Carbon farming


Carbon farming is a set of agricultural methods that aim to store carbon in the soil and biomass. The technical term for this is carbon sequestration. The overall goal of carbon farming is to create a net loss of carbon from the atmosphere. This is done by increasing the rate at which carbon is sequestered into soil and plant material. The increase of biomass from roots and the soil's microbiome leads to an increase in the organic matter content of the soil. Increasing organic matter content in soils aids plant growth, improves soil water retention capacity and reduces fertilizer use. Sustainable forest management is another tool that is used in carbon farming. Carbon farming is one component of climate-smart agriculture. It is also one way to remove carbon dioxide from the atmosphere.
Agricultural methods for carbon farming include adjusting how tillage and livestock grazing is done, using organic mulch or compost, working with biochar and terra preta, and changing the crop types. Methods used in forestry include reforestation and bamboo farming. As of 2016, variants of carbon farming reached hundreds of millions of hectares globally, of the nearly of world farmland.
Carbon farming tends to be more expensive than conventional agricultural practices. Depending on the region, carbon farmings costs US$3-130 per tonne of carbon dioxide sequestered. Some countries provide subsidies to farmers to use carbon farming methods. While the implementation of carbon farming methods can reduce/sequester emissions, it is important to also consider the effects of land use changes with respect to the conversion of forests to agricultural production.

Aims

The overall aim of carbon farming is to store carbon in the soil, crop roots, wood and leaves. It is one of several methods for carbon sequestration. It can be achieved by modification of agricultural practices because soil can act as an effective carbon sink and thus offset carbon dioxide emissions.
Agricultural sequestration practices may have positive effects on soil, air, and water quality, be beneficial to wildlife, and expand food production. On degraded croplands, an increase of one ton of soil carbon pool may increase crop yield by 20 to 40 kilograms per hectare of wheat, 10 to 20 kg/ha for maize, and 0.5 to 1 kg/ha for cowpeas.

Mechanism

Compared to natural vegetation, cropland soils are depleted in soil organic carbon. When a soil is converted from natural land or semi natural land, such as forests, woodlands, grasslands, steppes and savannas, the SOC content in the soil reduces by about 30–40%. The loss of carbon through agricultural practices can eventually lead to the loss of soil suitable for agriculture. The carbon loss from the soil is due to the removal of plant material containing carbon, via harvesting. When land use changes, soil carbon either increases or decreases. This change continues until the soil reaches a new equilibrium. Deviations from this equilibrium can also be affected by varying climate. The decrease can be counteracted by increasing carbon input. This can be done via several strategies, e.g. leaving harvest residues on the field, using manure or rotating perennial crops. Perennial crops have a larger below ground biomass fraction, which increases the SOC content. Globally, soils are estimated to contain >8,580 gigatons of organic carbon, about ten times the amount in the atmosphere and much more than in vegetation.
In part, soil carbon is thought to accumulate when decaying organic matter was physically mixed with soil. Small roots die and decay while the plant is alive, depositing carbon below the surface. More recently, the role of living plants has been emphasized where carbon is released as plants grow. Soils can contain up to 5% carbon by weight, including decomposing plant and animal matter and biochar.
About half of soil carbon is found within deep soils. About 90% of this is stabilized by mineral–organic associations.

Scale

Carbon farming can offset as much as 20% of 2010 carbon dioxide emissions annually. As of 2016, variants of carbon farming reached hundreds of millions of hectares globally, of the nearly of world farmland.
However, the effects of soil sequestration can be reversed. If the soil is disrupted or intensive tillage practices are used, the soil becomes a net source of greenhouse gases. Typically after several decades of sequestration, the soil becomes saturated and ceases to absorb carbon. This implies that there is a global limit to the amount of carbon that soil can hold.

Methods used in agriculture

All crops absorb during growth and release it after harvest. The goal of agricultural carbon removal is to use the crop and its relation to the carbon cycle to permanently sequester carbon within the soil. This is done by selecting farming methods that return biomass to the soil and enhance the conditions in which the carbon within the plants will be reduced to its elemental nature and stored in a stable state. Methods for accomplishing this include:
  • Use cover crops such as grasses and weeds as a temporary cover between planting seasons
  • Concentrate livestock in small paddocks for days at a time so they graze lightly but evenly. This encourages roots to grow deeper into the soil. Stock also till the soil with their hooves, grinding old grass and manures into the soil.
  • Cover bare paddocks with hay or dead vegetation. This protects soil from the sun and allows the soil to hold more water and be more attractive to carbon-capturing microbes.
  • Restore degraded, marginal, and abandoned land, which slows carbon release while returning the land to agriculture or other use. Degraded land with low soil carbon pool has particularly high potential to store soil carbon, which can be farther enhanced by proper selection of vegetation.

    Adjusting livestock grazing

Livestock, like all animals, are net producers of carbon. Ruminants like cows and sheep produce not only CO2, but also methane due to the microbes residing in their digestive system. A small amount of carbon may be sequestered in grassland soils through root exudates and manure. By regularly rotating the herd through multiple paddocks the paddocks can rest/recover between grazing periods. This pattern produces stable grasslands with significant fodder. Annual grasses have shallower roots and die once they are grazed. Rotational grazing leads to the replacement of annuals by perennials with deeper roots, which can recover after grazing. By contrast, allowing animals to range over a large area for an extended period can destroy the grassland.
Silvopasture involves grazing livestock under tree cover, with trees separated enough to allow adequate sunlight to nourish the grass. For example, a farm in Mexico planted native trees on a paddock spanning. This evolved into a successful organic dairy. The operation became a subsistence farm, earning income from consulting/training others rather than from crop production.
However, many researchers have argued the approach is unable to provide the benefits claimed. Moreover, several peer-reviewed studies have found that excluding livestock completely from semi-arid grasslands can lead to significant recovery of vegetation and soil carbon sequestration.

Adjusting tillage

Carbon farming minimizes disruption to soils over the planting/growing/harvest cycle. Tillage is avoided using seed drills or similar techniques. Livestock can trample and/or eat the remains of a harvested field. The reduction or complete halt of tilling will create an increase in the soil carbon concentrations of topsoil allowing for regeneration of the soil. Plowing splits soil aggregates and allows microorganisms to consume their organic compounds. The increased microbial activity releases nutrients, initially boosting yield. Thereafter the loss of structure reduces soil's ability to hold water and resist erosion, thereby reducing yield.

Using organic mulch and compost

ing covers the soil around plants with a mulch of wood chips or straw. Alternatively, crop residue can be left in place to enter the soil as it decomposes.
Compost sequesters carbon in a stable form. Carbon farmers spread it over the soil surface without tilling. A 2013 study found that a single compost application significantly and durably increased grassland carbon storage by 25–70%. The continuation sequestration likely came from increased water-holding and "fertilization" by compost decomposition. Both factors support increased productivity. Both tested sites showed large increases in grassland productivity: a forage increase of 78% in a drier valley site, while a wetter coastal site averaged an increase of 42%. and and emissions did not increase significantly. Methane fluxes were negligible. Soil emissions from temperate grasslands amended with chemical fertilizers and manures were orders of magnitude higher. Another study found that grasslands treated with.5" of commercial compost began absorbing carbon at an annual rate of nearly 1.5 tons/acre and continued to do so in subsequent years. As of 2018, this study had not been replicated.

Working with biochar and terra preta

Mixing anaerobically burned biochar into soil sequesters approximately 50% of the carbon in the biomass. Globally up to 12% of the anthropogenic carbon emissions from land use change can be off-set annually in soil, if slash-and-burn is replaced by slash-and-char. Agriculture and forestry wastes could add some 0.16 gigatonnes/year. Biofuel production using modern biomass can produce a bio-char by-product through pyrolysis sequestering 30.6 kg for each gigajoule of energy produced. Soil-sequestered carbon is easily and verifiably measured.

Adjusting crop type

are fast-growing species planted to protect soils from wind and water erosion during the off-growing season. The cover crop may be incorporated into the soil to increase soil organic matter. Legume cover crops can also produce a small amount of nitrogen. The carbon content of a soil should not be increased without also ensuring that the relative amount of nitrogen also increases to maintain a healthy soil ecosystem.
Perennial crops offer potential to sequester carbon when grown in multilayered systems. One system uses perennial staple crops that grow on trees that are analogs to maize and beans, or vines, palms and herbaceous perennials.