By Paul Davis
The development and expansion of agricultural has always required substantive changes to the local landscape whether through deforestation, wetland drainage or the conversion of grasslands for cultivation (IPCC 2000). In fact, it has been estimated that terrestrial carbon stocks have decreased by 170 Pg C since 1700 due to agricultural expansion alone (Houghton 1990). However today, there exists a great deal of interest surrounding the potential for agriculture to recapture a portion of this 170Pg C loss through the adoption of “carbon-focused” management practices. In this paper, a framework for analyzing agricultural carbon mitigation potential is outlined and then tested through the evaluation of nitrogen fertilizers as a carbon mitigation tool.
History of Agricultural Emissions
The development and expansion of agricultural has always required substantive changes to the local landscape whether through deforestation, wetland drainage and/or the conversion of grasslands for cultivation (IPCC 2000). This expansion of cultivated landscapes has historically led to substantive releases of carbon dioxide as agricultural soils and vegetation store significantly less carbon per area than the natural biomes they replace (See Table 1) (IPCC 2000).
Until the 1920s agricultural carbon emissions were the largest anthropogenic contributor to the global atmospheric CO2 reservoir (Houghton 1990). It has been estimated that terrestrial carbon stocks have decreased by 170 Pg since the 1700s, as the result of the conversion of native ecosystems such as forests, wetlands and grasslands to cultivated cropland (Houghton 1990). Of this 170 Pg loss in carbon, it is believed that 54 Pg can be attributed to the reduction of carbon content in soils.
Agriculture lands as a Carbon Sink
Today, there exists a great deal of interest surrounding how agriculture may recapture a portion of the C lost by traditional cultivation through the adoption of better management practices focused on carbon reduction.
Unfortunately annual crops, whose vegetation stocks come and go with the growing season, dominate our agricultural system. Therefore, the opportunity to develop carbon sinks in vegetation growth is minimal. Thus, when considering how to increase carbon storage on agricultural lands – the only real promising resource is the land itself. As a result, current efforts by the agricultural community to become a player in the carbon-offset market are largely focused on how to manipulate and maximize soil organic carbon (SOC) content (Paustian 1998).
In addition to soil carbon storage, Agriculture can reduce its greenhouse gas footprint through reductions in energy use. Agriculture is a substantial contributor to national greenhouse gas loads. In fact, a life-cycle assessment conducted by Michigan’s Center for Sustainable Systems in 2000, estimated that 10% of the United States fossil fuel consumption could be attributed to the cultivation and transport of food products (Keoleian 2000).
Therefore, in developing a framework though which to evaluate the potential of a new management practices to reduce the carbon footprint of agriculture, two main themes need to be considered (Paustian 1998), that is:
- Does the practice increase SOC stocks?
- Does the practice reduce the energy intensity of agricultural production?
In the following paper, we discuss the potential for nitrogen fertilizers to capture carbon through enhanced SOC content. The goal is to illustrate through analysis of synthetic fertilizer inputs as a carbon mitigation tool, the need to consider both SOC levels and life cycle energy consumption when attempting to determine the carbon reduction merit of a mitigation tool/practice.
Kyoto Protocol: Agriculture Practices and N Fertilization
The Kyoto protocol was the first major international accord to set binding emissions targets for Greenhouse gases. The Protocol set binding targets for GHG reductions for 37 industrialized nations and the European Union, with the aim of reducing global emissions from industrialized partners to five percent below 1990 levels by 2012 (UNFCCC Website).
As part of their annual assessment of carbon emissions, the Kyoto Protocol requires that participating nations report greenhouse gas emissions associated with changes in Land Use/Land Cover character (Kyoto 1998) as well as land use practices. Article 3.4 of the original Protocol states that guidelines governing how to inventory and monitor GHGs emissions generated by agricultural practices be determined at a future date by the Conference of the Parties (Kyoto 1998).
In 2000, the IPCC released a special report on Land Use, Land Use Change and Forestry (LULUCF). This report detailed how changes in land use impact global GHG emissions, as well as outlined potential carbon mitigation measures to reduce GHG levels through both land use conversion and better land-use management practices.
Section 18.104.22.168 of the IPCC Special Report on LULUCF states that the use of inorganic fertilizers (among other soil additives) can be an effective technique through which to increase crop yields. In turn, the report states that high crop yields produce higher crop residues and thus “favor enhanced soil carbon storage”. In Table 4.5 of the same report, the IPCC states that the USA could achieve soil carbon increases of 0.1-0.3 t C/ha-yr through better management practices, which includes “Fertilization, Crop Rotation and Organic Amendments” (IPCC LULUCF 2000, Lal et al. 1999). Below we will analyze the extent to which fertilizer application leads to SOC increases as well as its potential to decrease atmospheric C levels.
N Fertilization: Carbon Sink or Source
In Managing US Cropland to sequester Carbon in Soil, Dr. Lal notes that numerous studies have found a strong connection between the application of N fertilizer to agricultural soils and increases in SOC levels (Paustian 1992, Robinson 1996, Anderson 1990) While the literature is replete with examples of N induced SOC increases, we will focus on two particular studies in this area.
The first study was conducted by Varvel (1994) at the Agronomy Farm in Nebraska. Varvel found that over an 8-year period, the application of nitrogen fertilizers enhanced carbon sequestration by 10 to 20 gC/m2-y. Meanwhile, Ismail etal. in a 20-year study in Kentucky assessing the impact of fertilizer application on soil C content, found substantive SOC gains on the order of 40-60gC/m2-year as the result of fertilizer application. It is important to not however, that the carbon lenses of Varvel and Ismail only extended as far as their study plots’ border. That is to say, they both only considered the carbon gains and losses associated with the management of local soils.
Upstream Energy of N Fertilizer
While the aforementioned studies indicate that applications of nitrogen fertilizers can lead to substantive gains in SOC, we should be weary of promoting nitrogen fertilization as a GHG abatement measure prior to considering the upstream energy costs of the nitrogen inputs.
The fabrication of nitrogen fertilizers is an energy intensive process that consumes substantial quantities of natural gas feedstock. The mere stoichiometry of the Haber process releases 0.375 mol of C for every 1 mole of Nitrogen converted to N fertilizer (Schlesinger 2000).
- 3CH4 + 6H20 -> 3 CO2 +12H2
- 4 N2 + 12 H2 -> 8 NH3
Carbon Intensity: 3C/8N = 0.375 mol C released per mole of N fixed.
Beyond the large energy feedstocks required to produce nitrogen fertilizers, additional energy is utilized for the fabrication, transport and eventual application of these N additives to agricultural lands. Schlesinger (2000) estimates that full accounting of the upstream production and distribution of N fertilizers results in the release of 1.4 mol of C per mol of N additive generated.
If the upstream carbon emissions of N fertilizer production are accounted for, the carbon balance associated with synthetic fertilizer application is substantially altered. The energy utilized for N fertilizer feedstock, processing and transport translates into a vast “carbon-debt” for fertilizers before they reaches the application stage (Schlesinger 2000, Fargione 2008). This upstream carbon footprint for N fertilizer substantially reduces the net GHG savings as it offsets the downstream increases in SOC.
Returning now to the Varvel and Ismail studies discussed earlier, we see that the enhanced SOC gains associated with fertilizer application are significantly offset by the “ hidden carbon cost” of N fertilizer production (Table 2, Figure 1). In fact, as Schlesinger notes if the 1.4 mol C to 1 mol N product-ratio is applied to the figures provided by Varvel, the SOC gains realized on the Nebraska farm plots are outpaced by the upstream C emissions of N fertilizer (Schlesinger 2000). Similarly, on Ismail’s corn plots in Kentucky, the upstream carbon costs of N fertilizer reduced the “carbon-savings” achieved by 30%. It is important to note however that in the case of Ismail’s Kentucky corn farms, the fertilizer application still produced net carbon gains, suggesting that the applicability and success of certain carbon mitigation practices may be regional in nature.
In his seminal work Land Clearing and the Biofuel Carbon Debt, Dr. Joseph Fargione illustrated the importance of considering the “carbon debt” of land-use change when determining where to site biofuels crop production. In Carbon Debt, Fargione etal. clearly demonstrated that the carbon-impact of biofuels should not be solely evaluated based on the use stage of the product. Rather, the life cycle impact of the biofuel from raw material to final product must be analyzed with respect to its carbon footprint. In a similar vein, this paper aims to illustrate that the same “carbon debt” analysis needs to be conducted when evaluating the carbon mitigation potential of land-use management techniques. In particular, this paper illustrates that while N fertilizers often produce enhanced SOC on agricultural lands, their elevated upstream energy costs may generate a “carbon-debt” that is higher than their downstream C mitigation potential. Thus, to return to the guiding framework laid out at the beginning of this paper, it is clear that when considering the carbon mitigation potential of a new agricultural-practice it is critical that both SOC levels and life cycle energy consumption be assessed in order to determine the true carbon-reduction potential of the practice.
Bruce, James P., Michle Frome, Eric Haites, Henry Janzen, Rattan Lal, Keith Paustian. Carbon sequestration in soils. Journal of Soil and Water Conservation. 1999: 54:1.
Elliot, E.T. “Aggregate structure and carbon, nitrogen, and phosphorous in native and cultivated soils”. Soil Science Society America Journal, 1986: 50.
Fargione, Joseph, Jason Hill, David Tilman, Stephen Polasky, and Peter Hawthorne. Land Clearing ad Biofuel Carbon Debt. Science Vol. 319 Feb 2008.
Houghton R.A., and Skole D.L., “Carbon”. The Earth as Transformed by Human Action, Cambridge University Press, New York, pp. 393-408
Heller, Martin C., and Gregory A. Keoleian. Life Cycle-Based Sustainability Indicators for Assessment of the U.S. Food System. Ann Arbor, MI: Center for Sustainable Systems, University of Michigan, 2000: 42.
International Fertilizer Industry Association (IFA) Website: http://www.fertilizer.org/ifa/Home-Page/ABOUT-IFA
IPCC Land Use, Land Use Change and Forestry, Summary for Policymakers. IPCC 2000.
Ismail, I. Blevins, R.L., Frye W.W., 1994. Long term no-tillage effects on soil properties and continuous corn yields. Soil Science Society of America Journal 193-198.
Lal, R., J.M. Kimble, and R.F. Follett, 1999a. Agricultural practices and policies for carbon sequestration in soil. Recommendation and Conclusions of the International Symposium, 19-23 July 1999, Columbus, OH, USA.
Lal R. R.F. Follett, J Kimble and C.V. Cole, 1999b. Managing U.S. Cropland to Sequester Carbon in Soil. Journal of Soil and Water Conservation, First Quarter, 1999 54, 1. Pp. 374
Schlesinger, William H. Carbon Sequestration in soils: some cautions amidst optimisim. Agriculture Ecosystems and Environment. 2000; 82.
Paustian, Keith and C. Vernon Cole. CO2 Mitigation by Agriculture: An Overview. Climatic Change 40. 135-162, 1998.
UNFCC Website: http://unfccc.int/kyoto_protocol/items/2830.php
Varvel, G.E. 1994. Rotation and nitrogen fertilization effects on changes in soil carbon and nitrogen. Agronomy Journal 86, 319-325