Chapter 22: Soil Organic Carbon Sequestration and Climate Change

This chapter was published on “Inuitech – Intuitech Technologies for Sustainability” on May 8, 2012.

Soil organic carbon (SOC) is an important component of soil organic matter that provides essential nutrients for crop growth, increases resilience against soil erosion and improves water conservation.  Increasing SOC storage, also known as Carbon Sequestration, helps offset carbon dioxide (CO2) emissions from farming activities such as cropping and livestock production, while enhancing soil quality, water retention, and decreasing nutrient losses.  Soil Carbon Sequestration (SCS) is the balance between carbon inputs to soil through plant biomass and the release of carbon from soil as CO2 through microbial activity and the decomposition of organic residues.

The process of plant sequester, removes carbon from the atmosphere through growth.  As a result, CO2 is converted into plant tissues through photosynthesis.  After a plant dies, primarily soil microorganisms decompose the plant material, and carbon is released back into the atmosphere through respiration or it is left behind as humus.  So plants and the microorganisms in the soil provide the link between the carbon in the atmosphere and how it can be stored or fixed to biological matter in the soils – this process is called SCS.Slide1In other words, SOC represents a significant pool of carbon within the biosphere.  Climatic shifts in temperature and precipitation have a major influence on the decomposition and amount of SOC stored within an ecosystem and that released into the atmosphere.

Soils used for growing crops and pastures are very important for storing carbon:

  • Soils can store large amounts of carbon, up to 50-300 tonnes per hectare, which is equivalent to 180-1100 tonnes of carbon dioxide;
  • The above-ground component (or above-ground biomass) of pastures and crops store between 2 and 20 tonnes of carbon per hectare;
  • The above ground component of plantation forests can store 250 tonnes of carbon per hectare.

It is important to understand the carbon cycle in order to appreciate how valuable the soil is in storing carbon.  Here is a brief description of the carbon cycle:

  • The carbon cycle refers to the transfer of carbon, in various forms, through the atmosphere, oceans, plants, animals, soils and sediments.  As part of the carbon cycle, plants and algae convert carbon dioxide and water into biomass using energy from the sun (photosynthesis). Living organisms return carbon to the atmosphere when they respire, decompose or burn. Methane is released through the decomposition of plants, animals and other hydrocarbon material (fossil fuels and waste) when no oxygen is present.

Here is a graphical representation of the carbon cycle:

Slide2The global soil carbon (C) pool of 2500 Gigatons (Gt) includes about 1550 Gt of SOC and 950 Gt of Soil Inorganic Carbon (SIC).  It is easy to think of soil as just dirt. In fact, soil is a living, dynamic entity that consists of organic material with living organisms and inorganic minerals. Both play an essential role in plant health.

The soil C pool is 3.3 times the size of the atmospheric pool (760 Gt) and 4.5 times the size of the biotic pool.  The SOC pool to 1-m depth ranges from 30 tons/ha in arid climates to 800 tons/ha in organic soils in cold regions, and a predominant range of 50 to 150 tons/ha. The SOC pool represents a dynamic equilibrium of gains and losses. Conversion of natural to agricultural ecosystems causes depletion of the SOC pool by as much as 60 percent in soils of temperate regions and 75 percent or more in cultivated soils of the tropics. The depletion is exacerbated when the output of C exceeds the input and when soil degradation is severe.

Some soils have lost as much as 20 to 80 tons C/ha, mostly emitted into the atmosphere.  Severe depletion of the SOC pool degrades soil quality, reduces biomass productivity, and adversely affects water quality, and projected global warming may exacerbate the depletion.

SOC, the major component of soil organic matter, is extremely important in all soil processes.  SOC refers to all the different carbon compounds found in soils that are, or were previously, a living organism whereas organic matter in soil fulfills many vital functions.  It provides food for growing plants, helps prevent erosion, increases moisture absorption and improves the quality of air and water. By paying attention to the amount of organic matter in soil, gardeners and farmers can directly influence the quality of their produce while simultaneously having a positive effect on the environment.

Soil organic matter is generally divided into three components:

  • Particulate material refers to the bits and pieces of plant material, or material that is available to soil organisms for decomposition. Soil organisms break down particulate matter to create humus, which is the final product of the decaying process;  it will break down no further;
  • Humus is important for binding soil particles together.  It improves the water and nutrient holding capacity of soils, and these are essential for plant growth.  Humus stores or sequesters carbon for decades, or even centuries; and
  • Charcoal is the result of incomplete burning of plant material or fossil fuels. It is believed to be biologically and chemically un-reactive compared with other soil organic matter components.  This means that the carbon stays locked in the charcoal in the soil and is not readily released or taken up by soil organisms.

Healthy soils with high organic carbon content contribute to the following soil characteristics, which are important for soil fertility:

  • Nutrient availability:  Decomposition of soil organic matter releases nitrogen, phosphorus and a range of other nutrients for plant growth;
  • Soil structure and soil physical properties:  Soil organic carbon promotes a healthy soil structure by holding the soil particles together, thereby improving soil physical properties such as water-holding capacity, water infiltration, gaseous exchange, root growth and ease of cultivation;
  • Biological soil health:  Soils contain microscopic plants and animals that live between, and feed on, the many soil particles. Soil organic matter plays an important role in the soil food web by controlling the number and types of soil inhabitants. These inhabitants serve important functions such as cycling nutrients, making nutrients available for other organisms, assisting root growth, assisting plant nutrient uptake, creating burrows, and even suppressing crop diseases; and
  • Buffer for toxic and harmful substances:  Soil organic matter can lessen the effect of harmful substances, for example toxins such as heavy metals, by acting as a buffer. Heavy metal toxins can bond very tightly to soil particles, preventing their release into waterways where contamination affecting the food chain may occur. Soil organic matter also increases the adsorption of pesticides, thus reducing the amount of chemicals that may enter groundwater.

The problem with traditional farming practices is that where farmers have used the land for many years for growing crops, the soils typically have low levels of organic carbon content, due to disturbance, erosion and regular periods when very few very few organic nutrients have been added to the soil, for example during fallow (leaving the soil bare between crops) and during the early stages of crop growth.

Harvesting a crop leaves little organic material for decomposition, and this, often coupled with traditional farming methods, results in what little soil carbon is present in the soils being released.  The tillage method of farming uses machinery to turn over the soil to reduce weeds, loosen, and aerate the soil, allowing the deeper penetration of roots.  In doing this, however, it is now known that more soil carbon may actually be released through disturbance of the soil structure and soil microorganisms.

The carbon sink capacity of the world’s agricultural and degraded soils is 50 to 66 percent of the historic carbon loss of 42 to 78 gigatons of carbon. The rate of soil organic carbon sequestration with adoption of recommended technologies depends on soil texture and structure, rainfall, temperature, farming system, and soil management. Strategies to increase the soil carbon pool include soil restoration and woodland regeneration, no-till farming, cover crops, nutrient management, manuring and sludge application, improved grazing, water conservation and harvesting, efficient irrigation, Agroforestry practices, and growing energy crops on spare lands. An increase of 1 ton of soil carbon pool of degraded cropland soils may increase crop yield by 20 to 40 kilograms per hectare (kg/ha) for wheat, 10 to 20 kg/ha for maize, and 0.5 to 1 kg/ha for cowpeas. As well as enhancing food security, carbon sequestration has the potential to offset fossil fuel emissions by 0.4 to 1.2 gigatons of carbon per year, or 5 to 15 percent of the global fossil-fuel emissions.

Quantifying the extent of CO2 released from soil and identifying its source can help determine management factors that affect soil processes influencing CO2 release.  It is true that over time, a change in land use from forest or grassland to cropping generally leads to a loss of 50 percent or more in soil carbon.  While it is possible that there is a significant potential to increase carbon stocks in the carbon-poor soils by improving land management practices, it is recognized that stable isotopes of carbon (carbon-13 and carbon-12) in CO2 released from soil are used to assess organic matter dynamics, carbon sequestration potential and the stabilization of carbon in soils.  It helps assess the influence of land-use change on SOC.Slide3Here is an explanation for stable isotopes.  Isotopes are atoms of the same element that differ in atomic mass, due to differences in the number of neutrons contained in the atoms’ nuclei.  For example, the three most abundant isotopes of carbon are carbon-12 (12C), which contains 6 protons, 6 electrons, and 6 neutrons; carbon-13 (13C), which also has 6 protons and electrons, but has 7 neutrons; and carbon-14 (14C), which contains 6 protons and electrons, but has 8 neutrons.  Having too few or too many neutrons compared to protons causes some isotopes, such as 14C, to be unstable.  These unstable ‘radioisotopes’ will decay to stable products. Other isotopes, such as 12C and 13C do not decay, because their particular combinations of neutrons and protons are stable.   These are referred to as stable isotopes.

Some of the most exciting advances in ecology and environmental sciences in the past decade have relied on stable isotopes.  Stable isotopes can be used to address many questions in ecology and environmental sciences.

Stable isotope techniques allow the investigation of these questions quantitatively and non-intrusively (without the environmental hazards of radioisotopes), and thus offer considerable advantages over other techniques.  Indeed, many of these questions can only be addressed by using stable isotopes.

Many of these techniques rely on natural differences in the ways that ‘heavy’ and ‘light’ isotopes are processed in the environment through chemical, biological, and physical transformations. These are referred to as natural abundance isotope techniques. For example, plants preferentially take up carbon dioxide containing the lighter carbon isotope (12C-CO2) in photosynthesis, but the degree of preference depends on water availability and on the photosynthetic pathway, which is a major distinguishing characteristic of plants from hot, xeric environments versus more mesic environments.  Thus, the13C composition of plants provides a time-integrated measure of the efficiency with which plants use water.

It is also possible to tell how much a plant depends on surface water compared to deep sources of water by measuring the 2H and 18O composition of water in a plant’s stem.  This is possible simply because:

  • “Heavier” water will evaporate at the soil surface more slowly than ‘lighter’ water, causing surface soil water to be isotopically enriched compared to deeper water, such as groundwater; and
  • The rain and snow that falls during winter (precipitation that contributes to deeper water sources in the soil) will be isotopically ‘lighter’ than summer precipitation.

Slide4Other stable isotope techniques rely on adding trace amounts of compounds that are artificially enriched in the rare (heavy) isotope of the element of interest.  These are referred to as isotope tracer techniques. For example, without isotopes, measuring independently the processes of microbial production of ammonium (NH4+) through mineralization and the consumption of NH4+ through immobilization and nitrification was not possible, because all these processes occur simultaneously. By adding 15NH4+ to soil and monitoring the rate at which it is ‘diluted’ by the more abundant 14NH4+, one gets a measure of the rate of mineralization of soil organic matter, a rate that is independent of nitrification and immobilization (the NH4+-consuming processes).  By adding 15N as NH4+ or NO3 and monitoring both 15N and 14N in the soil, it is possible to quantify each of these microbial transformations, in situ.

Resources:

  1. IAEA – Nuclear Technology Review 2011;
  2. LandLearn NSW – The Journey Begins Here;
  3. The Carbon Cycle;
  4. Soil Carbon and Climate Change;
  5. Science – Soil Carbon Sequestration Impact;
  6. eHow – Definition of Soil Organic Matter; and
  7. Colorado Plateau Stable ISOTOPE Laboratory.

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