The debate on the EU agricultural policy in recent months has involved greenhouse gases (the infamous GHG, i.e. Green House Gas) and the contribution in terms of Carbon Sink (carbon storage) that agriculture could provide. Carbon sink is an ecological service that good agriculture offers to society: it stores CO2 in the soil in the form of stable humus, removing it from the atmosphere. It is talked about because there is the hope that the farmer can find a remuneration for this service or at least a further justification in defense of the subsidies received.
There is, however, some confusion on this subject that comes from a static vision of the “carbon” system rather than the dynamic and cyclical one that it actually is. Clarifying this issue provides the key to evaluating not only the political proposal, but also those business proposals that aim to involve the farmer as an active economic subject in the Carbon market. In other words, it is necessary to understand if, or in what situations, agricultural and forestry activities remove carbon from the atmosphere, if this function can be quantified and therefore verified and attested, and finally, if all this is true, if it can be evaluated and sold.
Through photosynthesis, vegetation removes carbon dioxide (CO2) from the atmosphere and, through a chemical reduction process, releases part of the oxygen (O2) into the air, replacing it with carbon (C) and hydrogen (H) atoms to form a molecule of glucose, a sugar with six carbon, twelve hydrogen and six oxygen atoms. Photosynthesis produces glucose as long as the chloroplast receives light, water, and other useful elements in the expected quantities. All these glucose molecules inside the plant are used, depending on the momentary needs, as a source of energy to sustain the metabolism of the cells, or as bricks to build the different parts of the plant. In this case, glucose, through chemical processes including condensation, forms long chains of cellulose, hemicellulose and lignin. These substances are important carbon sinks because they are not easily degradable; in particular lignin, one of the main constituents of wood, is “digested” only by some species of fungi. When a plant dies, the vegetal matter is degraded at different rates: simple sugars, proteins and fats are degraded quickly, whereas the above mentioned substances are degraded more slowly. Furthermore, the degradation of the latter is not perfect and remains, especially in the case of lignin, some undigestible compound which constitutes the most stable part of the humus in the soil. Therefore a corn field is a temporary carbon reservoir and only a small part of the vegetable matter that constituted it remains in the soil for a longer time. Conversely, in a permanent meadow or in a forest a good quantity of carbon remains “trapped” in the living plants while in the soil, undisturbed, large amounts of organic matter accumulate, contributing significantly to the Carbon Sink. In the forest, the amount of CO2 sequestered is greater than in the meadow because it is composed of a greater mass of vegetation. In fact, both the meadow and the forest behave like a cornfield when the soil is cleared and used for annual crops.
Therefore, from all of the above, we deduce that there are crops (and cultivation practices) that are more virtuous than others in terms of CO2 sequestration, but if I change management, I (more or less) quickly put the accumulated carbon back into the atmosphere. This aspect explains why it may be unprofitable to invest public money in agricultural systems capable of sequestering greater quantities of carbon if, once the subsidy ends, the farmer is left free to re-adopt the previous cultivation models. There is also another consideration: storage capacity is not infinite. For example, a forest continues to accumulate new CO2 until it reaches a condition that we can define as maturity, in which the amount of carbon sequestered becomes equal to that released. Forests, like all surfaces on which plants live, accept new CO2 until the “reservoir” is filled, that is, until a certain climax is reached, and remain in this equilibrium as long as there are no perturbations that alter this state.
From this we understand that on the carbon market, an agricultural or forest environment can offer a carbon sequestration that starts from the current situation and ends when the climax is reached, but the contractor will ask for it to be maintained indefinitely. The Carbon market in agriculture is therefore not easy to implement.
Lorenzo Benvenuti
