The Role of Soils in Sequestration

Soils are an important part of the carbon cycle with some two to three times the terrestrial biosphere carbon than plants. Soil processes influence carbon sequestration and transport. The dynamics of carbon transformations and transport in soil are complex and can result in sequestration in the soil as organic matter or in groundwater as dissolved carbonates, increased emissions of CO2 to the atmosphere, or export of carbon in various forms into aquatic systems. The key to sequestration in soils is good management of them and well managed forests and farmlands sequester significant amounts of carbon and as they are then also more productive, sequestering carbon in this way is important[1].

"It is life that gives soil its structure. It is life that provides fertility and balanced nutrition. It is life that retains soil moisture, restoring water balance and reversing the effects of dry land salinity. It is life that retains carbon and nitrogen from the atmosphere and balances the greenhouse equation."[2]

According to Dr Christine Jones [3] "In a healthy ecosystem, vibrant, living soils are a dynamic part of the carbon cycle. The carbon compounds added to soil as exudates from active plant roots and the decomposition of plant and animal residues, fuel the biological processes that improve soil structure, which in turn increases oxygen and moisture retention and creates better conditions for more life." The process of creating and improving top soils sequesters more carbon than is lost to the atmosphere and as a consequence soils are the largest carbon sink over which we have control. According to Dr Jones "Groundcover management is the prime determinant of whether agricultural soils act as a source (net loss) or a sink (net gain) for atmospheric carbon. Organic carbon (such as humus) has many benefits in soils, making effective carbon management the key factor for productive farms, revitalised catchments and a greener planet." "Around 50-80% of the carbon has been lost from the topsoil in many farmed soils, often as a direct result of the loss of the soil itself. Even today, most farming businesses continue to lose soil carbon - their most valuable asset. As a result, landholders invest a great deal of time and effort in forcing ‘dead’ soils to be productive using chemicals that further compound the problem. Carbon equilibrium levels in soil are determined by carbon inputs and outputs, which in turn are influenced by temperature, rainfall and management. In general terms, soil carbon accumulation is positively correlated with rainfall and negatively correlated with temperature. That is, more carbon can be stored in soil in cold, moist environments than in hot, dry ones. Landholders cannot alter rainfall or ambient temperature regimes, but they can markedly improve water infiltration rates, soil moisture retention, the buffering of soil temperatures and carbon inputs and outputs, through changes in groundcover management. Carbon cannot be sequestered in soils if we continue with the same forms of land management that caused the carbon losses in the first place. People cannot function without a skin. Soil cannot function without cover." We need to improve groundcover using mulches and other methods like leaving crop stubble in the ground. "Managing groundcover for increased soil carbon levels results in improved soil structure, lower bulk density, greater porosity, higher infiltration rates, more effective use of rainfall, enhanced water quality, higher cation exchange capacity, greater sequestration of nitrogen and sulphur, enhanced availability of phosphorus and trace elements, reduced costs, reduced inputs, improved biodiversity and increased productivity."

 

Soil Sequestration Processes [4]

What Land Management Practices are Best for Increasing Soil Sequestration?

Soil conservation practices not only reduce soil erosion but also increase the organic matter content of soils. Principal conservation strategies, which sequester carbon, include converting marginal lands to compatible land use systems, restoring degraded soils, and adopting best management practices. For example, removing agriculturally marginal land from annual crop production and adopting an ecologically compatible land use, such as livestock grazing and/or wildlife habitat, can lead to increases in total biomass production and an increase in carbon content in the soil.

The following best management practices have been proven to sequester soil carbon:

  1. conservation tillage practices like direct seeding systems;
  2. elimination of summer fallow;
  3. extending crop rotations to include perennial forage crops for hay or pasture.
  4. management of crop residue and application of organic materials and manures;
  5. soil fertility optimization through improved fertilizer placement and site-specific management;
  6. reductions in the use of chemicals.
  7. other techniques that may improve crop yields, and reduce on-site and off-site production risks.

If soils are completely devoid of carbon and life because of the use of chemicals or for some other reason that are a number of new techniques and products emerging including "agrichar"[5] and various new fertilizers made of bio waste with the pathogens removed.

How Much Carbon Sequestration Would Occur as a Result of Improving Soils?

The Permanence of Carbon Sinks [6]

There seems to be no clear understanding of exactly how much carbon has been lost around the world from bad agricultural practices but common figures in the literature are in the range 50-80%.

If 50% is assumed and currently there are some 1600 gigatonnes of carbon in soils[6] then at least a further 1600 gigatonnes could be taken up over say a 20 year period of dramatic improvement in soil management practices amounting to 80 gigatonnes a year which is much much more than the current annual net flux of CO2 into the atmosphere of around 8 gigatonnes. We could achieve safe levels of CO2 of say 300 ppm in this way provided nothing goes wrong. In spite of better agricultural practices that result in soil sequestration having many other advantages Murphy's law predicts that only 1/4 of what could easily be possible will be achieved.

Our conclusion is that raising the level of carbon in soils is very important but given obstacles including widespread ignorance and the large chemical company lobbies should not be totally relied on. The TecEco Gaia Engineering opion of putting carbon away as man made carbonate also has no downsides and is easily implemented through building approval systems. Both alternatives have much in common and mean changing the way we live.


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[1] DOE, 1999: Carbon Sequestration Research and Development [Reichle, D., J. Houghton, B. Kane, J. Ekmann (eds.), S. Benson, J. Clarke, R. Dahlman, G. Hendrey, H. Herzog, J. Hunter-Cevera, G. Jacobs, R. Judkins, J. Ogden, A. Palmisano, R. Socolow, J. Stringer, T. Surles, A. Wolsky, N. Woodward, and M. York]. Department of Energy, Oak Ridge, TN.

[2] Jones, C. E. (2006). Healthy Soils through Communication. Symposium, Federation of Biological Farmers Inc.,. Seymour, Victoria, Australia.

[3] Jones, C. E. (2007). Australian Soil Carbon Accreditation Scheme (ASCAS). Managing the Carbon Cycle. Katanning Workshop.

[4] DOE, 1999: Carbon Sequestration Research and Development [Reichle, D., J. Houghton, B. Kane, J. Ekmann (eds.)

[5] Agrichar is char produced in low intensity fires and when added to soil mimics the practice of Brazilians in the formation of Terra Preta over the last few thousand years. See Newsletter 67

[6] Ziock, H. J. and D. P. Harrison. "Zero Emission Coal Power, a New Concept." from http://www.netl.doe.gov/publications/proceedings/01/carbon_seq/2b2.pdf.

Reading and Resources

Putting Carbon back into our Soils to Help Reverse Global Warming?

The Wonderful World of Humus and Carbon

Carbon Farmers of America