Greenhouse gas mitigation potential of the world’s grazing lands:Modeling soil carbon and nitrogen fluxes of mitigation practices

Benjamin B. Henderson a,b,*, Pierre J. Gerber a, Tom E. Hilinski c, Alessandra Falcucci a,Dennis S. Ojima c, Mirella Salvatore a, Richard T. Conant c,d

aUN Food and Agriculture Organization, Rome, ItalybCommonwealth Scientific and Industrial Research Organization, St.Lucia, Queensland, AustraliacNatural Resource Ecology Laboratory, Colorado State University, Fort Collins, CO, United Statesd International Livestock Research Institute, Nairobi, Kenya

A R T I C L E I N F O

Article history:Received 21 August 2014Received in revised form 24 March 2015Accepted 27 March 2015Available online 11 April 2015

Keywords:Grazing managementLegumeFertilizationCenturyDaycent

A B S T R A C T

This study provides estimates of the net GHG mitigation potential of a selected range of managementpractices in the world’s native and cultivated grazing lands. The Century and Daycent models are used tocalculate the changes in soil carbon stocks, soil N2O emissions, and forage removals by ruminantsassociated with these practices. GLEAM is used in combination with these models to establish grazingarea boundaries and to parameterize links between forage consumption, animal production and animalGHG emissions. This study provides an alternative to the usual approach of extrapolating from a smallnumber of field studies and by modeling the linkage between soil, forage and animals it sheds new lighton the net mitigation potential of C sequestration practices in the world’s grazing lands. Three differentmitigation practices are assessed in this study, namely, improved grazing management, legume sowingand N fertilization. We estimate that optimization of grazing pressure could sequester 148Tg CO2 yr�1.The soil C sequestration potential of 203Tg CO2 yr�1 for legume sowing was higher than for improvedgrazingmanagement, despite being applied over amuch smaller total area. However, N2O emissions fromlegumes were estimated to offset 28% of its global C sequestration benefits, in CO2 equivalent terms.Conversely, N2O emissions from N fertilization exceeded soil C sequestration, in all regions. Ourestimated potential for increasing C stocks though in grazing lands is lower than earlier worldwideestimates (Smith et al., 2007; Lal, 2004), mainly due to themuch smaller grazing land area overwhichweestimate mitigation practices to be effective. A big concern is the high risk of the practices, particularlylegumes, increasing soil-based GHGs if applied outside of this relatively small effective area.Morework isneeded to develop indicators, based on biophysical and management characteristics of grazing lands, toidentify amenable areas before these practices can be considered ready for large scale implementation.The additional ruminant GHG emissions associated with higher forage output are likely to substantiallyreduce the mitigation potential of these practices, but could contribute to more GHG-efficient livestockproduction.

ã 2015 Elsevier B.V. All rights reserved.

1. Introduction

There is widespread enthusiasm for harnessing the large soilcarbon (C) sequestration potential of grazing lands to offset globalgreenhouse gas (GHG) emissions, owing to their vast land area,widespread history of degradation, and potential for improvedmanagement. Their capacity for soil C storage is estimated to be asimilar order ofmagnitude as the potential in croplands and forests

(Smith et al., 2007). Consequently, grazing land C sequestration isbeing considered as an important component of national GHGmitigation programs by countries, including Brazil and China.Nevertheless, many practical challenges remain, chief among themis uncertainty in the magnitude of the potential and costsassociated with the adoption of sequestration practices.

In grazing lands that have experienced the excessive removal ofvegetation and soil C losses from sustained periods of overgrazing,historical C losses can at least be partially reversed by reducinggrazing pressure (Conant and Paustian, 2002). Conversely, there isalso scope to improve grass productivity and sequester soil C byincreasing grazing pressure in grazing lands that are only lightlygrazed (Holland et al., 1992). There are several other practices that

* Corresponding author. Present address: Commonwealth Scientific and Indus-trial Research Organization, St. Lucia, Queensland, Australia. Tel.: +61 7 321 42208.

E-mail address: ben.henderson@csiro.au (B.B. Henderson).

http://dx.doi.org/10.1016/j.agee.2015.03.0290167-8809/ã 2015 Elsevier B.V. All rights reserved.

Agriculture, Ecosystems and Environment 207 (2015) 91–100

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could be used to further augment grazing land C stocks, includingthe sowing of legumes and more productive grass species, firemanagement and fertilization (Lal, 2004; Smith et al., 2007; Follettand Reed, 2010; Eagle et al., 2012). All of these measures can raiseforage production, increase returns of plant litter and dung (wheremore animals are introduced to make use of additional forage) tothe soil, and can thereby raise the amount of organic C that isincorporated into soils (Frank et al., 2012; Piniero et al., 2010). Theaugmentation of soil C stocks can also provide several agronomicand environmental co-benefits by raising soil fertility, increasingwater holding capacity, soil aggregation and reducing erosion(Conant and Paustian, 2002). The improvements to soil waterholding capacity, in particular, can increase the resilience of forageresources production to climate change. This is especiallyimportant for arid and semiarid grazing systems found in lowlatitudes where higher temperatures and lower rainfall areanticipated (Hoffman and Vogel, 2008). Further, where practicesfor augmenting soil C stocks in grazing lands can be profitabilityimplemented, the strong link between forage production and soil Cstorage offer scope for the joint delivery of production, economicand environmental benefits. Thus, these practices can providemuch needed development opportunities and increase foodsecurity for the many impoverished and marginalized pastoralistcommunities, which much of the world’s grazing lands support.

Givenwidespread interest in themitigation potential of grazinglands among policy makers and practitioners, this study seeks toestimate the effectiveness of mitigation practices in grazing lands,using biophysical and process-based models, and detailed spatialinformation. All assessments of GHG mitigation potential ingrazing lands are based on the concept that a change inmanagement practices can lead to a change in C stocks and/orN2O emissions (Conant, 2011). Thus all estimates of mitigationpotential are constructed using (1) information about C storageand N2O emission rates given a change in land management and(2) information about where land management changes arefeasible (Paustian et al., 1997). It is clear that not all managementchanges are appropriate or possible for all grazing lands, as theirapplicability and effectiveness depend on a range of factors such asaccessibility, soil conditions, climate, and current and pastmanagement. To-date, limited data on grazing land managementhave constrained the ability of researchers to delineate areasamenable to improved management from those that are not(Conant and Paustian, 2004). Another limitation of most broad-scale assessments is that they have relied on emission factorsgenerated from the synthesis ormeta-analysis of published studies

(Ogle et al., 2004; Conant et al., 2001; Smith et al., 2007). Whilethese are often the most sophisticated approaches possible(analogous to Tier 1 and Tier 2 approaches in the IPCC guidelines),they are inherently reliant on a small set of observations under arestricted set of biophysical/management conditions that is widelyextrapolated.

The work we present here contributes to the current body ofevidence about mitigation practices in the world’s grazing landsin three important ways. First, we have applied process-basedmodels – the Century and Daycent models (Parton et al., 1987,1998) – that represent the effects of a variety of managementpractices on C and N cycling in agroecosystems. These models arecapable of representing the multiple interactions betweenbiophysical processes and management at a landscape scale.Second, by using observations of past and current land use wehave confined our assessment to areas where livestock produc-tion is present and where practice changes are likely to beeffective, rather than assuming the blanket application ofmanagement practices across all or most of the world’s grazinglands. Finally, by modeling the linkage between forage, animalproduction, and animal GHG emissions we aim to shed new lighton the ‘net’mitigation potential of C sequestration practices in theworld’s grazing lands.

2. Methods

We separate grazing lands into rangelands (where we onlyconsider grazing management) and pasturelands (where, inaddition to grazingmanagement, we consider agronomic practicessuch as fertilization and legume planting). In this study we definerangelands as uncultivated land on which the native vegetation ispredominantly grasses, grass-like plants, forbs or shrubs suitablefor grazing or browsing, primarily managed through the manipu-lation of grazing (NRCS, 1997). Pasturelands, on the other hand, aredefined as those areas onwhich there is the periodic cultivation ofgrasses and other agronomic inputs such as irrigation andfertilization (NRCS, 1992; Eagle et al., 2011).

We used the Century and Daycent models to estimate soil Cstocks, N2O emissions and forage production from grazing landsglobally at 0.5� resolution. The Century model was originallydeveloped to describe ecosystem processes in grassland systems,which is reflected in model parameters and the variety manage-ment practices relevant to grazing lands. The Century and Daycentmodels are commonly used for project- and national-levelgreenhouse gas accounting because they have both been

Table 1Description of plant communities, grazing management, and fire history for rangeland biomes used for Century modeling.

Biome Grass type Tree type Grazing Fire

Season Frequency Season Intensity Frequency(in yrs)

GrasslandsShortgrass Warm season N/A Late spr-sum Annual – None –

Tall/medium Warm/cool N/A Late spr-sum Annual Fall Hot 4

SavannasTemperate Warm season Decid. trees Spr-mid sum 3/4 yrs Wet Hot 4Tropical Warm season Shrub/tree mix Spr-mid sum 3/4 yrs Wet Hot 4Wet Warm season Spr-mid sum 3/4 yrs Wet Hot 4

ShrublandsArid shrubland Warm season Sagebrush-like Spr-early sum Annual Spring Hot 30Mediterranean scrub Warm season Chapparal-type Spr-early sum Annual Spring Hot 30Xeromorphic forest Warm season Tree/shrub mix Spr-early sum Annual Spring Cool 30Desert Warm season Temp. shrubs Spr-early sum Annual Spring Hot 30

92 B.B. Henderson et al. / Agriculture, Ecosystems and Environment 207 (2015) 91–100

extensively validated against observations of changes in forageproduction, soil C stocks, and N2O fluxes in response to changes ingrazing land management from field sites around the world. TheCentury model was initiated with 2000 year spin-ups using meanmonthly climate from the Climate Research Unit (CRU) of theUniversity of East Anglia (Mitchell and Jones, 2005) withvegetation for each grid cell, except cells dominated by rock, ice,water, forest and croplands, which were excluded. The purpose ofthe model spin-up runs is to generate a stable state variables,particularly for the soil organic matter pools, which can take 1000sof years to turnover. All of the spin-ups were sufficiently long thatthese pools reached a stable equilibrium that varied from place-to-place as a function of vegetation, soil and climate. Soils data werederived from the FAO Soil Map of the World, as modified byReynolds et al. (2000). For rangelands, information about nativevegetation was derived for the Potsdam model inter-comparisonstudy (Melillo et al., 1993), and descriptions of the plantcommunities, fire frequency, land types and general assumptionsof grazing seasonality used in the Century model, are shown inTable 1. Production in pasturelands was simulated using highproductivity plant parameterizations based on cool-season (highlatitudes), warm-season (low latitudes), or mixed (mid-latitudes)grasses. Pastureswere assumed to be replanted in latewinter everyten years, with grazing starting in the second year.

In order to confine our analysis to those areas that are subject tograzing,we area-corrected the results by scaling them tomatch thearea of grazing land within each half-degree pixel. First, themaximum spatial extent of the world’s grazing lands was definedby selecting the grassland and woodland land cover classes in theGlobal Agro-Ecological Zone (GAEZ) data layers produced by theUN Food and Agricultural Organization and the InternationalInstitute for Applied Systems Analysis Global (IIASA/FAO, 2012).This area was then adjusted to match the national area ofpermanent pastures andmeadows reported in FAOSTAT in the year2005 (FAOSTAT, 2013). As a result of this step, our grazing landareas differ from those reported in the GAEZ data layer. This isjustified for two reasons. Firstly, the GAEZ areas includewoodlandsas well as grasslands. So by scaling the data to match FAOSTATreported statistics on grassland areas we end up with a morereasonable estimate of grazing area. Secondly, because theFAOSTAT figures represent the country’s officially reportedstatistics on grassland area, we considered it necessary to conformas closely as possible to this data. Next, areas where animals werenot present, based on data from FAO (2007, 2011), were excluded.The resulting total grazing land area following this procedure wasapproximately 2.6 billionha. Finally, to separate this total grazingland area into rangelands and pasturelands, rangelands wereidentified as the portion of the grazing lands that included nativevegetation (Melillo et al., 1993) with pasturelands residuallyidentified as the remainder of the total grazing land area. TheCentury and Daycent results were in the form of densities (e.g.,grams of soil C per square meter) within each pixel. Therefore,these results can easily be scaled to match the area grazing areaboundaries above without introducing inconsistencies related tospatial mismatching of the data layers.

2.1. Grazing management scenarios

Grazing management is a key determinant of C and N cyclingwithin ecosystems and it is the main management variable thatcan be altered to affect C stocks in grazing lands. Correspondingly,forage offtake, defined as the proportion of aboveground live anddeadmaterial removed by livestock, is a keymanagement driver inthe Century and Daycent models.

Forage consumption by ruminants was based on data from theGlobal Livestock Environmental Assessment Model (GLEAM)

(Gerber et al., 2013), which is a spatial model of livestockproduction systems that represents the biophysical relationshipsbetween livestock populations (FAO, 2007, 2011) and feed inputs(including the relative contribution of feed types including forages,crop residues and concentrates to animal diets) for each livestockspecies, country and production system. The production param-eters and data in GLEAM have been drawn from an exhaustivereview of the literature, and validated through consultation withexperts during several workshops and meetings. Consistencybetween GLEAM production data and FAOSTAT production datahas also been checked and affirmed. The emission intensities havealso been cross validated for ruminants across a range of regionsand studies, and published reports on GLEAM have also beenthrough rigorous peer review (Opio et al., 2013; Gerber et al., 2013).We translated a map of forage consumption from GLEAM into anestimate of forage removal rates by ruminants for each grid cell torepresent offtake rates in the Century model.

We ran the Century model for a set of grazing offtake scenariosto explore the soil C and forage benefits that producers mightrealize by shifting to grazing management that optimizes forageproduction. Since it is more feasible and beneficial for producers totry and maximize forage production than soil C sequestration(because forage production is easier to observe and it benefits farmincome), we defined the optimum as the offtake rate that led tomaximum forage production within each pixel. This optimum candiffer fromone based onmaximized soil C, because it can result in ashift away from C inputs to soil toward C offtake by livestock(Pineiro et al., 2010). All grazing was restricted to the growingseason excluding the month in which plant growth initiated. Weidentified optimum offtake rates by conducting a set of global runsfor a range of offtake rates (ranging from 0to 100% in 10%increments) and selecting the offtake rate that maximized forageproduction averaged between 1987 and 2006. In most cases thisoptimum offtake rate was different than the baseline (1901–1986)offtake rates, with baseline rates being greater than or less than ourcomputed optima. On the assumption that climate change-induced changes in GHG fluxes over the next decades will bemodest in comparisonwith the simulatedmanagement effects, thefindings from this assessment are considered to reflect the futuresequestration potential over the same 20-year time frame.

2.2. Legume planting and fertilization scenarios

Improved grazingmanagement was applied to all grazing lands(i.e., native rangelands and pasturelands), but legume planting andfertilization were only considered to be feasible in pasturelandswhich are more amenable to agronomic inputs, because of theiragroecological conditions (e.g., soil moisture availability). TheDaycent model (Parton et al., 1998) was used to simulate N2Oemissions from pasturelands under the baseline scenario andscenarios with legume sowing and fertilization. The Daycentmodel runs required daily climate data, also from CRU TS3.0(Mitchell and Jones, 2005), but otherwise relied on the same soil,plant and grazing management drivers as the Century soil C runsfor pasturelands.

Legumes were represented within the same warm/cool seasongrass mixtures as described above for grasses, and were assumedto be oversownon grass to achieve approximately 20% cover, and topersist over the course of the simulation without re-sowing oradditional inputs. The effects of different application rates ofammonium-nitrate fertilizer ranging from 0 to 140 kg Nha�1 in20kg Nha�1 increments on grass forage production, soil C stocks,and soil N2O emissions were also evaluated in a range offertilization scenarios. The impact of the legume sowing andfertilization scenarios on forage production, soil C stocks, andsoil N2O emissions were compared with the “no-legume” and

B.B. Henderson et al. / Agriculture, Ecosystems and Environment 207 (2015) 91–100 93

“no-fertilizer” baseline, using the same driving data and param-eterizations as described above. An extensive search of FAO andother global institute’s data sources revealed a severe lack of globaldata on the distribution of management practices in grazing lands.There is spatial information on crop types and fertilization forcroplands, but for grazing lands data availability is extremely pooreven at country level, even for most developed countries. Theimplications of the simplified baseline assumptions for ourfindings are discussed later. The net GHG impacts were estimatedby subtracting increases in soil N2O emissions from the amount ofsoil C sequestered, for both the legume sowing and nitrogenfertilization measures. Soil carbon stock changes and N2Oemissions were converted into CO2-equivalent emissions for thispurpose. A global warming potential of 298 was used to convertN2O emissions in CO2-equivalent emissions (IPCC, 2007). For mostof our analyses, we confined our estimation of the mitigationpotential of the entire suite ofmeasures to those grazing land areaswhere the changes in soil C stocks were positive.

2.3. Changes in ruminant GHG emissions

As the practices assessed in this study aim to increase forageproduction, related increases in ruminant production and GHGemissions from enteric digestion, manure deposition and man-agement need to be accounted for. The additional ruminant GHGemissions, associated with improved grazing management andlegume sowing, were calculated by converting the changes inforage consumption from Century and Daycent into changes inanimal numbers within the framework of GLEAM, on the basis ofaverage dry matter (DM) consumption and GHG emissions forruminant animals in each region, as specified in Opio et al. (2013).For improved grazing management we assumed no changes inanimal productivity. For example, if the increase in net forageconsumption from the Century model equated to a 10% increase inthe GLEAM-based total dry matter intake for the ruminant herd ina particular region, we used the simple assumption that both theGHG emissions and animal productionwould also increase by 10%.Nonetheless, we expect the GHG emission intensity (Ei) ofproduction within each region to fall due to the sequestration ofcarbon offsetting some of the additional emissions. For legumeplanting, we used the same approach with the additionalassumption (based on findings from Rochon et al., 2004; Minet al., 2003; Coates andMannetje, 1990; Mannetje and Jones,1990;MacLeod and Cook, 2004) that the improved additional nutritivevalue of legumes, increased animal growth rates by 15% and milkyields by 10%.

3. Results

3.1. Grazing management

We estimate that adjustments in grazing pressure, fromcurrent forage offtake rates to rates that maximize forageproduction, can sequester 148.4 Tg CO2 yr�1 (Tables 2 and 3) ingrazing lands worldwide. Of the total 2.6 billion ha of grazing landover which the Century simulations were carried out, thispractice was only found to be effective (i.e., changes in C stockswere estimated to be positive) in 28% of this area (26% ofrangeland and 33% of pastureland area). The area in whichpractices have positive mitigation benefits is described as the“amenable” area throughout the rest of this paper. Most of the Csequestration potential (�74%) was in rangelands, which containmost of the grazing land area and for which average sequestrationrates were greater than in pasturelands (0.23 versus 0.16MgCO2 ha�1 yr�1). The amount of sequestration in rangelands variedsubstantially between regions. The regions with the largestsequestration potentials were Central/South America (26.7 TgCO2 yr�1), Sub-Saharan Africa (24.3 Tg CO2 yr�1), Oceania (15.6 TgCO2 yr�1), and East/Southeast Asia (13.7 Tg CO2 yr�1), collectivelyaccounting for 73% of the total potential and 65% of the amenablerangeland area. There was moderate variation in the regionalsequestration rates, ranging from 0.13 to 0.32Mg CO2 ha�1 yr�1

(coefficient of variation = 26%), however most of the difference intotal potential between regions was due to variations in area(coefficient of variation = 72%). There was also a relationshipbetween the agroecological zones (AEZs: temperate, humid andarid) within which rangelands are located and sequestrationpotential, at both a global and regional level. At the global averagelevel, the humid rangelands have the highest C sequestrationrates, followed by arid and temperate rangelands. However, totalsequestration potentials followed a different pattern, with aridareas accounting for just over half of the total sequestrationpotential due to their dominant share of the total amenablerangeland area (Table S1). The global distribution of sequestrationrates and potentials by AEZ, was generally reflected at theregional level, with the large sequestration potentials in Central &South America, Sub Saharan Africa and Oceania, corresponding totheir large areas of rangelands in the humid and arid AEZs.Similarly, the low per hectare potentials in Central Asia, EasternEurope & Russia, East & Southeast Asia, reflect the very dominantshares of temperate rangelands in these areas.

Regional variation in sequestration potentials for pasturelandswas similar to that observed in rangelands, with Central & SouthAmerica (16.0 Tg CO2 yr�1), Sub-Saharan Africa (9.0 Tg CO2 yr�1),

Table 2Rangeland area and annual C sequestration potential by region for grid cells with positive C sequestration rates in response to changes in grazing management.

Region Amenable area C sequestration potential DForage consumption(Tg DM)

(Mha) (%) (Tg CO2) (Mg CO2ha�1)

Central & S. America 95.9 29.8 26.7 0.28 37.3Central Asia 73.8 48.5 9.8 0.13 13.1Eastern Europe & Russia 5.1 39.3 0.9 0.17 0.8East & South-East Asia 71.0 46.8 13.7 0.19 5.9Middle East & N. Africa 30.4 18.2 6.4 0.21 14.4North America 43.9 34.5 9.3 0.21 5.1Oceania 57.6 15.9 15.6 0.27 28.2South Asia 11.9 20.3 2.9 0.24 4.5Sub-Saharan Africa 79.1 16.6 24.3 0.31 76.0Western Europe 1.4 15.1 0.4 0.32 0.2

World total 470.2 25.5 110.1 0.23 185.7

94 B.B. Henderson et al. / Agriculture, Ecosystems and Environment 207 (2015) 91–100

and East & Southeast Asia (6.3 Tg CO2 yr�1) again among the topfour contributors to total global potential. However, the pasture-land potentials are more skewed, with these three regionscontributing 82% of the total 38.3 Tg CO2 yr�1 of C sequesteredglobally. As with rangelands, the large potentials in these regionscan mostly be ascribed to their larger areas, with these threeregions accounting for 75% of the total amenable pastureland area(Table 3). In addition to the sequestration rates being generallylower for pasturelands than for rangelands, they also display morevariation between regions (coefficient of variation (CV) of 53%).The difference in the rates between rangelands and pasturelandsreflect differing physiological potential and the alignment betweencurrent and forage-optimized management practices. For bothpasturelands and rangelands, higher sequestration rates wereestimated in overgrazed areaswhere grazing pressurewas reducedthan in underutilized areas where grazing pressure was raised(0.26Mg CO2ha�1 versus 0.14Mg CO2ha�1 for pasturelands, and0.26Mg CO2ha�1 versus 0.21Mg CO2ha�1 for rangelands).However, whereas rangelands have comparable shares of area in

which offtake rates are increased (40%) or lowered (44%), thearea share in which offtake rates were raised in pasturelands wasmuch larger (83%). Another notable differencewas that therewas amuch higher proportion of land in which the optimum forageofftake rates were within �30% points of the baseline rates inpasturelands (99%) than in rangelands (69%). The sequestrationrateswere estimated to be lower on these areas than on landwherethe optimum offtake was beyond �30% of the baseline rates.As with rangelands, the sequestration rate in the humid grazingareas was the highest, although for pasturelands temperate areashad an equally high global average rate. A further differencewith rangelands, was that the sequestration rate in the arid AEZwas far lower than either the temperate or humid AEZs. Aroundhalf of total C sequestration potential and amenable pastureland area was found in the humid pasturelands, with near equalshares of the remaining sequestration potential in temperateand arid pasturelands (Table S2). Amap of the global distribution ofthe soil C sequestration potential in grazing lands is provided inFig. 1.

Table 3Pastureland area and annual C sequestration potential for grid cells with positive C sequestration rates in response to changes in grazing management.

Region Amenable area C sequestration potential DForage consumption(Tg DM)

(Mha) (%) (Tg CO2) (Mg CO2ha�1)

Central & S. America 69.7 55.6 16.0 0.23 246.9Central Asia 2.9 27.6 0.4 0.15 0.9Eastern Europe & Russia 2.0 4.1 0.1 0.03 7.5East & South-East Asia 46.1 21.4 6.3 0.14 112.1Middle East & N. Africa 14.5 27.6 1.3 0.09 9.6North America 9.4 22.1 1.3 0.14 8.5Oceania 16.3 55.7 1.5 0.09 12.9South Asia 6.2 31.0 2.0 0.32 0.1Sub-Saharan Africa 65.5 43.2 9.0 0.14 275.9Western Europe 8.8 21.1 0.5 0.05 11.6

World total 241.4 32.7 38.3 0.16 686.1

[(Fig._1)TD$FIG]

Fig. 1. Global distribution of soil C sequestration potential, from improved grazing management in the world’s grazing lands (rangelands and pasturelands combined).

B.B. Henderson et al. / Agriculture, Ecosystems and Environment 207 (2015) 91–100 95

Increases in the forage offtake rate in under-grazed areas almostinvariably resulted in more forage consumption. Perhaps moresurprising is that increases in forage consumption were registeredin 41% of the 276 millionha over overgrazed areas inwhich offtakerates were lowered. Taken together, forage consumption wasestimated to increase in 77%, and decrease in 23%, of the totalamenable grazing land area in which offtake rates were adjusted,leading to a total net gain in forage consumption of 871Tg DMyr�1,with the vast majority coming from pasturelands (79%). Corre-spondingly, the overwhelming majority of the sequestrationpotential (80% in both rangelands and pasturelands) was in areaswhich registered an increase in the amount of forage consumed byruminants.

Given that improved grazing management causes losses of soilC stocks over much of the total grazing area, we have presentedmean sequestration rates from the application of this practiceacross both the amenable rangeland area and the entire rangelandarea, in Table 6. These results provide a range of the expectedmeansequestration rates for each region, based on (a) the perfecttargeting of the practice (i.e., applying them only in areas wherethey result in positive sequestration) to (b) the untargetedapplication of the practice within the entire rangeland area. Thus(a) and (b) can be thought of as upper and lower boundsequestration potentials, respectively. We have also includedcoefficients of variation to show the variance of sequestrationoutcomes in each region. Notably, in the absence of any spatialtargeting, this practice leads to negative sequestration outcomes inall but two of the ten regions, namely, Central Asia and East &South-East Asia, with marginally negative rates in North Americaand Eastern Europe & Russia. The coefficients of variation from thetargeted scenario, range from low (<100%) to high (>100%),whereas they are consistently high in the untargeted scenario. Theglobal distribution of both the amenable and non-amenable areasfor this practice, for all grazing lands, is shown in Fig. 1.

3.2. Sowing legumes

Sowing of legumes in pasturelands increased soil C stocks inhalf of the global pastureland area, but net GHGmitigation (whereincreased soil C stocks were greater than increased N2O emissions)was estimated to occur in only 10% of the global pastureland area(Table 4). Despite the relatively small area, we estimated global Csequestration potential on the pastureland areas where netmitigation was positive (203.4 Tg CO2 yr�1) to be greater thanfor improved grazing management on both rangelands andpasturelands combined. The largest potentials were in Central &S. America (61.7 Tg CO2 yr�1), Western Europe (53.8 Tg CO2 yr�1),

Oceania (27.3 Tg CO2 yr�1), and the Middle East & North Africa(19.6 Tg CO2yr�1). Sequestration rates were estimated to be muchlarger than those for grazing management in rangelands andpasturelands, averaging 2.8Mg CO2ha�1 yr�1.

In the pastureland areas where net mitigation was positive,legume sowing increased N2O emissions by 122Gg N2O��Nyr�1

(56.9 Tg CO2-eqyr�1) offsetting 28% of the global GHG mitigationbenefits of C sequestration, resulting in the global net mitigation of146.5TgCO2yr�1(Table4).FluxesofN2Oincreasedbyanaveragerateof 1.7kg N2O��Nha�1 yr�1, with substantial variation betweenregions ranging from 0.01 to 9.1 kg N2O��Nha�1 yr�1. Despite thesehigher emissions, the net mitigation rate from sowing legumes,estimated to be 2.0Mg CO2-eqha�1 yr�1, remained much greaterthan for grazing management (0.21Mg CO2-eqha�1 yr�1 for allamenablegrazing land). Aswith thefindings for soil C sequestration,the same four regionsWesternEurope (46.3 TgCO2-eqyr�1), Central& S. America (30.8Tg CO2-eqyr�1), Oceania (18.4 Tg CO2-eqyr�1),and the Middle East & North Africa (17.7Tg CO2-eqyr�1) accountedfor more than three quarters of the total net mitigation potential. Amap displaying the global distribution net mitigation from legumesowing is provided in Fig. 2, and the corresponding soil Csequestration and N2O emissions from this practice are providedin the Supplementary information for this paper (Fig. S1, Fig. 2).

As with improved grazing management, the average global rateof C sequestration (in the areas where net mitigation occurs) ishighest in the humid pasturelands (Table S3). However, humidareas comprised the smallest share of the total net mitigationpotential, owing to their relatively small amenable area. Forlegume sowing, the temperate pasturelands dominate, accountingfor nearly half (49%) of the total amenable area and similar amount(43%) of the total net mitigation potential. These patterns arereflected to some extent at the regional level. For example most ofthe amenable area and mitigation potential in Western Europeoccurs in temperate pasturelands, despite this AEZ having thelowest net mitigation rate in this region. The pattern differs forOceania, where most the amenable area is found in the humidpasturelands, which also have the region’s highest net mitigationrate. However, for Central & South America, there is very littledifference between AEZs with regard to both amenable pasture-land area and net mitigation rates.

As with grazingmanagement, the sowing of legumes also led tosubstantial increases in forage consumption, averaging 0.44Mgbiomass ha�1 yr�1, globally. The regional distribution of theseincreases tended to mirror those of the increases in net mitigationand sequestration, with most of the increased forage located inCentral & S. America, Oceania and Western Europe. Interestingly,while the share of soil C sequestrationwas very low in Sub Saharan

Table 4Pastureland area and annual C sequestration for cells with positive net mitigationwhen sownwith legumes, along with annual changes in N2O flux rates, net GHGmitigationpotential, and change in forage consumed by livestock.

Region Amenable area C sequ.potential(Tg CO2)

DN2O flux(TgCO2-eq)

Net GHG mitigation(TgCO2-eq)

Net GHG mitigation rate(MgCO2-eqha�1)

DForage consumption(Tg DM)

(Mha) (%)

Central & S. America 7.3 5.8 61.7 30.9 30.8 4.2 14.7Central Asia 1.0 9.0 0.5 0.0 0.5 0.5 0.3Eastern Europe & Russia 4.0 8.2 4.5 0.0 4.4 1.1 0.6East & South-East Asia 13.7 6.4 18.6 5.2 13.4 1.0 2.6Middle East & N. Africa 9.0 17.2 19.6 1.9 17.7 2.0 1.9North America 5.6 13.2 7.5 0.4 7.2 1.3 0.4Oceania 6.6 22.6 27.3 8.9 18.4 2.8 4.7South Asia 3.1 15.7 4.6 0.7 3.9 1.2 1.0Sub-Saharan Africa 5.9 3.9 5.3 1.4 3.9 0.7 2.6Western Europe 15.5 37.1 53.8 7.5 46.3 3.0 3.1

World total 71.8 9.7 203.4 56.9 146.5 2.0 31.8

96 B.B. Henderson et al. / Agriculture, Ecosystems and Environment 207 (2015) 91–100

Africa (3%), it accounted for a relatively larger share of totalincrease in forage (8%). In aggregate, our results show that sowinglegumes would increase annual forage production on pasturelandsby about 31.8 Tg DM from 71.8 millionha.

As indicated in Tables 2 and 4 and Fig. 2, the relative area overwhich legume sowing risks increasing, instead of mitigating, soil-based GHGs is much higher than for the improved grazingmanagement practice. Further details about this risk can begleaned from Table 6, which displays ranges of net mitigation ratesfor each region based on (a) the perfect targeting of the practiceand (b) the untargeted application of the practice within the entirepastureland area. Not only is the relative amenable area for legumeplanting much smaller than grazing management, the risks ofmassive increases in soil-based GHGs through poor targeting aremuch higher for legume sowing, as shownby the large negative netmitigation rates for most regions in the untargeted scenario(Table 6).

In order to develop information that could facilitate identifica-tion of regions (cells) with positive GHG balance responses tochanges in management, we carried out regression and

discrimination analyses to evaluate whether rainfall, temperature,soil edaphic properties, or baseline grazing pressure could explainpositive and negative mitigation outcomes. The results from theseanalyses did not clearly indicate any predictive variables thatwould enable us to enhance targeting at this scale, though it didconfirm regional variation (as described above and in Table 6).

3.3. Fertilization

Fertilization led to increased forage production, but rarely led toincreased soil C stocks and always led to increased N2O emissions.Moreover, net GHG emissions increased for all fertilization rates inall regions, because increases in N2O emissions always exceededsoil C sequestration in CO2 equivalent terms. Very few fertilizationtreatments led to increased soil C stocks in comparison with theno-fertilization control, with the only exceptions in Central/SouthAmerica and Western Europe (Table 5). Even in those regions,sequestration rates were low, ranging from 0.001 to 0.002MgCO2ha�1 yr�1. Almost all (�99%) of the potential sequestration inthose regions was realized with low rates of N inputs (20 kg

[(Fig._2)TD$FIG]

Fig. 2. Global distribution of the net mitigation potential from sowing legumes in pasturelands.

Table 5Pastureland area and annual C sequestration, along with annual changes in N2O flux rates, net greenhouse gas mitigation potential, and change in forage consumed bylivestock (all averaged across multiple N fertilization rates ranging from 20 to 140 kgNha�1 yr�1), and emission factors normalized by unit of N added.

Region Area(Mha)

C sequestration(Mg CO2ha�1)

D N2O emissions(kg N2O-Nha�1)

N2O emission factor(kg N2O-N/kg fert. N)

Net GHG mitigation(Tg CO2-eq)

DForage consumption(t DMha�1)

Central & S. America 12.9 0.002 0.89 0.011 �5.4 0.1Central Asia 6.4 �0.112 0.62 0.008 �2.6 2.9Eastern Europe & Russia 1.4 �0.007 0.29 0.004 �0.2 0.5East & South-East Asia 28.4 �0.005 0.25 0.003 �3.5 0.4Middle East & N. Africa 30.5 �0.127 0.28 0.003 �7.9 4.3North America 3.8 �0.005 0.15 0.002 �0.3 0.3Oceania 18.1 �0.015 0.44 0.005 �4.0 0.6South Asia 13.6 �0.085 0.47 0.006 �4.2 2.8Sub-Saharan Africa 32.3 �0.039 0.29 0.004 �5.6 4.7Western Europe 8.0 0.001 0.14 0.002 �0.5 0.4

World total 155.3 �0.048 0.37 0.005 �34.1 2.4

B.B. Henderson et al. / Agriculture, Ecosystems and Environment 207 (2015) 91–100 97

Nha�1 yr�1) and estimated sequestration rates (and amounts)changed minimally with increasing N inputs. These results mayappear counterintuitive given that N fertilization can increaseplant production and subsequent organic C inputs to soil. However,N fertilization can also lead to reductions in soil C stocks byaccelerating decomposition of soil C, particularly when applied inexcess of plant requirements (Kuzyakov et al., 2010; Khan et al.,2007) or by shifting biomass allocation from roots to shoots(Farrior et al., 2013). Our results suggest the processes that led tosoil C losses exceeded those that led to C gains inmost cases.Whennormalized for N input rate, N2O emissions increases were greaterat higher fertilization rates in all regions. Increases in forageconsumption were correlated with increases in fertilization rates,and in the two regions where soil C stocks increased, fertilizationincreased forage production by an average of 0.2Mg biomass ha�1

yr�1, summing to a total increase of 3.7 Tg biomass yr�1.

3.4. Changes in ruminant GHG emissions

The net increases in forage production from the practicesassessed in this study involve tradeoffs with higher ruminant GHGemissions, because the consumption of this additional forage bygrazing ruminants would require higher numbers of ruminants. Asshown in Table 7, the impacts of these higher animal emissions(CH4 and N2O from enteric fermentation and manure) would fullyoffset all of the C sequestration gains from improved grazingmanagement in both rangelands and pasturelands. For legumes, onthe other hand, increases in forage consumption and associatedruminant emissions are estimated to only offset 26% of the net soilC sequestration potential of this practice at the global level.

However, thanks to the C sequestration potential of thesepractices, the additional animal output is possible with a loweremission intensity than in the baseline, within each region andAEZ. Improved grazing management resulted in a 10% reduction inthe emission intensity of ruminant production in rangelands. Forpasturelands, there is a 10% increase in emission intensity, despitereductions in each region, because production increases by muchmore in emission intensive regions (e.g., Sub-Saharan Africa andCentral & South America) relative to other regions in this scenario.Because this practice results in a net increase in animal production,it could deliver netmitigation benefits if its applicationwere scaledback in some of the areas in which production increases, withoutchanging the total baseline production in each region. Althoughthis possibility was not assessed in this study. For legume sowing, arelatively smaller increase in forage production and animalnumbers, combined much larger per hectare rates of C sequestra-tion and slight improvements in animal productivity, resulted in asubstantial 59% reduction in emission intensity (Table 7).

4. Discussion

The amount of C that could be stored in the world’s grazinglands is considerable and presents a potentially large mitigationopportunity. We estimate the global potential for C sequestrationin the soils of the world’s grazing lands is 352Tg CO2 yr�1 throughimproved grazing management in rangelands and pasturelands,and the sowing of legumes in pasturelands. When subtracting theincrease in soil N2O emissions associated with legume sowing, theglobal net sequestration potential of the assessed practices falls to295 Tg CO2yr�1. However, the realization of this potential woulddepend on being able to, a priori, identify and target areas whichare amenable to the selected practices. As shown in Table 6, therisks of increasing rather than mitigating GHG emissions are high,particularly for legumes. More work is needed to developindicators, based on biophysical and management attributes, toidentify amenable grazing areas and ameliorate these risks.

Notwithstanding these challenges it is useful to put our resultsinto perspective by comparing them with those from other globalassessments. Evenwhen restricting our focus to amenable areas foreach practice, our total global potential tends to be smaller thancomparable global studies in the literature. This is due todifferences in the blend of practices considered and because wetend to apply the practices to a smaller aggregate area, according toa priori considerations about the types of grazing lands in whichthey can be implemented and according to outputs from ourprocess-based models about where they are effective. Ourestimated potential for increasing C stocks in grazing lands issubstantially less than the previous global estimate of 1.4GtCO2 yr�1 by Smith et al. (2007, 2008), but that study used astatistical approach to derive sequestration potentials for differentclimatic zones from a different combination of measures (grazingmanagement, nutrient management, irrigation, improved speciesand fire management) and assumed these potentials could be

Table 6The mitigation potential, expressed as means with coefficients of variation (CV),from targeted versus untargeted application of grazing management in rangelandsand legume sowing in pasturelands.

Region Improved grazingmanagement(Mg CO2ha�1)

Legume sowing(Mg CO2-eqha�1)

Targeted Untargeted Targeted Untargeted

Mean CV(%)

Mean CV(%)

Mean CV(%)

Mean CV(%)

Central & S. America 0.28 117 �0.18 300 4.2 79 �57.3 75Central Asia 0.13 78 0.03 306 0.5 71 0.5 71Eastern Europe &Russia

0.17 65 �0.02 4145 1.1 95 �9.8 135

East & South-East Asia 0.19 136 0.01 1780 1.0 132 �27.5 107Middle East & N. Africa 0.21 125 �0.32 156 2.0 115 �1.8 421North America 0.21 116 �0.07 749 1.3 70 �17.8 131Oceania 0.27 98 �0.25 170 2.8 70 �13.9 167South Asia 0.24 100 �0.11 393 1.2 121 �17.7 148Sub-Saharan Africa 0.31 104 �0.39 146 0.7 223 �78.4 58Western Europe 0.32 115 �0.26 186 3.0 53 �5.7 160

World total 0.23 �0.21 2.0 �41.9

Table 7Summary of mitigation effects at global level, including direct emissions from animals.

Practice change Landtype

Net mitigation (excl.animal)(Tg CO2-eq yr�1)

DForageproduction(Tg DMyr�1)

Baseline animalGHGs(Tg CO2-eq yr�1)

D animalGHGs(Tg CO2-eq yr�1)

Net mitigation (incl.animal)(Tg CO2-eqyr�1)

Baseline Ei(kgCO2-eq/kgprotein)

DEi(%)

Grazingmanagement

Range 110 186 542 156 �46 128 �10Pasture 38 686 490 565 �526 126 10

Legumes Pasture 147 32 216 38 109 64 �59

98 B.B. Henderson et al. / Agriculture, Ecosystems and Environment 207 (2015) 91–100

applied to a much larger global grazing land area (2.4 billionhacompared to our total amenable area of 0.7 billionha). The meanper hectare sequestration potentials reported by Smith et al.(2007) of between 0.11 and 0.81Mg CO2ha�1, are generally higherbut of the same order ofmagnitude as those estimated in our study.On the other hand, our total global potential of 0.35Gt CO2 yr�1

falls close to the middle of the range of 0.04–1.1Gt CO2 yr�1

estimated by Lal (2004) in another global assessment of the soil Csequestration potential in grazing lands. While Lal (2004) alsoconsidered a broader range practices (grazing management,improved species, nutrient management and fire management),he reported sequestration rates specific to grazing management ofbetween 0.18 and 0.55Mg CO2ha�1, which are again higher butsimilar to those estimated in our study. Our findings also showreasonable agreement with regional sequestration potentials fromother studies. Conant and Paustian (2002) considered reducinggrazing pressure on degraded land and estimated lower potentialsfor North America (8.1 Tg CO2 yr�1) and Europe/Asia (15.8 TgCO2 yr�1), a similar potential for Oceania (16.1 Tg CO2 yr�1), andhigher potentials for South America (66.4 Tg CO2yr�1) and Africa(61.2 Tg CO2 yr�1). Thornton and Herrero (2010) also assess theimpact of restoring degraded rangelands, and estimate highersequestration potentials than our study for Sub-Saharan African(96.7 Tg CO2 yr�1) and Central and South America (53.6 TgCO2 yr�1). Lower rates of C storage could also reflect the 20 yeartime horizon of our study, which contrasts with other studies thathave not explicitly addressed how long sequestration endures.

It is clear that the N2O emissions associated with N fertilizationor N fixation offset much of the mitigation that can be achievedthrough the accumulation of soil C stocks, and that these emissionsneed to be accounted for (Schlesinger, 2010). However, our worksuggests that the C sequestration benefits of sowing legumes tendto outweigh the N-induced increases in N2O emissions for a smallbut still significant area of pasturelands. As with improved grazingmanagement, generalizations can be made about where, amongthe global regions and AEZs, legume sowing could be mosteffective. However, more work is needed to identify amenableareas, based on their biophysical and management attributes, toavoid sowing of legumes in pasturelands with the potential forlarge increases in soil-based GHG emissions (Table 6). Given thesize of these risks and the lack of baseline information areas sownwith legumes, it is difficult to recommend legume sowing as a largescale mitigation option without further research.

Perhaps the most encouraging result is the large combined netincrease in forage consumption resulting from improved grazingmanagement and legume sowing (totaling 904 Tg DMworldwide).This finding demonstrates the strong synergistic role that grazingmanagement can have with regard to the delivery of environmen-tal, economic and food security benefits. However, this study(which is the first global assessment that we are aware of thatexplicitly models the linkage between soil C sequestration, animalproduction and GHG emissions) reveals that these productionbenefits involve significant tradeoffs with higher ruminant GHGemissions. More importantly, the implied growth in the animalnumbers for each practice presents a useful developmentopportunity for extensive grazing systems, and has to be placedin the context of expected demand growth for animal products.Increased outputs can have significant food security benefits forpastoral communities, and these productivity gains are particular-ly desirable given the limited potential for the further expansion ofgrazing lands, worldwide (Asner et al., 2004).

In addition to the need for further modeling work to discernattributes and indicators for grazing areas with high mitigationpotential, further on-ground research and piloting is needed toverify the long-term feasibility and economic viability of theassessed practices. Furthermore, concerns about the permanency

of C sequestration and challenges associated with measurementand project coordination (particularly on communal lands) canlower the demand for C sequestration projects in agricultural landsfrom investors and policy makers (Larson et al., 2011; Gerber et al.,2013). Despite these challenges, some countries have managed toeffectively integrate C sequestration activities in grazing lands intonational level mitigation policy instruments. Notable examples areBrazil’s ABC programme, which includes the large scale restorationof degraded grazing lands to restore soil C stocks (Ministéro daAgricultura, 2014), and the suite of government programs(Grassland Ecology Conservation Subsidy and RewardMechanism;Grassland Retirement Program) in China to incentivize the uptakeof sustainable grassland management practices (ADB, 2014).

5. Conclusions

Previous findings about the massive soil C sequestrationpotential of the world’s grazing lands have spurred muchenthusiasm among scientists and policy makers about the bigrole that these lands could play in offsetting global GHG emissions.In this study, findings from process-based models, Century andDaycent, show some agreement with the per hectare mitigationrates from previous studies. However, our results indicate thatgrazing management, legume sowing and fertilization practices,are likely to deliver net mitigation benefits in less than one third ofthe world’s total grazing land area. Consequently, while stillsubstantial, our total annual net soil C sequestration potential of295Tg CO2 yr�1 is lower than the global estimates from previousstudies (Smith et al., 2007; Lal, 2004). However, in the absence ofreliable indicators to help avoid non-amenable areas, the risks ofthe assessed practices increasing instead of mitigating GHGemissions is high, particularly for legumes. Given this risk, alongwith a lack of baseline management data, it is difficult torecommend legume sowing as reliable global-scale mitigationoption. Furthermore, the additional ruminant GHG emissionsassociated with the assessed practices, are likely to substantiallyreduce the mitigation potential of the assessed practices,particularly for improved grazing management. Nevertheless,the growth in less emission intensive animal production associatedwith these practices can provide important development oppor-tunities for pastoralists, and has to be placed in the context ofexpected demand growth for animal products.

Acknowledgements

This research was supported by a Queensland Smart FuturesFellowship, and grants from the UN Food and AgriculturalOrganization, the Mitigation of Climate Change in Agriculture(MICCA) Programme, and the U.S. Environmental ProtectionAgency to Colorado State University.

Appendix A. Supplementary data

Supplementary data associatedwith this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.agee.2015.03.029.

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