By Erika Foster, a 2017-2018 Sustainability Leadership Fellow and Ph.D. Candidate in the Department of Soil and Crop Sciences and Graduate Degree Program in Ecology
Poor management and lost carbon. Since the dawn of agriculture, humans have cleared forested lands, razing all natural vegetation for the sake of food production. Land use change has now impacted over 40% of all land mass, upsetting natural global processes including the carbon cycle. Land use change causes carbon loss by removing plant biomass, causing the accumulation of CO2 in the atmosphere, and decreasing the soil carbon via lower inputs and exposed soil erosion. Forests naturally store carbon by pulling carbon dioxide (CO2) from the atmosphere for photosynthesis and storing it in their biomass. Tree roots and decaying leaf litter also add carbon into the soil, where organic carbon compounds stick to clay particles and remain there for decades to centuries.1 The reservoir of carbon in the soil, or soil carbon “pool,” accounts for 1500-2400 petagrams (Pg) of carbon globally (see global carbon figure).2
With agriculture and grazing, we have now lost 133 Pg of carbon from the top two meters of soil,3 resulting in more carbon in the atmosphere contributing to global warming.
Since soil naturally stores more carbon than the atmosphere and terrestrial biomass combined, soil plays a critical role in the global carbon balance. Despite its importance, soil only recently made a debut on the international climate policy stage, but not without controversy.
Resetting the carbon cycle. International negotiations focused on reducing greenhouse gas emissions and rebalancing the carbon cycle have occurred at the Conference of the Parties (COP) to the United Nations Framework Convention on Climate Change for the last two decades. The most recent COP21 in Paris produced the Climate Agreement in which over 190 countries pledged to reduce their greenhouse gas emissions and keep global warming under 2°C. In conjunction with the primary agreement, a new French initiative proposed to mitigate climate change by storing carbon in non-atmospheric pools, particularly in soil.
In 2015, the French minister of Agriculture, Stéphane Le Foll, introduced a unique international program: the “4 per 1000 Soils for Food Security and Climate.”4 The idea is simple. Human activities emit 8.9 Pg of carbon into the atmosphere annually; if we add an equivalent 8.9 Pg of carbon into the soil each year, we can offset our carbon emissions. The 8.9 Pg of C only is 0.4% of the total 2400 Pg of soil carbon. We can represent this tiny number either as 0.4% (using the ‘per cent’ symbol, out of 100) or 4‰ (using the ‘per mille’ symbol, out of 1000), hence the colloquial name of the ‘4 per mille’ initiative.
The initiative proposes that we add carbon at a rate of 4‰ annually in the top 30-40 cm of agricultural soils. At first this seems like a relatively small amount and an achievable goal, but opposition developed as soon as soil scientists began to crunch the numbers. Is ‘4 per mille’ possible?
The science behind ‘4 per mille’. While the ‘4 per mille’ initiative set a concrete goal, scientists quickly began to investigate and question its feasibility. In a collaborative international paper scientists Minasny et al. examined the possibility of adding 4‰ soil carbon in agricultural soils in 20 distinct world regions (Geoderma, 2017).5 Misany et al. calculate that we need to store carbon at a rate of 0.6 metric tons per hectare per yr. (One hectare (ha) equals roughly two football fields). To sequester or increase carbon storage in soils, we can use the following practices:
- Plant forests (afforestation) (0.6 t/ha/yr),
- Convert land to pasture (0.5 t C/ha/yr),
- Add organic amendments (0.5 t/ha/yr),
- Keep crop residues (0.35 t c/ha/yr),
- Use reduced or no tillage (0.2 t C/ha/yr), and
- Rotate crops (0.2 t C/ha/yr).5
Even after implementing all of these possible practices, Minasny et al. calculated sequestration rates fell in between 0.2-0.5 t C/ha/yr, shy of the target rate 0.6 t C/ha/yr.5 Since ‘4 per mille’ number was based on a global calculation, it does not account for certain soils that cannot store any more carbon, such as high carbon peat and bog soils. Also, Minasny et al. point out that smaller countries cannot do their part offset their own emissions. For example Belgium and South Korea do not have enough agricultural land available to offset their own greenhouse gas emissions at the ‘4 per mille’ rate. The scientists go on to explain that the initiative also ignores the ‘lifespan’ of the added carbon in the soil. In wet, hot, tropical environments the decomposition rates in soils are high, meaning that the soil critters (worms, beetles, bacteria and fungi) rapidly use the added carbon as food, breaking it down and releasing CO2 on a relatively short timescale. The research by Minasny et al. concluded that only 20-35% of global anthropogenic emissions could be sequestered in the top 1m of agricultural soils, due to initial high carbon content of some soils, the limited available agricultural lands, and climate factors.5 The authors include a positive note, stating that dry regions with degraded soil, like much of Australia, may sequester more than 4‰ , potentially even up to 10‰ annually. The researchers posit that the ‘4 per mille’ initiative, although aspirational, remains an important mechanism to put soils on the map and delineate goals for climate-smart management.
The discussion of ‘4 per mille’ heated up when scientists Baveye et al. submitted a Letter to the Editor of Geoderma: “The ‘4 per 1000’ initiative: A credibility issue for the soil science community?”6 The scathing pieces called out the initiative for being “deceptively simple,” having “considerable uncertainty,”
and misrepresenting farmers heroically as part of the solution, rather than the problem. Baveye et al. again question the ‘lifespan’ of new carbon addition in soils, stating that Minasny et al. glossed over their graph showing that new carbon may remain in soil for only a couple of years to decades,6 not exactly a long-lasting climate change mitigation strategy. Furthermore, Baveye et al. emphasize that governments need to calculate the cost of inputs and monitoring programs, before implementation. The authors reiterate that the initiative is “aspirational,” and add that “no matter how many times one tries…to say that the value of 0.4% should not be taken literally…there is a clear risk that policy-makers…will simply take the misleading message…at face value.”6 So, how can we discriminate between the aspirational global “4 per mille” initiative and the actual regional 4‰ soil carbon sequestration potential?
Can soil save us? The conversation continues between soil scientists in meetings, conferences, and peer-reviewed journals such as Geoderma and Environmental Science and Technology. We know that any blanket calculation will not hold true in every ecosystem, but quick estimates provide numbers necessary for strategy comparisons and decision making. Although the ‘4 per mille’ initiative boldly proposes that soils are the solution, the proposed management practices at least bring critical soil carbon into the discussion. Full understanding of nuanced soil processes, such as nutrient balance, temperature effects on decomposition, and uncertainty of global soil carbon estimates, proves challenging even for scientists. Policy-makers require concrete numbers on which to base decisions. The problem with ‘4 per mille’ lies in the fact that climate change is a global issue, but not all soils around the world are created equally; a single number will never be attainable. Scientists recognize the potential of agricultural soils to sequester carbon using a myriad of regionally-specific techniques. Climate change mitigation requires motivating global policies, such as the laudable ‘4 per mille’ initiative. However, lofty aspirational goals, in practice, require numerous local solutions. Shifting to climate-smart agricultural practices may mitigate climate change, but only in conjunction with improved practices in other sectors (e.g., transportation, industry, energy generation). Ultimately, can soil save us? Certainly not soil alone.
- Baveye, P.C., Berthelin, J., Tessier, D., Lemaire, G., 2018. The “4 per 1000” initiative: A credibility issue for the soil science community? Geoderma 309, 118–123.
- IPCC, 2014. Climate Change 2014: Synthesis Report, Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change.
- Minasny, B., Arrouays, D., McBratney, A.B., Angers, D.A., Chambers, A., Chaplot, V., Chen, Z.-S., Cheng, K., Das, B.S., Field, D.J., Gimona, A., Hedley, C., Hong, S.Y., Mandal, B., Malone, B.P., 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., Wesemael, B. van, Winowiecki, L., 2018. Letter to Editor: Rejoinder to Comments on Minasny et al. Geoderma 309, 124–129.
- “4 per 1000 Initiative.” Webpage, 2017. https://www.4p1000.org/. Accessed Jan 2018.
- Paul, E.A., 2016. The nature and dynamics of soil organic matter: Plant inputs, microbial transformations, and organic matter stabilization. Soil Biology and Biochemistry 98, 109–126.
- Sanderman, J., Hengl, T., Fiske, G.J., 2017. Soil carbon debt of 12,000 years of human land use. Proceedings of the National Academy of Sciences 114, 9575–9580.