“In agriculture, the most cost‐effective mitigation options are cropland management, grazing land management, and restoration of organic soils” (IPCC Fifth Assessment Report 2014).
1) Agriculture C-Cycle
The balance of emissions in any particular agriculture system depends on the carbon stored in two major pools: soil organic matter and biomass. While sequestration and emissions depend on specific crop types, management plays a fundamental role, as it influences deeply the emissions profile of any agriculture system.
Soil organic matter contains about 60% of carbon, and it is a crucial factor in soil’s influence on the global carbon cycle. With around 1,500 Gigatons of carbon (GtC) stored as organic matter in the soil globally, soils are the second largest active reservoir of carbon after the oceans (40,000 GtC). There is more carbon stored in soil than in the atmosphere (760 GtC) and in vegetation (560 GtC) combined. The photosynthetic process allows the plants to absorb CO2 from the atmosphere and use it to build their roots, stems or leaves. Carbon is transferred into the soil through the release of organic compounds by plant roots or through the decomposition of plant material by soil organisms. The microbial breakdown of the organic matter releases the nutrients that plants use for their growth. During the decomposition process, a part of carbon is released as carbon dioxide (CO2) through soil respiration, while the remaining C is converted into organic compounds that can stay in soil for a period of time ranging from few years to centuries. The rapidity of decomposition depends on many factors including temperature and rainfall, the soil-water balance, and the composition of the organic material.
In general the processes leading to soil carbon losses occurs more rapidly and easily than the processes of storing carbon.
Source: EU 2011. Soil the Hidden Part of the Climate Cycle
Biomass is composed of both living plants and dead plant materials. Living plants are responsible for photosynthesis, i.e. the natural process by which CO2 is removed from the atmosphere. Respiration of living plants and the decay of dead plant materials are responsible for CO2 emissions. Unlike forests, biomass stocks in agriculture are usually very low, as the crops tend to grow and be harvested within very short time periods, often lower than 1 year.
Exceptions include woody perennial crops, such as olive trees, vineyards or fruit trees, and agri-forest systems.
Land management is the second largest contributor to CO2 emissions on terrestrial ecosystems. Agriculture is the only sector that has the ability to transform from a net emitter of CO2 to a net sequester of CO2 — there is no other human managed realm with this potential. Common agricultural practices, including the use of a tractor, tillage, over-grazing, using fossil fuel based fertilizers, pesticides and herbicides result in significant releases of CO2 and other greenhouse gases. Alternatively, a spectrum of activities can be promoted: from increasing efficiency of farming processes (to allow more agricultural output per unit of input); reducing or changing the amount of fertilizer or the types of fertilisers used; reducing or changing the tillage and irrigation practices; to soil carbon sequestration to allow the carbon to be stored for a long term in soils. Carbon farming involves implementing practices that are known to improve the rate at which CO2 is removed from the atmosphere and converted to plant material, to be finally sequestered as soil organic matter.
Mitigation and adaptation are different but complementary strategies. While mitigation practices are designed with the aim of reducing emissions from agricultural practices and increasing sequestration, adaptation practices are designed to allow agricultural systems to better adapt to a changing climate. Agriculture is one sector where many strategies serve both objectives.
Source: International Institute for Applied System Analysis (IIASA 2007)
Other Greenhouse Gases: Carbon dioxide (CO2), is the main greenhouse gas and it is involved in all processes leading to the accumulation of biomass and soil organic matter (sequestration) and to all processes leading to the loss of biomass and soil organic matter (emissions). In addition, other gases are also relevant, such as Nitrous oxide (N2O), a greenhouse gas 298 times more powerful than CO2 associated with the decomposition of organic matter and the use of nitrogen fertilisers, and Methane (CH4) a greenhouse gas about 25 times powerful than CO2 that is produced in soils under anaerobic conditions. N2O and CH4 are also emitted during burning of agricultural wastes or wildfires.
There are different suitable strategies to maximise soil carbon sequestration for different land uses.
On cropland, carbon stocks can be increased by:
improve biomass management and agro-forestry systems
return of biomass to the soil
eliminate or reduce tillage intensity
improve residue management
improve resource-use efficiency and productivity
improve water-use efficiency and management
On grassland, soil carbon stocks can be increased by:
improve biomass management and forestry-pasture systems
match grazing intensity with grassland productivity
improve grassland composition and species management
improve resource-use efficiency and productivity;
improve water-use efficiency and management
2) Emissions reporting
Emissions reporting deals with how to organise, calculate and report emissions and sequestration in any given sector or activity. The official guidance on how to perform this reporting is given by the Intergovernmental Panel on Climate Change, in its “Guidelines for National Greenhouse Gas Inventories” from 2006.
Emissions reporting in cropland and grassland is organised in carbon pools:
Living Biomass: the Carbon contained in all living plants, both woody and non-woody. Usually further divided in above-ground (wood, branches, stems, bark, leaves) and below-ground biomass (large roots, fine roots).
Dead Biomass: the Carbon contained in dead plant materials, be it entire dead plants, parts of plant materials (e.g. fallen leaves) or crop residues (halm, dead roots). It may also include the leftovers of management activities such as pruning, if these materials are left on the soil to decay. Usually further divided in dead wood (larger woody plant materials) and litter (all other dead biomass).
Soil: the Carbon contained in soil organic matter. Usually further divided in Mineral Soils and Organic Soils, depending on soil characteristics and organic matter content.
The emissions in any particular year result from the sum of all additions (i.e. new plant growth, new dead biomass, new inputs of soil organic matter) and subtractions (i.e. respiration, decay of dead biomass, decay of soil organic matter) to each of the pools.