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Carbon sequestration

What is carbon sequestration?
It is widely believed that reducing the atmospheric concentration of CO2 could reduce future climate-related damage and would be more beneficial than costly. Options for achieving this goal include reducing future emissions as well as sequestering CO2 that has already accumulated. Carbon sequestration involves "removing C from the atmosphere and depositing it in a reservoir" (UNFCCC 2006).

Carbon can be sequestered in two major ways: biological and geological. Geological sequestration, which has a fairly large technological potential, has not yet been demonstrated on a scale anticipated to mitigate CO2 emissions. It is also more costly. Thus current discussions are focused on biological methods for sequestering C.

What is biological carbon sequestration?
Biological C sequestration transfers C (from CO2) in the atmosphere to biomass through photosynthesis and ultimately stores it in plants (foliage, wood, and roots) and soils. Biological sequestration encompasses various ways of using agricultural and forest land to enhance the natural storage of atmospheric CO2. Examples include planting or preserving trees, altering crop production practices, planting vegetation in areas prone to soil erosion, and changing the way grazing lands are managed. When forests, croplands, and grazing lands sequester C, they are referred to as C “sinks".

Forest and agricultural lands have received considerable attention as potential C sinks. In most cases, atmospheric CO2 removal through C sequestration exceeds CO2 emissions from these land-use types, particularly from forests. According to the U.S. GHG Inventory 2006 (EPA 2008), land use, land-use change, and forestry activities comprise a net sequestration of 883.7 million metric tons (mmt) of CO2-Eq. This represents an offset of approximately 14.8 percent of total U.S. CO2 emissions, or 12.5% of total greenhouse gas emissions. Over 84% of this net sink occurs on forest lands. Between 1990 and 2006, total land use, land-use change, and forestry net C flux resulted in a 20% increase in CO2 sequestration, primarily due to an increase in the rate of net C accumulation in forest C stocks, particularly in aboveground and belowground tree biomass. In contrast, the agriculture sector is a net emitter of GHGs (EPA 2008). In 2006, CH4 and N2O were the primary greenhouse gases emitted by agricultural activities. Methane emissions from enteric fermentation and manure management represented about 23% and 7% of total CH4 emissions from anthropogenic activities, respectively.

The use of biomass and biofuels to replace CO2 emitting petroleum based fuels as energy sources has great potential for mitigating GHG emissions in the southeastern United States (EPA 2005). Over the coming decades, Florida forestry and agriculture could significantly offset and reduce the projected emission increases in the state. Florida’'s forest cover declined 36% between 1945 and 2002 (although it still covers 43% of the 34.3 million land acres); during the same period, crop and pasture lands increased by 22% (USDA/ERS, 2006). Improved management of forests and agricultural lands could provide an effective tool to help stabilize atmospheric GHGs.

What are the major pools of sequestered carbon in agricultural and forest lands?
In a forest ecosystem, C sequestration occurs in four components of the system: soil, trees, forest floor, and understory vegetation. The total amount sequestered in each part varies greatly depending on the region, type and age of the forest, the quality of the site, and land use history. On average, the soil and above-ground parts of trees hold the major portions, roughly 60% and 30% respectively, of the total C stored in a forest; the rest is mostly in forest litter (9%) and understory vegetation (1%) (Birdsey, 1992).

According to the IPCC (2000), potential increases in C storage may occur in agricultural and forest lands via (1) improved management within a land use, (2) conversion to a land use with higher C stocks, or (3) increased C storage in harvested products. Achieving those increases will vary according to the new land use and management practices, net emissions of GHGs associated with additional management activities, and land use policies. The scientific literature to evaluate diverse scenarios for increasing C storage is currently limited. However, one such scenario, presented in Table 1, illustrates the potential range for C stock increases through some broadly defined activities. It provides data and information on C stock changes for some candidate activities for the year 2010. The greatest potential for C sequestration occurs when land-use becomes more sustainable, with the largest dividend estimated when arable land is changed to agroforestry (Table 1).

Integrated production systems like silvopastoral agroforestry systems, where trees were integrated into pasture animal production, can increase net C storage. When both the tree and grass components are well-managed, an increase in net C storage compared with pasture or forest alone can be achieved. Sharrow and Ismail (2004) reported from their studies in Oregon that the silvopastoral system accumulated approximately 299 kg acre-1yr-1 more C than forests and 210 kg acre-1 yr-1 more C than pastures. The agroforests were silvopastures of 11-yr-old Douglas fir (Pseudotsuga menziesii) with perennial ryegrass (Lolium perenne) and clover (Trifolium sp.) pasture. The combination of pasture and trees also stored 214 kg acre-1 more N aboveground than the forest, and the pasture stored 486 kg N acre-1 more N aboveground than the forest. More efficient sharing of site resources between tree and pasture plants and microclimate modification by trees may increase overall net production of phytomass available for storage (Table 2).

Table 1. 
Global estimates of potential net change in carbon storage through improved management within land-use and change in land-use activities. Values shown are average rates during this period of accumulation.

Table 2. 
Estimates of carbon dioxide sequestered in three different land-use systems in Oregon, USA

Improved forest management
Carbon storage can also be improved by changing silvicultural practices. Since these practices are usually developed and applied for purposes other than C sequestration, it may be difficult to quantify the magnitude of increased total C storage when practices change. For instance, increasing timber growth will not necessarily increase biomass growth and soil C storage. Nevertheless, estimates of such increases range from 2.1 to 3.1 metric tons of CO2–eq acre-1 yr-1. Twenty to 45% of the C in the salable portion of harvested timber is sequestered in wood and paper products during their usable lives and afterwards in landfills (Gorte, 2007). Thus, increases in C from timber management do not have a fixed time horizon.

Reduced conversion of forest land to non-forest use (Avoid deforestation)
Conversion of forest land to non-forest use usually means permanent loss of all or a substantial part of live biomass and reduction of organic matter in soils and in the forest floor. Some C may be sequestered in wood products if the harvested biomass is utilized. Carbon dioxide and other GHGs are emitted when the remaining biomass and organic matter is burned or decomposed. Protecting and conserving forests should maintain or increase C pools in the short term. Carbon in wood and paper products remains sequestered and is emitted to varying degrees depending on how products are made, used, and disposed. Sequestration in products and uses can be increased by altered processing methods, shifts in products used, end-use durability, and landfill management. Sequestration in forests and products can be maximized by coordinated understanding of forest ecosystems and product utilization.

Agricultural and grassland practices
Compared with forests, above-ground biomass stocks in agricultural and grassland ecosystems are fairly small (typically less than 15 metric tons of CO2-eq acre-1). Prospects for C sequestration in agricultural and grassland ecosystems are largely centered on the soil. Hence, strategies for increasing C stocks in these systems revolve around maximizing the amount of C that can be delivered to the soil and maximizing its residence time in the soil. Carbon sequestration in agricultural and grass lands can be enhanced by intensifying prime agricultural lands, land conservation, and restorations. These options include a variety of land use practices that could be adapted as best management practices by landowners.

Prospects for C sequestration in grasslands include (1) optimizing grazing intensity, because C accrual on optimally grazed lands is often greater than on non-grazed or over-grazed lands (Liebig et al. 2005); (2) increasing grass productivity through increased fertilization and improved soil quality; and (3) introducing grass species that are more productive or have higher C allocation to the roots.

In the case of croplands, C sequestration strategies include (1) improved agronomic practices that increase yield and generate higher crop residue; and (2) improved tillage and residue management such as reduced tillage, or no-till, that reduces soil disturbance, consequently reducing C losses through enhanced decomposition or soil erosion. Table 3 lists the various strategies that can be used for C sequestration in U.S. croplands. Lal et al. (1999) estimated that 302 to 763 mmt CO2-eq yr-1 could be sequestered in arable lands of the United States by adopting these improved practices, of which about 50% is due to conservation tillage and residue management, 6% to supplemental irrigation and water table management, and 25% to adoption of cropping systems.

The practices listed above, if adopted by Florida’'s agricultural and forestry sectors hold great potential to increase the rate of C sequestration in the state. However, further advances in developing cost-effective ways to monitor and quantify the amount of C sequestered in agricultural and forest lands is required. In addition, developing C best management practices (BMPs) that provide incentives for adopting strategies to enhance C sequestration is crucial. Eventually, these combined with the growing C market can lead to an increased contribution of agricultural and forest sectors to the mitigation of climate change.