Carbon farming

• 1. Carbon Farming

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  Carbon farming begins with a measurement that surprises most people. When forests, grasslands, or savannas are converted to cropland, the soil's organic carbon content typically falls by about 30-40%. Carbon farming is the collective name for agricultural and forestry methods designed to reverse that depletion. Its technical term is carbon sequestration. The overall goal is a net loss of carbon from the atmosphere. Farmers pursue it by increasing the rate at which carbon is drawn into soil and plant material. As of 2016, variants of carbon farming were practiced on hundreds of millions of hectares globally. Improving soil water retention and reducing fertilizer use are two documented side effects. They make the practice attractive to farmers beyond its direct climate value.

• 2. The Carbon Below

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  Globally, soils are estimated to hold more than 8,580 gigatons of organic carbon. That is roughly ten times the carbon circulating in the atmosphere, and far more than all vegetation holds. Soils can contain up to 5% carbon by weight, drawn from decomposing plant and animal matter as well as from biochar.

  Small roots die and decay while the plant above them is still alive, depositing carbon beneath the surface without any human intervention. Living plants also release carbon into the soil as they grow, a process researchers have increasingly recognized as significant. About half of all soil carbon is found deep below the surface. Roughly 90% of that deep carbon is held in place through associations between minerals and organic matter.

  Perennial crops work with this mechanism. Their below-ground biomass fraction is larger than that of annual crops. More carbon is deposited below the surface and persists there even after harvest. On degraded croplands, rebuilding the soil's carbon pool by a single ton per hectare can lift wheat yields by 20-40 kilograms per hectare. Maize responds with gains of 10-20 kilograms per hectare, and cowpeas with around half a kilogram.

• 3. Methods on the Ground

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  Ruminants like cows and sheep are net producers of carbon. They release not only carbon dioxide but also methane, generated by the microbes in their digestive systems. Rotational grazing turns this into a partial management tool. By moving herds through multiple paddocks, sometimes as often as daily, farmers give each paddock time to recover. Annual grasses give way over time to perennials with deeper roots, which can recover after grazing rather than dying off. Allowing animals to range freely over a large area for an extended period can destroy the grassland instead.

  Silvopasture takes a different approach. Livestock graze under tree cover, with trees spaced to allow adequate sunlight to reach the grass. One farm in Mexico planted native trees across a 22-hectare paddock and eventually became a successful organic dairy. Some researchers dispute the claimed benefits of silvopasture. Several peer-reviewed studies have found that excluding livestock completely from semi-arid grasslands produces greater recovery of vegetation and soil carbon.

  Plowing splits soil aggregates and lets microorganisms consume the organic compounds inside. The initial burst of microbial activity releases nutrients and temporarily boosts yields. Over time, the disrupted soil structure loses its ability to hold water and resist erosion, and yields fall. Carbon farming addresses this by replacing the plow with seed drills and letting livestock trample the remains of harvested fields.

  A 2013 study found that a single compost application significantly and durably increased grassland carbon storage by 25-70%. At a drier valley test site, forage production rose by 78%. A wetter coastal site averaged a 42% gain in grassland productivity. Methane fluxes at both sites were negligible. Another study found that half an inch of commercial compost caused grasslands to absorb carbon at nearly 1.5 tons per acre annually. As of 2018, that result had not been replicated.

  Biochar, produced by burning organic material without oxygen, sequesters roughly 50% of the carbon in the original biomass when mixed into soil. If slash-and-burn land clearing were replaced globally by slash-and-char, up to 12% of annual carbon emissions from land use change could be offset. That represents roughly 0.21 gigatonnes per year. Biofuel production using modern biomass can generate biochar as a by-product through pyrolysis. That process sequesters 30.6 kilograms of carbon for each gigajoule of energy produced.

• 4. The Forest Option

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  Forestry and agriculture together account for roughly a third of global greenhouse gas emissions. Carbon farming in forested land includes both reforestation, restoring trees to once-deforested areas, and afforestation, planting forests where none historically existed. Most reforestation for carbon farming will occur in small patches, with individual landowners planting in exchange for benefits from carbon credit programs.

  Biodiversity, often assumed to be a side benefit of forest restoration, can directly accelerate carbon sequestration in forest ecosystems. Not all forests store carbon at the same rate. The amount depends on forest age, type, management practices, and local climate. In forest ecosystems, treating biodiversity as a tool rather than a bonus can measurably improve outcomes.

  A bamboo plantation sequesters carbon faster than a mature forest or conventional tree plantation, provided it is actively managed and selectively harvested. Among fast-growing tree species, bamboo's edge in carbon sequestration holds only when harvesting is selective. More than half of bamboo's total carbon sequestration ends up stored underground. Rhizomes and roots remain in the soil after above-ground material is cut, preserving that carbon long-term. A bamboo forest stores less total carbon than most mature forests. It can, however, match what is stored in rubber plantations and tree orchards, and it exceeds the totals held in palm oil plantations, grasslands, and shrublands. Bamboo can be planted on sub-optimal land unsuitable for other crops. Farmers who do so can earn carbon credits through emissions trading on otherwise uncultivated land. When harvested bamboo is turned into durable products, the carbon remains locked in the material rather than returning to the atmosphere through decomposition.

• 5. Costs and Country Programs

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  Sequestering a tonne of carbon dioxide through carbon farming costs between US$3 and US$130, depending on the region. That range reflects differences in soil quality, transaction costs, and externalities including environmental damage and leakage. Reducing atmospheric carbon dioxide is a long-term concern. Farmers can be reluctant to adopt expensive techniques when there is no clear benefit to their crop, soil, or income. Governments have responded with subsidies, credit systems, and mandates.

  Australia launched a cap-and-trade program in 2011, allowing farmers to sell carbon credits to companies that need to offset their emissions. The country's Direct Action Plan described bio-sequestration as the single largest opportunity for emissions reduction in Australia. Studies of test plots over 20 years showed increased microbial activity when farmers incorporated organic matter or reduced tillage. Between 1990 and 2006, soil carbon levels under continuous cropping declined by 30% on average. Incorporating organic matter alone was not enough; nitrogen, phosphorus, and sulphur also had to be added.

  By 2014, more than 75% of Canadian Prairies cropland had adopted conservation tillage. More than 50% practiced no-till. Canada later committed CAD$885 million for climate solutions in agriculture between 2021 and 2031. That sum is part of a larger CAD$2 billion Natural Climate Solutions Fund. Twenty-five countries pledged to adopt conservation tillage at the December 2015 Paris climate talks.

  France leads the largest international carbon farming effort, called "four per 1,000." Its goal is to increase soil carbon globally by 0.4% per year through agricultural and forestry changes. In 2016, France capped the share of energy crops usable for biofuel production, limiting competition with food crops. Cover crops were exempted from that cap, creating a financial incentive for their adoption.

  In the French-speaking part of Switzerland, a public-private partnership called Agroimpact pays farmers based on the carbon dioxide emissions they avoid. Stakeholders in the agri-food sector fund the premiums. Each premium corresponds to a specific quantity of low-carbon products, which the farmer must commit to purchasing from the same supplier.

  In 2016, Chevrolet partnered with the US Department of Agriculture to purchase 40,000 carbon credits from ranchers on 11,000 no-till acres. The transaction was equivalent to removing 5,000 cars from the road and was the largest carbon-credit deal in US history at that point. California appropriated $7.5 million for its Healthy Soils Program, supporting composting, mulching, cover crops, hedgerows, and buffer strips. In Hawaii, a 250-acre demonstration project attempted to produce biofuels from the pongamia tree, which also adds nitrogen to the soil.

• 6. Limits and Trade-offs

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  Carbon farming can potentially offset as much as 20% of 2010 global carbon dioxide emissions annually. That ceiling comes with conditions. After several decades of sequestration, soils typically become saturated and stop absorbing carbon. There is a global limit to the total amount of carbon soil can hold. Disrupting the soil before that point, through intensive tillage or land use change, releases the stored carbon. The soil becomes a net source of greenhouse gases rather than a sink.

  No-till farming carries its own complication. Critics note that avoiding the plow often increases herbicide use, which can diminish or eliminate the climate benefit. Composting faces a separate constraint: commercial compost supplies are limited and cannot realistically cover large areas. Composting is not an approved technique under the US Natural Resources Conservation Service. Its effects on native species and greenhouse gas emissions during production have not been fully resolved.

  Ecosystem services such as water filtration, habitat, and soil health are often ignored in project planning. Unlike carbon sequestration, they are not a global commodity that can be traded, so they rarely carry financial weight. When policy focuses narrowly on carbon sequestration, farmers may inadvertently clear land and plant monocultures. Species diversity becomes a casualty when it is not explicitly written into a project's goals.

  Critics of regenerative agriculture argue it cannot be adopted fast enough to matter at scale. Others warn that widespread adoption could suppress commodity prices. The effect of increased soil carbon on crop yields remains scientifically unsettled. Maryland's Healthy Soils Program supports research, education, and technical assistance for carbon farming. Measuring what any individual farm actually sequesters remains one of the field's most basic unresolved challenges.