N2O and NOx emissions from nitrogen fertilizer

A major source of CO2-equivalent emission in the corn-to-ethanol fuelcycle is N2O produced from nitrogen fertilizers. This emission of N2O usually is expressed as grams of nitrogen lost as N2O per gram of nitrogen in applied fertilizer. The lost N2O has two components: N2O lost from the field, from fertilizer in the soil, and N2O lost offsite, from fertilizer carried away in groundwater. In a recent review and analysis of the literature, Bouman (1996) estimates that the first component, "direct" emission of N, is 1.25% of applied nitrogen fertilizer, for all crops. However, the evidence reviewed in Delucchi and Lipman (1996) indicates that the emission rate for corn is higher than the emission rate for other crops. Consistent with this, the several experiments reviewed in Volume 2 (DeLuchi, 1993) estimate a direct emission rate of about 1.3% for corn. I assume a baseline rate of 1.3% for corn, and 1.1% for soybeans.

N2O emissions can be reduced by improving the efficiency of plant utilization of nitrogen (IPCC, 1996c). The recent IPCC (1996c) report reviews a number of ways to mitigate N2O emissions, and estimates that all of the mitigation measures have the potential to reduce N2O emissions by 20%. However, they point out, properly, that "farmers...will not volunteer to implement practices proposed to mitigate greenhouse-forced climate change," and will adopt such practices only if they are convinced that they will be profitable (p. 765). Nevertheless, many of the mitigation measures may indeed be attractive economically. I therefore assume that the N2O emission rate declines by 0.5% per year, with the result that emissions are reduced by 10% -- half of the "potential" estimated by the IPCC -- after 20 years.

Until recently, there was virtually no data on the second emission-rate component, N-N2O/N-nitrogen from groundwater offsite. On the basis of brief discussions in two studies, the IPCC assumed that the offsite emission rate was equal to the measured on-site rate. In Volume 2 (DeLuchi, 1993) of the original report, I adopted this assumption also. Since then there have been a few more relevant studies, which are reviewed in the recent IPCC reports on climate change (IPCC, 1996c). The IPCC now concludes that the off-site emission rate is about 0.75% of the applied N. This factor is consistent with data in the literature that indicates that some 20-30% of applied nitrogen leaves the site, and that some 0.05% to 5% of this off-site nitrogen evolves as N2O (see Delucchi and Lipman, 1996, for further discussion), if one picks the average of the 0.05% to 5% range for evolution of nitrogen off site (30% x 2.5% = 0.75%). However, it seems to me more reasonable to assume that the fraction of off-site N that evolves as N2O is equal to the fraction for on-site N -- about 1.3%, as discussed above. Thus, if 30% of applied N leaves the site, and 1.3% of that evolves as N2O, then an additional 0.4% of applied nitrogen is emitted as N in N2O offsite. I use this factor here, but assume that the fraction of N lost off site declines by 0.5% per year (relative terms) as well.

Nitrogen in fertilizer also can be emitted as NOx. On the basis of the recent literature review and analysis by Stohl et al. (1996), I have increased the N-NOx/N-nitrogen-fertilizer emission rate from 0.008 to 0.04, (see Delucchi and Lipman, 1996, for further discussion). Although this is a factor of 5 increase in the input parameter, fuelcycle GHG emissions change by less than 1%, because of the relatively low CEF for NOx.

For wood plantations, I assume that 0.6% of the added N fertilizer is evolved as N in N2O (Appendices K and N). For grass I assume 1.0%, partly on the basis of new data.

CO2 from soil and biomass

It is well established that cultivation reduces the carbon content in the soil, and in the standing above-ground biomass, especially if the climax ecosystem replaced by crops is a mature forest (IPCC, 1996c; Volume 2, Appendix K). The recent IPCC (1996c) report provides an excellent review of studies of the long-term loss of loss of carbon from soil to the atmosphere, as a result of cultivation. The loss depends on the type of soil, the type of crop and the intensity of cultivation, the type of ecosystem displaced, and other factors, and spans a range of two orders of magnitude, from 0.1 to 5 kg C per square meter. The present global mean loss, however, is closer to the upper end: about 3 kg C/m2 (IPCC, 1996c). The loss of carbon sequestered in the biomass can be of the same order of magnitude (Volume 2, Table K.12), with the result that the overall soil + biomass loss for the present global agricultural system may be on the order of 6 kg-C/m2/year. The loss in the U. S. probably is lower than the global average, on account of better agricultural management practices. The loss for grass and tree energy-crop systems presumably would be less than the present global agricultural-system average, too.

The loss of soil and biomass carbon occurs over a few decades (IPCC,1996c). This short-term, one-time loss must be converted to an equivalent perpetual annual loss, for proper comparison with, and addition to, the other emissions streams in the analysis. The best way to do this is to convert the short-term loss to an equivalent one-time initial loss in year zero, and then to annualize the equivalent initial loss over infinite time. The initial loss equivalent to the actual short-term loss is the present value of the actual short-term loss stream, and the equivalent annual carbon loss (g-C/m2/yr) is equal to the equivalent "initial" loss (g-C/m2) multiplied by the discount rate. Cline (1992) provides the most comprehensive analysis of the appropriate discount rate to use in the context of global warming, and settles on a rate of a bit under 2%. This is a relatively low rate -- lower than rates used by other analysts of the costs of global warming -- and consequently results in relatively low annualized emissions. Note that annualizing at a 2% discount rate over an infinite time period is equivalent to annualizing over a 50-year time period at zero discount rate.

The loss of carbon from the soil can be reduced considerably, by 50% or more, by various conservation management practices. Because some of these practices are likely to be introduced for reasons other than the desire to mitigate CO2 emissions, I will assume a gradual reduction in the areal loss.

Finally, it is possible that an increase in demand for corn (due to increased demand for ethanol) will spur an increase in per-acre yields that would not have happened otherwise, with the result that at least some of the additional corn will be grown on existing rather than new land. Unfortunately, it is not clear to what extent this might occur. That corn production has grown somewhat while harvested acreage has not over the past 20 years might be taken as evidence in favor of this proposition. However, there is much year-to-year variation: often, harvested acreage has increased with production, and in a few cases, harvested acreage has increased by a greater percentage than has production. Moreover, it is not necessarily the case that increases in yields are driven by increases in demand (outward shifts of the demand curve). The reverse proposition -- that increases in yield shift the supply curve out, reduce price, and spur additional consumption -- is at least as plausible. Indeed, the long-term decline in the real price of corn from 1951 to 1996 is evidence that supply-side improvements have reduced price and stimulated consumption. (If the market were driven primarily by shifts in demand, real prices would have risen.) Consistent with the proposition that increased output results from improved yields rather than the other way around, the World Agricultural Outlook Board (WAOB, 1997) projects declining real prices and increasing harvested acreage for corn through the year 2005.

Nevertheless, it is possible that, on balance, some fraction of the additional corn demanded for ethanol production will be grown on land already used for corn production. Moreover, the net effect of the increase in the price of corn, due to the shift in demand induced by extra demand for ethanol (see the discussion elsewhere regarding the co-products of the ethanol production process), might be a slight reduction in land devoted to agriculture. With these considerations, I assume that 35% of the corn used to make ethanol is grown on "old" land, and 65% on land newly brought into production. I assume the same percentages for soybeans. Because wood and grass energy crops are, by definition, grown to produce energy, one should assume that the "margin" is all production, and hence that all production occurs on new land.

We now can specify our formal model of CO2 emissions from soil and biomass:

where:

NA = of total additional corn demanded for ethanol production, the fraction that is grown on new land (see discussion in text)

ECY = the energy conversion yield, in BTUs of fuel per bushel or ton of feedstock (Table XII)

FL = fraction of fuel production lost due to evaporation or spillage (Appendix B of Volume 2 [DeLuchi, 1993], and updates thereto in this report)

These parameter values result in a very large contribution to fuelcycle CO2-equivalent emissions: on the order of 60 g/mi in the corn/ethanol fuelcycle, or about 15% of the fuelcycle total. Of course, this estimate is quite uncertain, because all of the key parameter values are uncertain.

It is possible that the application of fertilizer changes the rate of emission of carbon from soils. In light of this possibility, I added a g-CO2/g-N parameter to the model. However, in the literature there is no strong relationship between the amount of nitrogen fertilizer added and CO2 emissions from soil. Thus, the value of the g-CO2/g-N parameter presently is zero. See Delucchi and Lipman (1996) for further discussion.

Methane emissions from soil

Cultivation reduces the oxidation of methane in aerobic soils, and thereby increases the concentration of methane in the atmosphere (IPCC, 1996c). Some of the reduction in soil uptake of methane is related to the use of nitrogen fertilizer, and some is related to cultivation per se, independent of the use of fertilizer. The reduction in methane uptake is equivalent to an emission of methane from cultivated soils. Delucchi and Lipman (1996) review some of the available data, and estimate effective methane emissions from soils as follows:

Although there is considerable uncertainty in the parameters SEMF and SEMH, even the upper-end of the plausible range (reviewed in Delucchi and Lipman, 1996) results in negligible fuelcycle CO2-equivalent emissions (less than 0.5 g/mi). The best-guess assumptions result in less than 0.1 g/mi CO2-equivalent emissions.

Environmental impacts of corn farming

Pitstick (1992) reviews a study by the Economic Research Service (ERS) that estimates the shifts in agricultural production and changes in soil erosion as a result of increased production of ethanol from corn. The ERS finds that increased production of ethanol from corn will cause a decrease in the number of acres planted in soybeans, because the ethanol co-products (corn gluten feed and meal, distillers dried grains, and corn oil) will displace soybean products in the animal-feed and vegetable-oil markets. This finding suggests that the appropriate way to handle the ethanol co-products is to deduct from total ethanol-production emissions the emissions foregone from the production of soybean products (see discussion below). This is co-product method 1 in Appendix K of the original report.