Carbon farming is a name for a variety of agricultural methods aimed at sequestering atmospheric carbon into the soil. Increasing the carbon content of soil can aid plant growth, increase soil organic matter (improving agricultural yield), improve soil water retention capacity, and reduce fertilizer use (and the accompanying emissions of greenhouse gas nitrous oxide (N
2O). As of 2016, variants of carbon farming affected hundreds of millions of hectares globally, of the nearly 5,000,000,000 hectares (1.2×1010 acres) of world farmland. Soils can contain up to five per cent carbon by weight, including decomposing plant and animal matter and biochar.
Potential sequestration alternatives to carbon farming include scrubbing the air with machines; fertilizing the oceans to prompt algal blooms that after death carry carbon to the sea bottom; storing the carbon dioxide emitted by electricity generation; and crushing and spreading types of rock such as basalt that absorb atmospheric carbon. 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.)
In 2011 Australia started a cap-and-trade program. Farmers who sequester carbon can sell carbon credits to companies in need of carbon offsets. 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. However, soil carbon levels from 1990-2006 declined by 30% on average under continuous cropping. Incorporating organic matter alone was not enough to build carbon. Nitrogen, phosphorus and sulphur had to be added as well to build soil carbon. By 2014 more than 75% of the Canadian Prairies' cropland had adopted "conservation tillage" and more than 50% had adopted no till. Twenty-five countries pledged to adopt the practice at the December 2015 Paris climate talks. In California multiple Resource Conservation Districts (RCDs) support local partnerships to develop and implement carbon farming, In 2015 the agency that administers California's carbon-credit exchange began granting credits to farmers who compost grazing lands. 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. In 2017 multiple US states passed legislation in support of carbon farming and soil health.
- 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. Nearly half of California counties have farmers who are working on carbon-farming.
- Maryland's Healthy Soils Program supports research, education and technical assistance.
- Massachusetts funds education and training to support agriculture that regenerates soil health.
- Hawaii created the Carbon Farming Task Force to develop incentives to increase soil carbon content. A 250-acre demonstration project attempted to produce biofuels from the pongamia tree. Pongamia adds nitrogen back into the soil. Similarly, one ranch wrangles 2,000 head of cattle on 4,000 acres, using regenerative grazing to build soil, store carbon, restore hydrologic function and reduce runoff.
Other states are considering similar programs.
Four per 1,000Edit
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.
Traditionally, soil carbon was thought to accumulate when decaying organic matter was physically mixed with soil. 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. Further, as plants grow, their roots inject some carbon into the soil, feeding mycorrhiza. An estimated 12,000 miles of their hyphae live in every square meter of healthy soil.
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 (e.g., composting) have been recommended. Carbon farming may consider related issues such as groundwater and surface water degradation.
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.
Grazing livestock sequesters carbon when the animal eats the grass, causing its roots to release carbon into the soil. However, these animals also produce significant methane, potentially offsetting the benefits of sequestration. Livestock is regularly rotated through multiple paddocks (as often as daily) allowing the paddocks to rest/recover between grazing periods. This pattern produces stable grasslands with significant fodder. Annual grasses have shallower roots and die once they’re grazed. Rotational grazing led to the replace of the annuals perennials with deeper roots, which can recover after limited 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 reach the ground. For example, a farm in Mexico planted native trees on 22 hectares (54 acres) grazing cattle on the paddock. This evolved into a successful organic dairy. The operation became a subsistence farm, earning income from consulting/training others rather than from crop production.
Compost sequesters carbon in a stable (not easily accessed) form. Carbon farmers dust 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 the drier valley site, while the 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. Another study found that grasslands treated with a single treatment .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.
With row crops such as corn and wheat, fast-growing ground cover is grown between the stalks (e.g., clover or vetch). They protect the soil from carbon loss through the winter and may be planted together with cash crops to compensate for carbon lost when those crops are harvested. Forage crops such as grasses, clovers and alfalfa develop extensive root systems that can become soil organic matter. Crops with poor root systems (corn, soybeans) do not increase organic matter in the soil.
Perennial crops offer the highest potential to sequester carbon, when grown in multilayered systems. These systems can be challenging and require farmers to cultivate potentially unfamiliar crops. One system uses perennial staple crops that grow on trees that are analogs to maize and beans, or vines, palms and herbaceous perennials.
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.
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 resolved. Another criticism says that no-till practices may increase herbicide use, diminishing or eliminating the benefits. Composting is not an NRCS-approved technique and its impacts on native species and the greenhouse emissions during production have not been fully resolved. Further, commercial compost supplies are too limited to cover large amounts of land.
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