Abstract
Abstract. The relationship, on a global basis, between the amount of N fixed by chemical, biological or atmospheric processes entering the terrestrial biosphere, and the total emission of nitrous oxide (N2O), has been re-examined, using known global atmospheric removal rates and concentration growth of N2O as a proxy for overall emissions. For both the pre-industrial period and in recent times, after taking into account the large-scale changes in synthetic N fertiliser production, we find an overall conversion factor of 3–5% from newly fixed N to N2O-N. We assume the same factor to be valid for biofuel production systems. It is covered only in part by the default conversion factor for "direct" emissions from agricultural crop lands (1%) estimated by IPCC (2006), and the default factors for the "indirect" emissions (following volatilization/deposition and leaching/runoff of N: 0.35–0.45%) cited therein. However, as we show in the paper, when additional emissions included in the IPCC methodology, e.g. those from livestock production, are included, the total may not be inconsistent with that given by our "top-down" method. When the extra N2O emission from biofuel production is calculated in "CO2-equivalent" global warming terms, and compared with the quasi-cooling effect of "saving" emissions of fossil fuel derived CO2, the outcome is that the production of commonly used biofuels, such as biodiesel from rapeseed and bioethanol from corn (maize), depending on N fertilizer uptake efficiency by the plants, can contribute as much or more to global warming by N2O emissions than cooling by fossil fuel savings. Crops with less N demand, such as grasses and woody coppice species, have more favourable climate impacts. This analysis only considers the conversion of biomass to biofuel. It does not take into account the use of fossil fuel on the farms and for fertilizer and pesticide production, but it also neglects the production of useful co-products. Both factors partially compensate each other. This needs to be analyzed in a full life cycle assessment.
Highlights
N2O, a by-product of fixed nitrogen application in agriculture, is a “greenhouse gas” with a 100-yr average global warming potential (GWP) 296 times larger than an equal mass of CO2 (Prather et al, 2001)
For bio-diesel derived from rapeseed, this analysis indicates that the global warming by N2O is on average about 1.0–1.7 times larger than the quasi-cooling effect due to “saved fossil CO2” emissions
We have shown that the yield of N2O-N from fixed nitrogen application in agro-biofuel production can be in the range of 3–5%, 3–5 times larger than assumed in current life cycle analyses, with great importance for climate
Summary
N2O, a by-product of fixed nitrogen application in agriculture, is a “greenhouse gas” with a 100-yr average global warming potential (GWP) 296 times larger than an equal mass of CO2 (Prather et al, 2001). The increasing use of biofuels to reduce dependence on imported fossil fuels and to achieve “carbon neutrality” will further cause atmospheric N2O concentrations to increase, because of N2O emissions associated with N-fertilization. We propose a global average criterion for the ratio of N to dry matter in the plant material, which indicates to what degree the reduced global warming (“saved CO2”) achieved by using biofuels instead of fossil fuel as energy sources is counteracted by release of N2O. This study shows that those agricultural crops most commonly used at present for biofuel production and climate protection can readily lead to enhanced greenhouse warming by N2O emissions
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