Sinking Carbon into Soils

The increasing release of greenhouse gases (GHG) into the atmosphere is prompting considerable interest in the role that natural sinks have in the uptake of carbon. Soils are the third largest sink of the five global carbon pools. The other four reservoirs are the ocean, the biosphere, the atmosphere, and the earth’s crust (sedimentary rock deposits and fossil fuels). Though soils are a major carbon sink, storing about 2,500 gigatonnes of carbon (GtC), there has been a historic loss of soil carbon due to human activities such as land use conversion and agricultural practices. Since the spread of agriculture around 9,000 years ago, soils have lost at least 320 Gt of soil organic carbon. Also, since the beginning of the industrial era, circa 1850, the “conversion of forest to agricultural land has depleted the soil C pool by [about] 22 %.” Though human activities are major drivers of the loss of soil carbon, there are practices that can prevent this impact.

Several agricultural practices increase the levels of carbon in the soil and they are usually referred to as best management practices (BMPs) or recommended management practices. These BMPs do not only provide benefits as a GHG mitigation strategy, but they also have valuable ecological, social, and economic impacts. Some of these practices include, but are not limited to, no-till farming, planting cover crops, implementing improved grazing, and rotating crops with legumes. Before I delve into the importance of increasing the levels of carbon in global soils, I will briefly mention where soil organic carbon (SOC) and soil organic matter (SOM) come from and why they are important.

SOC originates from the decomposition of plant residues, soil microorganisms and fauna. A large portion of the carbon from the decomposition process is emitted into the atmosphere in the form of carbon dioxide (CO2). About 10-20% of that carbon forms SOM, which is also composed of essential elements like nitrogen (N), phosphorous (P), and sulfur (S). SOM is a strong indicator of healthy and productive soils, making it a vital component of the terrestrial biosphere. Organic matter improves soil structure, which leads to higher productivity. Good soil structure leads to greater porosity and water infiltration resulting in less soil erosion. SOM also enhances aggregation which makes soils less apt to crusting and compaction. Additionally, SOM improves cation exchange, increases plant root growth, and supports important soil microorganisms and fauna.

In the book Soil Carbon Management, Rice et al. dedicate a chapter of the book to the benefits of SOC to physical, chemical, and biological properties of the soil. This chapter includes the relation between SOM supporting ecosystem services like improved drought tolerance, root growth, plant production, decreased fertilizer inputs, improved water quality and decreased erosion. Furthermore, SOM plays an essential role in soil functions such as water holding capacity, soil biodiversity, nutrient reserves, bulk density, and soil structure. Therefore, adopting practices that decrease SOM and SOC depletes a source of nutrients that would have to be offset by applying higher amounts of fertilizers, and causes soils to be more susceptible to erosion. Lower SOM and SOC levels disrupt the sustainability of soils.

In the same book (Soil Carbon Management), Kimble et al. present the social and economic benefits from the adoption of practices that increase soil carbon sequestration. Some on-farm advantages include lower production costs, labor cost savings, reduced energy costs, reduced use of machinery, reduced maintenance and machinery repair needs, and potential eligibility to receive payments for sequestering carbon through federal programs or carbon markets. Additionally, some of the off-site benefits are better air quality from lower soil erosion, less runoff which is closely connected to better water quality, flood mitigation, protection of wildlife, and increased recreation activities like fishing. The promotion of agricultural practices that increase soil carbon content could be further promoted by either social, economic, or policy constructs. In this article, I would like to make a special emphasis on a recent effort launched at COP21.

In December of 2015, the French Minister of Agriculture, Stéphane Le Foll, launched the “4 per 1000” Initiative at COP21. This strategy is inspired by the extensive body of research showing that best management practices can lead to higher content of soil carbon, and its purpose is to increase global soil carbon content by 0.4 percent per year. Signatories to this initiative commit to a voluntary action plan to implement farming practices that increase the levels of carbon in global soils. The value 4 per 1000 is the ratio of annual global anthropogenic emissions from fossil fuels and the estimated content of global soil carbon stock to 2 m of soil depth (2,400 GtC).  If these best management practices are implemented in accordance with the 4 per 1000 initiative, global soils could sequester 1.2 GtC per year in global soils. Though 1.2 GtC seem to be a small number relative to the annual rate of carbon emissions from burning fossil fuels (almost 10 GtC per year), keeping 1 GtC in the soil sink is equivalent to 0.47 ppm of carbon dioxide (CO2) in the atmosphere.

The adoption of this initiative at a global scale presents many challenges. There are significant gaps in the research and literature to answer if the “4 per 1000” initiative is feasible in every country. Additionally, many countries have large numbers of farmers and small-holders with limited access to resources, which requires that these countries engage in “careful planning.” In order to address this knowledge gap, a recent study explored the question of whether the initiative was feasible in 20 regions/countries. This type of study can encourage researchers, and different stakeholders, to ask whether the initiative can be used as a framework to increase carbon sequestration rates, and how it could help pursue more targeted strategies. Additional studies could also lead to explore the several co-benefits from increasing the levels of C such as: the adaptation of the global food and agricultural systems to climate change, improvement of the health of soils, and increasing agricultural productivity. Although soil carbon sequestration may present a short-term, and small-scale, solution to the mitigation of GHG, it could provide a long-term approach to transforming our agricultural system into a more sustainable food system.


Image courtesy of Flickr. Originally published by S&S on October 3, 2017.

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