Geoderma 195–196 (2013) 201–206

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Geoderma

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Review

Soil organic carbon sequestration, storage, retention and loss in U.S. croplands: Issuespaper for protocol development

Kenneth R. Olson ⁎Department of NRES, Colleges of ACES, University of Illinois, USA

⁎ Tel.: +1 217 333 9639; fax: +1 217 244 3219.E-mail address: krolson@illinois.edu.

0016-7061/$ – see front matter © 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.geoderma.2012.12.004

a b s t r a c t

a r t i c l e i n f o

Article history:Received 11 July 2012Received in revised form 2 December 2012Accepted 9 December 2012Available online 9 January 2013

Keywords:Soil organic carbon sequestrationComparison studiesPre-treatment SOC valuesSoil organic carbon

The atmospheric levels of carbon dioxide (CO2) have been due largely to the burning of fossil fuels, deforestation,cultivation of the grasslands, drainage of the land, and land use changes. This has led to increase in greenhousegases, created concerns about the potential for long-term climate change and interest in developing methods tosequester some of this atmospheric carbon. In agricultural land areas no-tillage (NT) systems have been pro-posed, to replacemoldboard plow and chisel systems as away to sequester soil organic carbon (SOC). Numerousestimates have been published of soil organic carbon (SOC) sequestration total and rates as a result of a switch toNT systems. Other researchers have proposed the use of cover crops, synthetic fertilizers, organic fertilizer, ma-nure, liming, agricultural systems and management, agroforestry, forages, compost, crop rotations, and reducedrow crop use asways to sequester SOC. For SOC sequestration to occur as a result of a treatment applied to a landunit, all of the SOC sequesteredmust have come fromatmosphere and be transferred into the soil humus throughthe unit plants, plant residues and other organic solids. The amount of SOC present in the soil humus at the end ofthe study has to be greater than the pre-treatment SOC levels in the same land unit and there needs to be a netdepletion of atmospheric CO2 as a result. The objectives of this paper are to: (1) determine long-term studySOC levels and trends in agricultural lands, (2) application of the SOC sequestration concept to a specific site,(3) identify appropriate experimental designs for plot area use in determining SOC sequestration, (4) develop aprocedure, such as pre-treatmentmeasurements of SOC levels in the plots before treatments are applied, to verifySOC sequestration at a site (5) equivalent soil mass samplingmethod, (6) compare laboratorymethods for quan-tifying SOC content, and (7) account for the loading of C rich amendments. To unequivocally demonstrate SOC se-questration at a specific site has occurred, a temporal increasemust be documented relative to pre-treatment SOClevel and linked to a net depletion of atmospheric CO2.

© 2013 Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2012. Determination of long-term SOC levels and trends in agricultural lands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2023. Application of the SOC sequestration concept to a specific site. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2034. Identify appropriate experimental designs for plot area use in determining SOC sequestration . . . . . . . . . . . . . . . . . . . . . . . 2035. Develop a procedure, such as pre-treatment measurements of SOC levels in the plots before treatments are applied . . . . . . . . . . . . . 2046. Equivalent soil mass sampling method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2047. Compare laboratory methods for quantifying SOC content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2058. Account for the loading of C rich amendments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2059. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205

rights reserved.

1. Introduction

Forest and grassland soils tend to lose 20 to 50% of the original SOCcontent in the zone of cultivation (surface 15–20 cm) within the first

202 K.R. Olson / Geoderma 195–196 (2013) 201–206

40 to 50 years of cultivation and drainage (Campbell and Souster,1982; Houghton, 1995; Johnson and Kern, 1991; Mann, 1985, 1986;Rasmussen and Parton, 1994; Schimel, 1986; Tiessen et al., 1982).Plowing was the primary cause of oxidation of SOC and emission ofCO2 to the atmosphere. Land drainage of poorly drained, nearlylevel soils and eroded, well drained sloping soils often resulted inthe higher rates of oxidation of SOC and emission of CO2 to the atmo-sphere than for nearly level and well drained soils without a signifi-cant water or wind soil erosion hazard. Kern (1994) estimated thehistorical soil carbon losses in the surface 30 cm after cultivation formajor field crops in the contiguous U.S. to be 16% primarily as a resultof the plowing which caused oxidation of SOC and emissions of CO2 tothe atmosphere. Many researchers (Clark et al., 1985; Lal, 1995; Paulet al., 1997) have found accelerated soil erosion exacerbates carbonemissions. Lal (1995) suggested that 20% of the carbon dislocatedby erosion may be released eventually into the atmosphere and 10%to the water.

Franzluebbers and Follett (2005) reported the SOC content of tim-berland and prairie soils declinedwith cultivation inNorth America. Therate of decline as a result of cultivationwas 22%±10% for the northeast;no value was reported for the central, 25%±33% for southwest and36%±29% for southeast. Ismail et al. (1994) observed a decrease inSOC in the 0- to 30-cm silt loam layer of soil during the first 5 years,no change in the next 5 years and an increase in SOC in the last10 years in both NT and MP in comparison with sod plots.

Conservation tillage systems have beenproposed as away of achiev-ing SOC sequestration. Franzluebbers (2010) summarized results forthe southeastern region of USA (0.45±0.04 MgCha−1 yr−1)which in-cludedmany individual tillage treatments, experiments or studies sam-pled to a 20-cm depth, with average individual study duration of morethan 10 years and located in 8 states. Previously, other researchers(Franzluebbers, 2005; Johnson et al., 2005; Liebig et al., 2005) hadreviewed and synthesized the results for the central (0.40±0.61 Mg Cha−1 yr−1), the northwestern (0.27±0.19 Mg C ha−1 yr−1), andsoutheastern regions (0.42±0.46 Mg C ha−1 yr−1), respectively.Franzluebbers and Follett (2005) summarized the SOC sequestrationrates for no-tillage (conservation tillage) versus conventional tillagefor all regions of North America and added values for the northeasternregion (−0.07±0.27 Mg C ha−1 yr−1) and a value for the southwestregion (0.30±0.21 Mg C ha−1 yr−1). These specific regional SOC se-questration rates were apparently determined based on many compar-ison studieswithin each regionwith the SOCmeasured at the end of thetillage study and reported. SOC was higher in NT than in MP. Lal et al.(1998) suggested that the conversion of a conventional system to con-servation tillage could result in a 0.50 MT (or Mg) ha−1 yr−1 C seques-tration rate for the USA. Previously eroded soils with low SOC contentwere identified as having significant potential to sequester SOC (Lal,1999; Lal et al., 1998). Del Grosso et al. (2005) used model projectionsto suggest that conversion to NT at the national scale couldmitigate ap-proximately 20% of theUSA agricultural greenhouse gas emissions. Con-servation tillage systems have been proposed as a way of achieving SOCsequestration.

Due largely to the burning of fossil fuels, cultivation and drainageof grasslands, deforestation, and land use conversions, atmosphericthere has been increased interest in developing methods to sequestersome of this atmospheric carbon (Baker et al., 2007; Sundermeier, etal., 2005). In agricultural land areas no-tillage (NT) systems havebeen proposed, to replace moldboard plow and chisel systems as aways to sequester soil organic carbon (SOC) (Luo et al., 2010; Ogleet al., 2012). Many studies have suggested converting from MP orCP systems to NT has great potential for SOC sequestration. Otherstudies have shown that the anti-erosion benefits of NT systemsmakes it a disable strategy, it does not differ from CP and MP interms of its impact on SOC sequestration (Baker et al., 2007; Luo etal., 2010; Ogle et al., 2012; Sundermeier et al., 2005; VandenBygaartet al., 2003).

According to Baker et al. (2007), this difference in SOC distributioncan be attributed to different properties of tilled vs. no till soils. No-tillsoils are more compacted and have a higher bulk density and feweraggregates than plowed soils. This prevents roots from penetrating asdeeply and concentrates the root biomass at amore shallow depth. Fur-thermore, any crop residues left on the soil in no-till promote a lowersoil temperature, which limits root growth (Baker et al., 2007). LessSOC is lost due to a lack of plowing in no-till, but less is producedthrough root growth.

West and Post (2002) reviewed 137 paired studies and showedmore SOC storage in NT than MP, but in the studies considered, SOCwas only measured in the top 15 cm or 30 cm of soil (Baker et al.,2007). Another review by VandenBygaart et al. (2003), however, includ-ed studies where SOC was measured to a depth greater than 30 cm. Inthese cases, there was no significant difference between the volumes ofSOC in plowed vs. no-till regimes. The only difference found was the lo-cation of the carbon within the soil profile. In no-till plots SOC was con-centrated in the top 30 cm, but was dispersed to greater depths in tilledplots (Baker et al., 2007; VandenBygaart et al., 2003).

Khan et al. (2007) found that use of the comparison method withSOC content measurements only taken either in the middle or at theend of a variable rate N fertilizer, crop yield and SOC sequestration stud-ies resulted in an overestimation of the magnitude and rate of SOCsequestration in response to N fertilizer. Comparison studies with onetreatment as the baseline or control should not be used to determineSOC sequestration if soil samples are only collected and tested once dur-ing or at the end of study. Only experimental designswith pre-treatmentSOC measurements (baseline data) made before or at the start oflong-term field studies should be used. Mulvaney et al. (2009) did notfind an increase soil organic matter build-up in response to increasedcrop residue input as a result of fertilizer N applications when apre-treatment SOC datawere collected and used as the baseline to deter-mine SOC change over time in long-term cereal cropping experiments.

Attempts to take a gas, such as carbon dioxide, from the atmosphereand store it in the earth have been referred to as carbon sequestration,organic carbon sequestration or soil organic carbon sequestration.When the gases are taken from the atmosphere by plants and storedin the soil humus, this process is defined as SOC sequestration. Howev-er, the science of C sequestration in soils is not well established andpublished findings report conflicting SOC sequestration results whenapplying treatments (such as conservation tillage, no-tillage, covercrops, synthetic fertilizers, organic fertilizer, manure, liming, land usechange, agricultural management systems, agroforestry, compost, for-ages, crop rotations, and reduced row crop production).

Government agencies and other groups such as the EnvironmentalDefense Fund (Fynn et al., 2009) are aware of the need to establish“pre-project soil carbon stocks” as the baseline. They have suggestedthat the term “additionality” be applied which means that C or SOCsequestration must be achieved by project activity over and aboveany that would have occurred in the absence of the projects. Fynnet al. (2009) also introduced the term “permanence” which refers tothe idea that C or SOC sequestration that was achieved as a result ofproject activity must have “permanence” which suggests the C orSOC gain must be secured over the lifetime of the study.

2. Determination of long-term SOC levels and trends inagricultural lands

U.S. researchers have reported the SOC content of timberland andprairie soils decline 20 to 30% with cultivation (Houghton, 1995;Johnson and Kern, 1991; Olson, 2010; Rasmussen and Parton, 1994).Rapid losses frequently occur in the first few years after clearingand cultivation; and land use change can result in additional SOClosses for many more years with the most significant being withinthe first 40 years.

203K.R. Olson / Geoderma 195–196 (2013) 201–206

The causes depend on site specific situations. For example much ofthe flat wet prairie soils (Mollisols) in the north central region of U.S.lost SOC as a result of land drainage when deep ditches were createdto drain excess water (Olson, 1995). The drainage systems aeratedthe soils and microbial decomposition occurred with significant (20to 30%) CO2 losses to atmosphere. The primary reason for drainagewas to produce crops. Subsequent cultivation and tillage resulted inmixing, aeration and soil structure disruption which further reducedthe SOC levels of the soils. Initially, crop rotations in north centraland other regions of the U.S. included forages and small grainswhich were grown for an animal based agriculture. By the 1920s, soy-bean was introduced and eventually became a row crop which dra-matically changed the crop rotation mix (Olson, 1995). Since the1970s much of the North Central region crop rotations have shiftedalmost exclusively to a corn-soybean rotation and these changesresulted in reduced SOC levels.

In the 1960s, tillage practices changed from MP to chisel plow (CP)with more of the residue remaining on the surface to reduce wind andwater induced soil erosion. About this time weed control methods forrow crops started to change from between row cultivators to chemicalweed control which also reduced mixing and disturbance of the soils.By the 1970s, NT systems further reduced the mixing and disturbanceby eliminating primary tillage which resulted in less break-down ofsoil structure and aggregation, reduced aeration and resulted in greaterretention and protection of SOC (Olson, 2002; Olson et al., 2005).

In addition to theMollisols of the North Central and other regions ofthe U.S., there are many timber soils (Alfisols) which often occur onsloping lands along a major river or stream or a glacial moraine. Thesesoils are well drained and do not require land drainage. However,they do have a much greater soil erosion potential when cleared andcultivated. Soil erosion, including sheet, rill and gully erosion damagedmuch of the land in the North Central region of the U.S. by the 1930s(Dust Bowl) and often had to be abandoned, leaving behind large gullies(Olson, 1995). Since the topsoil contains most of the SOC in Alfisols theerosion process transported considerable SOC rich sediment from thesloping land units (fields, farms or tracts) and deposited the SOC sedi-ment on the lower landscape positions or in the natural drainageways.This removal of SOC rich sediment from plots, fields and farms remainsa problem today but various conservation practices have been intro-duced during the past 150 years to reduce the soil erosion and theSOC losses from the plots, fields and farms. These practices includebuilding terraces, waterways, conservation tillage, NT, filter strips,wind breaks and cover crops (Olson, 2002). Most of these conservationpractices helped reduce the soil erosion and SOC rich sediment lossesfrom the eroding landscapes.

All of these land use and agricultural system changes over time havehad an impact on the SOC levels and trends (gains, steady state or loss)in the U.S. Each change affected the SOC levels and trends. Some man-agement practices raised the SOC levels and some lowered them butthe SOC of the soils seldom reached a true “steady state” since manyof these land use and practice changes were adopted at various timesand had an impact over different time periods. It usually takes yearsto reach a steady state, especially if the soils are sloping and erodingand SOC rich sediment is being transported from the land unit.

While our agricultural farming systems have supplied a lot of C inthe form of food, feed, fiber, and fuel they have also released vastamounts of carbon previously stored in the terrestrial (soil) pool. TheC was released it to either the water or atmospheric pools and contrib-uted for 150 years to the increase in greenhouse gas emissions from ag-ricultural lands. Recently tillage researchers have proposed alternativeagricultural systems (such as no-till (NT)) to increase the storage ofSOC in the terrestrial pool and claimed the greater storage or retentionof SOC as a result of the adoption of NT as a method to sequester SOC.Their theory is that if the SOC in terrestrial pool is greater as a resultof everyone using the NT instead of everyone using MP system. TheSOC stored in the soil or retained by NT would have to be more than

MP and therefore would be releasing less carbon dioxide to the atmo-sphere. While this could be true there is also the possibility that NTfarming systems used to produce crops are still lowering the SOC inthe terrestrial system but at a slower rate than the MP agricultural sys-tem. Annual losses in SOC storage could be as a result of oxidation ofSOC and emissions of CO2 to atmosphere could at specific sites be great-er than the additional amount of annually stored SOC (in humus) by NTagricultural system. If an agricultural system such as NT is used tosequester SOC in the terrestrial pool there should be more SOC in thesoil after each year of production than at the start and if not then the at-mospheric and water pools have gained CO2 or C. Consequently, therewould not be a net increase in SOC storage as a result of SOC sequestra-tion. For most researchers it is difficult to measure the total terrestrial,water and atmospheric pools and determine the net change as a resultof the adoption of a NT agricultural system for many years on the netSOC storage balance. While modelers could attempt this it usually re-quires data to develop the models and to validate them. Some of thisterrestrial data should come from specific field sites and therefore theSOC sequestration concept needs to be applied to a specific land unit(plot, plot area, parcel, tract, field, farm, landscape position, landscape,wetland, forest or prairie) with boundaries to allow measurements.

3. Application of the SOC sequestration concept to a specific site.

One source of the conflicting findings relate to the general nature ofthe definition of SOC sequestration. Soil organic carbon (SOC) sequestra-tion was defined by Olson (2010) as “Process of transferring CO2 fromthe atmosphere into the soil through plants, plant residues and otherorganic solids, which are stored or retained as part of the soil organicmatter (humus). The retention time of sequestered carbon in the soil(terrestrial pool) can range from short-term (not immediately releasedback to atmosphere) to long-term (millennia) storage. The sequestratedSOC process should increase the net SOC storage during and at the end ofa study to above the previous pre-treatment baseline levels and result ina net reduction in the CO2 levels in atmosphere.” The phrase “of a landunit” needs to be added to the definition proposed by Olson (2010) toadd clarity and to prevent the loading or adding SOC to the land unitsoil naturally or artificially from external sources. Carbon not directlyfrom atmosphere and from outside the land unit should not be countedas sequestered SOC. These external inputs could include organic fertil-izers, manure, plant residues, or topsoil or natural input processes suchas erosion of a sloping soil and sediment rich C deposition on a soil locat-ed on a lower landscape position or in awaterway. The land unit could bea plot, plot area, parcel, tract, field, farm, landscape position, landscape,wetland, forest or prairie with defined and identified boundaries. Thispaper only discusses SOC sequestration as defined in the proposed defi-nition and not soil inorganic carbon (SIC), OC or C sequestration. Atmo-spheric carbon is cycled to the plant by photosynthesis, the plant cyclesthe organic C to the soil as residue and it becomes humus or soil organicmatter. It is impossible for most researchers, with the possible exceptionof modelers, to quantify changes in both the terrestrial and atmosphericpools. Therefore the soil sequestration definition needs meaningfulboundaries to be used by researchers who want to measure actualchanges in a specific part of a terrestrial (soil) pool. The proposed defini-tion of soil sequestration is the “process of transferring CO2 from the at-mosphere into the soil of a land unit through unit plants, plant residuesand other organic solids, which are stored or retained in the unit aspart of the soil organic matter (humus).”

4. Identify appropriate experimental designs for plot area use indetermining SOC sequestration

Muchof the literature (Franzluebbers, 2005, 2010; Franzluebbers andFollett, 2005; Johnson et al., 2005; Lal et al., 1998; Liebig et al., 2005) sug-gests that paired comparison between various tillage treatments withone treatment, such asMP, used as baseline or control. These comparison

204 K.R. Olson / Geoderma 195–196 (2013) 201–206

plots are often sampled only once during or at the end of a long-termstudy to determine the amount and rate of SOC sequestration. Theseresearchers apparently assumed that the treatment used as baseline(often MP) at the end of study had the same SOC level it had prior tothe treatment application or at the start of a long-term study and theMP baseline level of SOC was maintained during the study at a steadystate. If true then any increase in SOC of the comparison treatment(NT) above the baseline treatment (MP) at the end of study wouldrepresent the amount of SOC sequestered. However, these studiesoften lack or do not report the pre-treatment SOC content of the baselinetreatment (MP). Without such pre-treatment SOC data for the baselinetreatment (MP), the SOC sequestrationmagnitude and ratefindings can-not be verified (Fynn et al., 2009; Olson, 2010; Sanderman and Baldock,2010a, 2010b).

If SOC in comparison treatment (NT) at the end of experiment washigher than the baseline treatment (MP) before the tillage treatmentswere applied, then SOC sequestration occurred. Alternatively, if thecomparison treatment (NT) did not have more SOC content at the endof the study than the baseline treatment (MP) at the start and end ofthe study, then no SOC sequestration occurred. If the SOC sequestrationrate based on a NT comparison with SOC level of the baseline (MP)treatment in the final study year (West and Post, 2002) when baseline(MP) treatment is not at a steady state, findings might not be correct.It is possible that all treatments including baseline treatment (MP)lost SOC over time and if so the differences between treatments at theend of the study only means that one treatment (NT) lost less SOC orretained more SOC in storage or is losing SOC at a lower rate. If thepre-treatment SOC for baseline treatment (MP) at the beginning ofthe long-term experiment is higher than at end, no SOC sequestrationcan be claimed for any other comparison treatment (NT) withoutsubtracting this baseline treatment (MP) SOC level decrease.

5. Develop a procedure, such as pre-treatment measurements ofSOC levels in the plots before treatments are applied

The use of the comparison method with MP as baseline withoutestablishing the pre-treatment SOC content of the baseline treatment be-fore establishment of the experiment could, in some cases, overestimatethe amount of SOC sequestration, the SOC sequestration rate, and under-estimate the amount of greenhouse gas released to the atmosphere fromSOC during the study. SOC losses can occur from water erosion, MP till-age mixing, disturbance during planting in NT, disturbance when nitro-gen injection occurs in corn years, aeration, and mineralization. Thiscan and often does offset the SOC gains from plant and root growthand residue being returned to the humus or soil organic matter of soil.

Previously eroded soilswith lowSOC contentwere identified ashav-ing significant potential to sequester SOC (Lal, 1999; Lal et al., 1998). For20-years, Olson (2010) studied previously eroded soils on 6% slopeswith low SOC content in an attempt to quantify the amount and ratesof SOC storage and retention as a result of a conversion to NT system.Pre-treatment SOC baseline of the plot area was used to determine

Table 1Twenty-year effects of tillage treatments (6 replications) on the SOC content (Mg C ha−1 laye1988 baseline methods.

Tillagetreatment

Depth September 1988 pre-treatment baseline June 2009 Pbo

cm Mg C ha−1 layer−1 Mg C ha−1 layer−1 M

NT 0–15 28.5aa 25.2aa

15–75 23.6a 20.1a0–75 (all) 52.1a 45.3a

MP 0–15 28.3a 17.3b −15–75 23.1a 18.9a0–75 (all) 51.4a 36.2b −

a Mean of six replications with the same letter and in the same year and depth with a di

SOC loss fromNT andMP plots (Table 1). No SOC sequestration actuallyoccurred in the NT andMP plots since the SOC level of the plot area washigher at the start of the experiment than at the end of the study(Table 1). The NT plots did retain or storemore SOC after 20-year tillagestudy than MP. However, there was no increase in sequestered SOC inNT system.

Findings suggest a pre-treatment SOC baseline is essential in all till-age comparison studies to determine the amount and rate of SOCsequestration, steady state or loss. A pre-treatment SOC baseline wasneeded in these comparison studies when determining the amountand rate of SOC sequestration, storage, retention or loss, especially onsloping and eroding soils with more intensive cropping rotations(more row crops and fewer years of forages) during the study thanin previous years preceding the study. Clearly, MP treatment wasnot at steady state during the 20-yr tillage experiment. In fact MPplots lost −15.2 Mg C ha−1 from the root zone as a result of mixing,intensity of crop rotation, aeration, and eroded SOC rich sedimentsbeing transported off the plot area (Olson, 2010). NT treatmentonly reduced the magnitude and rate of SOC loss over time. Thepair comparison method used by many researchers with MP as base-line suggested+9.1 MgC ha−1 of SOC sequestration occurred (Table 1)during the 20-yr experiment. However, the 1988 baseline method didnot validate the SOC sequestion value. At this site the sloping and erod-ingNT plot actually lost−6.8 MgC ha−1 during the 20-year study sonoactually SOC sequestration occurred. The assumption that theMPwas ata steady state, made by researchers using the comparison method withone 1 year of SOC sampling near the end,was incorrect and resulted in aSOC sequestration finding that was invalid.

It is true that if a researcher had decided to use MP instead of NTthe amount of SOC retained in the soil after 20-years of MP treatment(Table 1) would be significantly less (−9.1 Mg C ha−1) than after20-years of NT treatment and therefore the greenhouse gas emissionsfrom the SOC in storage or retained in the NT plots would be lowerthan from the SOC in storage or retained in the MP plots.

6. Equivalent soil mass sampling method

Most soil sampling techniques use distance from the soil surfaceas a primary metric. The soil surface, however, is a reliable datumonly for measurement of C concentration characteristics directly re-lated to distance from the soil surface at the time of sampling(Wuest, 2009). Deep SOC profiles differ between the tillage treat-ments of interest. Deeper sampling will not completely overcome abias caused by bulk density variations and resultant change in soilsurface elevation except when the SOC constituent is universally ab-sent of lower depths. Equivalent soil mass (mass-depth) instead oflinear depth can be used to correct for tillage treatment differencesin soil bulk density, allowing more precise and accurate quantitativecomparison of SOC constituents (Lee et al., 2009; Wuest, 2009). Sam-pling soils to a depth that the SOC constituent is universally absent atlower depths is recommended. In most soils SOC constituent is

r−1) of the Grantsburg soil. Paired comparison with 2009 MP baseline and pre-treatment

re-treatment 1988aseline method 20-yr SOC lss (below pre-treatment 1988 baseline)

Paired comparison method with MPbaseline NT vs. MP 20-yr total SOCretention difference (above 2009 MP baseline)

g C ha−1 layer−1 Mg C ha−1 layer−1

−3.3 +7.9−3.5 +1.2−6.8 +9.111.0−4.215.2

fferent tillage treatment are not significantly different at P=0.05.

205K.R. Olson / Geoderma 195–196 (2013) 201–206

uniformly absent at depth ranging from 0.5 to 1.0 m. The SOC datashould be expressed on an equal soil mass per unit area to the appro-priate depth where SOC constituent is absent. An example of suchunits would be Mg C ha−1 to a 1 m depth.

7. Compare laboratory methods for quantifying SOC content

The laboratorymethods used to quantify SOC concentrationmust beable to distinguish soil inorganic carbon (SIC) from soil organic carbon(SOC). Soil organic carbon can be measured by wet combustion (modi-fied acid-dichromate organic carbon procedure) (Soil Survey Staff,2004) or dry combustion methods (Soil Survey Staff, 2004) if an HCLpre-treatment is used to eliminate the inorganic carbon prior to ignitionand measurement where the total carbon becomes the SOC value(Harris et al., 2001). Soil pH of above 7.1 has been used by manyresearchers to identify the samples with inorganic C and selected forthe pre-treatment with HCl. An alternative approach, when carbonatesare high, would be to measure inorganic C or carbonate C and subtractfrom total C using dry combustion to determine the SOC by difference(Soil Survey Staff, 2004). When SOC is measured multiple times in thecourse of a long-term study, the same laboratory method should beused (Mulvaney et al., 2010). If researcherswant to use the dry combus-tion methods then they should report total C (both SOC and SIC) beforethe treatments are applied, during the study and at the end of the study.Researchers should not assume soil samples do not have inorganic C(from past liming or naturally formed carbonates) even if soil sampleshave a low pH and then report their total C findings (dry combustion)as SOC when no pre-treatment with HCl was used (Mulvaney et al.,2010). They could however report the total C but need to realize thatliming and other amendments during the study could be raising totalC levels but not SOC levels.

If inorganic C is added to a land use as an amendment from an exter-nal source, it would not be measured in the recommended laboratorymethods and would not count as SOC. However, if SOC from externalsources such as in manure, organic fertilizer or plant residues wereadded they would contribute to the measured SOC (wet combustion)in the soils (in un-digested manure form, in decomposed form or inthe humus fraction). Current laboratory methods cannot distinguishOC in an amendment from OC in humus. Therefore, the amount of Caddedmust be accounted for and should not be credited to SOC seques-tration total. It was not sequestered in the land unit from the atmo-sphere, was not transferred to the soil humus by unit plants and didnot result in a depletion of CO2 in the atmosphere. However, the SOCrich amendments could eventually raise the amount of SOC storage inthe humus fraction of land unit soils and could be reported that way.This increase in SOC storage, as a result of C amendments from outsidethe unit, is not SOC sequestration.

8. Account for the loading of C rich amendments

The loading of C rich amendments by researcher or the natural de-position of SOC rich sediment (Mulvaney et al., 2010) from outsidethe land unit needs to be accounted for when claiming SOC sequestra-tion. The proposed definition of SOC sequestration requires the landunit used have specified boundaries and only C gains from the atmo-sphere to the soil humus be based on C inputs from the land unit plantsand roots and their residue only. Any loading of external C rich amend-ments (not directly from the atmosphere through unit plants to the soilhumus) such asmanure and organic fertilizers must be accounted for inthe analysis and should be deducted from the amount claimed as SOCsequestration since it was not created in the land unit and did notlead to a reduction in the atmospheric carbon dioxide levels. The treat-ment applied could still result in SOC sequestration. C loading on a landunit would most likely increase the amount of CO2 given off, from theland unit during decomposition, to the atmosphere even if some

additional humus formation were to occur on the land unit as a resultof the C loading.

The loading of C rich amendments from an external source onto aland unit creates a number of issues. Some researchers will want toclaim part of the C rich amendment as sequestered SOC. This could bein the form OC amendments (manure, saw dust, or organic fertilizers);IC amendments (lime or dolomite) and SOC amendments (new topsoilor sediment deposition). These amendment cases illustrate the necessityof specifying the land unit and origin of the C sources in order to accu-rately measure the change in SOC derived from the atmosphere. Thisspecification prevents the loading of C rich amendments from outsidethe land unit boundaries and claiming the C additions as SOC sequestra-tion when the SOC was not sequestered in the land unit and the atmo-sphere was not depleted of CO2 as a result. The addition of land unitboundaries in the definition of SOC sequestration prevents over or un-derestimation of the SOC sequestered. C rich amendments from externalsources could in some cases eventually raise the land unit humus or SOCstorage levels but that C addition to the humuswas not sequestered SOC.

9. Conclusions

It is important to determine SOC sequestration rates for variousagricultural land treatments and to establish a protocol to validate theamount and rate of SOC sequestration. Previous soil science researchhas built an important beginning foundation to assess the capacity ofsoil to store and sequester carbon. However, there are inconsistenciesand errors in the application of SOC sequestration concept, plot area ex-periment designs andmeasurements as well as the laboratory methodsused to determine SOC sequestration. Most critical is the inability to ac-curately verify C drawn from the atmosphere and sequestered in thesoil. In this paper, it has been proposed that land unit boundaries be in-corporated into the definition of SOC sequestration when applied to aspecific site, that pre-treatment baseline measurements are essentialto all studies, and that field experiments be designed to more carefullymeasure, monitor and assess internal and external inputs. That amountof SOC loss from the soil storage during the time of experiment needs tobe subtracted from SOC sequestration amount to determine the changein net SOC storage. Further, soil laboratory and field methods forquantifying SOC concentration must be refined to reduce under andover-estimation bias. Additional investments in SOC research is neededto better understand the agricultural management practices that aremost likely to sequester SOC. The proposed protocols that have beendiscussed above are necessary to move the science forward and to at-tempt to address future predicted climate trends. The amount of SOCsequestered as a result of alternative agricultural systems such as NTand its effects of on the net SOC storage changes in terrestrial pooland SOC released to the water and atmospheric pools need to be mea-sured or calculated. If the losses from the terrestrial pool are greaterthan the gains in SOM during the time of the experiment then no netSOC sequestration would have occurred and the release of CO2 gaswould have increased rather than being depleted in the atmosphere.

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