Crop Production |
Environment & Pollution :: GHE :: Carbon Sequestration |
Weather |
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Carbon sequestration What is carbon sequestration? 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? 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? 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.
Table 2.
Strategies for enhancing carbon sequestration Forestry practices: . Improved forest management Reduced conversion of forest land to non-forest use (Avoid deforestation) Agricultural and grassland practices 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. |
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