Data and conclusions selected from research presented in A. J. Franz­lueb­bers and J. A. Stuedemann, “Soil-Profile Organic Carbon and Total Nitrogen During 12 Years of Pasture Management in the Southern Piedmont USA,” Agri­culture, Ecosystems and Environment 129 (2009): 28–36 (henceforth referred to as “F&S”) are examined within a broader environmental context than is ad­dressed within that article. Specifically, F&S found that low grazing pressure (LGP) proved superior to unharvested pasture (UH) with regard to the con­cen­tra­tion and change rate of soil organic carbon (SOC). Although this result sug­gests that LGP is also superior to UH in reducing atmospheric car­bon, ac­count­ing for enteric-fermentation-produced methane (CH4) emitted by the steers in the LGP protocol reveals that the opposite is true. Specifically, when the an­nu­al mass of CH4 emitted by those steers is balanced against the SOC soil se­ques­tra­tion rate (to 30 cm soil depth) the net atmospheric loading of carbon dioxide (CO2) equivalents is 13.47 Mg ha-1 year-1 under Global Warm­ing Po­ten­tial (GWP) value of 86 (20-year duration) and 2.324 Mg ha-1 year-1 under GWP value of 34 (100-year duration). Based on these values, if the LGP pro­to­col was applied across the 13.8 Mha of the Southeastern U.S. (as F&S rec­om­mend), the greenhouse gas impact (as calculated from a U.S. EPA formula) would be equivalent in the first case to 54 coal-fired power plants op­era­ting for 20 years, and in the second case to 9.3 such power plants op­er­ating for 100 years. As the pasture on which F&S conducted their experiments was most likely originally forest, I then compared F&S’s concentration of SOC (to 150 cm soil depth) and carbon in above-ground biomass (totaling 77.63 Mg ha-1) with above- and below-ground carbon mass for nearby regenerating and un­dis­turbed forests reported in Thomas G. Huntington, “Carbon Sequestration in an Aggrading Forest Ecosystem in the Southeastern USA,” Soil Science Society of America Journal 59(5) (1995): 1459–67. Huntington reported the ecosystem sequestered carbon (to 100 cm soil depth) of the regenerating forest as 185 Mg ha-1 (more than 2-and-a-third times the carbon mass in the LGP-managed pasture) and of the un­dis­turbed forest as 326 Mg ha-1 (more than 4 times the carbon mass in the LGP-managed pasture).

I wrote this essay for two primary reasons. First, I wanted to demonstrate how a research project about mitigating global climate change, though pre­sumably truthful in its methods and findings, could nevertheless be false (as regards the mitigation) within a broader environmental context. And second, I wanted to provide a tutorial, primarily for animal and environmental ad­vo­cates, that shows essential factors to consider when evaluating any research that purports to demonstrate that cattle grazing can mitigate global climate change.

Agricultural publications proclaim the good news: “Cattle Pastures May Im­prove Soil Quality”1 and “USDA Weighs In: Grazing Good for Soil & Envi­ron­ment.”2 Both headlines refer to the findings published in Franzluebbers and Stuedemann (2009)3 (henceforth referred to as “F&S”)—research dem­on­strat­ing that soil sequesters more atmospheric carbon (C) as pasture managed under “low grazing pressure” (LGP) than as “unharvested” pasture (UH) left ungrazed by cattle, or as pasture subjected to “high grazing pressure” (HGP).

While this result may be correct, the study neglects to account for the meth­ane (CH4) (a short-acting, but potent greenhouse gas) that is produced through enteric fermentation4 and emitted by the study’s cattle into the global climate system. Without such an accounting, it is impossible to conclude that any of F&S’s grazing management prescriptions are superior to land man­age­ment that excludes cattle (as regards reducing the heat trapping capacity of atmospheric C).

F&S arrived at their conclusion by evaluating the factorial combination of nutrient source and forage utilization on soil-profile distribution (0–150 cm) of soil organic carbon (SOC) during 12 years of management on Typic Kanhap­lu­dult (Acrisol) in Georgia, USA. In measuring the concentration of SOC (p. 31, Table 2) and change rate of SOC (p. 33, Table 4), the authors found that graz-ing’s superiority (compared to ungrazed management) was greatest at soil depth 0–30 cm, with statistical significance decreasing as soil depth ap­proached 150 cm.

Based on F&S’s finding of lessening statistical significance below 30 cm, I will primarily focus on soil depth data in the range of 0–30 cm when com-paring the change rate of SOC sequestration to CH4 emitted by cattle grazing on LGP test plots.

With management averaged over three nutrient source treatments ac­cessed to a soil depth of 30 cm, I compute from the data (p. 33, Table 4) a rate of change for SOC of 1.40 Mg ha-1 year-1 for LGP management compared to only 0.797 Mg ha-1 year-1 for UH management. At face value this result ap­pears to support the conclusion that grazing is superior to non-grazing in mit­i­gat­ing global climate change, as the grazed pasture sequesters ap­prox­i­mately 1.76 times as much C as the ungrazed one.

But as noted above, there’s a significant omission in this analysis as re­gards CH4, which in the short term has even greater potential than CO2 to pro­duce global warming. F&S address neither the mass of CH4 produced by their cattle through enteric fermentation nor the mass of atmospheric CH4 that is being absorbed by the soil upon which the cattle graze. Despite the absence of this information in their article, reasonable estimates can be made.

Let’s first consider the mass of C emitted as CH4 by the steers that grazed the pasture. The LGP trials consisted of 5.8 steers per hectare grazing for 140 days per year during the first 5 years of the study, and for approximately 310 days per year during the remaining 7 years. Although CH4 emitted by a typical steer ranges from 60 to 71 kg per year,5 as a concession to ranching ad­vo­cates, I’ll calculate CH4 emissions based on the low end of the range (60 kg year-1). And I’ll attribute to the steers the CH4 emitted only during the time they were present on the test plots during the 12 years of the study.

The mass of CH4 emitted by the steers per hectare per year can be esti-mated by computing the weighted average of the annual CH4 emissions per steer over the two periods of the 12-year study, and then multiplying by the number of steers per hectare. This yields 0.228 Mg CH4 ha-1 year-1, of which about 75% by mass is C (0.170 Mg C ha-1 year-1). From the perspective of ranching advocates this still looks like a favorable result, as the mass of C se­ques­tered by the soil (1.40 Mg C ha-1 year-1) is more than 8.2 times the mass of C emitted by the steers. But this balance in favor of the soil seques­tering atmospheric C may be less significant than it appears at first glance, as there are two essential factors about CH4 yet to consider.

The relative ability of CH4 compared to CO2 to trap heat in the global cli­mate system over a given time frame is expressed by CH4’s “global warming potential” (GWP).6 Internationally accepted values for CH4’s GWP (with cli­mate-carbon feedback) are “34” over a 100-year interval (GWP100) and “86” over a 20-year interval (GWP20).7 Stated otherwise, over a 20-year interval, a given mass of CH4 would have the same effect in the global climate system as a mass of CO2 that is 86 times greater than that mass of CH4.8

Authors of climate-related articles have usually considered CH4’s impact over a 100-year period. But in 2013, the IPCC noted that “there is no scientific argument for selecting 100 years compared with other choices.”9 Moreover, the IPCC found that at the 20-year timescale, total global emissions of CH4 are equiv­a­lent to over 80% of global CO2 emissions.10 In that light, Howarth (2014) argued for focusing on the 20-year, rather than the 100-year, period based on “the urgent need to reduce methane emissions over the coming 15–35 years.”11

The second CH4-related factor to consider in regard to F&S’s research is the amount of sequestered SOC that is derived from CH4, rather than from CO2. Although I know of no results reported from the same region as their research, studies conducted elsewhere provide a reasonable upper bound for the mass of this CH4.

For example, Wang et al. (2015)12 examined soil sequestration of CH4 on land grazed by sheep at the Guyuan State Key Monitoring and Research Sta­tion of Grassland Ecosystem (China). Climate characteristics here differed sig­nif­i­cantly from those of the Georgia site studied by F&S. Whereas the latter site, at latitude 33° 22' N, has experienced long-term mean annual temper­ature of 16.5°C and rainfall of 1250 mm, the Wang et al. site at latitude 41° 44' N (more than 945 km farther north), has long-term mean annual temper­ature of 1.4°C and precipitation of 450 mm—hence much cooler, with shorter growing season and only one-third the precipitation of the F&S site.

Of the grassland management protocols that Wang et al. studied, “mod­er­ate­ly grazed” was found to sequester the greatest mass of CH4—an average daily uptake by soil of 0.02781 kg ha-1 day-1. To yield the most favorable out­come for the ranching industry, I’ll extrapolate this value to the duration of an entire year rather than to just the average of 239 days per year that the steers remained on the landscape over the 12 years of the F&S study. This yields a soil sequestration rate of 0.01015 Mg CH4 ha-1 year-1, approximately 4.4% of the average annual mass of CH4 emitted by the F&S steers through enteric fermentation.

Another study13 conducted in China examined CH4 sequestration on three types of steppe: desert, typical, and meadow. As in the previously mentioned study by Wang et al., all locations studied are at high latitude (more than 940 km north of the F&S site), and hence have much lower annual temperatures than the F&S site—only slightly above the freezing point of water. Similarly, all these steppe locations are characterized by annual mean precipitation ranging from a third (“typical” and “meadow” sites) to less than a fourth (“desert” site) that of the F&S site. Consistent with Wang et al., “low grazing” (among the graz­ing protocols examined) yielded the highest rate of CH4 sequestration. Extrapolating to a one-year duration the mean values thus obtained,14 yield sequestration rates as follows: desert steppe: 0.0114 Mg ha-1 year-1, typical steppe: 0.00860 Mg ha-1 year-1, and meadow steppe: 0.00601 Mg ha-1 year-1.

Allen et al. (2009)15 examined soil sequestration of CH4 in pasture and for­est sites in three different climate regions (Temperate, Mediterranean, Sub­tropical) of Australia. Of the three regions, the Mediterranean climate region most closely approximates the F&S site in terms of mean annual temperature (15.7°C), but exhibits a much lower annual precipitation range (696–812 mm) and hence much lower mean annual precipitation. In each climate region Allen et al. examined three paired pasture-forest sites representing three key stages of forest stand development. For the purposes of my analysis of the F&S study, I need only consider the CH4 sequestration in the pasture portion of each site. As reported in Table 4, p. 453 of Allen et al., the annual CH4 flux (mg m-2
year-1) for the Mediterranean climate region sites are -96, -52, and -47. (Note that Allen et al. never uses the word “sequestration” in regard to the transfer of a greenhouse gas between the atmosphere and soil. Instead the “flux” of a gas is stated as being either positive or negative, “negative” meaning that the gas is passing from the atmosphere into the soil. For my examination of results from Allen et al., I will retain their terminology.) The average of these values is -65 mg CH4 m-2 year-1 which can be equivalently written as -0.00065 Mg CH4 ha-1 year-1. This mass of CH4 represents only about 0.285% of that which is annually emitted by the F&S steers.

Of the three climate regions studied, Allen et al. report (Table 4, p, 453) that the Temperate region pasture sites exhibited the highest rates of CH4 sequestration by soil: -107, -187, and -119 (mg m-2 year-1). Averaging to -137 mg CH4 m-2 year-1 (equivalently written: -0.001377 Mg CH4 ha-1 year-1), this mass represents approximately 0.6% of the CH4 annually emitted by the F&S steers.

Due to greater similarly of climate, it is more likely that the soil seques-tration rate of CH4 at the F&S site in Georgia is closer to that of the Mediter-ranean climate region site studied by Allen et al., than it is to the desert steppe site studied by Tang et al. (2013). Indeed the latter study even notes that “Where soil pore spaces are filled by water, anoxic conditions increase and CH4 diffusion to the methanotrophs in the subsurface is restricted.”16 In other words, other conditions being equal, the soil of a region with higher annual pre­cip­i­tation (such as the F&S site) is likely to exhibit a lower rate of CH4 se­ques­tra­tion than the soil in a region with lower annual precipitation.

To establish an upper bound for the mass of CH4 that may be soil seques-tered at the F&S site, I will choose the highest rate of sequestration found in the above-cited studies regardless of its region’s climatological similarity to the F&S site. That value is 0.0114 Mg ha-1 year-1 found in desert steppe reported by Tang et al. (2013)—approximately 5.0% of the CH4 mass produced by the F&S steers.

Subtracting this desert-steppe-site mass from that which is emitted by the steers yields 0.217 Mg CH4 ha-1 year-1 (i.e., 0.228 - 0.0114) added to the atmosphere.

With a CH4 GWP20 of 86, the atmospheric CH4 remaining from the steers per hectare has a CO2 equivalency of 18.66 Mg CO2 ha-1 year-1 (i.e., 86 × 0.217 Mg CH4 ha-1 year-1). But the C sequestered by the soil (1.40 Mg C ha-1 year-1) represents only 5.19 Mg CO2 ha-1 year-1 (as C represents only 27% of a CO2 molecule’s mass). On balance, the CH4 emitted by the steers and the CO2 contributing to SOC yields a net atmospheric loading equivalent to 13.47 Mg CO2 ha-1 year-1 (over a 20-year interval).17

What other sources might annually produce 13.47 Mg (over a 20-year in­ter­val) of atmospheric CO2 pollution? For answers, I consulted the U.S. Envi­ronmental Protection Agency’s website18 which provides a number of possibil-ities. Among them we find that this quantity of CO2 is equivalent to consuming 31.3 barrels of oil, or burning 14,370 pounds of coal, or driving an average passenger vehicle 32,302 miles. And this is the air pollution generated by a mere 5.8 steers grazing in accord with the experimental design of F&S on only one hectare of land over the course of approximately two-thirds of a year. This is the prescription these authors tout (p. 28) as an environmentally beneficial land use to be replicated on 13.8 Mha of pasture across the eastern coastal and southeastern states of the U.S. Were such replication to occur, annual CO2-equivalent pollution of 185,886,000 Mg (over a 20-year interval) would ensue, which the just-cited EPA website equates to the CO2 pollution annually spewing from more than 54 coal-fired power plants.

For completeness, I’ll consider the net C change rates for management under the LGP and UH protocols assessed to the maximum soil depth in­ves­ti­gat­ed by F&S (i.e., 150 cm). With the measurements averaged over their three nutrient source treatments, I compute from data in F&S (p. 33, Table 4) an LGP change rate of 0.796 Mg SOC ha-1 year-1 compared to a UH change rate of 0.28 Mg SOC ha-1 year-1.19 Again, as C represents only 27% of the mass of a CO2 molecule, the LGP value of 0.796 Mg SOC ha-1 year-1 yields a soil se­ques­tra­tion value of 2.95 Mg CO2 ha-1 year-1. When balanced against the CH4 emitted by the steers, the LGP treatment yields an atmospheric increase in CO2 equivalency of 16.09 Mg CO2 ha-1 year-1 (i.e., 19.04 - 2.95) when the impact of CH4 is assessed with GWP20 at 86.

The CH4 emitted annually by cattle in F&S’s study would generate far more atmospheric heat trapping over a 20-year period than that which would be reduced by the soil sequestration of atmospheric CO2. Mitigation of global cli­mate change would have been achieved within the experimental design only by foregoing even light grazing (LGP) and instead settling for the lower rate of soil C sequestration afforded by the UH management. This conclusion prevails regardless of whether the change rate of SOC sequestration is measured to a soil depth of 30 or 150 cm, or whether the impact of CH4 is measured over 20 or 100 years.20

An even more fundamental issue to consider than whether domesticated land, such as that studied by F&S, would better reduce atmospheric green­house gases by being grazed or not grazed by cattle is whether the landscape would sequester more atmospheric carbon in its domesticated or in its natural state. Although the region of the F&S test plots has been cropland since the early 19th century, from what sort of biome was that land converted? Was it grassland, similar to the pasture studied in these experiments? Or was the land originally forest? The close proximity (only 16 km distant) of the Oconee National Forest strongly suggests the latter. With sufficient time (and perhaps reforestation) that land would likely revert to forest. And if so, how much atmospheric C might the land then sequester?

That question has been addressed in research reported in Huntington (1995),21 conducted less than 75 km distant and approximately due west from the site of the F&S experiments. For abandoned cropland regenerating as for­est over a 70-year period, Huntington found the rate of soil C sequestration to range from 0.34 to 0.79 Mg ha-1 year-1 22 (1.06 to 0.61 Mg ha-1 year-1 less than results of F&S for LGP management averaged over three nutrient treatments and measured to a depth of 30 cm). But both rates of forest soil C sequestration are certainly superior to net C sequestration of LGP management when CH4 emissions of the cattle are considered.23 If the greater goal is to maximize the mass of atmospheric C that is sequestered over a long period, rather than to maximize the rate at which C is sequestered, then additional data from Huntington strongly suggests that regenerating the landscape as native forest is superior to any management of the landscape as pasture.

Consider that in that same regenerating forest, Huntington found 82.1 Mg C ha-1 (soil depth to 100 cm)24 compared to that of 69.9 Mg SOC ha-1 ob­tained with the best grazing management (i.e., LGP) evaluated by F&S on pasture, averaged over three nutrient treat­ments to the greater (by 50%) depth of 150 cm (p. 31, Table 2). When above- and below-ground tree bio­mass is included, the total forest ecosystem sequestration soars to 185 Mg C ha-1.25 This value is more than two-and-a-third times the total mass of C (i.e., 77.63 Mg ha-1) annually stored in F&S’s pasture ecosystem under LGP man­age­ment when accounting for C, not just in soil (i.e., 69.9 Mg ha-1) but also in the below-ground biomass (6.3 Mg ha-1)26 and above-ground residual biomass (1.43 Mg ha-1).27

Finally, consider Huntington’s report of C sequestration in nearby “mildly disturbed” native forest (Fernbank Forest, Atlanta, GA). There, soil C was measured at 122 Mg C ha-1. Carbon in above- and below-ground tree biomass was 203.9 Mg C ha-1, and C sequestered in the total ecosystem was 326 Mg C ha-1.28 Would managed pasture (that was once native forest, such as that described in F&S) ever sequester C in this amount? Most likely not, as the con­ver­sion of a grassland to a coniferous forest (the “Potential Natural Vege­ta­tion” of the region) has been estimated to yield an increase within standing biomass of 157.5 MT (equivalently “Mg”) C ha-1.29

Comparing the findings of F&S to those of Huntington reveals that if the highest objective is to reduce the greenhouse gas impacts of atmospheric C, then forestland should remain undisturbed. Forestland that has become unproductive cropland should be returned to forest if possible, not maintained as pasture. And because the heat-trapping properties of enteric fermentation-emitted CH4 will far outweigh any benefits associated with increased soil-sequestered C, the least desirable option would be to manage unproductive cropland as cattlegrazed pasture, even under the best grazing management.

The author thanks E. Patch, T. Shuman, and E. Walsh for valuable comments on earlier versions of this essay.