Nitrous Oxide Emissions Associated with Nitrogen Use on Arable Crops in England

Keywords: nitrous oxide emissions, crop yield, greenhouse gas intensity, ammonium nitrate, arable cropping, emission factor.

Arable farms in Northern Europe are likely to play an increasingly vital role in food production; most crops receive significant amounts of manufactured nitrogen (N) fertilisers, which can be associated with large losses of the greenhouse gas (GHG) nitrous oxide (N₂O). In national GHG inventories where direct N₂O emissions from soil are calculated using the standard Tier 1 Intergovernmental Panel on Climate Change (IPCC) methodology and in current commercial GHG accounting procedures, direct N₂O emissions from soil are assumed to be linearly related to N inputs. This assumption implies that drastic reductions in N fertiliser use and crop productivity would be required to minimise N₂O emission intensities of crop products (kg N₂O-N per kg product). We hypothesise that the response of annual N₂O emissions to N supply must, to some extent, be related to the surplus of N supply over crop N uptake. If so, fertiliser N application strategies to minimise emission intensities may have much less severe implications for crop productivity.

Ten experiments were carried out on crops harvested in 2010 and 2011 on commercial farms in England with contrasting soil texture growing cereals, sugar beet and oilseed rape. Direct N₂O emissions were monitored, using the static chamber technique, for 12 months following spring ammonium nitrate fertiliser applications in the range nil to 240% of recommended N.

Cumulative direct N₂O emissions were small and emissions did not generally respond immediately to N fertiliser application due to very dry spring conditions and low soil moisture contents. The mean annual N₂O emission factor (EF) at 120% of the recommended N rate was 0.26% total-N applied compared to the IPCC EF of 1.0%. Most responses of N₂O emission to N application rate were linear, but it was evident that this relationship was more complicated than our hypothesis assumed and depended on interactions between fertiliser N supply, crop N uptake and soil conditions. It is unknown whether more typical weather conditions would change the dynamics of crop N uptake and N₂O emission and lead to non-linear responses. Current processing of results from the wetter 2012 season should enable a more generally appropriate N applications strategy to be devised to minimise N₂O emission intensities. Notwithstanding the influential role of yield in shaping the emission intensity curve, if linear responses prove to be the norm, it may have to be accepted that high crop yields are incompatible with minimising emission intensities, although if the EF for applied N is small (e.g. 0.50% total-N applied) extra emissions may be slight.

Thorman, R.E.¹, Smith, K.E.¹, Rees, R.M.², Chauhan, M.¹, Bennett, G.³, Malkin, S.⁴, Munro, D.G.¹, Sylvester-Bradley, R.¹

¹ ADAS UK Ltd., Battlegate Road, Boxworth, Cambridge, CB23 4NN, UK.

² SRUC, West Mains Road, Edinburgh, EH9 3JG, UK.

³ ADAS UK Ltd., Gleadthorpe, Meden Vale, Mansfield, Nottingham, NG20 9PF, UK.

⁴ ADAS UK Ltd., Bentinck Farm, Rhoon Road, Terrington St Clement, King’s Lynn, Norfolk, PE34 4HZ, UK.

42 pages, 10 figures, 8 tables, 65 references.