Soil organic carbon

July 7, 2012, 10:38 am

Introduction

Soils contain carbon (C) in both organic and inorganic forms. In most soils (with the exception of calcareous soils) the majority of C is held as soil organic carbon (SOC). The term soil organic matter (SOM) is used to describe the organic constituents in the soil (tissues from dead plants and animals, products produced as these decompose and the soil microbial biomass). The term ‘soil organic carbon’ refers to the C occurring in the soil in SOM.

The constituents of SOM can be divided into non-humic substances, which are discrete identifiable compounds such as sugars, amino acids and lipids, and humic substances, which are complex largely unidentifiable organic compounds. As organic compounds, both humic and non-humic substances contain carbon, oxygen (O) and hydrogen (H) and can also contain nitrogen (N), phosphorus (P) and sulfur (S).

Amounts of organic carbon in soils

The amount of SOC depends on soil texture, climate, vegetation and historical and current land use/management. Soil texture affects SOC because of the stabilizing properties that clay has on organic matter. Organic matter can be trapped in the very small spaces between clay particles making them inaccessible to micro-organisms and therefore slowing decomposition. In addition, clay offers chemical protection to organic matter through adsorption onto clay surfaces, which again prevents organic matter from being decomposed by bacteria. Soils with high clay content therefore tend to have higher SOC than soils with low clay content under similar land use and climate conditions. Climate affects SOC amount as it is a major determinant of the rate of decomposition and therefore the turnover time of C in soils. In a temperate grassland, high organic matter inputs combined with slow decomposition rates (determined by climate) lead to high SOC amounts, whereas in tropical areas, decomposition and the turnover of SOC tend to be faster.

The content of SOC in soils ranges from less than 1% in sandy soils to almost 100% in wetland soils.

Measurement of SOC content

A simple way to determine the SOM content of a soil is to burn off the organic matter in a furnace and determine the mass lost. To determine SOC an assumption can then be made that SOM is, on average, 58% C. This method is called ‘Loss on Ignition’. It is a very approximate method which varies in accuracy depending on the clay content of the soil.

Another method is to use chemical means to oxidize the C in SOM to carbon dioxide (CO2) and organic acids. The amount of the oxidizing agent used gives a measure of the mass of C oxidized. This method is called ‘The dichromate method’ as it involves oxidation by boiling the soil sample with an acid dichromate solution. It is also referred to as the Walkley Black method, after the scientists who first documented its use.

Today, many labs use rapid C analysis equipment such as LECO for the determination of C content in soils. LECO elemental analysers use combustion methods. The sample is combusted at high temperature and resulting gases are passed through detectors to measure each element.

Management of SOC

Soil organic carbon is important for the function of ecosystems and agro-ecosystems having a major influence on the physical structure of the soil, the soil's ability to store water (water holding capacity), and the soil’s ability to form complexes with metal ions and supply nutrients. Loss of SOC can, therefore, lead to a reduction in soil fertility, land degradation and even desertification.

Soils under natural forests or grassland tend to have higher SOC content than soils under cropland. When forest or grasslands are converted to cropland there is a large initial loss of SOC followed by slower losses until eventually a new equilibrium is reached. The conversion of cropland back to forest or grassland is, therefore, a land use strategy that is likely to increase SOC.

After conversion to cropland, appropriate management can maintain or increase SOC thereby improving the productivity and sustainability of the soil. The content of SOC in a soil is determined by losses of organic carbon (through decomposition, erosion of particles and losses through dissolved organic matter) and inputs of organic C (e.g. crop residues, manures and other organic wastes). Management of soils to increase SOC content therefore involves measures that reduce losses and/or measures that increase inputs (Table 1.).

Table 1. Possible cropland management practices for the maintenance or increase of soil organic carbon.

SOC loss reduction

SOC input increase

Reduced tillage/no-tillage

Organic manures


Composts

Cover crops

Cover crops

Erosion reduction measures (mulching, contour cultivation on sloping land)

Crop residue returns

Reduced fallow periods

Increase crop productivity through irrigation/chemical fertilizer

Reduced burning of crop residues

 

SOC and the global carbon cycle

Excluding carbonate rocks, soils represent the largest terrestrial stock of C, holding approximately 1,500 Pg (1015 g) C in the top metre. This is approximately twice the amount held in the atmosphere and three times the amount held in terrestrial vegetation.

Recently, soils and SOC have received attention in terms of the role they can play in mitigating the effects of elevated atmospheric CO2 and associated global warming. Lal, estimated that with appropriate use and management soils have the potential to sequester ~ 0.9 Pg C per year. This is roughly equal to 13 % of the anthropogenic CO2-C produced annually. Changing land use and management practices to sequester C in soils can have additional environmental benefits (reduced erosion, increased water holding capacity, increased below-ground biodiversity, etc.). Many therefore see C sequestration in soils as a ‘win-win’ strategy. However, attention has also been draw to the fact that the amount of C that can be stored in soils is finite and can be reversed by a change back to previous land management practices or a change in climate. Efforts to sequester C in soils need to take into account all greenhouse gas (GHG) emissions associated with the proposed change in land management to ensure these do not negate C sequestration benefit.

Estimating SOC stocks

When trying to assess the stock of C that is held in a given area of soil (for example in a field, a region or even a country), account has to be taken of the depth of the soil and the compaction of the soil. Estimates of SOC stock will generally refer to a given depth of soil (e.g. the top 30 cm or top 100 cm). The amount of soil in a given depth depends on how compacted the soil is, which is measured by determining the soil’s bulk density (BD). The SOC stock of a given area of soil with the same soil type can then be expressed by equation 1.

Equation 1.    SOC stock = SOC content of the soil x BD x area x depth
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where SOC stock is given in Pg (1015 g), SOC content is in g C g-1, BD is in Mg m-3, area is in Mha and depth is in m.

Further Reading

  • Batjes, N.H., 1996. Total carbon and nitrogen in soils of the world. European Journal of Soil Science 47 (2): 151-163.
  • Batjes N.H. and Sombroek, W.G., 1997. Possibilities for carbon sequestration in tropical and sub-tropical soils. Global Change Biology 3 (2), 161-173.
  • Lal. R., 2004. Soil carbon sequestration to mitigate climate change. Geoderma 123: 1
  • Nelson, D.W., and L.E. Sommers. 1982. Total carbon, organic carbon, and organic matter. p. 539–580. In A.L. Page et al. (ed.) Methods of soil Analysis. Part 2. 2nd ed. Agron. Monogr. 9. ASA and SSSA, Madison, WI.
  • Smith P., 2004. Soils as carbon sinks: the global context. Soil Use and Management 20: 212-218 Suppl. S.
Glossary

Citation

Milne, E. (2012). Soil organic carbon. Retrieved from http://www.eoearth.org/view/article/156087

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