Carbon Footprint Of Organic Fertilizer

The Carbon Footprint of Fertilization with Manure and Composted Manure Climate change is increasingly accepted as a significant threat and there is building consensus that steps should be taken in all industries to reduce greenhouse gas emissions. The proponents of Organic farming often make the claim that organic farming is superior in terms of its impact on “Global Warming” or more appropriately, “Climate Change.” The Rodale Institute, the organization that introduced the US to the concept of organic in the 1950s, has published a white paper claiming that Organic farming can be “a solution” to climate change (LaSalle 2008), http://www.rodaleinstitute.org/global_warming . It is widely believed that crops grown under the rules of “USDA Organic” have a more desirable “carbon footprint” because they have not received synthetic fertilizers or crop protection chemicals that are manufactured using fossil fuels. The marketers of Organic foods often make this claim http://www.horizonorganic.com/health/envben.html. http://members.seemonterey.com/earthboundfarms/

as do it’s non farming supporters (http://www.organicconsumers.org/organic/globalwar, http://www.dearjac.com/baby/onesies/wrybaby-organic-global-warmingonesie.htmlming101003.cfm , http://www.mosesorganic.org/attachments/broadcaster/roger17.1obama.html ,

“Personal carbon footprint calculators” often give positive credit for consumption of Organic food including the one on the web site of the Nature Conservancy, a respected environmental advocate. (http://www.nature.org/initiatives/climatechange/calculator/).

Are these assertions accurate? There are several reasons to question the superiority of Organic (land use efficiency, dependency on tillage…) but probably the least recognized issue has to do with fertilization. “Embedded Carbon” in Fertilizers Synthetic nitrogen fertilizer contains “embedded carbon dioxide emissions” based on the fossil energy used in the Haber-Bosch process to produce Ammonia from atmospheric di-nitrogen gas, later chemical steps to generate other forms of nitrogen, and from fuel used to

transport fertilizers from production sites to farms. For most forms of nitrogen this “footprint” represents approximately .8 to 1.2 lbs of CO2carbon per pound of nitrogen in the fertilizer (West 2002, Robertson 2000, Snyder 2007). Fertilizers which include ammonium nitrate have higher “footprints” of 2.6 lbs CO2-C/lb because some nitrous oxide is released during their manufacture and this gas is 310 times as potent as CO2 in terms of global warming potential (Snyder 2007, EPA 2004). Many “Life Cycle Analyses” of Organic production have assumed no greenhouse gas emissions for the manures and composts that are major sources of nitrogen fertilization for Organic production (LaSalle 2008, Teasdale 2007, Robertson 2000). The Rodale Institute document cited above (LaSalle 2008) does not even mention the words “methane” or “nitrous oxide.” The zero emission assumption in these publications fails to consider methane emissions that are well documented from the storage and composting of manure. Since methane is 21 times as potent as carbon dioxide as a greenhouse gas (EPA 2004), even small emissions can represent a significant “footprint.” Even though the carbon that gives rise to this emission is potentially “carbon neutral,” that is only the case if it is eventually released as CO2, not if it is converted to a more potent greenhouse gas. It could be argued that this particular “carbon footprint” should be assigned to the “life cycle analysis” of the animal production system rather than to Organic (or conventional) farming, but that logic fails to recognize the fact that the handling practices for manure intended for application to crops is specifically oriented to that use as opposed to other manure management options. Since animals produce manure on a daily basis, and crop fertilization is only practiced at specific times during the year, manure must be stored for crop use. If the manure is going to be used to fertilize a crop directly consumed by humans, it is necessary to compost it to reduce the risk of contamination of that food with human pathogens. Thus greenhouse gasses emitted during the storage or composting process should appropriately be considered as “embedded carbon emissions” in the fertilizer. The best practice for animal manures is not to use them as fertilizers but to use an anaerobic digester to convert the manure carbon to a clean, carbon neutral fuel (Voell 2008). The IPCC (International Panel on Climate Change) assumes that for most reasonably careful storage conditions, on the order of 1-2% of the original carbon in the manure is converted to methane though much higher conversions are possible (IPCC 2006). Thus, for a typical strawbedded bovine manure (Hao 2004) with 1.99% nitrogen and 330.5 kg carbon/Mg, the methane emissions during storage would represent between 3.3 and 6.6 kg methane-carbon/Mg (Table 1). Converting that

to CO2 equivalents per kg of nitrogen demonstrates that fertilization with stored manure has a carbon footprint many times as large as that from many synthetic sources (Table 1).

Table 1. Calculation of embedded carbon in stored manure Methane conversion assumption (% of original 1% 1.5% 2% carbon converted) Nitrogen content kg N/Mg dry 19.92 19.92 19.92 weight of manure* Kg Manure to supply 1 kg N 50.2 50.2 50.2 (dry weight) Starting Carbon kg C/Mg 330.5 330.5 330.5 Manure (dry weight basis)* Methane emissions Kg CH43.31 4.96 6.61 C/Mg Methane emissions as kg .0694 .1041 .1388 CO2-C equiv/kg manure** GHG emissions on a nitrogen 3.48 5.23 6.97 basis, kg CO2-C equiv/kg N Ratio to synthetic Urea at 1.2 2.9 4.4 5.8 kg CO2-C equiv/kg N * Hao 2004 **Conversion factor for methane to carbon dioxide equivalents 21 (IPCC 2006) Hao et al (Hao 2004) measured the greenhouse gas emissions from a commercial-scale composting operation in Canada, which involved eight “turns” over a 99-day composting process. 53% of the original carbon was released during the composting process. 8.92 kg of methane-carbon was released for every Mg of original manure (dry weight). 30.1 percent of the original mass was lost. Between methane, nitrous oxide and fuel use, 202.6 kg CO2-C equiv were emitted for every kg of original manure on a dry weight basis. The nitrogen content of the composted manure was 16.6 kg N/Mg dry weight. On a dry weight basis it would take 86.2 kg of original manure to deliver 1 lb of Nitrogen fertilizer. That would mean that for every 1 kg of N fertilizer, the “embedded” carbon in the compost represented 17.46 kg CO2-C equiv, 14.6 times as much as that for synthetic urea.

This issue is not limited to manure composting. A study of composting grass and other green waste found that 0.5% of the nitrogen was emitted as nitrous oxide and 1.7% of the carbon was emitted as methane. These would also translate into a very substantial “carbon footprint.” (Hellebrand 1998). This analysis suggests that major sources of nitrogen for Organic crops, manure and composts, entail far more “embedded carbon emissions” than synthetic nitrogen sources. These emissions are more than sufficient to cancel-out carbon sequestration gains that might be achieved by the use of organic fertilizers (Robertson 2000, Teasdale 2007). Based on the unexpectedly large “carbon footprint” of manure and compost fertilization, the widely held assumption that Organic agriculture is better for climate change must be questioned. It is also appropriate to reconsider whether an administration that respects science and is finally trying to do something about climate change should simultaneously be striving to increase organic farm acreage. Steven D. Savage, Ph.D. [email protected]

References

EPA, Unit Conversions, Emissions Factors, and Other Reference Data (2004). Hao, X., Chang, C., Larney F.J. Carbon, nitrogen balances and greenhouse gas emission during cattle feedlot manure composting. J. Environ. Qual. 33:37-44 (2004). Hellebrand, H.J. Emission of nitrous oxide and other trace gases during composting of grass and green waste. Engineering Research 69:365375 (1998). IPCC. Emissions from Livestock and Manure Management. 2006 IPCC guidelines for National Greenhouse Gas Inventories. Table 10.17 (2006). LaSalle, T.J., Hepperly, P.H. Regenerative Organic Farming: A Solution to Global Warming. ©2008, Rodale Institute.

Phillips, R Organic agriculture and nitrous oxide emissions at sub-zero soil temperatures. J. Environ. Qual. 36:23-30 (2007) Reicosky, D.C., Hatfield, J.L., Sass, R.L. Agricultural Contributions to greenhouse gas emissions. Chapter 3 in Climate Change and Global Crop Productivity, eds K.R. Reddy, H.F. Hodges, ©CAB International 2000 Robertson, G.P., Paul, E.A., Harwood, R.R. Greenhouse Gases in Intensive Agriculture; Contributions of Individual Gases to the Radiative Forcing of the Atmosphere. Science 289: 1922-1925 (2000). Teasdale, J.R., Coffman, C.B., Magnum, R.W. Potential long-term benefits of no-tillage and organic cropping systems for grain production and soil improvement. Agro. J. 99:1297-1305 (2007). Voell, C. Anaerobic digesters for manure methane capture and use. US. EPA. http://airquality.ucdavis.edu/pages/events/2008/green_acres/VOELL.pdf (2008) West, T.O., Marland, G. A synthesis of carbon sequestration, carbon emissions and net carbon flux in agriculture: comparing tillage practices in the United States. Agriculture, Ecosystems and Environment 91:217-232 (2002) Wagner-Riddle, C., Thurtell, G.W. Nitrous oxide emissions from agricultural fields during winter and spring thaw as affected by management practices. Earth and Environment Science 52:151-163 (2004).