of changes in French SOC stocks linked to changes in Land Cover during the 20th century. In particular, we saw the evidence that the bare-soil fallow period from the European CAP had been a catastrophe. We were looking for a way to emphasize that SOC stocks were enormous if compared to anthropogenic GHG fluxes to the atmosphere and thus getting a small relative change of the stocks can have a significant effect. We did a approximate calculation and came up with the 4 per Mille ratio between world’s soil carbon stocks and C-CO2 emissions. Actually, we wrote that if such a relatively small change of soil C stock were to occur, it would offset GHG emissions. We did not elaborate whether it was plausible to achieve that rate.
Is 4 per mille a magic number?
As I explained earlier, no matter how the calculation is made, 4 per mille is the result which is claimed. But I don’t believe it is a magic number. 4 per mille is a well-intentioned aspirational target that has also become a slogan in helping the promotion of sustainable soil management.
Is there a scientific discussion behind the 4 per mille initiative?
Of course, there are discussions that took place before and during the launching of this programme (http://newsroom.unfccc.int/lpaa/agriculture/join-the-41000-initiative-soils-for-food-security-and-climate/). The 4 per Mille initiative includes a proposal for a research programme which is guided by a scientific committee (http://www.agropolis.fr/pdf/actu/4-per-1000-comite.pdf ). The 4 per Mille initiative was also discussed under the auspices of the International Union of Soil Sciences (IUSS).
Isn’t the 4 per mille target too optimistic?
There have been various estimates of potential soil carbon sequestration potential from agricultural and manage lands ranging from 0.5 to 2.5 Gt C yr-1. If we only stay to currently managed soils and rely only on atmospheric deposition and fertilizers for nutrient management, then the potential is low. If we believe, that there is a large technical potential in soils that are currently not managed or poorly managed and new technologies were developed, then the 4 per mille aspiration is realistic.
Some said that it is implausible to upscale experimental results of SOC sequestration to the global scale because of nutrient limitations.
I agree that some limitations to C sequestration may occur because of lack of nutrients such as N and P. However, we should not fully rely on chemical fertilizer and N deposition to solve this nutrient issue. Nutrient management may be improved by many techniques, such as the use of legumes, grazing management, agroforestry, etc.. In many parts of the world, crop residues are not well managed and frequently burnt. There is also an enormous potential for waste recycling, especially from large cities. SOC sequestration should be the outcome of improved farming practices that increased productivity, which in turn build up organic matter. The main feedback of N fertilization is through increased crop yields. However, the relationship between fertilizer N and SOC is also not direct. The missing argument from this is that increased SOC has a positive feedback on soil quality and crop productivity. A recent study by Sanderman et al. (2017) demonstrated that the management system which returns more carbon in the soil increases its SOC, which in turn, supplies more nutrients back to the crop, and increases crop productiion. This positive feedback thus eases the nitrogen dilemma.
In many places, organic amendments are not considered as sequestration.
Many would exclude organic amendments as C sequestration, as it assumes that it is merely transferring C material from one place to another. It is right in cases such as farmyard manure in intensive cattle production systems. However, when managed properly, this may lead to sequestration. More importantly, a large part of potential organic amendments in the world is largely under-utilized, such as for instance urban wastes and sewage sludges and thus have the potential for contribution to abating GHG.
But true abatement should consider net changes in all GHG.
Yes, that is true, we need to work further on this topic. But there are also examples where reduced or no-till systems lead to a lower fuel consumption, and the reduction in N fertilizer application when legumes are used in crop rotation.
There is a limit of C sequestration with time?
Absolutely, the capacity of soils to sequester carbon is time constrained, in our paper of Minasny et al. (2017) , studies from France, New Zealand and Chile showed some constraint. Some countries reported that a new equilibrium will be reached (e.g., UK, Canada) and some others indicate that for some soils the maximum has already been attained (e.g., Scotland, New Zealand, Chile, USA, Belgium) and that the main challenge for these soil is not to lose the accumulated carbon. We also need to be aware that soil carbon sequestration is reversible.
Some simulation studies show that climate change (increasing temperature) can enhance soil C lost.
Increase temperature from climate change may have adverse effects on soil carbon storage, especially in the extreme conditions, by accelerating mineralization in the cold climates such as in permafrosts and northern hemisphere peats and by reducing net primary production in hot-and-dry areas. However, in the intermediate situations, which are areas where increasing SOC is practicable, the effect of rising temperature is still under debate. The effect is convoluted by the fact that not only temperature but precipitation and extreme events will change, and also changes in land use may have larger effect on C stocks. Several regional studies for instance in China, Italy, and Korea found that management practices superseded the increase in temperature. The overall effects of land use and soil management will be much more important for the next decades.
Where SOC sequestration should be targeted?
Restoring degraded soils by management is one of the main challenges of the developing world. We acknowledge that the sink related to unmanaged rangeland or degraded soils, because of their relative large areas within the cultivated lands category, deserves more scientific attention in order to decrease current uncertainties. SOC sequestration can also be seen as a way to improving the resilience of the soil to future climate change, that is, improving adaptation rather than mitigation. In some parts of the world where food security is threatened, the benefit of soil carbon management for adaptation should be stressed more than for mitigation. This is the reason why the 4 per mille initiative explicitly includes food security.
You chose to be optimistic?
I believe it is better to be optimistic with an aspirational target and use the best empirical evidence to achieve this rather than being pessimistic, refuse to do anything, and lament that it is unachievable. The paper by Minasny et al. (2017) lists potential and challenges from 20 counties and regions worldwide, representing many biomes and landuses. The authors conclude that 4 per mille sensu lato is a challenge worth pursuing. Optimised practices for enhancing soil carbon will be region and site-specific and will raise new questions and generate new soil knowledge. The economic and social aspects need further investigation and hope that the initiative will create the opportunity to join these scientific communities.
This interview was originally published in Pedometron
References
Ademe, 2015. Organic Carbon in Soils, Meeting climate change and food security challenges. ADEME, France.
Balesdent, J. and Arrouays, D. 1999. Usage des terres et stockage de carbone dans les sols du territoire français. Une estimation préliminaire des flux nets annuels pour la période 1900-1999. Comptes Rendus de l’Académie d’Agriculture de France 85, 265-277.
Batjes, N.H., 1996. Total carbon and nitrogen in soils of the world. European Journal of Soil Science, 47(2), 151-163.
Minasny, B., Malone, B.P., McBratney, A.B., Angers, D.A., Arrouays, D., Chambers, A., Chaplot, V., Chen, Z.S., Cheng, K., Das, B.S., Field, D.J., Gimona, A, Hedley, C.B., Hong, S.Y., Mandal, B., Marchant, B.P., Martin, M., McConkey, B.G., Mulder, V.L., O’Rourke S., Richer-de-Forges A.C., Odeh, I., Padarian, J., Paustian, K., Pan, G., Poggio, L., Savin, I., Stolbovoy, V., Stockmann, U., Sulaeman, Y., Tsui, C-C., Vågen, T.-G., van Wesemael B., and Winowiecki, L., 2017. Soil carbon 4 per mille. Geoderma 292, 59–86.
Minasny, B., Arrouays, D., et al., 2017. Rejoinder to Comments on Minasny et al., 2017 Soil carbon 4 per mille Geoderma 292, 59-86: Opportunities Prevail over Limitations. Geoderma (In Press)
Sanderman, J., Creamer, C., Baisden, W.T., Farrell, M. and Fallon, S., 2017. Greater soil carbon stocks and faster turnover rates with increasing agricultural productivity. Soil 3, 1-16. doi:10.5194/soil-3-1-2017.
Soussana, J.F., Saint-macary, H., Chotte, J.L. 2015. Carbon sequestration in soils: the 4 per mil concept. Agriculture and agricultural soils facing climate change and food security challenges: public policies and practices conference. Paris, Sept. 16, 2015. http://www.ag4climate.org/programme/ag4climate-session-2-3-soussana.pdf
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