December 11, 2017 Blog

This year has seen numerous unprecedented weather events, many of which scientists say are driven or exacerbated by climate change1. The vast majority of the scientific community are well aware that climate change is occurring due to excess greenhouse gas emissions, resulting in increased carbon in the atmosphere. However, did you know that soils actually contain more than 2 times the amount of carbon than is in the atmosphere?

Soils are the second largest carbon sink on earth (the ocean is the largest). It is estimated that soils have lost more than 130 billion tonnes, or 8% of their total carbon in the last 12,000 years due to deforestation, intensive agriculture and land use change2. On December 5th, we celebrated World Soil Day, which reminds us how important soils are (see our recent blog post on soil health here), not only for growing food, fibre and fuel, but also for their role in climate regulation.

A recent article in The Economist3 highlighted the fact that reducing our carbon emissions alone will not be enough to halt global warming – carbon also must be extracted from the atmosphere. Increasing soil carbon stocks could help remove some atmospheric carbon. The “4 per 1000: Soils for Food Security and Climate” is a movement that began at the United Nations Paris Climate Conference in 2015, which promoted the idea that increasing worldwide soil organic matter by 0.4% (4 per 1000) could help to offset significant quantities of anthropogenic emissions4. A recent study evaluated the feasibility of increasing soil carbon by 0.4% for 20 different countries worldwide. The study found that some soils could actually sequester even more carbon, and concluded that globally, soils had the capacity to sequester about 2-3 Gigatonnes of carbon per year, which could offset between 20-35% of global anthropogenic emissions5. While there is debate in the scientific community about the feasibility of meeting the 0.4% goal worldwide, there is agreement that focusing on increasing soil carbon stocks could help offset emissions, as well as improve soil quality.

One strategy for increasing soil carbon stocks is through the addition of organic amendments (such as biosolids), which contain carbon, stimulate microbial activity, and promote the growth of below ground biomass. Biosolids contain carbon in different forms and quantities depending upon the treatment process. If we landfill biosolids, this carbon is emitted as methane (CH4), a greenhouse gas that is approximately 25 times more potent than carbon dioxide (CO2). When we add biosolids and other carbon containing organic amendments to the soil, plants and microbes sequester some of the carbon. Studies have found that adding biosolids to soil can sequester carbon, however the level of treatment of the biosolids prior to application, as well as soil properties, affect how much is sequestered. Some of this carbon comes from the biosolids itself, but the majority of the sequestered carbon comes from the improvements in crop growth and the stimulation of soil microbes6. Municipalities can use the Biosolids Emissions Assessment Model (BEAM) to evaluate different management options based on their greenhouse gas emissions7. The BEAM model assigns land application a credit of 25 Mg CO2e per 100 tonnes of dry biosolids for carbon sequestration and avoiding the use of inorganic fertilizers8.

The understanding of greenhouse gas emissions in biosolids management has become increasingly relevant in the age of carbon taxing and cap-and-trade: British Columbia, Alberta, Ontario, and Quebec all have evolving carbon markets, and many states in the US have voluntary markets, or have introduced proposals for carbon pricing. Policies vary, but some of the existing frameworks include large emitters purchasing offsets from those who have been able to reduce their emissions or sequester carbon. The Climate Action Registry and the American Carbon Registry are two groups who have developed and validated protocols for US markets by addressing both organics and nitrogen management in agriculture. The Marin Carbon Project registered a protocol called Compost Additions to Grazed Grasslands in 2014 with the American Carbon Registry9. In this protocol, farmers earn tradeable credits for adding composts to their rangeland. The credits are for the carbon sequestration from added compost, increase in carbon from stimulating plant growth, and for avoidance of greenhouse gas emissions by diverting organic waste from landfills to composting.

There is still a lot of research and validation required to gain a better understanding of the carbon sequestration potential of different types of organic amendments in different soil, climatic and management conditions. We are continually learning more about the benefits of adding biosolids and other organic amendments to soil. Carbon sequestration and climate change mitigation is one more benefit that can be achieved when organic residuals are land applied.


  1. Nuccitelli D. “Battered by extreme weather, Americans are more worried about climate change”. The Guardian. 20 November 2017.
  2. Sanderman J., T. Hengl and G.J. Fiske. 2017. Soil carbon debt of 12,000 years of human land use. Proc. Natl. Acad. Sci. U.S.A. 114(36): 9575-9580.
  3. The Economist. “What they don’t tell you”. November 18, 2017. []
  4. 4 Per 1000 Initiative.
  5. Minasny B. 2017. Soil carbon 4 per mille. Geoderma 292: 59-86.
  6. Tian G., C.Y. Chiu, A.J. Franzluebbers, O.O. Oladeji, T.C. Granato, A.E. Cox. 2015. Biosolids amendment dramatically increases sequestration of crop residue-carbon in agricultural soils in western Illinois. Appl. Soil Ecol. 85: 86-93.
  7. Canadian Council of Ministers of the Environment. Biosolids Emissions Assessment Model.
  8. Brown S., N. Beecher and A. Carpenter. 2010. Calculator tool for determining greenhouse gas emissions for biosolids processing and end use. Environ. Sci. Technol. 44(24): 9509-9515.
  9. Haden V.R. and De Gryze S. 2014. Methodology for Compost Additions to Grazed Grasslands. Version 1.0. American Carbon Registry. [].

Authored by Samantha Halloran, Project Coordinator, Lystek