What is open burning?
Open burning in agriculture is the process of using fire to burn materials such as crop residues, releasing smoke directly into the atmosphere without going through a stack or chimney. According to IPCC guidelines, burning savanna and crop residues is considered under agriculture within the agriculture, forestry, and other land use (AFOLU) sector. In contrast, burning biomass is considered under forestry and other land use (FOLU).
The burning of savanna, crop residues, and biomass produces emissions of methane (CH4) and nitrous oxide (N2O), and, only in the case of organic soils (i.e., histosols), also carbon dioxide (CO2). Five land cover types are considered under emissions from the burning of savanna biomass vegetation: savanna, woody savanna, open shrubland, closed shrubland, and grassland. Crop residue burning emissions are those from residues burnt in agricultural fields. Emissions from burning biomass can be categorized as humid tropical forests, other forests, and organic soils (FAO, 2014).
Table 1. Global emissions from the burning of savanna, crop residues, and biomass in 2017. Data from FAOSTAT. Mt = million tons.
|Burning type||Land-use Type||CO2e (Mt)||% of Category Emissions (%)||% of Total Emissions|
|Biomass||Humid tropical forest||119.6||9%|
Contributions to Emissions
In 2017, open burning of savanna, crop residues, and biomass accounted for 1683.7 Mt CO2e. Biomass burning far exceeded burning crop residues and savanna, making up 81.4% of total emissions from open burning, while burning savanna and crop residues make up 16.7% and 1.8% of associated emissions, respectively. See Table 1 for sub-category emissions within open burning.
Burning biomass is largely related to deforestation activities for livestock production and fires in temperate and boreal forests.
Open burning can also increase black carbon particulate matter (soot), directly contributing to warming by absorbing and scattering heat from the sun.
The products of complete combustion of organic matter are CO2 and water vapor. However, the combustion process is often incomplete, preventing some biomass carbon from being completely oxidized to CO2. Every biomass fire has four phases of combustion, flaming, pyrolysis, smoldering, and glowing. These processes coexist in fires, and the magnitude and duration of each phase depends on the kind of biomass and conditions during the fire. The heat from the flaming phase initiates pyrolysis that provides fuel gasses, including CH4. During the smoldering phase, where charcoal is produced, gaseous products can escape oxidation. As crop residues can contain high levels of nitrogen, burning such material can produce a relatively high percentage of N2O (van Amstel and Swart, 1994).
Factors affecting emissions from open burning
- Elemental composition and moisture content of the debris being burned.
- Weather conditions, such as wind, may affect one or more of the four phases of combustion (see Mechanisms above).
- Fuel loading, or how much debris material is burned per unit of land area.
- Burning strategies, such as headfire or backfire in the case of field crops. Headfires are started at the upwind side of a field and allowed to progress in the direction of the wind, whereas backfires are started at the downwind edge and forced to progress in the opposite direction of the wind.
- How the debris is arranged, such as in piles, rows, or spread out.
Mitigation strategy and options
Promote “no-burn” alternatives
- Retain crop residues on the field with a no-till practice or incorporate residues into the soil with low-till practices.
- Process residues to enable decomposition and soil incorporation.
- For crop residues that do not readily integrate into the soil, such as rice stubble, export residues for bioenergy, compost, or bedding for livestock.
- Promote forestry mulching to return nutrients and organic matter to the soil.
Avoid land conversion
- Reduce land expansion over natural woodlands and other sensitive areas (e.g., forests, shrublands, mangroves, grasslands, and peatlands).
- Use woodfuel from agroforestry systems to meet energy needs in a carbon-neutral way (see Case Study below).
- Reduce the demand for livestock commodities, the primary driver of deforestation.
- Conduct farmer outreach and education through study tours and field demonstrations on the negative impacts of burning, country- and crop-appropriate alternatives, and how to integrate these practices.
- Implement policies to prohibit burning, such as in India. When smoke affects areas of dense populations and neighboring countries or causes severe health problems, policies are often necessary.
The Quesungual system is an indigenous agroforestry system in Lempira, Honduras. This system provides an alternative land use practice for smallholder farmers as land becomes scarcer in this region due to population increases and land distribution inequality. The major production system of the region is subsistence agriculture (maize, beans, and sorghum). Maize is the first crop, intercropped with both sorghum and beans. Mineral fertilizer is expensive in the region and is only used when maize and sorghum are both grown first.
Here, smallholder farmers (1-3 ha) use a combination of naturally regenerated pruned trees and shrubs with more traditional agroforestry components, such as high-value timber and fruit trees. Before sowing, vegetation is cleared by hand rather than burned. In the dry season, the trees and shrubs are pollarded to eliminate the branches and provide light for the subsequent crop rotation. The pollarded material is used for mulching the soil, whereas the branches and trunks are used as firewood and as poles to build infrastructure. A typical smallholder plot consists of numerous pollarded trees and shrubs and about 15-20 large trees (timber and fruit species), promoting high species diversity in the system. Overall, the Quesungual system has proven to meet the community’s food and fuel requirements sustainably.