Enriching Soil Carbon

Soil has four components: minerals, water, air, and organic materials—both nonliving and living. The former comes from dead plant, animal, and microbial matter while the living organic material is from flora and fauna of the soil biota, including living roots and microbes. Together, living and nonliving organic materials account for only 1-6 percent of the soil's volume, but they contribute much more to its

Farming and Land Use to Cool the Planet productivity. The organic materials retain air and water in the soil and provide nutrients that the plants and the soil fauna depend on for life. They are also reservoirs of carbon in the soil.10

In fact, soil is the third largest carbon pool on the planet. In the long term, agricultural practices that amend soil carbon from year to year through organic matter management rather than depleting it will provide productive soils that are rich in carbon and require fewer chemical inputs. New mapping tools, such as the 2008 Global Carbon Gap Map produced by the Food and Agriculture Organization, can identify areas where soil carbon storage is greatest and areas with the physical potential for billions of tons of additional carbon to be stored in degraded soils.11

Enhance soil nutrients through organic methods. Current use of inorganic fertilizers is estimated at 102 million tons worldwide, with use concentrated in industrial countries and in irrigated regions of developing nations. Soils with nitrogen fertilizers release nitrous oxide, a greenhouse gas that has about 300 times the warming capacity of carbon dioxide. Fertilized soils release more than 2 billion tons (in terms of carbon dioxide equivalent) of greenhouse gases every year. One promising strategy to reduce emissions is to adopt soil fertility management practices that increase soil organic matter and siphon carbon from the atmosphere.12

Numerous technologies can be used to substitute or minimize the need for inorganic fertilizers. Examples include composting, green manures, nitrogen-fixing cover crops and intercrops, and livestock manures. Even improved fertilizer application methods can reduce emissions. In one example of organic farming, a 23-year experiment by the Rodale Institute compared organic and conventional cropping systems in the United States and found that organic farming increased soil carbon by 15-28 percent and nitrogen content by 8-15 percent. The researchers concluded that if the 65 million hectares of corn and soybean grown in the United States were switched to organic farming, a quarter of a billion tons of carbon dioxide could be sequestered.13

The economics and productivity implications of these methods vary widely. In some very intensive, high-yield cropping systems, replacing some or all inorganic fertilizer may require methods that use more labor or require costlier inputs, but there is commonly scope for much more efficient use of fertilizer through better targeting and timing. In moderately intensive systems, the use of organic nutrient sources with small amounts of supplemental inorganic fertilizer can be quite competitive and attractive to farmers seeking to reduce cash costs.14

Improvements in organic technologies over the past few decades have led to comparable levels of productivity across a wide range of crops and farming systems. The question of whether organic farming can feed the world, as some claim, remains controversial. And more research is needed to understand the potentials and limitations of biologically based soil nutrient management systems across the range of soil types and climatic conditions. But there is little question that farmers in many production systems can already profitably maintain yields while using much less nitrogen fertilizer—and with major climate benefits.

Minimize soil tillage. Soil used to grow crops is commonly tilled to improve the conditions of the seed bed and to uproot weeds. But tilling turns the soil upside down, exposing anaerobic microbes to oxygen and suffocating aerobic microbes by working them under. This disturbance exposes nonliving organic matter to oxygen, releasing carbon dioxide. Keeping crop residues or mulch on

Farming and Land Use to Cool the Planet the surface helps soil retain moisture, prevents erosion, and returns carbon to the soil through decomposition. Hence practices that reduce tillage also generally reduce carbon emissions.15

A variety of conservation tillage practices accomplish this goal. In nonmechanized systems, farmers might use digging sticks to plant seeds and can manage weeds through mulch and hand-weeding. Special mechanized systems have been developed that drill the seed through the vegetative layer and use herbicides to manage weeds. Many farmers combine no-till with crop rotations and green manure crops. In Paraná, Brazil, farmers have developed organic management systems combined with no-till. No-till plots yielded a third more wheat and soybean than conventionally ploughed plots and reduced soil erosion by up to 90 percent. No-till has the additional benefit of reducing labor and fossil fuel use and enhancing soil biodiversity—all while cycling nutrients and storing carbon.16

In Paraná, Brazil, no-till plots yielded a third more wheat and soybean than conventionally ploughed plots and reduced soil erosion by up to 90 percent.

Worldwide, approximately 95 million hectares of cropland are under no-till management—a figure that is growing rapidly, particularly as rising fossil fuel prices increase the cost of tillage. The actual net impacts on greenhouse gases of reduced emissions and increased carbon storage from reduced tillage depend significantly on associated practices, such as the level of vegetative soil cover and the impact of tillage on crop root development, which depends on the specific crop and soil type. It is projected that the carbon storage benefits of no-till may plateau over the next 50 years, but this can be a cost-effective option to buy time while alternative energy systems develop.17

Incorporate biochar. Decomposition of plant matter is one way of enriching soil carbon if it takes place securely within the soil; decomposition on the surface, on the other hand, releases carbon into the atmosphere as carbon dioxide. In the humid tropics, for example, organic matter breaks down rapidly, reducing the carbon storage benefits of organic systems. Another option, recently discovered, is to incorporate biochar— burned biomass in a low-oxygen environment. This keeps carbon in soil longer and releases the nutrients slowly over a long period of time. While the burning does release some carbon dioxide, the remaining carbon-rich dark aromatic matter is highly stable in soil. Hence planting fast-growing trees in previously barren or degraded areas, converting them to biochar, and adding them to soil is a quick way of taking carbon from the atmosphere and turning it into an organic slow-release fertilizer that benefits both the plant and the soil fauna.

Interestingly, between 500 and 2,500 years ago Amerindian populations added incompletely burnt biomass to the soil. Today, Amazonian Dark Earths still retain high amounts of organic carbon and fertility in stark contrast to the low fertility of adjacent soils. There is a global production potential of 594 million tons of carbon dioxide equivalent in biochar per year, simply by using waste materials such as forest and milling residues, rice husks, groundnut shells, and urban waste. Far more could be generated by planting and converting trees. Initial analyses suggest that it could be quite economical to plant vegetation for biochar on idle and degraded lands, though not on more highly productive lands.18

Most crops respond with improved yields for biochar additions of up to 183 tons of carbon dioxide equivalent and can tolerate more

Farming and Land Use to Cool the Planet without declining productivity. Advocates calculate that if biochar additions were applied at this rate on just 10 percent of the world's cropland (about 150 million hectares), this method could store 29 billion tons of CO2-equivalent, offsetting nearly all the emissions from fossil fuel burning.19

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Negotiating Essentials

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