Are the benefits of yield responses to nitrogen fertilizer application in the bioenergy cropMiscanthus ×giganteusoffset by increased soil emissions of nitrous oxide?

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A field trial was carried out on a 15 year old Miscanthus stand, subject to nitrogen fertilizer treatments of 0, 63 and 125 kg-N ha−1, measuring N2O emissions, as well as annual crop yield over a full year. N2O emission intensity (N2O emissions calculated as a function of above-ground biomass) was significantly affected by fertilizer application, with values of 52.2 and 59.4 g N2O-N t−1 observed at 63 and 125 kg-N ha−1, respectively, compared to 31.3 g N2O-N t−1 in the zero fertilizer control. A life cycle analyses approach was applied to calculate the increase in yield required to offset N2O emissions from Miscanthus through fossil fuel substitution in the fuel chain. For the conditions observed during the field trial yield increases of 0.33 and 0.39 t ha−1 were found to be required to offset N2O emissions from the 63 kg-N ha−1 treatment, when replacing peat and coal, respectively, while increases of 0.71 and 0.83 t ha−1 were required for the 125 kg-N ha−1 treatment, for each fuel. These values are considerably less than the mean above-ground biomass yield increases observed here of 1.57 and 2.79 t ha−1 at fertilization rates 63 and 125 kg-N ha−1 respectively. Extending this analysis to include a range of fertilizer application rates and N2O emission factors found increases in yield necessary to offset soil N2O emissions ranging from 0.26 to 2.54 t ha−1. These relatively low yield increase requirements indicate that where nitrogen fertilizer application improves yield, the benefits of such a response will not be offset by soil N2O emissions.

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