GLOBAL CHANGE: RESPONSES AND MANAGEMENT STRATEGIES FOR SEMI-ARID RANGELANDS
Location: Rangeland Resources Research
Title: Supporting Evidence for Greenhouse Gas Mitigation in Agriculture
| Dumanksi, Julian - RETIRED SOIL SCIENTIST |
| Desjardins, R - AG AND AGRI-FOOD |
| Lal, R - OHIO STATE UNIVERSITY |
| DE Freitas, Pedro - EMBRAPA SOILS |
| Landers, John - APDC BRAZIL |
| Gerber, Pierre - FAO |
| Steinfeld, Henning - FAO |
| Verchot, Louis - CIFOR |
| Schuman, Gerald - RETIRED SOIL SCIENTIST |
Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: June 9, 2009
Publication Date: July 2, 2010
Citation: Dumanksi, J., Desjardins, R.L., Lal, R., De Freitas, P.L., Landers, J.N., Gerber, P., Steinfeld, H., Verchot, L., Schuman, G.E., Derner, J.D. 2010. Supporting Evidence for Greenhouse Gas Mitigation in Agriculture. In: Stigter, K. (ed.), Applied Agrometeorology. Springer, Heidelberg, Germany. pp 1101. Book Chapter.
There are many opinions on the potentials for GHG mitigation in agriculture, but it is not always clear which among these are the most reliable and useful. The issues are complex, and the opinions as many and varied as those who have been brave enough to put their ideas forward. This collection of case studies and supporting documentation, prepared by world authorities in their field, is an attempt to move us towards some resolution of these complex questions.
The options discussed are farming systems with the highest potentials for GHG mitigation in agriculture. These include agroforestry, rangeland management, zero tillage, and livestock production. Undoubtedly, other options may provide benefits in local situations, but recent evidence indicates that these farming systems provide the best opportunities. Recommendations are also provided on procedural and institutional changes needed to enable farmers to capitalize on the opportunities in the carbon market, particularly for farmers in developing countries.
The case studies and documentation are discussed below:
GHG Mitigation in Agroforestry Systems (Prepared by Louis Verchot, Principle Scientist, CIFOR)
Agroforestry systems in the humid tropics include various types of tree-based production systems. Research in these areas (Palm et al. 2002) showed that conversion of primary tropical forests to cropland or grassland resulted in the loss of about 310 Mg C ha-1, with managed or logged forests having only about half the C stocks of primary forests. Agroforestry systems contained 50 to 75 Mg C ha-1 compared to row crop systems with < 10 Mg C ha-1. These results show that converting row crops or pastures to agroforestry systems can greatly enhance the C stored in above and below ground biomass.
Agroforestry also compares well with other land-uses with respect to other GHGs. In Sumatra, a jungle rubber system had lower N2O emissions than a primary forest, but also lower CH4 uptake (Tsuruta et al., 2000). However, agroforestry systems such as multi-story coffee with a leguminous tree shade canopy had N2O emissions five times higher than open-grown coffee and about half the CH4 uptake (Verchot et al., 2007). In Peru, agroforestry systems (multistrata coffee and a peach palm plantation) with leguminous cover crops had lower N2O emissions than both intensive and low-input agriculture and similar emissions to a nearby secondary forest (Palm et al., 2002). Soil uptake of CH4 was similar to other land-use systems, with the exception of intensive agriculture site, which became a net source to the atmosphere.
Agroforestry also has an important carbon sequestration role to play in the sub-humid tropics. Improved tree-based fallow rotations between cereal crops and tree-legume fallows have high potential to sequester C in both the aboveground biomass and the soil. Belowground C storage in these systems represents the potential for long-term C storage, as long as trees remain in the rotation, but the storage capacity is largely dependent upon soil texture and total rainfall. Coppicing fallows are newer, but follow similar trends. While these systems are cut frequently, the average aboveground carbon stocks exceed stocks in degraded land, cropland or pastures. Nitrous oxide emissions following the leguminous tree fallows was found to be almost 10 times that of unfertilized maize (Chikowo et al., 2003) but these levels were still extremely low in comparison to the amount of C stored.
Restoration of degraded land using improved tree fallows has the potential not only to sequester significant amounts of C from the atmosphere, it also offers opportunities for improving rural livelihoods by turning unproductive land into productive land that can produce food, wood and other tree products, and generate income. Typically, there are tradeoffs between carbon stored and on-farm profitability, and while high carbon and high profit land uses have not yet been identified, several no regret options with medium to high profit and medium carbon stocks are already available, and could be implemented as a component in climate change mitigation schemes.
Greenhouse Gas Mitigation in Rangeland Ecosystems (Prepared by Gerald E. Schuman and Justin D. Derner, Agric. Res. Service, Cheyenne, WY)
Rangelands have a large potential for GHG mitigation because of the large global land areas represented, even though the increase in of soil carbon per unit of land is small (0.02 to 0.20 Mg C ha-1 yr-1, Lal, 2000). Globally, rangelands are estimated to contain 10 to 30 % of the global SOC (Scurlock and Hall 1998). Schuman et al. (2001) estimated that improved management on 113 Mha of poorly managed rangelands in the U.S. could sequester an additional 11 Tg of C annually. In addition, they estimated that the loss of 43 Tg C yr-1 could be avoided through the continued use of sustainable grazing practices, conservation of undisturbed native rangelands, and restoration of marginal croplands to perennial grasslands.
Soil C storage in rangelands is influenced by climate, biome type, rangeland management including grazing, N inputs, and restoration, and environmental conditions such as drought, and climate change. Grazing management can influence rates of soil C sequestration by facilitating physiological breakdown, soil incorporation and rate of decomposition of plant materials. Grazing on a shortgrass steppe increased SOC in the top 30 cm of the soil compared to adjacent nongrazed exclosures by 0.12 and 0.07 Mg C ha-1 yr-1 for moderate and heavy stocking rates, respectively (Derner et al. 1997, 2006; Reeder et al. 2004). Also, grazing at light or heavy stocking rates in a northern mixed grass prairie increased SOC in the top 30 cm of the soil by 0.30 Mg C ha-1 yr-1 compared to nongrazed exclosures (Schuman et al. 2004). Improvement of soil N status in rangelands can be achieved by interseeding legumes into these systems; for example, Mortenson et al. (2004, 2005) reported that interseeding of alfalfa (Medicago sativa ssp. falcata) into northern mixed grass rangelands significantly increased total forage production and increased SOC from 0.33 to 1.56 Mg C ha-1 yr-1. The use of legumes to enhance the N status of the soil can be achieved without the risk of increased N20 emissions (Schuman et al. 2004). However, reduced SOC sequestration can be expected with longevity in grazing practices (Derner and Schuman, 2007), consistent with other observations that ecosystems reach a new ‘steady-state’ normally at levels lower than the original.
Climate, especially precipitation, can significantly impact C sequestration in rangelands, with SOC generally increasing with increasing precipitation; SOC in mesic rangelands can be 2-3 times higher than those in semiarid rangelands (Derner et al. 2006). However, changes in precipitation and droughts may change rangelands from sinks to sources of CO2. Ingram et al. (2008) reported that several years of severe drought resulted in a loss of SOC from soil of a northern mixed grass prairie that had been storing SOC for the previous 10 years. Also, C sequestration rates (in the top 30 cm of soil) have been shown to go from positive to negative with approximately 440 mm of precipitation (Derner and Schuman, 2007).
Impacts of Zero Tillage on GHG Mitigation in the Tropics and Sub-Tropics (Prepared by Pedro Luiz de Freitas (Embrapa Soils) and John N. Landers (APDC), Brazil)
Zero tillage (ZT) involves direct placement of seeds into the residues of the previous crop. However, refined procedures of ZT, called integrated ZT (Conservation Agriculture), includes maintenance of crop rotations, integrated pest and weed management, use of modern varieties and cultivars, careful and selective crop fertilization systems, and many other conservation technologies (Machado and Freitas, 2004). The collective impacts of these technologies are increased soil carbon sequestration, reduced emission of non-CO2 gases,