Agricultural Greenhouse Gas Inventory Research Platform - InveN2Ory. Fertiliser experimental site in Devon, 2013

At Rothamsted Research - North Wyke near Okehampton, south-west England (clay loam topsoil texture), direct N2O-N emissions were measured from replicated (x3) plots (12 x 3m) following spring applications of manufactured nitrogen (N) fertilisers to grassland. A control treatment was included where no N fertiliser was applied. Ammonium nitrate (AN) fertiliser (34.5% N) was applied at 5 different rates; 80, 160, 240, 320 and 400 kg N/ha, and urea fertiliser (46% N) at a rate of 320 kg N/ha, in four split applications (late-February, early-April, late-May and late-June). In separate AN and urea treatments, a commercially available nitrification inhibitor was tested; dicyandiamide (DCD) was sprayed at a rate equivalent to 10 kg DCD/ha onto the plots immediately after each N fertiliser application. The N supplied by the DCD was accounted for in the total N application of 320 kg N/ha, which was applied in four splits. Additionally, AN fertiliser was also applied in six splits in late-February, mid-March, early-April, mid-April, late-May and late-June; the first four splits were of 30-40 kg/N ha i.e. ‘little and often’. Following N fertiliser application, measurements of direct N2O-N were made over c.12 months, using 5 static chambers (0.8 m2 total surface area) per plot and analysed by gas chromatography. Grass yields and N offtakes were also measured following grass cuts in late-May, mid-June, late-July and late-September 2013. The Devon, 2013 experiment contains data sets of; annual nitrous oxide emission, annual nitrous oxide emission factor, soil moisture, top soil mineral nitrogen (selected dates), temperature, rainfall and associated crop (grass yield and nitrogen offtakes) and soil measurements.

Direct N2O emissions were measured with five static flux chambers (40 cm wide x 40 cm long x 25 cm high) per plot, covering a total surface area of 0.8 m2. The chambers were of white (i.e. reflective) PVC and un-vented with an expanded neoprene rubber strip (Portmere rubber, SO14 5QZ) to form an air-tight seal following chamber enclosure with a lid (Cardenas et al., 2010). Chambers were pushed into the soil up to a depth of 5 cm and remained in place throughout the experiment, except during fertiliser application and grass cutting 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) a headspace sample was taken using a 50-ml syringe. Using a double needle system the sample was flushed through 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 three 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 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 and not stored for more than 2 days (to minimise potential leakage) using gas chromatographs fitted with an electron-capture detector and an automated sample injection system. The gas chromatographs were calibrated on a daily basis using certified N2O standard gas mixtures. An exchange of samples of chamber air and standard gas mixtures between labs from the different research organisations involved in the InveN2Ory programme of experiments who operated the GCs were carried out, to avoid the possibility of any bias in the results towards high or low values. Following each fertiliser application, N2O flux measurements were carried out in line with the following sampling schedule until the next fertiliser application or for the final application until the end of the monitoring period; measurements were taken for 5 days immediately following fertiliser application, daily for a further 5 days during the next week, twice weekly for the next two weeks, every other week over the next c.four months, decreasing in frequency to monthly until the end of the 12 month sampling period. Prior to the first fertiliser application N2O measurements were taken to provide baseline information. This sampling schedule resulted in an annual total of c.70 sampling days starting from the day of the first fertiliser application. 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 five replicate chambers per plot were averaged. Cumulative fluxes were calculated using the trapezoidal rule to interpolate fluxes between sampling points.