Rangelands make up a large proportion of the Earth’s surface, and the soils hold a significant amount of sequestered carbon. Rangelands are estimated to contain more than one-third of the world’s above and below ground carbon reserves.[i] As a consequence, there is interest in determining the potential for soil carbon sequestration in rangeland soils, and whether livestock grazing helps or hinders this sequestration.
The potential for sequestering more carbon varies tremendously, however, based upon a number of factors including existing carbon storage (there is a finite amount of carbon that soils can hold before they are “saturated”), plant productivity, grazing management, and climate.
Annual rates of soil organic carbon (SOC) accumulation decline as the soil approaches equilibrium. Sometimes overly optimistic predictions result when SOC accumulations increase in the early years after a change in grazing management, but these increases cannot be extrapolated indefinitely.[ii]
The issue of whether livestock grazing can sequester carbon in soils has gotten greater attention in recent years. Some proponents of livestock grazing assert that grazing can lead to the sequestering of significant amounts of carbon in soils, and thus reduce GHG emissions.
In the most optimistic claims, some, like Allan Savory, suggest that livestock grazing can reduce carbon to pre-Industrial levels. In particular, Savory’s unverified claims have generated a number of responses that dispute his assertions.[iii][iv]
While there may be circumstances under which grazing could increase carbon in soil, most rangeland soils have a limited ability to store additional carbon, and under most conditions livestock grazing will reduce carbon storage, rather than increase it.
Efforts to maintain and increase carbon storage in rangelands should focus on reducing livestock grazing in areas where it ecologically inappropriate and causing degradation. Those promoting the climate benefits of livestock grazing must account for effects beyond soil carbon. Livestock are significant sources of methane emissions, so any speculative benefits from increased soil carbon storage are likely offset by increased methane emissions.
SOIL CARBON IN RANGELANDS
There is a tremendous amount of carbon tied up in agricultural soils. The amount of carbon bound up in soils is approximately three times the amount of C found in aboveground biomass. The argument goes that a small increase in soil carbon pools could have a major impact on reduction of global GHG emissions.
The basic way that carbon is sequestered in soil is by plant growth, primarily through roots, in the soil, along with micro bacteria, soil microbes and other soil life that are able to live on plant material.
Given the existing condition of many rangelands, the biggest concern is maintaining current carbon, and avoiding losses through soil erosion, degradation of plant productivity and other changes that lead to soil carbon losses. In other words, the best way to reduce CO2 emissions from rangelands globally is to reduce rangeland degradation. Since livestock grazing is frequently the major source of rangeland degradation, a reduction in grazing pressure, can in many ecosystems, potentially preserve more soil carbon.
While increasing soil carbon storage may be theoretically possible in some circumstances, the ability of soil to absorb carbon is limited. Many ecosystems are at or near equilibrium and cannot store additional carbon. To bind more carbon in the soil requires a continuous input of new organic matter. The soils that have the best opportunities for additional carbon storage are those that have been depleted by overgrazing and ecosystem degradation, but these landscapes frequently require a long period of time to recover. They are also the most difficult to recover.[v]
This gets into the time factor regarding global GHG emissions. Reducing emissions is more important now than in future decades. Due to the slow accumulation rate of carbon in soils, even if certain grazing practices could enhance carbon storage in some situations, the process may not help reduce global CO2 levels in a useful time frame.
By contrast, methane emissions from livestock are a major contributor to GHG warming now, and these emissions are one of the easiest (relatively speaking) human anthropogenic sources of CO2 equivalents to reduce.
LIVESTOCK AS A SOURCE OF METHANE
Livestock are among the largest sources of global anthropogenic methane. Depending on which study is used, anywhere from 14-51% of the global GHG emissions in CO2 equivalences are due to livestock production (UN Livestock’s Long Shadow [vi]or Climate Change and Livestock, Worldwatch Institute).[vii] Even if livestock grazing could generate some carbon sequestration over the long term (and as we shall see below, this is not a proven given), the presence of livestock emissions occurring in the interim would still be a significant problem. One would have to balance any carbon sequestration against the methane emissions to see if livestock grazing were a net benefit.
A study in China found that the uptake of CH4 (methane) by grassland soils only offset 3.1-8.6% of methane emissions from grazing sheep.[viii]This suggests that even if livestock grazing could promote the sequestration of methane, it would not be significant enough to outweigh the emissions resulting from livestock digestion.
One problem to keep in mind is that methane is 84 to 86 times more effective than CO2 in trapping heat during the first 20 years after its release into the atmosphere (IPCC, Fifth Report, 2013.) (Over a 100 year time scale, methane breaks down into CO2; the IPCC currently considers methane to be approximately 28 to 34 times more effective than CO2 in trapping heat over the 100 year time scale.) Since reducing global heating is a priority now, not 100 years from now, the more effective heat trapping properties of methane in the first decade or two after emission make it especially dangerous.
VARIATION IN CARBON STORAGE
When viewing the contribution that livestock grazing may make to carbon sequestration there are a few other considerations that must be part of an informed decision.
The first is that there is tremendous variation in reported soil carbon storage due to variation in ecosystems, grazing methods and management, the soil profile and depth analyzed, study duration, grass type, and precipitation. Accounting for this variation is context-based and making comparative statements difficult as a result.[ix]
GRAZING INFLUENCES ON SOIL CARBON STORAGE
Grazing can affect carbon storage losses by shifting plant species dominance in some communities. Ecosystems where heavy grazing by native ungulates was historically common (like the Great Plains) have an ability to facilitate soil carbon sequestration by a shift in plant species.
For example, a long-term study of the Northern Great Plains in North Dakota found that moderate grazing resulted in 17% less soil carbon sequestered than in ungrazed exclosures; however, heavy grazing resulted in a shift towards blue grama, a grazing tolerant species with shallow, but dense roots. The dominance by blue grama resulted in levels of sequestered soil carbon similar to those found in the ungrazed exclosure.[x]
A similar situation was found in a study of alpine meadows in China where medium to heavy grazing increased soil carbon in the top soil layers as a result of a shift in plant species to grasses tolerant of heavy grazing. These grasses have dense roots, thereby increasing soil organic carbon.[xi]
However, even where a shift in plant dominance may appear to improve soil carbon storage potential, there is a great variation in rates of sequestration due to the influence of other climate variables, such as drought and increased soil temperature.
A study in a mixed grass ecosystem on the Great Plains found that heavy grazing (50 percent utilization) over a 10 year period resulted in a 30% loss in soil organic carbon (SOC). This was a consequence of shifting plant dominance from mixed prairie grasses to blue grama, a grass that tolerates heavy grazing. Blue grama has dense but shallow roots; thus, SOC accumulates closer to the surface where it is more easily lost.[xii] Under conditions of drought, gradually increasing soil temperatures, and heavy grazing, the blue grama root system was unable to retain carbon that had previously been accumulated under the wetter, cooler-soil conditions of the previous decade. (Interestingly, this study also documented that there was little change in SOC in the no grazing and light grazing [10 percent utilization] treatment areas over this same dry, warmer-soil period. It also documented that increases in total nitrogen accumulated in the no grazing and light grazing treatments over this period while nitrogen stores declined in the heavy grazing treatments.)
Climatic influences are more likely to affect shallow SOC deposits like those found in blue grama grasslands. For instance, another study on the Great Plains found that in a wet year carbon storage by cool season grasses improved, and there was greater carbon storage in an exclosure, while the following year a grazed pasture dominated by blue grama had a higher CO2 exhange rate.[xiii]
This variability in results also points out the danger of short term studies that are the most common in the research world. Depending on what the climatic conditions were in the years measurements were taken, may greatly influence the findings and conclusions.
Many of the studies of soil carbon storage have been done in ecosystems where there were significant evolutionary grazing influence from native species such as bison like on the Great Plains which may bias conclusions. In these areas, plants exist that have developed tolerance for heavy grazing, while in areas where native grazers were less abundant (like the Great Basin and many desert areas) the ability of plants to adapt to livestock grazing appears to be less resilient. However, there have far fewer studies conducted grazing influence on soil carbon in these ecosystems.
Livestock grazing is the primary factor in tipping some ecosystems over ecological thresholds such as the rangelands found in the Great Basin. For instance, exotic shallow rooted cheatgrass has replaced deep-rooted native bunchgrasses and shrubs on some western rangelands. Cheatgrass sequesters very little carbon and increases SOC rate of turnover.[xiv] .
As the author notes: “The elimination of perennial understory vegetation and cryptobiotic crusts is a nearly inevitable consequence of livestock grazing in deserts. This opens these systems to annual grass invasion, subsequent burning, and loss of a major carbon sink, a heavy price to pay for the minimal economic gains derived from direct use of these intrinsically unproductive lands for livestock production” [xv]
Changing disturbance intensity (grazing) in an ecosystem with low disturbance regimes can lead to a cascade of events. One study found that after a shift to high disturbance, photosynthesis decreased followed by a decline in root biomass and a change in plant community structure 1.5 months later. Those changes led to a decrease of soil fungi, a proliferation of Gram(+) bacteria and accelerated decomposition of old particulate organic C (<6 months). At last, accelerated decomposition released plant-available nitrogen and decreased soil C storage. The results indicate that intensified grazing triggers proliferation of Gram(+) bacteria and subsequent faster decomposition by reducing roots adapted to low disturbance.[xvi]
A study in China found that grazing exclusion resulted in greater aboveground biomass, root biomass and plant litter compared to grazed grasslands. Grazing exclusion significantly increased C and N stored in plant biomass and litter and increased the concentrations and stocks of C and N in soils. Furthermore, these differences were accentuated the longer grazing was excluded with the highest C and N stocks in a 17 year grazer excluded grassland.[xvii]
In another study in China, variation in precipitation had a greater effect on carbon uptake and release. The ungrazed plots had less variation and the authors concluded that ungrazed lands may have greater resistant to changing climate.[xviii]
A third study in China’s Inner Mongolia found standing dead plant and litter carbon (C) decreased significantly in light grazing conditions (when compared to a non-grazed exclosure that had been fenced for 26 years), but light grazing did not significantly affect live plant C, total aboveground plant C, total root C (0–60 cm), and soil C (at 0-100-cm depths). Heavy grazing extensively reduced carbon in three pools, total aboveground plant C, subsoil C (at 60–100 cm), and total soil C (at 0–100 cm), but did not affect topsoil C (at 0–60 cm). The lack of an effect on topsoil C can be explained by a slight increase in root C (0–60 cm) and a higher ratio of root to vegetation C in the heavy grazing site. The decrease in subsoil C under heavy grazing is attributable to the organic carbon decomposition due to increased root C as a source of fresh carbon. Total ecosystem C decreased from 150.62 Mg C/ha in the NX site to 143.78 Mg C/ha in the LG site (a 4.5% decrease) and to 122.43 Mg C/ha in the HG site (an 18.7% decrease).[xix]
Similarly, in a fourth study of the Loess Plateau in China, a twenty year exclusion of livestock grazing significantly increased above and below ground biomass, species richness, cover and height for five different communities. The authors concluded that “long-term exclusion of livestock grazing can greatly improve properties of typical steppe in the Loess Plateau.[xx]
The point of all conflicting results is that one cannot generalize and always assume that grazing will increase soil carbon stocks. Clearly in many areas grazing exclusion is the best way to store and even increase soil carbon.
Briske and colleagues question how much carbon can be sequestered in rangelands in general, due to the low productivity of rangelands ecosystems. As they note, rangelands are known to be very weak sinks for atmospheric C because plant production is water limited and more C is often released into the atmosphere from soil respiration than is take up by vegetation, especially during drought periods.[xxi]
A further complicating matter is how vegetation affects soil carbon storage. For instance, under heavy grazing in the Great Plains, blue grama, a sod-forming grass very resistant to grazing, tends to increase. Blue grama roots are denser and found in shallower soil profiles than other grasses. Hence measurements of soil carbon can be affected by the dominant species and the depth of soil profiles examined. Heavy grazing may increase soil carbon in the Great Plains by favoring blue grama, but at the expense of a greater diversity of deep-rooted grasses.[xxii]
Another problem is that even if livestock grazing could enhance carbon sequestration, it would take a long time to implement on a large landscape scale and there are also limits to how much carbon soils can absorb.[xxiii]
Beyond the methane production from livestock, in particular, cattle, one must also look at the collateral damage from livestock grazing. Livestock production does not occur in isolation. Cattle, in particular, produce a large amount of manure that is a major source of water pollution. Cattle destroy biocrusts, particularly in arid ecosystems, fostering the spread of weeds and exotic plants like cheatgrass. Cattle trample riparian areas in arid ecosystems that are critical habitats for 70-80% of all wildlife. Cattle hooves compact soils, reducing infiltration of water. In most of the world, protecting livestock from predators is one of the primary factors contributing to the decline and endangerment of many predator species. And growing of forage for livestock is a major reason tropical forests are cleared (for hay and soy production,) with a sequent loss of carbon to the atmosphere. It is also the reason for much of the destruction of native vegetation in places like the Midwest of the US, where livestock forage crops like corn and soy dominate. Since much of the hay/alfalfa grown in the arid West requires irrigation, the impoundment of rivers with dams is yet another consequence of livestock production.[xxiv]
Grazing can also alter plant communities, with ecological consequences not only for livestock production, but also wildlife. For instance, a heavily grazed short grass steppe in Colorado shifted from mixed prairie with cool season plants to a plant community dominated by the warm season grass blue grama. This reduces the available forage earlier in the season, which can affect livestock productivity, but which also has obvious impacts on native wildlife that may depend on early green up of cool season grasses.[xxv]
In yet another study in New Mexico, grazing shifted grasslands to mesquite. The deep-rooted mesquite had far more carbon storage than the grasslands.[xxvi] However, many ecologists see the shift to mesquite as a degraded ecosystem.
Exclosures tended to have a more diverse flora, including more forbs (flowers) and cool season grasses. In particular, the presence of forbs may be important to pollinating insects like butterflies and flies, and the effects on total biodiversity should be considered, not just whether there are greater soil carbon accumulations.[xxvii]
Thus, focusing on carbon storage without considering other ecosystem values may be counterproductive.
Grass-fed beef is not a panacea either, as grass is nutritionally poor and requires greater transit time in the cow’s rumen, resulting in anywhere from 2-4 times as much methane production. For instance, one study reported a 48% increase in methane production by cows feeding on natural grasslands.[xxviii] In another study comparing CAFO farmed animals with natural pasture feed cattle, the grass fed beef had significantly greater methane emissions.[xxix]
Furthermore, any increase in cattle production (as advocated by Allan Savory and others) would likely come at the expense of forests, since the majority of new livestock pasturage is carved from forested landscapes. (Most natural grasslands are already under livestock production and have little space available for increasing animal numbers.) Since forests capture and store far more carbon than any grassland pastures that replace them, expanding livestock production would likely result in a net loss in carbon storage.
In their assessment of the full life cycle estimate of GHG emissions attributable to livestock (which includes collateral impacts like forest clearing), Goodland and Anhang suggest that nearly 51% of the annual worldwide anthropogenic GHG emissions are attributable to livestock.[xxx]
A reduction in livestock numbers and production would permit the reforestation of millions of acres of land that were cleared for livestock pasture. This would effectively store far more carbon than livestock grazing could achieve through any stimulation of plant production and SOC storage.[xxxi]
There is no clear evidence that livestock grazing can significantly enhance soil carbon stores. Conflicting evidence exists that demonstrates greater carbon storage with no grazing, while in other ecosystems, grazing may enhance soil carbon. But there are many cautionary remarks on how to measure and interpret findings. Climatic conditions year to year, for instance, can shift carbon storage in grazed areas from a positive to a negative. Furthermore, any storage is gradual and takes years to accumulate, while carbon uptake by soils is finite and slows over time. And compared to almost all other ecosystems, arid rangelands are among the least productive ecosystems—hence have little potential for soil carbon storage compared to other ecosystems like forests.
Because of this time factor and the need to reduce CO2 levels now, the use of rangelands as a carbon sink—even if it were proven effective—is a poor strategy for a host of reasons. One cannot look at the soil carbon storage issue out of context. Livestock are among the greatest source of GHG emissions now—and reducing livestock numbers is the quickest and perhaps the most effective means of significantly altering GHG emissions.[xxxii] Furthermore, there are a host of collateral damages created by livestock production, from the destruction of soil biocrusts, killing of predators, water pollution, clearing of forests for pasture, and so on. One cannot look at the carbon-livestock-soil issue in isolation. Taken as a whole, the production of livestock has many significant ecological impacts, not the least of which is its contribution to global GHG emissions.
George Wuerthner is coeditor of Keeping the Wild: Against the Domestication of the Earth (Island Press 2014).
[i] Ingram L.J. et al. 2008. Grazing impacts on soil carbon and microbial communities in a mixed grass ecosystem. Soil Sci. Soc. Am. J. 72:939-948
[ii]Powlson et al. 2014. Limited potential of no-till agriculture for climate change mitigation. Natuer Climate Change July 30 2014
[iii] Carter, J. et al. 2014. Holistic Management: Misinformation on the Science of Grazed Ecosystems. International Journal of Biodiversity Volume 2014, Article ID 163431, 10 pages
[iv] Briske, D. et al. 2014. The Savory Method cannot green deserts or reverse climate change. A response to Allan Savory’s TED video. Rangelands 35(5):72–74 doi: 10.2111/RANGELANDS-D-13-00044.1
[v] Sommer R. and D Brossio. 2014. Dynamics and climate change mitigation potential for soil carbon sequestration J of Environmental Management 144 pgs. 83-87.
[vii] Goodland, R Anhang, J (2009) Livestock and Climate Change: What if the key actors in climate change were pigs, chickens and cows? World Watch, November/December 2009. Worldwatch Institute, Washington, DC, USA. Pp. 10–19. Available at: http://www.worldwatch.org/files/pdf/Livestock%20and%20Climate%20Change.pdf
[viii] Wang, x. et al 2015.Methane uptake and emissions in a typical steppe grazing system during the grazing season. Atmospheric Environment Vol. 105, pgs. 14-21.
[ix] Mcsheery M. and Mark Richie. Effects of grazing on grassland soil carbon: a global review. Global Change Biology (2013) 19, 1347–1357, doi: 10.1111/gcb.12144
[x] Frank, A.B. et al. 1995. Soil carbon and nitrogen of Northern Great Plains grasslands as influenced by long term grazing. J. Range Management. 48: 470-474.
[xi] Gao. Y.H. et al. 2007. Grazing intensity impacts on carbon sequestration in an alpine meadow of the eastern Tibetan Plateau. Research Journal of Agriculture and Biological Sciences, 3(6): 642-647, 2007
[xii] Ingram, LJ. Et al. 2008. Grazing impacts on soil carbon and microbial communities in a mixed-grass ecosystem. Soil Sci. Soc. Am. J. 72:939-948
[xiii] LeCain et al. 2004. Carbon exchange and species composition of grazed pastures and exclosures in the shortgrass steppe of Colorado. Agriculture, Ecosystems and Environment 93 (421-435.
[xiv] Meyers, S. 2011. Is climate change mitigation the best use of desert shrublands? Natural Resources and Environmental Issues: Vol. 17, Article 2. Available at: http://digitalcommons.usu.edu/nrei/vol17/iss1/2
[xv] Meyers, S. 2011. Is climate change mitigation the best use of desert shrublands? Natural Resources and Environmental Issues: Vol. 17, Article 2. Available at: http://digitalcommons.usu.edu/nrei/vol17/iss1/2
[xvi] Klumpp, K., Fontaine, S., Attard, E., Le Roux, X., Gleixner, G. and Soussana, J.-F. (2009), Grazing triggers soil carbon loss by altering plant roots and their control on soil microbial community. Journal of Ecology, 97: 876–885. doi: 10.1111/j.1365-2745.2009.01549.x
[xvii] Qiu, L. et al. 2013. Ecosystem carbon and nitrogen accumulation after grazing exclusion in semiarid grassland. PLOS ONE | www.plosone.org 1 January 2013 | Volume 8 | Issue 1
[xviii] Shao, C. et al. 2013. Grazing alters the biophysical regulation of carbon fluxes in a desert steppe. Environ. Res. Lett. 8 (2013) 025012 (14pp)
[xix] Fang Fei, Chang Rui-ying1, Tang Hai-ping 2014 Effects of Grazing on Carbon Sequestration in Temperate Grassland, Inner Mongolia of North China Vegetos- An International Journal of Plant Research Volume : 27, Issue : 3 pgs. 126-134.
[xx] Cheng, J. et al. Cumulative effects of 20 year exclusion of livestock grazing on above and belowground biomass of typical steppe communities in arid areas of the Loess Plateau, China. Plant Soil Environ. 57, 2011 (1): 40–44
[xxi] Briske, D. et al. 2013. The Savory Method cannot green deserts or reverse climate change. A response to Allan Savory’s TED video. Rangelands 35(5):72–74 doi: 10.2111/RANGELANDS-D-13-00044.1
[xxii] Ingram L.J. et al. 2008. Grazing impacts on soil carbon and microbial communities in a mixed grass ecosystem. Soil Sci. Soc. Am. J. 72:939-948
[xxiii] Sommer R. and D Brossio. 2014. Dynamics and climate change mitigation potential for soil carbon sequestration J of Environmental Management 144 pgs. 83-87.
[xxiv] Wuerthner G. and M. Matteson eds. 2002. Welfare Ranching. Island Press
[xxv] LeCain et al. 2004. Carbon exchange and species composition of grazed pastures and exclosures in the shortgrass steppe of Colorado. Agriculture, Ecosystems and Environment 93 (421-435.
[xxvi] Bird, S.B. et al. 2002 Spatial heterogeneity of aggregate stability and soil carbon in semi-arid rangelands. Environmental Pollution 116 pages 445-455
[xxvii] Reeder, J.D. and G.E. Schuman. 2001. Influence of livestock grazing on C sequestration in semi-arid mixed grass and short grass rangelands. Environmental Pollution 116 457-463.
[xxviii] Grobler, S.M. et al. 2014. Methane Production in dfferent breeds, grazing different pastures or fed a stotal mixed ration as measures by a Laser Methane Detector. South African Journal of Animal Science 2014, 44 (Issue 5, Supplement 1)
[xxix] Pelletier, N. et al. 2010. Comparative life cycle environmental impacts of three beef production strategies in the Upper Midwestern United States. Agricultural System 103 (2010) 380–389.
[xxx] Goodland, R Anhang, J (2009) Livestock and Climate Change: What if the key actors in climate change were pigs, chickens and cows? World Watch, November/December 2009. Worldwatch Institute, Washington, DC, USA. Pp. 10–19. Available at: http://www.worldwatch.org/files/pdf/Livestock%20and%20Climate%20Change.pdf
[xxxi] Goodland, Robert. January 2014. Happier Meals. Climate Alert. Climate Institute. http://www.climate.org/publications/Climate%20Alerts/2014-january/happier-meals.html
[xxxii] Ripple, W. et al. 2014. Ruminants, Climate Change and Climate Policy. Nature Climate Change Vol. 3