Conant, richard t et al 2015 greenhouse gas mitigation potential of the world’s grazing lands modeli

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

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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 a

UN Food and Agriculture Organization, Rome, Italy Commonwealth Scientific and Industrial Research Organization, St.Lucia, Queensland, Australia Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, CO, United States d International Livestock Research Institute, Nairobi, Kenya b c

A R T I C L E I N F O

A B S T R A C T

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

This study provides estimates of the net GHG mitigation potential of a selected range of management practices in the world’s native and cultivated grazing lands. The Century and Daycent models are used to calculate the changes in soil carbon stocks, soil N2O emissions, and forage removals by ruminants associated with these practices. GLEAM is used in combination with these models to establish grazing area boundaries and to parameterize links between forage consumption, animal production and animal GHG emissions. This study provides an alternative to the usual approach of extrapolating from a small number of field studies and by modeling the linkage between soil, forage and animals it sheds new light on the net mitigation potential of C sequestration practices in the world’s grazing lands. Three different mitigation practices are assessed in this study, namely, improved grazing management, legume sowing and N fertilization. We estimate that optimization of grazing pressure could sequester 148 Tg CO2 yr 1.

The soil C sequestration potential of 203 Tg CO2 yr 1 for legume sowing was higher than for improved grazing management, despite being applied over a much smaller total area. However, N2O emissions from legumes 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. Our estimated potential for increasing C stocks though in grazing lands is lower than earlier worldwide estimates (Smith et al., 2007; Lal, 2004), mainly due to the much smaller grazing land area over which we estimate mitigation practices to be effective. A big concern is the high risk of the practices, particularly legumes, increasing soil-based GHGs if applied outside of this relatively small effective area. More work is needed to develop indicators, based on biophysical and management characteristics of grazing lands, to identify 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 substantially reduce the mitigation potential of these practices, but could contribute to more GHG-efficient livestock production. ã 2015 Elsevier B.V. All rights reserved.

Keywords: Grazing management Legume Fertilization Century Daycent

1. Introduction There is widespread enthusiasm for harnessing the large soil carbon (C) sequestration potential of grazing lands to offset global greenhouse gas (GHG) emissions, owing to their vast land area, widespread history of degradation, and potential for improved management. Their capacity for soil C storage is estimated to be a similar order of magnitude as the potential in croplands and forests

* Corresponding author. Present address: Commonwealth Scientific and Industrial 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.2015.03.029 0167-8809/ ã 2015 Elsevier B.V. All rights reserved.

(Smith et al., 2007). Consequently, grazing land C sequestration is being considered as an important component of national GHG mitigation programs by countries, including Brazil and China.

Nevertheless, many practical challenges remain, chief among them is uncertainty in the magnitude of the potential and costs associated with the adoption of sequestration practices.

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

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

could be used to further augment grazing land C stocks, including the sowing of legumes and more productive grass species, ďŹ re management and fertilization (Lal, 2004; Smith et al., 2007; Follett and Reed, 2010; Eagle et al., 2012). All of these measures can raise forage production, increase returns of plant litter and dung (where more animals are introduced to make use of additional forage) to the soil, and can thereby raise the amount of organic C that is incorporated into soils (Frank et al., 2012; Piniero et al., 2010). The augmentation of soil C stocks can also provide several agronomic and environmental co-beneďŹ ts by raising soil fertility, increasing water holding capacity, soil aggregation and reducing erosion (Conant and Paustian, 2002). The improvements to soil water holding capacity, in particular, can increase the resilience of forage resources production to climate change. This is especially important for arid and semiarid grazing systems found in low latitudes where higher temperatures and lower rainfall are anticipated (Hoffman and Vogel, 2008). Further, where practices for augmenting soil C stocks in grazing lands can be proďŹ tability implemented, the strong link between forage production and soil C storage offer scope for the joint delivery of production, economic and environmental beneďŹ ts. Thus, these practices can provide much needed development opportunities and increase food security for the many impoverished and marginalized pastoralist communities, which much of the world’s grazing lands support.

Given widespread interest in the mitigation potential of grazing lands among policy makers and practitioners, this study seeks to estimate the effectiveness of mitigation practices in grazing lands, using biophysical and process-based models, and detailed spatial information. All assessments of GHG mitigation potential in grazing lands are based on the concept that a change in management practices can lead to a change in C stocks and/or N2O emissions (Conant, 2011). Thus all estimates of mitigation potential are constructed using (1) information about C storage and N2O emission rates given a change in land management and (2) information about where land management changes are feasible (Paustian et al., 1997). It is clear that not all management changes are appropriate or possible for all grazing lands, as their applicability and effectiveness depend on a range of factors such as accessibility, soil conditions, climate, and current and past management. To-date, limited data on grazing land management have constrained the ability of researchers to delineate areas amenable to improved management from those that are not (Conant and Paustian, 2004). Another limitation of most broadscale assessments is that they have relied on emission factors generated from the synthesis or meta-analysis of published studies

(Ogle et al., 2004; Conant et al., 2001; Smith et al., 2007). While these 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 a restricted set of biophysical/management conditions that is widely extrapolated.

The work we present here contributes to the current body of evidence about mitigation practices in the world’s grazing lands in three important ways. First, we have applied process-based models – the Century and Daycent models (Parton et al., 1987, 1998) – that represent the effects of a variety of management practices on C and N cycling in agroecosystems. These models are capable of representing the multiple interactions between biophysical processes and management at a landscape scale.

Second, by using observations of past and current land use we have conďŹ ned our assessment to areas where livestock production is present and where practice changes are likely to be effective, rather than assuming the blanket application of management practices across all or most of the world’s grazing lands. Finally, by modeling the linkage between forage, animal production, and animal GHG emissions we aim to shed new light on the ‘net’ mitigation potential of C sequestration practices in the world’s grazing lands.

2. Methods We separate grazing lands into rangelands (where we only consider grazing management) and pasturelands (where, in addition to grazing management, we consider agronomic practices such as fertilization and legume planting). In this study we deďŹ ne rangelands as uncultivated land on which the native vegetation is predominantly grasses, grass-like plants, forbs or shrubs suitable for grazing or browsing, primarily managed through the manipulation of grazing (NRCS, 1997). Pasturelands, on the other hand, are deďŹ ned as those areas on which there is the periodic cultivation of grasses and other agronomic inputs such as irrigation and fertilization (NRCS, 1992; Eagle et al., 2011).

We used the Century and Daycent models to estimate soil C stocks, N2O emissions and forage production from grazing lands globally at 0.5 resolution. The Century model was originally developed to describe ecosystem processes in grassland systems, which is reected in model parameters and the variety management practices relevant to grazing lands. The Century and Daycent models are commonly used for project- and national-level greenhouse gas accounting because they have both been

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

Biome

Grass type

Tree type

Grazing

Fire

Season

Frequency

Season

Intensity

Frequency (in yrs)

Grasslands Shortgrass Tall/medium

Warm season Warm/cool

N/A N/A

Late spr-sum Late spr-sum

Annual Annual

– Fall

None Hot

– 4

Savannas Temperate Tropical Wet

Warm season Warm season Warm season

Decid. trees Shrub/tree mix

Spr-mid sum Spr-mid sum Spr-mid sum

3/4 yrs 3/4 yrs 3/4 yrs

Wet Wet Wet

Hot Hot Hot

4 4 4

Shrublands Arid shrubland Mediterranean scrub Xeromorphic forest Desert

Warm Warm Warm Warm

Sagebrush-like Chapparal-type Tree/shrub mix Temp. shrubs

Spr-early Spr-early Spr-early Spr-early

Annual Annual Annual Annual

Spring Spring Spring Spring

Hot Hot Cool Hot

30 30 30 30

season season season season

sum sum sum sum

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

extensively validated against observations of changes in forage production, soil C stocks, and N2O fluxes in response to changes in grazing land management from field sites around the world. The Century model was initiated with 2000 year spin-ups using mean monthly climate from the Climate Research Unit (CRU) of the University of East Anglia (Mitchell and Jones, 2005) with vegetation for each grid cell, except cells dominated by rock, ice, water, forest and croplands, which were excluded. The purpose of the model spin-up runs is to generate a stable state variables, particularly for the soil organic matter pools, which can take 1000s of years to turnover. All of the spin-ups were sufficiently long that these pools reached a stable equilibrium that varied from place-toplace as a function of vegetation, soil and climate. Soils data were derived from the FAO Soil Map of the World, as modified by Reynolds et al. (2000). For rangelands, information about native vegetation was derived for the Potsdam model inter-comparison study (Melillo et al., 1993), and descriptions of the plant communities, fire frequency, land types and general assumptions of grazing seasonality used in the Century model, are shown in Table 1. Production in pasturelands was simulated using high productivity plant parameterizations based on cool-season (high latitudes), warm-season (low latitudes), or mixed (mid-latitudes) grasses. Pastures were assumed to be replanted in late winter every ten years, with grazing starting in the second year.

In order to confine our analysis to those areas that are subject to grazing, we area-corrected the results by scaling them to match the area of grazing land within each half-degree pixel. First, the maximum spatial extent of the world’s grazing lands was defined by selecting the grassland and woodland land cover classes in the Global Agro-Ecological Zone (GAEZ) data layers produced by the UN Food and Agricultural Organization and the International Institute for Applied Systems Analysis Global (IIASA/FAO, 2012).

This area was then adjusted to match the national area of permanent pastures and meadows reported in FAOSTAT in the year 2005 (FAOSTAT, 2013). As a result of this step, our grazing land areas differ from those reported in the GAEZ data layer. This is justified for two reasons. Firstly, the GAEZ areas include woodlands as well as grasslands. So by scaling the data to match FAOSTAT reported statistics on grassland areas we end up with a more reasonable estimate of grazing area. Secondly, because the FAOSTAT figures represent the country’s officially reported statistics on grassland area, we considered it necessary to conform as closely as possible to this data. Next, areas where animals were not present, based on data from FAO (2007, 2011), were excluded.

The resulting total grazing land area following this procedure was approximately 2.6 billion ha. Finally, to separate this total grazing land area into rangelands and pasturelands, rangelands were identified as the portion of the grazing lands that included native vegetation (Melillo et al., 1993) with pasturelands residually identified as the remainder of the total grazing land area. The Century 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 area boundaries above without introducing inconsistencies related to spatial mismatching of the data layers.

2.1. Grazing management scenarios Grazing management is a key determinant of C and N cycling within ecosystems and it is the main management variable that can be altered to affect C stocks in grazing lands. Correspondingly, forage offtake, defined as the proportion of aboveground live and dead material removed by livestock, is a key management driver in the Century and Daycent models.

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

93

(Gerber et al., 2013), which is a spatial model of livestock production systems that represents the biophysical relationships between 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 livestock species, country and production system. The production parameters and data in GLEAM have been drawn from an exhaustive review of the literature, and validated through consultation with experts during several workshops and meetings. Consistency between GLEAM production data and FAOSTAT production data has also been checked and affirmed. The emission intensities have also been cross validated for ruminants across a range of regions and studies, and published reports on GLEAM have also been through rigorous peer review (Opio et al., 2013; Gerber et al., 2013).

We translated a map of forage consumption from GLEAM into an estimate of forage removal rates by ruminants for each grid cell to represent offtake rates in the Century model.

We ran the Century model for a set of grazing offtake scenarios to explore the soil C and forage benefits that producers might realize by shifting to grazing management that optimizes forage production. Since it is more feasible and beneficial for producers to try and maximize forage production than soil C sequestration (because forage production is easier to observe and it benefits farm income), we defined the optimum as the offtake rate that led to maximum forage production within each pixel. This optimum can differ from one based on maximized soil C, because it can result in a shift away from C inputs to soil toward C offtake by livestock (Pineiro et al., 2010). All grazing was restricted to the growing season excluding the month in which plant growth initiated. We identified optimum offtake rates by conducting a set of global runs for a range of offtake rates (ranging from 0to 100% in 10% increments) and selecting the offtake rate that maximized forage production averaged between 1987 and 2006. In most cases this optimum offtake rate was different than the baseline (1901–1986) offtake rates, with baseline rates being greater than or less than our computed optima. On the assumption that climate changeinduced changes in GHG fluxes over the next decades will be modest in comparison with the simulated management effects, the findings from this assessment are considered to reflect the future sequestration potential over the same 20-year time frame.

2.2. Legume planting and fertilization scenarios Improved grazing management was applied to all grazing lands (i.e., native rangelands and pasturelands), but legume planting and fertilization were only considered to be feasible in pasturelands which are more amenable to agronomic inputs, because of their agroecological conditions (e.g., soil moisture availability). The Daycent model (Parton et al., 1998) was used to simulate N2O emissions from pasturelands under the baseline scenario and scenarios with legume sowing and fertilization. The Daycent model 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 runs for pasturelands.

Legumes were represented within the same warm/cool season grass mixtures as described above for grasses, and were assumed to be oversown on grass to achieve approximately 20% cover, and to persist over the course of the simulation without re-sowing or additional inputs. The effects of different application rates of ammonium-nitrate fertilizer ranging from 0 to 140 kg N ha 1 in 20 kg N ha 1 increments on grass forage production, soil C stocks, and soil N2O emissions were also evaluated in a range of fertilization scenarios. The impact of the legume sowing and fertilization scenarios on forage production, soil C stocks, and soil N2O emissions were compared with the “no-legume” and

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“no-fertilizer” baseline, using the same driving data and parameterizations as described above. An extensive search of FAO and other global institute’s data sources revealed a severe lack of global data on the distribution of management practices in grazing lands.

There is spatial information on crop types and fertilization for croplands, but for grazing lands data availability is extremely poor even at country level, even for most developed countries. The implications of the simplified baseline assumptions for our findings are discussed later. The net GHG impacts were estimated by subtracting increases in soil N2O emissions from the amount of soil C sequestered, for both the legume sowing and nitrogen fertilization measures. Soil carbon stock changes and N2O emissions were converted into CO2-equivalent emissions for this purpose. A global warming potential of 298 was used to convert N2O emissions in CO2-equivalent emissions (IPCC, 2007). For most of our analyses, we confined our estimation of the mitigation potential of the entire suite of measures to those grazing land areas where 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 forage production, related increases in ruminant production and GHG emissions from enteric digestion, manure deposition and management need to be accounted for. The additional ruminant GHG emissions, associated with improved grazing management and legume sowing, were calculated by converting the changes in forage consumption from Century and Daycent into changes in animal numbers within the framework of GLEAM, on the basis of average dry matter (DM) consumption and GHG emissions for ruminant animals in each region, as specified in Opio et al. (2013).

For improved grazing management we assumed no changes in animal productivity. For example, if the increase in net forage consumption from the Century model equated to a 10% increase in the GLEAM-based total dry matter intake for the ruminant herd in a particular region, we used the simple assumption that both the GHG emissions and animal production would also increase by 10%.

Nonetheless, we expect the GHG emission intensity (Ei) of production within each region to fall due to the sequestration of carbon offsetting some of the additional emissions. For legume planting, we used the same approach with the additional assumption (based on findings from Rochon et al., 2004; Min et al., 2003; Coates and Mannetje, 1990; Mannetje and Jones, 1990; MacLeod and Cook, 2004) that the improved additional nutritive value of legumes, increased animal growth rates by 15% and milk yields by 10%.

3. Results 3.1. Grazing management We estimate that adjustments in grazing pressure, from current forage offtake rates to rates that maximize forage production, can sequester 148.4 Tg CO2 yr 1 (Tables 2 and 3) in grazing lands worldwide. Of the total 2.6 billion ha of grazing land over which the Century simulations were carried out, this practice was only found to be effective (i.e., changes in C stocks were estimated to be positive) in 28% of this area (26% of rangeland and 33% of pastureland area). The area in which practices have positive mitigation benefits is described as the “amenable” area throughout the rest of this paper. Most of the C sequestration potential ( 74%) was in rangelands, which contain most of the grazing land area and for which average sequestration rates were greater than in pasturelands (0.23 versus 0.16 Mg CO2 ha 1 yr 1). The amount of sequestration in rangelands varied substantially between regions. The regions with the largest sequestration potentials were Central/South America (26.7 Tg CO2 yr 1), Sub-Saharan Africa (24.3 Tg CO2 yr 1), Oceania (15.6 Tg CO2 yr 1), and East/Southeast Asia (13.7 Tg CO2 yr 1), collectively accounting for 73% of the total potential and 65% of the amenable rangeland area. There was moderate variation in the regional sequestration rates, ranging from 0.13 to 0.32 Mg CO2 ha 1 yr 1 (coefficient of variation = 26%), however most of the difference in total potential between regions was due to variations in area (coefficient of variation = 72%). There was also a relationship between the agroecological zones (AEZs: temperate, humid and arid) within which rangelands are located and sequestration potential, at both a global and regional level. At the global average level, the humid rangelands have the highest C sequestration rates, followed by arid and temperate rangelands. However, total sequestration potentials followed a different pattern, with arid areas accounting for just over half of the total sequestration potential due to their dominant share of the total amenable rangeland area (Table S1). The global distribution of sequestration rates and potentials by AEZ, was generally reflected at the regional level, with the large sequestration potentials in Central & South America, Sub Saharan Africa and Oceania, corresponding to their large areas of rangelands in the humid and arid AEZs. Similarly, the low per hectare potentials in Central Asia, Eastern Europe & Russia, East & Southeast Asia, reflect the very dominant shares of temperate rangelands in these areas.

Regional variation in sequestration potentials for pasturelands was similar to that observed in rangelands, with Central & South America (16.0 Tg CO2 yr 1), Sub-Saharan Africa (9.0 Tg CO2 yr 1),

Table 2 Rangeland 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 (Mha)

Central & S. America Central Asia Eastern Europe & Russia East & South-East Asia Middle East & N. Africa North America Oceania South Asia Sub-Saharan Africa Western Europe World total

DForage consumption (Tg DM)

C sequestration potential (%)

(Tg CO2)

(Mg CO2 ha

1

)

95.9 73.8 5.1 71.0 30.4 43.9 57.6 11.9 79.1 1.4

29.8 48.5 39.3 46.8 18.2 34.5 15.9 20.3 16.6 15.1

26.7 9.8 0.9 13.7 6.4 9.3 15.6 2.9 24.3 0.4

0.28 0.13 0.17 0.19 0.21 0.21 0.27 0.24 0.31 0.32

37.3 13.1 0.8 5.9 14.4 5.1 28.2 4.5 76.0 0.2

470.2

25.5

110.1

0.23

185.7

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

95

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

Region

Amenable area (Mha)

Central & S. America Central Asia Eastern Europe & Russia East & South-East Asia Middle East & N. Africa North America Oceania South Asia Sub-Saharan Africa Western Europe World total

DForage consumption (Tg DM)

C sequestration potential 1

(%)

(Tg CO2)

(Mg CO2 ha

69.7 2.9 2.0 46.1 14.5 9.4 16.3 6.2 65.5 8.8

55.6 27.6 4.1 21.4 27.6 22.1 55.7 31.0 43.2 21.1

16.0 0.4 0.1 6.3 1.3 1.3 1.5 2.0 9.0 0.5

0.23 0.15 0.03 0.14 0.09 0.14 0.09 0.32 0.14 0.05

246.9 0.9 7.5 112.1 9.6 8.5 12.9 0.1 275.9 11.6

241.4

32.7

38.3

0.16

686.1

and East & Southeast Asia (6.3 Tg CO2 yr 1) again among the top four contributors to total global potential. However, the pastureland potentials are more skewed, with these three regions contributing 82% of the total 38.3 Tg CO2 yr 1 of C sequestered globally. As with rangelands, the large potentials in these regions can mostly be ascribed to their larger areas, with these three regions accounting for 75% of the total amenable pastureland area (Table 3). In addition to the sequestration rates being generally lower for pasturelands than for rangelands, they also display more variation between regions (coefficient of variation (CV) of 53%).

The difference in the rates between rangelands and pasturelands reflect differing physiological potential and the alignment between current and forage-optimized management practices. For both pasturelands and rangelands, higher sequestration rates were estimated in overgrazed areas where grazing pressure was reduced than in underutilized areas where grazing pressure was raised (0.26 Mg CO2 ha 1 versus 0.14 Mg CO2 ha 1 for pasturelands, and 0.26 Mg CO2 ha 1 versus 0.21 Mg CO2 ha 1 for rangelands).

However, whereas rangelands have comparable shares of area in

)

which offtake rates are increased (40%) or lowered (44%), the area share in which offtake rates were raised in pasturelands was much larger (83%). Another notable difference was that there was a much higher proportion of land in which the optimum forage offtake rates were within 30% points of the baseline rates in pasturelands (99%) than in rangelands (69%). The sequestration rates were estimated to be lower on these areas than on land where the optimum offtake was beyond 30% of the baseline rates.

As with rangelands, the sequestration rate in the humid grazing areas was the highest, although for pasturelands temperate areas had an equally high global average rate. A further difference with rangelands, was that the sequestration rate in the arid AEZ was far lower than either the temperate or humid AEZs. Around half of total C sequestration potential and amenable pasturel and area was found in the humid pasturelands, with near equal shares of the remaining sequestration potential in temperate and arid pasturelands (Table S2). A map of the global distribution of the soil C sequestration potential in grazing lands is provided in Fig. 1.

[(Fig._1)TD$IG]

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

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

Increases in the forage offtake rate in under-grazed areas almost invariably resulted in more forage consumption. Perhaps more surprising is that increases in forage consumption were registered in 41% of the 276 million ha over overgrazed areas in which offtake rates were lowered. Taken together, forage consumption was estimated to increase in 77%, and decrease in 23%, of the total amenable grazing land area in which offtake rates were adjusted, leading to a total net gain in forage consumption of 871 Tg DM yr 1, with the vast majority coming from pasturelands (79%). Correspondingly, the overwhelming majority of the sequestration potential (80% in both rangelands and pasturelands) was in areas which registered an increase in the amount of forage consumed by ruminants.

Given that improved grazing management causes losses of soil C stocks over much of the total grazing area, we have presented mean sequestration rates from the application of this practice across both the amenable rangeland area and the entire rangeland area, in Table 6. These results provide a range of the expected mean sequestration rates for each region, based on (a) the perfect targeting of the practice (i.e., applying them only in areas where they result in positive sequestration) to (b) the untargeted application of the practice within the entire rangeland area. Thus (a) and (b) can be thought of as upper and lower bound sequestration potentials, respectively. We have also included coefficients of variation to show the variance of sequestration outcomes in each region. Notably, in the absence of any spatial targeting, this practice leads to negative sequestration outcomes in all but two of the ten regions, namely, Central Asia and East & South-East Asia, with marginally negative rates in North America and Eastern Europe & Russia. The coefficients of variation from the targeted scenario, range from low (<100%) to high (>100%), whereas they are consistently high in the untargeted scenario. The global distribution of both the amenable and non-amenable areas for this practice, for all grazing lands, is shown in Fig. 1.

3.2. Sowing legumes Sowing of legumes in pasturelands increased soil C stocks in half of the global pastureland area, but net GHG mitigation (where increased 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 C sequestration potential on the pastureland areas where net mitigation was positive (203.4 Tg CO2 yr 1) to be greater than for improved grazing management on both rangelands and pasturelands 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 CO2 yr 1). Sequestration rates were estimated to be much larger than those for grazing management in rangelands and pasturelands, averaging 2.8 Mg CO2 ha 1 yr 1.

In the pastureland areas where net mitigation was positive, legume sowing increased N2O emissions by 122 Gg N2O N yr 1 (56.9 Tg CO2-eq yr 1) offsetting 28% of the global GHG mitigation benefits of C sequestration, resulting in the global net mitigation of 146.5 Tg CO2 yr 1 (Table 4). Fluxes of N2O increased by an average rate of 1.7 kg N2O N ha 1 yr 1, with substantial variation between regions ranging from 0.01 to 9.1 kg N2O N ha 1 yr 1. Despite these higher emissions, the net mitigation rate from sowing legumes, estimated to be 2.0 Mg CO2-eq ha 1 yr 1, remained much greater than for grazing management (0.21 Mg CO2-eq ha 1 yr 1 for all amenable grazing land). As with the findings for soil C sequestration, the same four regions Western Europe (46.3 Tg CO2-eq yr 1), Central & S. America (30.8 Tg CO2-eq yr 1), Oceania (18.4 Tg CO2-eq yr 1), and the Middle East & North Africa (17.7 Tg CO2-eq yr 1) accounted for more than three quarters of the total net mitigation potential. A map displaying the global distribution net mitigation from legume sowing is provided in Fig. 2, and the corresponding soil C sequestration and N2O emissions from this practice are provided in the Supplementary information for this paper (Fig. S1, Fig. 2).

As with improved grazing management, the average global rate of C sequestration (in the areas where net mitigation occurs) is highest in the humid pasturelands (Table S3). However, humid areas comprised the smallest share of the total net mitigation potential, owing to their relatively small amenable area. For legume sowing, the temperate pasturelands dominate, accounting for nearly half (49%) of the total amenable area and similar amount (43%) of the total net mitigation potential. These patterns are reflected to some extent at the regional level. For example most of the amenable area and mitigation potential in Western Europe occurs in temperate pasturelands, despite this AEZ having the lowest net mitigation rate in this region. The pattern differs for Oceania, where most the amenable area is found in the humid pasturelands, which also have the region’s highest net mitigation rate. However, for Central & South America, there is very little difference between AEZs with regard to both amenable pastureland area and net mitigation rates.

As with grazing management, the sowing of legumes also led to substantial increases in forage consumption, averaging 0.44 Mg biomass ha 1 yr 1, globally. The regional distribution of these increases tended to mirror those of the increases in net mitigation and sequestration, with most of the increased forage located in Central & S. America, Oceania and Western Europe. Interestingly, while the share of soil C sequestration was very low in Sub Saharan

Table 4 Pastureland area and annual C sequestration for cells with positive net mitigation when sown with legumes, along with annual changes in N2O flux rates, net GHG mitigation potential, 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-eq ha 1)

DForage consumption (Tg DM)

(Mha)

(%)

Central & S. America Central Asia Eastern Europe & Russia East & South-East Asia Middle East & N. Africa North America Oceania South Asia Sub-Saharan Africa Western Europe

7.3 1.0 4.0 13.7 9.0 5.6 6.6 3.1 5.9 15.5

5.8 9.0 8.2 6.4 17.2 13.2 22.6 15.7 3.9 37.1

61.7 0.5 4.5 18.6 19.6 7.5 27.3 4.6 5.3 53.8

30.9 0.0 0.0 5.2 1.9 0.4 8.9 0.7 1.4 7.5

30.8 0.5 4.4 13.4 17.7 7.2 18.4 3.9 3.9 46.3

4.2 0.5 1.1 1.0 2.0 1.3 2.8 1.2 0.7 3.0

14.7 0.3 0.6 2.6 1.9 0.4 4.7 1.0 2.6 3.1

World total

71.8

9.7

203.4

56.9

146.5

2.0

31.8

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

[(Fig._2)TD$IG]

97

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

Africa (3%), it accounted for a relatively larger share of total increase in forage (8%). In aggregate, our results show that sowing legumes would increase annual forage production on pasturelands by about 31.8 Tg DM from 71.8 million ha.

As indicated in Tables 2 and 4 and Fig. 2, the relative area over which legume sowing risks increasing, instead of mitigating, soilbased GHGs is much higher than for the improved grazing management practice. Further details about this risk can be gleaned from Table 6, which displays ranges of net mitigation rates for each region based on (a) the perfect targeting of the practice and (b) the untargeted application of the practice within the entire pastureland area. Not only is the relative amenable area for legume planting much smaller than grazing management, the risks of massive increases in soil-based GHGs through poor targeting are much higher for legume sowing, as shown by the large negative net mitigation rates for most regions in the untargeted scenario (Table 6).

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

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

3.3. Fertilization Fertilization led to increased forage production, but rarely led to increased soil C stocks and always led to increased N2O emissions.

Moreover, net GHG emissions increased for all fertilization rates in all regions, because increases in N2O emissions always exceeded soil C sequestration in CO2 equivalent terms. Very few fertilization treatments led to increased soil C stocks in comparison with the no-fertilization control, with the only exceptions in Central/South America and Western Europe (Table 5). Even in those regions, sequestration rates were low, ranging from 0.001 to 0.002 Mg CO2 ha 1 yr 1. Almost all ( 99%) of the potential sequestration in those regions was realized with low rates of N inputs (20 kg

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

Region Central & S. America Central Asia Eastern Europe & Russia East & South-East Asia Middle East & N. Africa North America Oceania South Asia Sub-Saharan Africa Western Europe World total

Area (Mha)

C sequestration (Mg CO2 ha 1)

D N2O emissions (kg N2O-N ha 1)

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

Net GHG mitigation (Tg CO2-eq)

DForage consumption (t DM ha 1)

12.9 6.4 1.4 28.4 30.5 3.8 18.1 13.6 32.3 8.0

0.002 0.112 0.007 0.005 0.127 0.005 0.015 0.085 0.039 0.001

0.89 0.62 0.29 0.25 0.28 0.15 0.44 0.47 0.29 0.14

0.011 0.008 0.004 0.003 0.003 0.002 0.005 0.006 0.004 0.002

5.4 2.6 0.2 3.5 7.9 0.3 4.0 4.2 5.6 0.5

0.1 2.9 0.5 0.4 4.3 0.3 0.6 2.8 4.7 0.4

155.3

0.048

0.37

0.005

34.1

2.4

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

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

Region

Improved grazing management (Mg CO2 ha 1)

Legume sowing (Mg CO2-eq ha 1)

Targeted

Untargeted

Targeted

Untargeted

Mean CV (%)

Mean

Mean CV (%)

Mean

CV (%)

Central & S. America Central Asia Eastern Europe & Russia East & South-East Asia Middle East & N. Africa North America Oceania South Asia Sub-Saharan Africa Western Europe

0.28 0.13 0.17

117 78 65

0.18 300 4.2 0.03 306 0.5 0.02 4145 1.1

79 71 95

0.19 0.21 0.21 0.27 0.24 0.31 0.32

136 125 116 98 100 104 115

0.01 1780 1.0 0.32 156 2.0 0.07 749 1.3 0.25 170 2.8 0.11 393 1.2 0.39 146 0.7 0.26 186 3.0

132 115 70 70 121 223 53

World total

0.23

0.21

2.0

CV (%)

57.3 75 0.5 71 9.8 135 27.5 1.8 17.8 13.9 17.7 78.4 5.7

107 421 131 167 148 58 160

However, thanks to the C sequestration potential of these practices, the additional animal output is possible with a lower emission intensity than in the baseline, within each region and AEZ. Improved grazing management resulted in a 10% reduction in the emission intensity of ruminant production in rangelands. For pasturelands, there is a 10% increase in emission intensity, despite reductions in each region, because production increases by much more in emission intensive regions (e.g., Sub-Saharan Africa and Central & South America) relative to other regions in this scenario.

Because this practice results in a net increase in animal production, it could deliver net mitigation benefits if its application were scaled back in some of the areas in which production increases, without changing the total baseline production in each region. Although this possibility was not assessed in this study. For legume sowing, a relatively smaller increase in forage production and animal numbers, combined much larger per hectare rates of C sequestration and slight improvements in animal productivity, resulted in a substantial 59% reduction in emission intensity (Table 7).

4. Discussion

41.9

N ha 1 yr 1) and estimated sequestration rates (and amounts) changed minimally with increasing N inputs. These results may appear counterintuitive given that N fertilization can increase plant production and subsequent organic C inputs to soil. However, N fertilization can also lead to reductions in soil C stocks by accelerating decomposition of soil C, particularly when applied in excess 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 to soil C losses exceeded those that led to C gains in most cases. When normalized for N input rate, N2O emissions increases were greater at higher fertilization rates in all regions. Increases in forage consumption were correlated with increases in fertilization rates, and in the two regions where soil C stocks increased, fertilization increased forage production by an average of 0.2 Mg 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 practices assessed in this study involve tradeoffs with higher ruminant GHG emissions, because the consumption of this additional forage by grazing ruminants would require higher numbers of ruminants. As shown in Table 7, the impacts of these higher animal emissions (CH4 and N2O from enteric fermentation and manure) would fully offset all of the C sequestration gains from improved grazing management in both rangelands and pasturelands. For legumes, on the other hand, increases in forage consumption and associated ruminant emissions are estimated to only offset 26% of the net soil C sequestration potential of this practice at the global level.

The amount of C that could be stored in the world’s grazing lands is considerable and presents a potentially large mitigation opportunity. We estimate the global potential for C sequestration in the soils of the world’s grazing lands is 352 Tg CO2 yr 1 through improved grazing management in rangelands and pasturelands, and the sowing of legumes in pasturelands. When subtracting the increase in soil N2O emissions associated with legume sowing, the global net sequestration potential of the assessed practices falls to 295 Tg CO2 yr 1. However, the realization of this potential would depend on being able to, a priori, identify and target areas which are amenable to the selected practices. As shown in Table 6, the risks of increasing rather than mitigating GHG emissions are high, particularly for legumes. More work is needed to develop indicators, based on biophysical and management attributes, to identify amenable grazing areas and ameliorate these risks.

Notwithstanding these challenges it is useful to put our results into perspective by comparing them with those from other global assessments. Even when restricting our focus to amenable areas for each practice, our total global potential tends to be smaller than comparable global studies in the literature. This is due to differences in the blend of practices considered and because we tend to apply the practices to a smaller aggregate area, according to a priori considerations about the types of grazing lands in which they can be implemented and according to outputs from our process-based models about where they are effective. Our estimated potential for increasing C stocks in grazing lands is substantially less than the previous global estimate of 1.4 Gt CO2 yr 1 by Smith et al. (2007, 2008), but that study used a statistical approach to derive sequestration potentials for different climatic zones from a different combination of measures (grazing management, nutrient management, irrigation, improved species and fire management) and assumed these potentials could be

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

Practice change

Grazing management Legumes

Land type

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

DForage production (Tg DM yr 1)

Baseline animal GHGs (Tg CO2-eq yr 1)

D animal

Range Pasture Pasture

110 38 147

186 686 32

542 490 216

156 565 38

GHGs (Tg CO2eq yr 1)

Net mitigation (incl. animal) (Tg CO2-eq yr 1) 46 526 109

Baseline Ei (kgCO2-eq/kg protein) 128 126 64

DEi (%)

10 10 59

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

applied to a much larger global grazing land area (2.4 billion ha compared to our total amenable area of 0.7 billion ha). The mean per hectare sequestration potentials reported by Smith et al.

(2007) of between 0.11 and 0.81 Mg CO2 ha 1, are generally higher but of the same order of magnitude as those estimated in our study.

On the other hand, our total global potential of 0.35 Gt CO2 yr 1 falls close to the middle of the range of 0.04–1.1 Gt CO2 yr 1 estimated by Lal (2004) in another global assessment of the soil C sequestration potential in grazing lands. While Lal (2004) also considered a broader range practices (grazing management, improved species, nutrient management and fire management), he reported sequestration rates specific to grazing management of between 0.18 and 0.55 Mg CO2 ha 1, which are again higher but similar to those estimated in our study. Our findings also show reasonable agreement with regional sequestration potentials from other studies. Conant and Paustian (2002) considered reducing grazing pressure on degraded land and estimated lower potentials for North America (8.1 Tg CO2 yr 1) and Europe/Asia (15.8 Tg CO2 yr 1), a similar potential for Oceania (16.1 Tg CO2 yr 1), and higher potentials for South America (66.4 Tg CO2 yr 1) and Africa (61.2 Tg CO2 yr 1). Thornton and Herrero (2010) also assess the impact of restoring degraded rangelands, and estimate higher sequestration potentials than our study for Sub-Saharan African (96.7 Tg CO2 yr 1) and Central and South America (53.6 Tg CO2 yr 1). Lower rates of C storage could also reflect the 20 year time horizon of our study, which contrasts with other studies that have not explicitly addressed how long sequestration endures.

It is clear that the N2O emissions associated with N fertilization or N fixation offset much of the mitigation that can be achieved through the accumulation of soil C stocks, and that these emissions need to be accounted for (Schlesinger, 2010). However, our work suggests that the C sequestration benefits of sowing legumes tend to outweigh the N-induced increases in N2O emissions for a small but still significant area of pasturelands. As with improved grazing management, generalizations can be made about where, among the global regions and AEZs, legume sowing could be most effective. However, more work is needed to identify amenable areas, based on their biophysical and management attributes, to avoid sowing of legumes in pasturelands with the potential for large increases in soil-based GHG emissions (Table 6). Given the size of these risks and the lack of baseline information areas sown with legumes, it is difficult to recommend legume sowing as a large scale mitigation option without further research.

Perhaps the most encouraging result is the large combined net increase in forage consumption resulting from improved grazing management and legume sowing (totaling 904 Tg DM worldwide).

This finding demonstrates the strong synergistic role that grazing management can have with regard to the delivery of environmental, economic and food security benefits. However, this study (which is the first global assessment that we are aware of that explicitly models the linkage between soil C sequestration, animal production and GHG emissions) reveals that these production benefits involve significant tradeoffs with higher ruminant GHG emissions. More importantly, the implied growth in the animal numbers for each practice presents a useful development opportunity for extensive grazing systems, and has to be placed in the context of expected demand growth for animal products.

Increased outputs can have significant food security benefits for pastoral communities, and these productivity gains are particularly desirable given the limited potential for the further expansion of grazing lands, worldwide (Asner et al., 2004).

In addition to the need for further modeling work to discern attributes and indicators for grazing areas with high mitigation potential, further on-ground research and piloting is needed to verify the long-term feasibility and economic viability of the assessed practices. Furthermore, concerns about the permanency

99

of C sequestration and challenges associated with measurement and project coordination (particularly on communal lands) can lower the demand for C sequestration projects in agricultural lands from investors and policy makers (Larson et al., 2011; Gerber et al., 2013). Despite these challenges, some countries have managed to effectively integrate C sequestration activities in grazing lands into national level mitigation policy instruments. Notable examples are Brazil’s ABC programme, which includes the large scale restoration of degraded grazing lands to restore soil C stocks (Ministéro da Agricultura, 2014), and the suite of government programs (Grassland Ecology Conservation Subsidy and Reward Mechanism; Grassland Retirement Program) in China to incentivize the uptake of sustainable grassland management practices (ADB, 2014).

5. Conclusions Previous findings about the massive soil C sequestration potential of the world’s grazing lands have spurred much enthusiasm among scientists and policy makers about the big role that these lands could play in offsetting global GHG emissions.

In this study, findings from process-based models, Century and Daycent, show some agreement with the per hectare mitigation rates from previous studies. However, our results indicate that grazing management, legume sowing and fertilization practices, are likely to deliver net mitigation benefits in less than one third of the world’s total grazing land area. Consequently, while still substantial, our total annual net soil C sequestration potential of 295 Tg CO2 yr 1 is lower than the global estimates from previous studies (Smith et al., 2007; Lal, 2004). However, in the absence of reliable indicators to help avoid non-amenable areas, the risks of the assessed practices increasing instead of mitigating GHG emissions is high, particularly for legumes. Given this risk, along with a lack of baseline management data, it is difficult to recommend legume sowing as reliable global-scale mitigation option. Furthermore, the additional ruminant GHG emissions associated with the assessed practices, are likely to substantially reduce the mitigation potential of the assessed practices, particularly for improved grazing management. Nevertheless, the growth in less emission intensive animal production associated with these practices can provide important development opportunities for pastoralists, and has to be placed in the context of expected demand growth for animal products.

Acknowledgements This research was supported by a Queensland Smart Futures Fellowship, and grants from the UN Food and Agricultural Organization, the Mitigation of Climate Change in Agriculture (MICCA) Programme, and the U.S. Environmental Protection Agency to Colorado State University.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.agee.2015.03.029.

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