What exactly is carbon sequestration?
In contrary to the big word [see + kwuh + stray + shn] it is actually a very simple equation!
When more carbon (C) is absorbed than released as carbon dioxide (CO2) or methane (CH4), and carbon accumulates in the soil, ocean, or earth's geology, this refers to C sequestration.
The Carbon Cycle visualised from Encyclopedia Britannica. The movement of carbon from its decomposed/fossilised state, to its natural and man-made sources and finally to the carbon sequesters and back into its decomposed/fossilised state.
Over the past 150 years, the amount of C in the atmosphere, from greenhouse gasses such as (CO2, and CH4) has increased by 30%. The increased levels of gases, particularly CO2, are strongly linked to global warming. Atmospheric C was approximately 280 ppm (parts per million) in the preindustrial era and had increased to 370 ppm by 2000. To stabilize future C concentrations at 550 ppm will require an annual reduction in worldwide CO2 emissions from the projected level of 21 to 7 billion tons (measured as C) by the year 2100.
There are several ways to reduce atmospheric CO2. These include the use of technology to develop energy-efficient fuels and the use of non-C energy sources such as solar, wind, water, and nuclear energy, which means that we will release less C into the atmosphere. Another way to reduce atmospheric CO2 is by carbon sequestration through agriculture, which will be the focus of this blog, by which we can remove C from the atmosphere.
Carbon sequestration is the long-term storage of C in oceans, soils, vegetation, and geological formations. The global carbon cycle is composed of both inputs (pools) and outputs (fluxes), as seen in the figure above. The ocean is by far the largest input, followed by the geological pool. The soil organic carbon (SOC) pool is the third largest, where carbon is stored primarily in the form of soil organic matter (SOM). The SOC pool, assuming an average content of 2400 billion metric tons to 2m depth, is 3.2 times the atmospheric pool and 4.4 times the biotic pool. Soils contain about 75% of the C pool on land, three times more than stored in living plants and animals. Although the SOC pool seems insignificant compared to the vast ocean and geological pool, we as humans have the greatest ability to increase or deplete this carbon pool.
How does carbon sequestration work?
Plants, through photosynthesis, are exceptional carbon sequesters. They absorb atmospheric CO2 and convert it into organic carbon, which is stored in their roots, stems, leaves, and other plant parts. As plants grow, they take in more carbon dioxide to produce the building blocks for C sequestration, while releasing oxygen back into the atmosphere.
Getting down to the roots! Although the source of C inputs from roots into soils is poorly understood and mostly ignored, plant roots play a vital role in determining whether the plant-tissue C will be mineralized to CO2 or accumulated as stabilized SOM. In the subsoil, plant roots and leached dissolved organic matter are the main sources of C inputs. In systems with annual crops, C from the roots, like biomass and less stable root exudates, adds almost twice as much C to stable SOC pools as C from aboveground residues. Fine root inputs have more opportunity for physiochemical interactions with soil particles and, therefore, chemical and physical protection. The importance of soil for carbon sequestration should not be underestimated, as it is a major reservoir for carbon storage.
The Problem
It has been estimated that land use changes and agriculture play an important role in the emission of CO2 and CH4 and account for 20% of the increase in radiative force. The rate of SOC loss due to conversion from natural to agricultural ecosystems, particularly cultivation, which enhances soil respiration, mineralization, and decomposition of SOC, is more significant in:
- Tropical soils vs. temperate region soils
- Cropland vs. pastures
- Soils with high initial SOC vs. soils with low initial SOC levels
Land use and crop & soil management have drastic effects on the level of the SOC pool and, thus, C sequestration. The loss of SOC stocks due to cultivation may be as high as 60–80 ton C per ha. Some soils may lose the SOC pool at a rate of 2–12% per year, with a cumulative decrease of 50–70% of the original pool. Declines in the SOC pool are due to (a) mineralization of soil organic carbon, (b) transport by soil erosion processes, and (c) leaching into subsoil groundwater.
The Solution
Regenerative Agriculture! Regenerative agriculture refers to a holistic farming approach that focuses on building and restoring soil health and the natural ecosystems present on farms. It involves practices such as cover cropping, crop rotation, reduced tillage, and the integration of livestock. These techniques promote soil carbon sequestration, leading to multiple benefits for the environment and agriculture.
- Reduced Tillage: Limited cultivation (tillage) such as no-till, can dramatically reduce C losses from soils. Reducing mineralization or rapid decomposition through the oxidation of organic matter leads to more mineral-bound (stable) SOM. Reduced soil disturbance and improved aggregate stability lead to reduced run-off and erosion, therefore promoting C sequestration. It has been estimated that extensive use of no-tillage in crop production could alone serve as a sink for 277 - 452 million ton C, about 1% of the fossil fuel emissions during the next 30 years. The effect of conservation tillage on C sequestration, however, is time-dependent and will only show after ± 10 years. No-till in an isolated system may probably not produce the positive results as expected from the accumulation of soil C. It is therefore important that no-till be combined with the establishment of diverse and high C input cropping systems.
- Crop Rotation: Conversion from monocropping or continuous cropping to crop rotations, especially with legumes, will allow farmers to sequester more carbon and reap the benefits of higher SOC levels much sooner.
- Cover Cropping: Planting cover crops, such as legumes or grasses, during fallow periods helps keep the soil covered and prevents erosion. These crops capture atmospheric carbon and store it in their biomass, enriching the soil with organic matter. The living roots of these plants are also essential in supporting the belowground communities of microbes.
- Stubble retention: Crop rotation and no-till management that retain crop residues near the surface of the soil, can increase the SOC through crop residue retention, which slows the decomposition of SOC in the top soil layers
- Livestock integration: Integrating livestock into farming systems can enhance carbon sequestration by humifying SOC and thereby stabilizing C which returns to the soil. Furthermore, managed grazing stimulates plant growth, promotes nutrient cycling, and increases organic matter deposition, leading to greater carbon storage in the soil
The potential for farmers to contribute towards carbon sequestration all depends on where they are on their journey of current farming practices or ideals towards change of practices. For conventional farmers it might be as simple as reducing tillage and stubble retention or for conservation farmers it might mean to include cover crops to their existing conservation tillage approach. Perhaps it might even be introducing livestock. It is important to remember that change starts small and doesn't occur overnight. The key is to implement change as soon as possible. The most important step towards carbon sequestration is by reducing the amount of residues or above-and below ground litter being removed from the farm. Furthermore the choice of crops, pastures or cover crops also determines the amount and rate of carbon sequestration. Maize for example produces high amounts of biomass (crop litter) which in turn can add more carbon to the soil through above ground residues over a shorter period. Legumes on the other hand are known to increase soil carbon content more rapidly due to below-ground inputs. For this reason choice of cropping system, current soil carbon status, correct cover crop mixes and and rotations all need to become part of farmers planning with their advisors. Contact Orizon to find out more on how carbon sequestration wont just benefit your production system, but also your pocket through carbon credits.