What is biomass carbon?
Biomass, the biological material of living organisms, is a mixture of organic molecules containing carbon (C), hydrogen, nitrogen, and small quantities of other elements. Biomass C from plants is of primary interest in the land-use sector. Carbon constitutes about half of the dry mass of plants. The C in plant biomass is derived from carbon dioxide (CO2) in the atmosphere, which plants take up through photosynthesis. Plant uptake of C is a major form of C sequestration in the land-use sector.
The IPCC identifies above-ground and below-ground biomass as two of the five agriculture, forestry, and other land-use (AFOLU) C pools: above-ground biomass, below-ground biomass, deadwood, litter, and soil organic matter. Above-ground biomass is the “biomass of living vegetation, both woody and herbaceous, above the soil including stems, stumps, branches, bark, seeds, and foliage” (IPCC 2006). Below-ground biomass is live plant roots. The IPCC suggests that roots of less than 2 mm diameter can be excluded because of the difficulty of distinguishing them from soil organic matter or litter.
Microorganisms and environmental conditions can decompose biomass C to become part of the soil organic matter pool. At the global level, 19% of the C in the earth’s biomass is stored in plants and 81% in the soil.
Biomass can also be sequestered by slowing microbial decomposition and preserving, processing, or otherwise using the biomass as a biomaterial, e.g., in construction. Conversion of biomass for bioenergy can substitute for fossil fuel use.
Contributions to emissions
Biomass C is considered a C sink. However, human-induced activities, primarily deforestation and other land-use changes, have greatly reduced the biomass C pool. Within AFOLU, net emissions and removals from forestry in 2017 equated to 1.07 GtCO2e, with the emissions largely coming from deforestation and forest degradation, primarily in tropical regions (Fig. 1). Global forests are estimated to sequester 638 GtC, which is more than the amount of C in the atmosphere and points to the importance of avoiding further deforestation. Overall, net land-use change emissions, which primarily reflect forest land changes, were estimated at 4.11 GtCO2 year-1 from 2000-2007.
Net emissions and removals from other significant biomass sources such as trees on agricultural land, shrubland, and grassland are less well monitored and documented. In 2000, agricultural land sequestered about 45.3 GtC of biomass, 75% of which was biomass from trees (34 GtC). Agricultural biomass increased by ~2 GtC between 2000 and 2010.
Photosynthesis is the main mechanism plants use to store C from the atmosphere; although, plant roots can also absorb some C from the soil. When a plant, such as a tree, dies or is felled, decomposition releases C stored in the biomass into the atmosphere. The biomass may also be exported for energy, fiber, or other uses. Microbial decomposition of organic C produces CO2 or methane (CH4) under the presence or absence of oxygen, respectively. See Soil Carbon for a more in-depth explanation regarding the mechanisms regulating microbial decomposition of organic C. Carbon is returned to the atmosphere as CO2 (CH4 and nitrous oxide may also be released) if biomass is burned.
Factors affecting biomass carbon
- Net primary productivity is the rate at which CO2 is stored in plants through photosynthesis. It represents the CO2 plants take in, less the CO2 respired back to the atmosphere.
- Water availability, sunlight, and temperature affect the rate of net primary productivity. Abundant water, sunlight, and higher temperatures generally lead to higher productivity. The greater amount of annual precipitation and soil-water holding capacity, the larger the leaf area index, in general. The amount of foliage a plant has is directly related to its productivity and the amount of C sequestered in its biomass.
- Soil nutrient concentrations. Plants tend to optimize C allocation to maximize C gains. Biomass C levels are highest in plants grown on fertilized land.
- Carbon sequestration in biomass tends to decrease over the lifespan of the plant, thought to be a result of a decline in net primary productivity and allocation of C to different tissues.
- Plant species. This affects the mass of the plant and the rate of C sequestration.
- Climatic conditions, such as mean annual temperature, precipitation, and length of the growing season, regulate plant productivity (see above) as well as the rate at which deadwood and litter are decomposed.
- Elevated atmospheric concentrations of CO2 make more C available for plants to uptake, with the resulting CO2 fertilization effect leading to increased photosynthesis rates if other growth factors are not limiting.
- Human activities including deforestation, afforestation, harvesting crop residues, preservation of timber, and other land-use changes.
Mitigation strategy and options
Increase or maintain carbon stocks of forests, agroforests, and trees on farms
- Avoid deforestation due to agriculture.
- Intensify agricultural production on croplands to reduce expansion into forests while monitoring and enforcing forest boundaries where needed.
- Relocate new agriculture investments (e.g., oil palm plantations) where there is a high risk of expansion into forest areas from non-forest areas.
- Reduce cutting of trees on farms or in agroforestry.
- Implement sustainable forest management and agroforestry policies and practices.
- Promote afforestation and reforestation (see Case Study below). Afforestation or reforestation should not be conducted where land-use change will lead to a net reduction in C (e.g., land with high initial soil C stocks such as some grasslands).
- Avoid conversion of native habitat with significant C stocks (e.g., peatlands).
Minimize loss of landscape-scale C stocks
- Adopt timber harvest systems that maintain partial forest cover.
- Increase timber harvest rotation lengths.
- Reduce or eliminate land clearing by burning.
Increase bioenergy substitution of fossil fuels
- Replace fossil fuels with sustainably managed biomass fuels.
- Increase efficiency of bioenergy use.
- Reduce bioenergy energy demand without compromising development.
Challenges to mitigation of emissions and sequestration
- Tree planting can have a high initial investment and a several-decade delay until harvesting can generate revenue unless short-term benefits are available.
- Protecting forests from harvest reduces the wood and land supply needed to meet other societal needs, such as food and economic development.
- Avoided land-use change is challenging where agricultural expansion is needed to meet food and economic development needs.
- Bioenergy can compete with the use of land for food.
- The reasons for land use conversion are complex and driven by a range of factors such as (a) demographic (population growth, urbanization, and migration); (b) economic (changes in prices, shifts in demands, and infrastructure development); and (c) policy (tenure rights, access to loans) (Diao and Sarpong, 2007; Geist and Lambin, 2002).
Deforestation in Uganda affects biodiversity and watersheds, driving species toward extinction and reducing the quality and quantity of fresh water in rivers. It also contributes to climate change, which is already affecting smallholder farmers.
To reduce the unsustainable exploitation of forest resources and the decline in ecosystem quality, the Trees for Global Benefits project – led by Ecotrust – has created a cooperative C offsetting scheme that combines community-led activities to increase C sequestration with performance-based payments for farmers. In this scheme, farmers register and estimate the amount of C to be generated from their adopted climate-smart land-use practices (e.g., tree planting, improved forestry management, and assisted regeneration). Credits are then aggregated and sold on the voluntary carbon market using the Plan Vivo system. Income from the sale of C credits provides the financial capital required to sustain the adopted land-use practices.
From the 2019 Annual Report, the Trees for Global Benefits project has:
- Supported more than 8,996 farming households from 12 different districts.
- Included more than 7,644 ha of land being sustainably managed.
- Issued 1,590,170 tCO2 emission reduction credits.
- Issued more than $3 million (USD) in total payments to participants.
As a result, smallholder farmers have higher incomes. They are also able to use their profits to invest in other sustainable ways of making money (e.g., producing honey, oils, fruits, fodder, and medical extracts). They can use the C sale agreement as collateral on loans, helping them grow their businesses.