Mike Hudak 7 April 2015 Revised 29 April 2015, 26 June 2015

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1 Claims that Livestock Grazing Enhances Soil Sequestration of Atmospheric Carbon Are Outweighed by Methane Emissions From Enteric Fermentation: A Closer Look at Franzluebbers and Stuedemann (2009) 1 Mike Hudak mike.hudak@gmail.com April 2015 Revised 29 April 2015, 26 June 2015 Agricultural publications proclaim the good news: Cattle Pastures May Improve Soil Quality 2 and USDA Weighs In: Grazing Good for Soil & Environment. 3 Both headlines refer to the findings published in Franzluebbers and Stuedemann (2009) research demonstrating that soil sequesters more atmospheric carbon (C) as pasture under low grazing pressure (LGP) than as unharvested pasture (UH) left ungrazed by cattle, or as pasture subjected to high grazing pressure (HGP). Franzluebbers and Stuedemann arrived at this 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 grazing s superiority (compared to ungrazed management) was greatest at soil depth 0 30 cm, with statistical significance decreasing as soil depth approached 150 cm. Therefore I will primarily focus on soil depth data in the range of 0 30 cm when comparing the change rate of soil carbon sequestration to methane (CH 4 ) 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/yr for LGP management compared to only Mg/ha/yr for UH management. At face value this result lends significant support to the claim grazing is superior to nongrazing in mitigating global climate change, as the grazed pasture sequesters approximately 1.76 times as much C as the ungrazed one. But there s a significant omission in this analysis as regards another greenhouse gas, one that in the short term has even greater potential to produce global warming. That gas is methane (CH 4 ) produced by the cattle through enteric fermentation. Franzluebbers and Stuedemann address neither the amount of CH 4 produced by the cattle in their experiments nor the amount of CH 4 that is being absorbed from the atmosphere by the soil upon which their cattle graze. Despite the absence of this information in their article, reasonable estimates can be made. Copyright 2015 by Mike Hudak. All rights reserved.

2 Let s first consider the amount of C emitted as CH 4 by the steers that grazed the pasture. The LGP trials consisted of 5.8 steers/ha grazing for 140 days/yr during the first 5 years of the study, and for approximately 310 days/yr during the remaining 7 years. Although CH 4 emitted by a typical steer ranges from 60 to 71 kg/yr, 4 as a concession to ranching advocates, I ll calculate CH 4 emissions based on the low end of the range (60 kg/yr). And I ll also charge to the steers the CH 4 emitted only for the time they were present on the test plots during the 12 years of the study. The amount of CH 4 emitted by the steers per hectare per year can be estimated by computing the weighted average of the annual CH 4 emissions per steer over the two periods of the 12-year study, and then multiplying by the number of steers per hectare. This yields Mg/ha/yr of CH 4, of which about 75% by mass is C (0.170 Mg/ha/yr). From the perspective of ranching advocates this still looks like a favorable result, as the amount of C sequestered by the soil (1.40 Mg/ha/yr) is more than 8.2 times the amount 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 additional factors about CH 4 to consider. First, CH 4 has a characteristic called Global Warming Potential (GWP) that indicates the relative ability of CH 4 compared to to trap heat in the global climate system over a given time frame. For example, current studies peg the GWP of CH 4 at 34 over a 100- year interval and at 86 over a 20-year interval. 5 Stated otherwise, over a 20-year interval, a given mass of CH 4 would have the same effect in the global climate system as 86 times that mass of. Authors of climate-related articles have often chosen to consider CH 4 s impact over a 100-year period. But in 2013, the IPCC noted: At the 20-year timescale, total global emissions of methane are equivalent to over 80% of global carbon dioxide emissions. 6 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 years. 7 The second factor that must be considered in regard to Franzluebbers and Stuedemann s research is the amount of C in the form of CH 4 (rather than as ) that is being removed from the atmosphere by the landscape that is part of the grazing system. Although I know of no results reported from the same region as this research, other studies can provide a likely upper bound for the amount of this CH 4. One such study by Wang et al. (2015) 8 examined the soil sequestration of CH 4 emitted by sheep at the Guyuan State Key Monitoring and Research Station of Grassland Ecosystem (China). There, moderately grazed grassland was found to sequester the greatest amount of CH 4 during 120 days in 2011 and during 100 days in Calculation of the average daily uptake of CH 4 by soil during this period yields kg/ha/day. If this result is then extrapolated to the average duration per year spent by steers during the 12 years of the Franzluebbers and Stuedemann study (i.e., 239 days/yr), the CH 4 sequestered by the soil would be Mg/ha/yr, approximately 2.9% of the estimated average annual CH 4 emitted by the steers through enteric fermentation (i.e., Mg/ha/yr). In view of the possibility that the result obtained by Wang et al. (2015) is uncharacteristically low, let s consider the study by Allen et al. (2009) 9 that measured soil sequestration of 2

3 CH 4 in three regions of Australia: temperate, Mediterranean, and subtropical, using a network of paired pasture forest sites, representing three key stages of forest stand development: establishment, canopy closure, and mid to late rotation. They found that in pasture soils, CH 4 sequestration ranged from kg/ha/yr (a CH 4 emitter) to kg/ha/yr and in forest soils the sequestration ranged from 0.08 kg/ha/yr to 4.38 kg/ha/yr. In all cases, the amount of CH 4 sequestered by soil was less than that found in Wang et al. (2015). Were these amounts prorated for the average duration per year (239 days) that steers spent on the land during the Franzluebbers and Stuedemann study, the amount of CH 4 sequestered would be even less. These results sharply contrast with the conclusion of Machmuller et al. (2015) 10 regarding a dairy operation in Georgia a study concluding that on the basis of a whole farm C- cycle analyses, C accumulation appears to offset methane emissions during the rapid soil C accumulation phase yielding net C-sequestration rates of ~1.6 Mg C- equivalents /ha/ yr or ~5,805 kg equivalents/ha/yr. This assessment by Machmuller et al. is not applicable to my critique of Franzluebbers and Stuedemann for several reasons, among them that they rely on a complex model of dairy farm management 11 involving irrigation and other practices that are irrelevant to the much simpler pasture grazing found in Franzluebbers and Stuedemann. And more importantly, even though the model used by Machmuller et al. found the dairy system to be C neutral during the soil s rapid C accumulation phase, it is doubtful that this C neutrality (despite a loss of C as CH 4 emitted into the atmosphere amounting to 452 kg/ha/yr on average) would have prevailed had the authors not used the unrealistically low value of 25 for the GWP of CH 4 over a 100-year time period. 12 Consequently, I ll assume that the amount of CH 4 sequestered by the soil in the Franzluebbers and Stuedemann study is well represented by the amount found to be sequestered in the study by Wang et al., namely Mg/ha/yr. Subtracting this amount from that which is emitted by the steers yields: , or Mg (CH 4 )/ha/yr added to the atmosphere. Using a CH 4 GWP of 86 (20-year interval), the atmospheric CH 4 remaining from the steers/ha has a equivalency of Mg/ha/yr (i.e., Mg/ha/yr). But the C sequestered by the soil (1.40 Mg/ha/yr) represents only 5.19 Mg/ha/yr of (as C represents only 27% of a molecule). On balance, the CH 4 emitted by the steers and the absorbed by the soil yields a net atmospheric loading equivalent to Mg/ha/yr of. Note that were these calculations performed under the assumption of a 100-year (rather than 20-year) atmospheric impact of CH 4, with the correspondingly lower GWP (typically pegged at 28 to 34), the atmospheric loading of CH 4 would be reduced (relative to that found with the 20-year interval), but it would still not be offset by the additional amount of atmospheric C sequestered by the soil resulting from the presence of cattle. For completeness, let s consider the net C change rates for management under LGP and under UH assessed to the maximum soil depth investigated by Franzluebbers and Stuedemann: 150 cm. With the measurements averaged over three nutrient source treatments, I compute from data in Franzluebbers and Stuedemann (p. 33, Table 4) an LGP SOC change 3

4 rate of Mg/ha/yr compared to a UH SOC change rate of 0.28 Mg/ha/yr. 13 Again, as C represents only 27% of a molecule, the LGP SOC value of Mg/ha/yr yields a soil sequestration value of 2.95 Mg/ha/yr. When balanced against the CH 4 emitted by the steers, the LGP treatment yields an atmospheric increase in equivalency of (19.04 Mg/ha/yr Mg/ha/yr) or Mg/ha/yr when assessed over a 20-year interval (CH 4 with GWP of 86). In summary, putting cattle onto the landscape used in Franzluebbers and Stuedemann s study contributed to global climate change far in excess of the amount of additional C sequestration achieved by the soil on which the cattle grazed. It would have been better to forego even light grazing (LGP) and instead settle for the lower rate of soil C sequestration afforded by the UH (ungrazed) management if one s goal were to reduce the overall GWP of the gases in the atmosphere. This conclusion holds regardless of whether the change rate of SOC sequestration is measured to a depth of 30 or 150 cm or whether the impact of CH 4 is measured over 20 or 100 years. 14 But even selecting the UH option begs a larger question regarding land use if the highest objective is to reduce greenhouse gases through soil sequestration of C. Although the region of the Franzluebbers and Stuedemann test plots may have been converted to cropland by the early 19th century, the close proximity (only 16 km distant) of the Oconee National Forest strongly suggests that this region was originally a forest, not a grassland. Presumably, with sufficient time (and perhaps encouragement) that land would revert to forest. And if so, how much atmospheric C might the land sequester? That question has been addressed in research by Huntington, 15 conducted less than 75 km distant and approximately due west from the site of the Franzluebbers and Stuedemann 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/yr 16 (1.06 to 0.61 Mg/ha/yr less than results of Franzluebbers and Stuedemann 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 CH 4 emissions of the cattle are considered. 17 If the goal is to maximize the long-term sequestration of atmospheric C, rather than maximizing the rate of C sequestration, 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 total soil C of 82.1 Mg/ha (soil depth of cm) 18 compared to that of 69.9 Mg/ha of SOC obtained with the best grazing management (i.e., LGP) evaluated by Franzluebbers and Stuedemann 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 was included, the total forest ecosystem C sequestration soared to 185 Mg/ha, 19 more than double the amount stored in the pasture ecosystem that includes C in the below-ground 20 and above-ground biomass 21 annually remaining from the LGP management. Finally, it is worth noting Huntington s report of C sequestration in nearby native forest (Fernback Forest, Atlanta, GA) that has been only mildly disturbed. There, soil C was measured at 122 Mg/ha. Carbon in above- and below-ground tree biomass was

5 Mg/ha, and C sequestered in the total ecosystem was 326 Mg/ha. 22 Would managed pasture (that was once native forest, such as that described in Franzluebbers and Stuedemann) ever sequester C in this amount? The likely answer is no, as the conversion of a grassland to a coniferous forest (being the Potential Natural Vegetation of the region) has been estimated to entail an increase of C contained in standing biomass of MT (Mg)/ha. 23 Conclusion Comparing the findings of Franzluebbers and Stuedemann to those of Huntington reveals that if reducing the greenhouse gas impacts of atmospheric C is the highest objective, then forestland should be left undisturbed. Forestland that has been converted to now unproductive cropland should be returned to forest if possible, not maintained as ungrazed pasture. And because of enteric fermentation-emitted CH 4, the least desirable option would be to manage the land as cattle-grazed pasture even under the best grazing management. Acknowledgments The author thanks E. Patch, T. Shuman, and E. Walsh for valuable comments on earlier versions of this essay. References 1. 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): Dennis O Brien, Cattle Pastures May Improve Soil Quality, Agricultural Research (March 2011), (accessed 29 April 2015) 3. (accessed 9 March 2015) 4. K. A. Johnson and D. E. Johnson, Methane Emissions from Cattle, Journal of Animal Science 73(8) (1995): IPCC Climate change 2013: The Physical Science Basis. Intergovernmental Panel on Climate Change. Available at (accessed 8 June 2015) 6. Ibid. 5

6 7. Robert W. Howarth, A Bridge to Nowhere: Methane Emissions and the Greenhouse Gas Footprint of Natural Gas, Energy Science & Engineering, (2014) doi: /ese3.35, (accessed 8 June 2015) 8. Xiaoya Wang, Yingjun Zhang, Ding Huang, Zhiqiang Li, and Xiaoqing Zhang. Methane Uptake and Emissions in a Typical Steppe Grazing System during the Grazing Season, Atmospheric Environment 105 (2015): D. E. Allen, D. S. Mendham, Bhupinderpal-Singh, A. Cowie, W. Wang, R. C. Dalal, and R. J. Raison, Nitrous Oxide and Methane Emissions from Soil are Reduced Following Afforestation of Pasture Lands in Three Contrasting Climatic Zones Australian Journal of Soil Research 47(5) (2009): Megan B. Machmuller, Marc G. Kramer, Taylor K. Cyle, Nick Hill, Dennis Hancock, and Aaron Thompson, Emerging Land Use Practices Rapidly Increase Soil Organic Matter, Nature Communications, (2015) doi: /ncomms Jeff B. Belflower, John K. Bernard, David K. Gattie, Dennis W. Hancock, Lawrence M. Risse, and C. Alan Rotz, A Case Study of the Potential Environmental Impacts of Different Dairy Production Systems in Georgia, Agricultural Systems 108(C) (2012): Even the authors of Machmuller et al. (2015) acknowledge that under assumptions most favorable to the dairy industry, the time period during which the operation remains a C sink or even C neutral, is short. They state: As the C accumulation rate declines these farms will become net C-emitting similar to all dairy production because of ruminant methane emissions. 13. The lower change rates of SOC to depth of 150 cm relative to 30 cm result from a small but significant decline in SOC with depth below 30 cm observed by Franzluebbers and Stuedemann. Although lacking a definitive explanation for this phenomenon, the authors suggest a few possible causes. 14. Considering the cattle-emitted CH 4 impact over 100 years (rather than 20 years) with GWP of 34 (under LGP management averaged over three nutrient treatments) yields a equivalency of Mg/ha/yr. The atmospheric C (as ) sequestered by the soil to depth of 30 cm (i.e., 5.19 Mg/ha/yr) yields an atmospheric equivalency gain of Mg/ha/yr. If the atmospheric C sequestered by the soil is considered to depth of 150 cm, then removed from the atmosphere is 2.95 Mg/ha/yr, yielding an atmospheric equivalency gain of Mg/ha/yr. 6

7 15. Thomas G. Huntington, Carbon Sequestration in an Aggrading Forest Ecosystem in the Southeastern USA, Soil Science Society of America Journal 59(5) (1995): Carbon Sequestration, p Huntington s lower bound of forest soil C sequestration (0.34 Mg/ha/yr) is superior to LGP management which produces net atmospheric greenhouse gas loading equivalent to in the amount of 12.3 Mg/ha/yr (20-year interval) or Mg/ha/yr (100-year interval). 18. Carbon Sequestration, p. 1463, Table Ibid. 20. Franzluebbers and Stuedemann do not report the weight of below-ground biomass of their forage (i.e., Coastal Bermuda Grass) in LGP management, but note that their soil samples included roots (p. 30). In any case, below-ground temperate grassland biomass is estimated at only 6.3 metric tons (MT)/ha (equivalently 6.3 Mg/ha) which provides an upper bound on the amount of sequestered C. See T. M. Sobecki, D. L. Moffitt, J. Stone, C. D. Franks, and A. G. Mendenhall, A Broad-Scale Perspective on the Extent, Distribution and Characteristics of U.S. Grazing Lands, in The Potential of U.S. Grazing Lands to Sequester Carbon and Mitigate the Greenhouse Effect, ed. R. F. Follett, J. M. Kimble, and R. Lal (Boca Raton, Florida, Lewis Publishers, 2001), 44, Table Franzluebbers and Stuedemann report that the LGP management includes leaving 3 Mg/ha of forage, mostly Coastal Bermuda Grass. As this grass consists of approximately 44.6% C (as calculated from data presented in Broad-Scale Perspective, p. 53), the above-ground annual residual vegetation sequesters, at most, 1.34 Mg/ha of C. 22. Carbon Sequestration, p T. M. Sobecki, D. L. Moffitt, J. Stone, C. D. Franks, and A. G. Mendenhall, A Broad- Scale Perspective on the Extent, Distribution and Characteristics of U.S. Grazing Lands, in The Potential of U.S. Grazing Lands to Sequester Carbon and Mitigate the Greenhouse Effect, ed. R. F. Follett, J. M. Kimble, and R. Lal (Boca Raton, Florida, Lewis Publishers, 2001), 49 (Table 2.5). 7

Curbing Greenhouse Gases: Agriculture's Role

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