Enhancement of Carbon Sequestration in US Soils

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<ul><li><p>October 2004 / Vol. 54 No. 10 BioScience 895</p><p>Articles</p><p>Rising atmospheric carbon dioxide (CO2) concen-tration is a concern because of its potential for alteringclimate. Since the beginning of the Industrial Revolution inthe 18th century, atmospheric CO2 has increased by more than30%. The increase in fossil fuel burning and associated CO2emissions is expected to continue for the foreseeable future,and a doubling or even tripling of the preindustrial concen-tration of atmospheric CO2 is possible by the end of the 21stcentury (IPCC 2001a). Management of vegetation and soilsfor terrestrial carbon sequestration can remove significantamounts of CO2 from the atmosphere and store it as carbonin the organic matter of ecosystems. However, such man-agement changes will not happen unless there are economicincentives or penalties associated with CO2 management.For terrestrial carbon sequestration to be useful, it must notonly result in carbon accumulation in vegetation and soil butalso induce lower net release of CO2 or other greenhouse gases.</p><p>Many factors intervene between demonstrating that a par-ticular management practice can enhance carbon sequestra-tion in the soil and determining that widespread applicationof the method is useful, acceptable, and cost-effective. A gen-eral methodological approach is currently lacking for evalu-ating all aspects of a carbon sequestration practice. Here weoutline a complete and integrated methodology for evaluat-ing alternative approaches to increase terrestrial carbon se-questration. The methodology has six components:</p><p> Identifying promising technologies for soil carbonsequestration</p><p> Understanding the effects of technologies on carbon atthe site scale</p><p> Evaluating other environmental impacts</p><p> Including a full carbon and greenhouse gas accounting</p><p> Performing a sensitivity analysis over the range ofapplicable conditions (model, laboratory, or field experiments)</p><p> Performing an economic analysis of the practices costcompetitiveness, market implications, and other factors</p><p>Wilfred M. Post (e-mail: postwmiii@ornl.gov), Philip M. Jardine, and Jizhong</p><p>Zhou are senior scientists, and Tristram O. West is a research scientist, in the</p><p>Environmental Sciences Division, Oak Ridge National Laboratory (ORNL),</p><p>Oak Ridge, TN 37831. R. Cesar Izaurralde is a senior scientist at the Joint Global</p><p>Change Research Institute (Pacific Northwest National Laboratory [PNNL]</p><p>and University of Maryland), College Park, MD 20740. Julie D. Jastrow is a</p><p>scientist in the Environmental Research Division at Argonne National Lab-</p><p>oratory (ANL), Argonne, IL 60439. Bruce A. McCarl is a professor of agricultural</p><p>economics at Texas A&amp;M University, College Station, TX 77843. James E.</p><p>Amonette and Vanessa L. Bailey are research scientists at PNNL, Richland, WA</p><p>99352. This paper has been authored by a contractor of the US government</p><p>under contracts DE-AC05-00OR22725 for ORNL, W-31-109-ENG-38 for</p><p>ANL, and DE-AC06-76RLO 1830 for PNNL. 2004 American Institute of</p><p>Biological Sciences.</p><p>Enhancement of CarbonSequestration in US Soils</p><p>WILFRED M. POST, R. CESAR IZAURRALDE, JULIE D. JASTROW, BRUCE A. MCCARL, JAMES E. AMONETTE,VANESSA L. BAILEY, PHILIP M. JARDINE, TRISTRAM O. WEST, AND JIZHONG ZHOU</p><p>Improved practices in agriculture, forestry, and land management could be used to increase soil carbon and thereby significantly reduce the concen-tration of atmospheric carbon dioxide. Understanding biological and edaphic processes that increase and retain soil carbon can lead to specific ma-nipulations that enhance soil carbon sequestration. These manipulations, however, will be suitable for adoption only if they are technically feasibleover large areas, economically competitive with alternative measures to offset greenhouse gas emissions, and environmentally beneficial. Here wepresent the elements of an integrated evaluation of soil carbon sequestration methods.</p><p>Keywords: soil carbon sequestration, full carbon accounting, terrestrial ecosystems, land-use change, integrated assessment</p><p> at Gazi U</p><p>niversitesi on August 16, 2014</p><p>http://bioscience.oxfordjournals.org/D</p><p>ownloaded from</p><p>http://bioscience.oxfordjournals.org/</p></li><li><p>There is considerable uncertainty in each of these steps. In thisarticle, we review recent information on carbon sequestrationand clearly identify significant areas of uncertainty in each stepof the evaluation process. We concentrate on soil carbon sequestration, but most aspects of the methodology shouldbe applicable to other carbon sequestration methods.</p><p>Global carbon cycle context of soil carbon sequestration The concentration of CO2 in the atmosphere has risenat firstgradually and then at an exponentially increasing ratefromapproximately 280 parts per million by volume (ppmv) in themid-1800s to 371 ppmv in 2001. This increase is expected tocontinue, since current emission rates are far in excess ofrates that may lead to stabilization of CO2 concentration.The observed increase in atmospheric CO2 concentrationreflects the trend in CO2 emissions caused by fossil fuel com-bustion and land use. Emissions of CO2 from land use occurwhen land is converted from one vegetative cover to a lessdense vegetative cover, especially from native vegetation to agri-culture. About half of all emitted carbon remains in the atmosphere, which translates in an average increase of 3.2 peta-grams (Pg) carbon per year. The worlds oceans and terres-trial ecosystems take up the remainder (table 1).</p><p>Oceans take up and release CO2 at their interface with theatmosphere. The net flux is negative (i.e., into the oceans),because the rising atmospheric CO2 creates a partial pressuregradient that results in additional CO2 dissolving in surfaceseawater. The net land-atmosphere sink consists of two dif-ferent components. The first net exchange represents car-bon released to the atmosphere as a result of land-use changes(now mostly from tropical deforestation). This flux is esti-mated to contribute 0.6 to 1.0 Pg carbon to the atmosphereper year (Houghton 2003). The second component is thedifference between the inferred net land-atmosphere sink of2.1 to 0.7 Pg carbon per year and the flux caused by land-use change. This difference has been estimated to range be-tween 1.3 and 3.1 Pg carbon per year and has been termed theresidual carbon sink. Several lines of evidence indicate that theresidual carbon sink occurs in terrestrial ecosystems and rep-resents sequestration of carbon in soils and plants. Leadinghypotheses suggest that the processes involved in this resid-ual sink include recent climate change, atmospheric nitrogen</p><p>deposition, and the stimulation of photosynthesis resultingfrom higher CO2 concentrations (Pacala et al. 2001). Otherindirect effects of land-use change that may contribute to theresidual carbon sink include fire suppression and regrowth ofperennial vegetation, both of which increase total biomass; ero-sion that results in carbon burial in wetlands, lakes, and reser-voirs; the diversion of forest products to long-lived woodproducts or landfills; and changes in land management thatlead to increases in soil carbon. Estimates of the magnitudeof each of these possible carbon sinks appear in table 2.</p><p>Land management plays a small but significant role inoverall global carbon cycle fluxes (table 2). It may account forabout 4% to 14% of the residual carbon sink. Regrowth ofperennial vegetation accounts for only a small potential soilcarbon sink, because of the small amount of land area that isexpected to be involved. Furthermore, most of the fairly sub-stantial carbon sink caused by forest regrowth (70% to 90%)is in biomass rather than soil. Although land management isnot a dominant factor in the residual carbon sink, it involvesintentional soil and vegetation manipulation over large landareas. The carbon sink associated with land management(except for the amount associated with improved forestry,which is largely biomass) results from soil carbon sequestra-tion and may amount to more than 0.4 Pg carbon per year.With focused effort, the amount of carbon sequestered in soilby land management could be significantly increased. Vari-ous studies estimate that soil carbon sequestration may be in-creased to a rate of 0.44 to 0.88 Pg carbon per year andsustained over a 50-year time frame (Cole et al. 1997). Whilethese rates will offset only a fraction of the emissions from fossil fuels, results from integrated assessment analyses (Edmonds et al. 1999, Rosenberg and Izaurralde 2001) indi-cate that soil carbon sequestration may have an importantstrategic roledue to its low cost and potential for early deploymentwithin a portfolio of technologies to mitigateclimate change. The important aspect of land managementfor soil carbon sequestration is that, unlike many other tech-nologies to offset fossil fuel emissions (e.g., geologic carbonsequestration, carbon capture), it can be implemented im-mediately, provided there are economic and other incentivesto do so. Because of the cumulative effect of CO2 on climate,an immediate increase in carbon sequestration to offset CO2emissions provides a significant delay in the rise of atmo-spheric CO2 concentration. In addition, by the time the carbon sequestration resulting from land management beginsto saturate the soils capacity to store additional carbon, othermethods of reducing emissions or sequestering carbon maybe available or already in use.</p><p>Identifying promising technologies for soil carbon sequestrationLand managers can enhance soil sequestration by (a) in-creasing the rates of organic matter input, (b) partitioning car-bon to longer-lived pools, and (c) increasing the longevity ofall or selected carbon pools. Different methods of agriculturalmanagement use one or more of these pathways to enhance</p><p>896 BioScience October 2004 / Vol. 54 No. 10</p><p>Articles</p><p>Table 1. Average global carbon dioxide budget.</p><p>Carbon flux to atmosphereCarbon source or sink (Pg per year)</p><p>Emissions (fossil fuel, cement manufacture) 6.3 0.4Net oceanatmosphere flux 1.7 0.5Net landatmosphere flux 1.4 0.7</p><p>Total atmospheric increase 3.2 0.1</p><p>Pg, petagram (1 1015 g).Note: Positive values represent carbon fluxes to the atmosphere;</p><p>negative values represent carbon removal from the atmosphere. Errorestimates denote uncertainties ( 2 standard deviations) and notinterannual variability, which is much larger (Prentice et al. 2001).</p><p> at Gazi U</p><p>niversitesi on August 16, 2014</p><p>http://bioscience.oxfordjournals.org/D</p><p>ownloaded from</p><p>http://bioscience.oxfordjournals.org/</p></li><li><p>carbon sequestration (see table 3 for a description of themost widely employed management practices). Depending onthe process involved, variations are expected in the length oftime the enhancement method is effective (time to saturation)and in the average residence time and susceptibility to dis-turbances that would release this sequestered carbon back tothe atmosphere (permanence).</p><p>The amount of soil carbon in various forms is determinedby the balance between inputs of organic detritus and lossesof carbon through decomposition, leaching to groundwater,and erosion. At a basic level, the mechanisms controllingthese processes and their balance can be tied to soil-formingfactors (table 4) and thus are generally well known to soil sci-entists. Disturbance or management practices also exert con-siderable influence on the amount of soil carbon, boththrough direct effects on inputs and losses and through in-direct effects on the factors controlling these fluxes. However,the complex interactions and feedbacks among controllingmechanisms and processes lead to uncertainties in predict-ing changes in soil carbon. It is critically important to iden-tify, develop, and quantitatively evaluate appropriatetechnologies and their effects on processes that determine soilcarbon dynamics.</p><p>Understanding the effects of technologies on carbon at the site scaleTable 3 lists several widely used land-management practicesthat, when applied to plow-tilled cropland, have demon-strated significant carbon sequestration potential. Most of theincreases in soil carbon associated with these practices result</p><p>from reversing processes by which traditional managementhas depleted the soil carbon stocks that accumulated undernative perennial vegetation (Cole et al. 1997). There is suffi-cient information from intensive studies of these soil man-agement practices to gain insight into the processes that areinvolved in increasing soil carbon content.</p><p>Cropping intensification. Elimination of fallow periods (dryfallow in seasonally dry environments, winter fallow in coldenvironments), use of high-yielding crop varieties, and thewide-scale application of inexpensive fertilizer and other soilamendments greatly increase the amount of organic matterproduced, thereby increasing organic matter input into thesoil (Parton et al. 1995, Buyanovsky and Wagner 1998, Lal etal. 1998). Precision agriculture, an intensive approach to optimize crop yields relative to fertilizer and other inputs, re-sults in increased soil carbon inputs. A large component ofthe increased crop residue input resulting from cropping intensification is readily decomposed and does not add tolong-term soil carbon increases. A portion remains to beconverted into humus, contributing to increasing long-termsoil organic carbon pools. The increases in soil carbon due tocropping intensification alone are often limited in amount andresult in saturation within a few decades. Much greater in-creases in soil carbon are frequently obtained if manures(biologically altered inputs) are applied (Jenkinson 1990). Suchamendments not only increase the amount of organic carboninput but appear to contain a larger fraction of organic car-bon materials that are more resistant to decomposition thanunaltered plant material.</p><p>Conservation tillage. Part of the decreasein soil organic carbon that occurs when na-tive vegetation is plowed for row crops re-sults from mechanical disruption of soilaggregates. Soil structure plays a domi-nant role in the physical protection of soilorganic matter (SOM) by controlling mi-crobial access to substrates, microbialturnover processes, and decomposer foodweb interactions. Relatively labile mater-ial may become physically protected fromdecomposition if it is incorporated into soilaggregates or deposited in micropores thatare inaccessible even to bacteria (figure1). Macroaggregates (at least 0.25 mil-limeters [mm] in diameter) are sensitive tosoil disturbance, but microaggregates (lessthan 0.25 mm in diameter) are generallymore stable, appear to turn over moreslowly, and are more resistant to distur-bance (Tisdall and Oades 1982). Increasesin SOM can be tied to the linkages andfeedbacks between macroaggregateturnover, microaggregate formation, andcarbon stabilization within microaggre-</p><p>October 2004 / Vol. 54 No. 10 BioScience 897</p><p>Articles</p><p>Table 2. Estimates of the current magnitude of possible terrestrial carbon sinks.</p><p>Carbon sequestrationTerrestrial carbon sink (petagrams per year)...</p></li></ul>