Historical advances in the study of global terrestrial soil organic carbon sequestration

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<ul><li><p>ou</p><p>M</p><p>Bio,</p><p>Accepted 6 September 2007</p><p> 2007 Elsevier Ltd. All rights reserved.</p><p>and climate change have raised questions about the poten-tial role of soils as sources or sinks of C (Houghton, 2003).</p><p>2. A summarized scientic history of C sequestration for the</p><p>soil-plant system</p><p>The terms sequestration and C sequestration wererst proposed to dene the aptitude of terrestrial ecosys-tems to act as sinks for GHGs. The appearance, use and</p><p>* Corresponding author. Tel.: +33 261 20 22 330 98; fax: +33 261 20 22369 82.</p><p>E-mail address: christian.feller@ird.fr (C. Feller).</p><p>Available online at www.sciencedirect.com</p><p>Waste Management 21. Introduction</p><p>In recent years, increasing attention has been given tosoil organic matter (SOM) in relation to carbon (C) seques-tration. Concerns about increasing atmospheric green-house gas (GHG) concentrations (carbon dioxide (CO2),methane (CH4) and nitrous oxide (N2O)), global warming</p><p>In this paper, the authors briey dene the term soil Csequestration before (i) discussing the historical scienticroots of the present study of soil C sequestration, and (ii)highlighting the diculties encountered when estimatingsoil C sequestration balances for systems using organicwastes (henceforth referred to as wastes).Available online 26 November 2007</p><p>Abstract</p><p>This paper serves two purposes: it provides a summarized scientic history of carbon sequestration in relation to the soil-plant systemand gives a commentary on organic wastes and SOC sequestration.</p><p>The concept of soil organic carbon (SOC) sequestration has its roots in: (i) the experimental work of Lundegardh, particularly hisin situ measurements of CO2 uxes at the soil-plant interface (1924, 1927, 1930); (ii) the rst estimates of SOC stocks at the global levelmade by Waksman [Waksman, S.A., 1938. Humus. Origin, Chemical Composition and Importance in Nature, second ed. revised. Wil-liams and Wilkins, Baltimore, p. 526] and Rubey [Rubey, W.W., 1951. Geologic history of sea water. Bulletin of the Geological Societyof America 62, 11111148]; (iii) the need for models dealing with soil organic matter (SOM) or SOC dynamics beginning with a concep-tual SOM model by De Saussure (17801796) followed by the mathematical models of Jenny [Jenny, H., 1941. Factors of Soil Forma-tion: a System of Quantitative Pedology. Dover Publications, New York, p. 288], Henin and Dupuis [Henin, S., Dupuis, M., 1945. Essaide bilan de la matie`re organique. Annales dAgronomie 15, 1729] and more recently the RothC [Jenkinson, D.S., Rayner, J.H., 1977.The turnover of soil organic matter in some of the Rothamsted classical experiments. Soil Science 123 (5), 298305] and Century [Parton,W.J., Schimel, D.S., Cole, C.V., Ojima, D.S., 1987. Analysis of factors controlling soil organic matter levels in great plains grasslands.Soil Science Society of America Journal 51 (5), 11731179] models.</p><p>The establishment of a soil C sequestration balance is not straightforward and depends greatly on the origin and the composition oforganic matter that is to be returned to the system. Wastes, which are important sources of organic carbon for soils, are taken as anexample. For these organic materials the following factors have to be considered: the presence or absence of fossil C, the potential ofdirect and indirect emissions of non-CO2 greenhouse gases (CH4 and N2O) following application and the agro-system which is beingused as a comparative reference.Historical advances in the studycarbon seq</p><p>C. Feller *,</p><p>Institut de Recherche pour le Developpement (IRD), UR Seq0956-053X/$ - see front matter 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.wasman.2007.09.022f global terrestrial soil organicestration</p><p>. Bernoux</p><p>ENSAM, 2 Place Viala, 34060 Montpellier cedex 1, France</p><p>www.elsevier.com/locate/wasman</p><p>8 (2008) 734740</p></li><li><p>signicance of the term C sequestration are discussedbelow. In addition, the methods used to estimate C seques-tration in soil at dierent temporal and spatial scales andthe methods used to measure CO2 uxes in the soil-plantsystem are considered.</p><p>2.1. Appearance of the terms C sequestration and soil C</p><p>sequestration</p><p>A bibliometric search of the ISI-Web of Science databasecovering the period 19452005 suggests that the rst inci-dence of the terms soil, carbon and sequestrationbeing used together to present a concept, occurred in1991. The number of references using these terms increasedrapidly during the 15 years that followed (Bernoux et al.,2006) (Table 1). The concept of soil C sequestration is,therefore, a relatively new one.</p><p>Most denitions of C sequestration (whether soil specicor not) refer simply to CO2 removal from the atmosphereand storage in an organic form in the soil or plants. How-ever, CH4 and N2O are also involved in exchanges betweenthe soil-plant system and the atmosphere. The UnitedNations Framework Convention on Climate Change(UNFCCC) provides an overall framework for intergov-ernmental eorts to tackle the challenges posed by climate</p><p>C. Feller, M. Bernoux / Waste Machange. Under the UNFCCC, governments have to pro-vide national inventories of anthropogenic emissions (bysources) and removals (by sinks) of all GHGs. To facilitatethis, the Intergovernmental Panel on Climate Change(IPCC) published guidelines for the production of GHG</p><p>Table 1Number of references indexed in the ISI-Web of Science (19452006) forthe word queries soil, carbon, and sequestration (query 1) andsoil and carbon (query 2) in the topics and in the title (inparentheses), updated from Bernoux et al. (2006)</p><p>Years Number of referencesreturned by the Queries</p><p>Query 1/Query 2</p><p>Query 1 Query 2</p><p>19451990 0 719 01991 1a 643 1.61992 5 (1b) 694 7.21993 14 (1) 816 17.21994 7 908 7.71995 21 (1) 985 21.31996 24 1220 19.71997 36 (2) 1398 25.71998 47 (3) 1520 30.91999 42 (3) 1568 26.82000 78 (8) 1618 48.22001 107 (14) 1727 62.02002 148 (15) 1851 80.02003 174 (17) 2142 81.22004 229 (33) 2136 107.22005 255 (27) 2611 97.62006 (till December 6th) 265 (21) 2740 96.7</p><p>Queries performed on December 6, 2006.</p><p>a Thornley et al. (1991).b Dewar and Cannell (1992).inventories (IPCC/UNEP/OECD/IEA, 1997). Fluxes forall gases are expressed as equivalents of CO2 after applica-tion of a conversion factor that reects the global warmingpotential (GWP) of each gas in relation to CO2. Currentconventions yield a 100 year-GWP value of 23 for CH4and 296 for N2O. A recent review by Six et al. (2002) high-lights the importance of accounting for all GHGs whenconsidering C sequestration. The authors found that inboth tropical and temperate soils, C levels increased inno-tillage (NT) systems as compared to those under con-ventional tillage (CT), but that in temperate soils averageN2O emissions increased substantially under NT as com-pared to CT and the increase in N2O emissions (whenexpressed on a CCO2 equivalent basis) lead to a negativetotal GWP, even if a positive C storage was observed in thesoil. Even the case of systems that use organic forms of Nfertilizer can be a hazard when considered in terms of N2Oemissions (Flessa et al., 2002; Giller et al., 2002; Millaret al., 2004).</p><p>When the above issues are considered, it becomes appar-ent that soil C sequestration, as a concept, should not berestricted to a mere quantication of C storage or CO2 bal-ance. All GHG uxes must be computed at the plot level inCCO2 or CO2 equivalents, incorporating as many emis-sion sources and sinks as possible across the entire soil-plant system. Therefore, Bernoux et al. (2006), proposeda new denition of C sequestration applied to the soil orsoil-plant system:</p><p>Soil C sequestration or soil-plant C sequestration,for a specic agro-ecosystem, in comparison with a refer-ence one, should be considered as the result for a givenperiod of time and portion of space of the net balanceof all GHGs, expressed in CCO2 equivalents or CO2equivalents, computing all emissions sources at the soil-plant-atmosphere interface.</p><p>The confusion (as is often the case) between the notionof SOC storage (C stored in the soil irrespective of itsorigin) and soil C sequestration (GHGs, expressed inequivalent CCO2, stored in the soil and originating fromthe atmosphere) can thus be avoided.</p><p>2.2. Early measurements of soil CO2 concentration and</p><p>uxes</p><p>2.2.1. The rst in situ and in vitro measurements of soil CO2concentrations</p><p>The rst in situ measurements of soil CO2 concentra-tions were made by Boussingault and Levy (1852, 1853)at depths ranging from 40 to 240 cm. Using sophisticatedequipment, to avoid contamination of soil CO2 by atmo-spheric CO2, they showed that concentrations of CO2 insoils without farmyard manure (FYM) application were2223 times higher than those found in the atmosphere,and that applying FYM, could increase this concentrationby a factor of up to 245.</p><p>nagement 28 (2008) 734740 735According to Waksman (1938), the rst measurementsof soil CO2 emissions under laboratory controlled condi-</p></li><li><p>Mations were made by Ingen-Housz (17941796), who demon-strated the eect of organic inputs and the importance ofoxygenation, temperature and humidity. As early as1855, Corenwinder (1855, 1856) was using equipmentwhich was very similar to todays respirometry apparatus.</p><p>2.2.2. Measurements of CO2 uxes at the soil-plant-</p><p>atmosphere interface</p><p>2.2.2.1. Lundegardhs studies at the plot scale. The mainforerunner of modern plot scale measurements was theDanish ecophysiologist, Henrik Lundegardh (18881969),whose abridged biography was recently published byLarkum (2003). Between 1924 and 1930, Lundegardhpublished considerable data on CO2 uxes at the soil-plantinterface in two books (1924, 1930) and a large paper(1927).</p><p>In these three publications, Lundegardh reported animpressive number of quantitative data on in situ CO2uxes between atmospheric, plant and soil components.Data were collected using instruments for the sampling ofsoil atmosphere (equivalent to our present day static cham-bers) or continuous monitoring of CO2 uxes at the plantor atmosphere level. In his 1927 publication, he evendescribes eld equipment and experimental designs whichare analogous to those used in the present-day Free AirCO2 Experiments (FACE), which are probably the mostsophisticated experiments we have today for the study ofCO2 uxes at the eld level. FACE experimenters, how-ever, seldom refer to Lundegardhs remarkable forerunningwork.</p><p>2.2.2.2. From the square meter scale to the hectare scale. Theeddy covariance (or eddy correlation) technique iscommonly used to estimate CO2 uxes at the plot(P1 ha) scale in continuous natural or cultivated agroeco-systems. A recent and exhaustive historical review of theresults obtained by using this approach is given in Baldoc-chi (2003). The technique can also be used at the scale ofthe cultivated plot (100 m2) and indeed, has been used byReicosky et al. (1997) to study the eect of tillage on soilCO2 uxes.</p><p>2.3. Assessment of soil C stocks and dynamics at dierent</p><p>scales</p><p>The importance of the soils component of an ecosystem,in terms of its inuence on atmospheric GHG budgets,becomes apparent when land use, land use change and for-estry (LULUCF) are considered. The published guidelinesfor the estimation and reporting of GHG inventories(IPCC/UNEP/OECD/IEA, 1997) recommend calculationof the net uxes of CO2 from the various C stocks in thedierent ecosystem compartments. The rationale of theIPCC for making this choice was that there are largeuncertainties in all current methods for estimating uxes</p><p>736 C. Feller, M. Bernoux / Wasteof CO2 from forestry and land-use change. Direct measure-ments of changes in C stocks are extremely dicult sinceone must confront the diculty of determining small dier-ences in large numbers as well as the inherent heterogeneityof terrestrial systems. The soil may act as a sink (by SOCaccretion and CH4 absorption) or a source for CCO2 inthe medium term (050 year). There has thus emerged agrowing need to: (i) quantify present SOC stocks at dier-ent spatial scales (from the plot to the continental), and (ii)predict SOC dynamics in response to LULUCF by the useof simple and robust mathematical models.</p><p>2.3.1. Evaluation of SOC stocks</p><p>The content of OC, OM or humus in soil was deter-mined as early as the beginning of the 19th century, as evi-denced by Thaers Humus Theory (1809). The emergenceof soil C sequestration as an issue has resulted in a largeeort to compile databases of SOC stocks at scales rangingfrom the plot to the globe. Table 2 summarizes historicaldata on the evaluation of SOC stocks at the global scale.The rst publication was probably that of Waksman(1938) who evaluated SOC for topsoil. Later, Rubey(1951), a geologist, calculated the soil C stock for deepersoil proles using SOC contents for nine main soil typespublished by Twenhofel (1926), which were based onselected values reported by Lyon et al. (1915). Rubbeysestimate (710 Gt C for the 0100 cm layer) was reasonablyclose to Batjes modern (1996) result (based on 4353 soilproles) of 1500 Gt C for same depth. Similarly, the globalestimates of Waksman (1938) of 400 Gt C, for the upper30 cm of the soils, is also close to that of Batjes (1996) esti-mate of 684724 Gt C for the same soil layer.</p><p>2.3.2. The need to model SOM/SOC dynamics</p><p>The rst qualitative approach for modeling SOMdynamics was by H.B. de Saussure in his famous Voyagesdans les Alpes (17801796). Extracts were re-published byhis son, N.T. de Saussure, in his book Recherches chimi-ques sur la vegetation (1804). They were based on obser-vations made by his father during a journey through theplain between Turin and Milan, a region that has been cul-tivated since antiquity. His observations and reections canbe summarized as follows:</p><p> since no continuous accumulation of SOM occurs evenwith continuous organic inputs, some of these inputsmust be destroyed,</p><p> the amount which is destroyed must, to a certain extent,be proportional to the absolute existing amount,</p><p> limits to SOM accretion must vary depending on cli-mate, nature of mother bedrock, vegetation, croppingsystem and fertility of the land,</p><p> even if all conditions are favorable to SOM accumula-tion, there must be a maximum for the thickness ofthe humus layer beyond which destructive causes equalproductive ones.</p><p>nagement 28 (2008) 734740H.B. de Saussures conclusions (completely ignored byhistorians of soil science) thus convey the basic equilibrium</p></li><li><p>esul</p><p>100</p><p>ata</p><p>002095762076</p><p>62</p><p>02</p><p>wen</p><p>Maconcepts utilized by modern mathematical models of SOMdynamics, yet it was not until 137 years later that a math-ematical formulation of SOM (C or N) dynamics for thedecrease in organic N content with cultivation wasexpressed by Jenny (1941). This was followed by the moregeneral model on SOM dynamics of Henin and Dupuis(1945). Many models have now been published and arein use (Smith et al., 1997). The most famous ones are prob-ably the RothC model of Jenkinson and Rayner (1977) andthe Century model of Parton et al. (1987). These modelswere designed to run at the plot level. Coupling them withgeographical information systems (GIS) in order to simu-late ch...</p></li></ul>

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