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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). As a source for NOx ,
i.e. NO plus NO2, N2O also plays a major role in stratospheric
ozone chemistry (Crutzen, 1970). 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. Here 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.
The work is currently subject to open review in the journal Atmospheric Chemistry and Physics. Crutzen has declined to comment until that process is completed. The paper suggests that microbes convert much more of the nitrogen in fertilizer to nitrous oxide than previously thought—3 to 5 percent, compared to the widely accepted figure of 2 percent used by the International Panel on Climate Change (IPCC) to calculate the impact of fertilizers on climate change.
For rapeseed biodiesel, which accounts for about 80 percent of the biofuel production in Europe, the relative warming due to nitrous oxide emissions is estimated at 1 to 1.7 times larger than the relative cooling effect due to saved fossil CO2 emissions. For corn bioethanol, dominant in the US, the figure is 0.9 to 1.5. Only sugarcane bioethanol—with a relative warming of 0.5 to 0.9—looks like a better alternative to conventional fuels.