Carbon farming

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Carbon farming is a name for a variety of agricultural methods aimed at sequestering atmospheric carbon into the soil and in crop roots, wood and leaves. Increasing a soil's organic matter content can aid plant growth, increase total carbon content, improve soil water retention capacity[1] and reduce fertilizer use[2].[3] As of 2016, variants of carbon farming reached hundreds of millions of hectares globally, of the nearly 5 billion hectares (1.2×1010 acres) of world farmland.[4] Soils can contain up to five per cent carbon by weight, including decomposing plant and animal matter and biochar.[5]

Potential sequestration alternatives to carbon farming include scrubbing CO2 from the air with machines (direct air capture); fertilizing the oceans to prompt algal blooms that after death carry carbon to the sea bottom[6];storing the carbon dioxide emitted by electricity generation; and crushing and spreading types of rock such as basalt that absorb atmospheric carbon.[3] Land management techniques that can be combined with farming include planting/restoring forests, burying biochar produced by anaerobically converted biomass and restoring wetlands. (Coal beds are the remains of marshes and peatlands.)[7]

Techniques

Soil carbon

SRS1000 being used to measure soil respiration in the field.

In part, soil carbon is thought to accumulate when decaying organic matter was physically mixed with soil.[8] More recently, the role of living plants has been emphasized. Small roots die and decay while the plant is alive, depositing carbon below the surface.[9] Further, as plants grow, their roots inject carbon into the soil, feeding mycorrhiza. An estimated 12,000 miles of their hyphae live in every square meter of high quality healthy soil.[3]

Bamboo

Bamboo-houseplant

Although a bamboo forest stores less total carbon than a mature forest of trees, a bamboo plantation sequesters carbon at a much faster rate than a mature forest or a tree plantation. Therefore the farming of bamboo timber may have significant carbon sequestration potential.[10][11][12]

Seaweed farming

Women Working Seaweed

Large-scale seaweed farming (called "ocean afforestation") could sequester huge amounts of carbon.[13] Afforesting just 9% of the ocean could sequester 53 billion tons of carbon dioxide annually. The IPCC Special Report on the Ocean and Cryosphere in a Changing Climate recommends "further research attention" as a mitigation tactic.[14]

Wetland restoration

An example of a healthy wetland ecosystem.

Wetlands are created when water overflows into heavily vegetated soil causing plants to adapt to a flooded ecosystem.[15] Wetlands can occur in three different regions.[16] Marine wetlands are found in shallow coastal areas, tidal wetlands are also coastal but are found farther inland, and non-tidal wetlands are found inland and have no affects from tides. Wetland soil is an important carbon sink; 14.5% of the world's soil carbon is found in wetlands, while only 5.5% of the world's land is composed of wetlands.[17] Not only are wetlands a great carbon sink, they have many other benefits like collecting floodwater, filtering air and water pollutants, and creating a home for numerous birds, fish, insects, and plants.[16]

Climate change could alter soil carbon storage changing it from a sink to a source.[18] With rising temperatures comes an increase in greenhouse gasses from wetlands especially locations with permafrost. When this permafrost melts it increases the available oxygen and water in the soil.[18] Because of this, bacteria in the soil would create large amounts of carbon dioxide and methane to be released into the atmosphere.[18]

Peatlands hold approximately 30 percent of the carbon in our ecosystem.[19] When wetlands are drained for agriculture and urbanization, because peatlands are so vast, large quantities of carbon decompose and emit CO2 into the atmosphere.[19] The loss of one peatland could potentially produce more carbon than 175–500 years of methane emissions.[18]

While the link between climate change and wetlands is still not fully known, it will be soon determined through future removal of wetlands.[18] It is also not clear how restored wetlands manage carbon while still being a contributing source of methane. However, preserving these areas would help prevent further release of carbon into the atmosphere.[19]

Agriculture

Compared to natural vegetation, cropland soils are depleted in soil organic carbon (SOC). 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%.[20] This loss 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.[21] 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.[22] Perennial crops have a larger below ground biomass fraction, which increases the SOC content.[20] 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.[23]

Modification of agricultural practices is a recognized method of carbon sequestration as soil can act as an effective carbon sink offsetting as much as 20% of 2010 carbon dioxide emissions annually.[24] Organic farming and earthworms may be able to more than offset the annual carbon excess of 4 Gt/year.[25]

Carbon emission reduction methods in agriculture can be grouped into two categories: reducing and/or displacing emissions and enhancing carbon sequestration. Reductions include increasing the efficiency of farm operations (e.g. more fuel-efficient equipment) and interrupting the natural carbon cycle. Effective techniques (such as the elimination of stubble burning) can negatively impact other environmental concerns (increased herbicide use to control weeds not destroyed by burning).

Deep soil

About half of soil carbon is found within deep soils.[26] About 90% of this is stabilized by mineral-organic associations.[27]

At least thirty-two Natural Resource Conservation Service (NRCS) practices improve soil health and sequester carbon, along with important co-benefits: increased water retention, hydrological function, biodiversity and resilience. Approved practices may make farmers eligible for federal funds. Not all carbon farming techniques have been recommended.[3] Carbon farming may consider related issues such as groundwater and surface water degradation.[1]

Biochar/terra preta

Main article: Biochar

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 (0.21 gigatonnes) 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.[7]

Tilling

Carbon farming minimizes disruption to soils over the planting/growing/harvest cycle.Tillage is avoided using seed drills or similar techniques[28]. Livestock can trample and/or eat the remains of a harvested field.[2] 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.[5]

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 (as often as daily) the paddocks can rest/recover between grazing periods. This pattern produces stable grasslands with significant fodder.[2] Annual grasses have shallower roots and die once they’re 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.[3]

Silvopasture

Silvopasture involves grazing livestock under tree cover, with trees separated enough to allow adequate sunlight to nourish the grass.[2] For example, a farm in Mexico planted native trees on a paddock spanning 22 hectares (54 acres). This evolved into a successful organic dairy. The operation became a subsistence farm, earning income from consulting/training others rather than from crop production.[4]

Organic mulch

Mulching 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.[2]

Compost

Main article: Compost

Compost sequesters carbon in a stable (not easily accessed) form. Carbon farmers spread it over the soil surface without tilling.[2] 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%. CH
4 and N
2O and emissions did not increase significantly. Methane fluxes were negligible. Soil N
2O emissions from temperate grasslands amended with chemical fertilizers and manures were orders of magnitude higher.[29] 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.[3]

Cover crops

Cover crops 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.

Hybrids

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.[12]

History

Australia

In 2011 Australia started a cap-and-trade program. Farmers who sequester carbon can sell carbon credits to companies in need of carbon offsets.[2] The country's Direct Action Plan states "The single largest opportunity for CO
2 emissions reduction in Australia is through bio-sequestration in general, and in particular, the replenishment of our soil carbons." In studies of test plots over 20 years showed increased microbial activity when farmers incorporated organic matter or reduced tillage. Soil carbon levels from 1990–2006 declined by 30% on average under continuous cropping. Incorporating organic matter alone was not enough to build soil carbon. Nitrogen, phosphorus and sulphur had to be added as well to do so.[12]

North America

By 2014 more than 75% of Canadian Prairies' cropland had adopted "conservation tillage" and more than 50% had adopted no till.[30] Twenty-five countries pledged to adopt the practice at the December 2015 Paris climate talks.[2] In California multiple Resource Conservation Districts (RCDs) support local partnerships to develop and implement carbon farming,[1] In 2015 the agency that administers California's carbon-credit exchange began granting credits to farmers who compost grazing lands.[2] In 2016 Chevrolet partnered with the US Department of Agriculture (USDA) to purchase 40,000 carbon credits from ranchers on 11,000 no-till acres. The transaction equates to removing 5,000 cars from the road and was the largest to date in the US.[2] In 2017 multiple US states passed legislation in support of carbon farming and soil health.[31]

  • California appropriated $7.5 million as part of its Healthy Soils Program. The objective is to demonstrate that "specific management practices sequester carbon, improve soil health and reduce atmospheric greenhouse gases." The program includes mulching, cover crops, composting, hedgerows and buffer strips.[31] Nearly half of California counties have farmers who are working on carbon-farming.[3]
  • Maryland's Healthy Soils Program supports research, education and technical assistance.[31]
  • Massachusetts funds education and training to support agriculture that regenerates soil health.[31]
  • Hawaii created the Carbon Farming Task Force to develop incentives to increase soil carbon content.[31] A 250-acre demonstration project attempted to produce biofuels from the pongamia tree. Pongamia adds nitrogen to the soil. Similarly, one ranch husbands 2,000 head of cattle on 4,000 acres, using rotational grazing to build soil, store carbon, restore hydrologic function and reduce runoff.[32]

Other states are considering similar programs.[31]

Four per 1,000

The largest international effort to promote carbon farming is “four per 1,000”, led by France. Its goal is to increase soil carbon by 0.4 percent per year through agricultural and forestry changes.[3]

Challenges

Critics say that the related regenerative agriculture cannot be adopted enough to matter or that it could lower commodity prices. The impact of increased soil carbon on yield has yet to be settled.[citation needed]

Another criticism says that no-till practices may increase herbicide use, diminishing or eliminating carbon benefits.[3]

Composting is not an NRCS-approved technique and its impacts on native species and greenhouse emissions during production have not been fully resolved. Further, commercial compost supplies are too limited to cover large amounts of land.[3]

See also

References

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  6. ^ Ortega, Alejandra; Geraldi, N.R.; Alam, I.; Kamau, A.A.; Acinas, S.; Logares, R.; Gasol, J.; Massana, R.; Krause-Jensen, D.; Duarte, C. (2019). "Important contribution of macroalgae to oceanic carbon sequestration". Nature Geoscience. 12: 748–754. doi:10.1038/s41561-019-0421-8. hdl:10754/656768.
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  31. ^ a b c d e f "6 States Tapping Into the Benefits of Carbon Farming". EcoWatch. Center For Food Safety. 2017-07-12. Retrieved 2018-04-27.
  32. ^ Swaffer, Miriam (2017-07-11). "Turning dirt into climate goals via carbon farming". GreenBiz. Retrieved 2018-04-27.

External links

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