Experimental determination of nitrous oxide emission factors for arable land. Experimental site in Devon, 2007

At Rothamsted Research - North Wyke in south west England nitrous oxide (N2O) emissions were monitored following spring ammonium nitrate fertiliser applications at 6 different rates; 0 (i.e. untreated control), 70, 90, 130, 160 and 200 kg N ha-1. Up to two separate fertiliser applications were made in order to reach the target application rate. There were 3 replicate plots (4 x 12 m) of each treatment arranged in a randomised block design. The plots were established on a winter wheat crop sown the previous autumn, on a coarse sandy loam soil. Direct nitrous oxide emissions were measured from each treatment for 12 months using the static chamber technique (3 chambers per plot) and GC analysis. The Devon, 2007 experiment contains data sets of; annual nitrous oxide emission, annual nitrous oxide emission factor, soil moisture, temperature, rainfall and crop yield (from the 130 kg N ha-1 treatment only) and soil measurements.

6 files and explanatory metadata available at DOI.

Direct N2O emissions were measured with three static flux chambers (40 cm wide x 40 cm long x 25 cm high) per plot, covering a total surface area of 0.48 m2. The chambers were of white (i.e. reflective) PVC and un-vented with a water-filled channel running around the upper rim of the chamber allowing an air-tight seal to form following chamber enclosure with a lid (Smith et al., 2012). Chambers were pushed into the soil up to a depth of 5 cm and remained in place throughout the experiment, except during fertiliser application, drilling and harvesting when chambers were removed, locations were marked, and chambers were re-instated to the same position as prior to removal. Chambers remained open except for a short time on each sampling day. On that day, ten samples of ambient air were taken to represent time zero (T0) N2O samples. From each chamber, after a 40-minute enclosure period (T40) one headspace sample was taken using a 50-ml syringe and flushed though a pre-evacuated 20-22 ml glass vial fitted with a chloro-butyl rubber septum and held at atmospheric pressure. The N2O flux was calculated using an assumed linear increase in N2O concentration from the ambient N2O concentration (T0) to the N2O concentration inside the chamber after 40-minutes enclosure (T40) (Chadwick et al., 2014). Throughout each experiment, the linearity of emissions through time was checked routinely from two chambers located on the highest N rate plots. A minimum of five samples were taken from each chamber at 15 min intervals commencing at closure i.e. T0 and spanning the T40 sampling time. In order to permit sampling from a growing crop, at the time of sampling an additional chamber was stacked (using the water-filled channel) onto each permanent chamber and the chamber enclosure period extended. In order to minimise the effect of diurnal variation, gas sampling was carried out between 10:00 am and 14:00 pm and where possible between 10:00 am and 12:00 pm as suggested by IAEA (1992) and referred to in the IPCC good practice guidance (IPCC, 2000). Gas samples were analysed as soon as possible after collection (to minimise potential leakage) using gas chromatographs fitted with an electron-capture detector and an automated sample injection system. Following receipt in the laboratory, two replicates of one standard N2O gas were kept with the samples and were used to verify sample integrity during storage. The gas chromatographs were calibrated on a daily basis using certified N2O standard gas mixtures. Following fertiliser application, N2O flux measurements were carried out on the three consecutive days after the onset of fertiliser prill degradation and thereafter following a daily rainfall exceeding 10 mm or every 10-14 days (14-21 days November to March) giving a total of c.50 sampling days over the 12-month sampling period. Prior to the first fertiliser application N2O measurements were taken to provide baseline information. Measurements were taken over 12 months to follow IPCC good practice guidance and so that the results were directly comparable to the IPCC 2006 methodology default emission factor.

Nitrous oxide fluxes from the three replicate chambers per plot were averaged. Cumulative fluxes were calculated using the trapezoidal rule to interpolate fluxes between sampling points.