Are the benefits of yield responses to nitrogen fertilizer application in the bioenergy crop <i>Miscanthus</i> × <i>giganteus</i> offset by increased soil emissions of nitrous oxide?

Abstract

Abstract 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 N 2 O emissions, as well as annual crop yield over a full year. N 2 O emission intensity (N 2 O emissions calculated as a function of above‐ground biomass) was significantly affected by fertilizer application, with values of 52.2 and 59.4 g N 2 O‐N t −1 observed at 63 and 125 kg‐N ha −1 , respectively, compared to 31.3 g N 2 O‐N t −1 in the zero fertilizer control. A life cycle analyses approach was applied to calculate the increase in yield required to offset N 2 O 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 N 2 O 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 N 2 O emission factors found increases in yield necessary to offset soil N 2 O 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 N 2 O emissions.