Integrated Strategies to Minimise Slurry Nitrogen Losses – Application Rates and Method. Experimental site Devon, 2004

Cattle slurry was applied by trailing shoe and surface broadcasting at five rates, viz: 20, 35, 50, 65 and 80 m3 ha-1, using a purpose built small-plot applicator to grassland, on a coarse sandy loam soil in autumn 2004 at a site close to North Wyke, Devon. There were 3 replicate plots (5 x 12 m) of each application treatment arranged in a randomised block design. Ammonia emissions were measured from each treatment for 7 days after application using wind tunnels. Direct nitrous oxide (N2O) emissions were measured from the 35 m3 ha-1 slurry rate only for about 12 months after application from both application techniques and an untreated control, using the static chamber technique (five chambers per plot). Nitrous oxide samples were analysed using gas chromatography. Crop (fresh weight and dry matter) yields and N offtakes were measured at harvest. The Devon, 2004 experiment contains data sets of; annual nitrous oxide emission, annual nitrous oxide emission factor, total ammonia loss, overwinter nitrate leaching loss, soil moisture, temperature, rainfall and associated crop, soil and manure 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 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 slurry 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, eight 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, reduced to a 20 ml sample which was drawn into a pre-evacuated 20 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). Previous field experiments in the UK have demonstrated a linear increase in N2O concentration within a chamber and this was supported by the results from a desk study which reviewed UK experimental data where the increase in N2O concentration within a field chamber had been measured during enclosure (Chadwick et al., 2014). More than 90% of the 1,970 measurements of chamber headspace accumulation demonstrated a linear increase in N2O concentration (Chadwick et al., 2014). In order to minimise the effect of diurnal variation, gas sampling was carried out where possible at the same time of day. 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 slurry application, N2O flux measurements were carried out for 5 days immediately following slurry application, daily for a further 5 days during the next week, weekly for the next three weeks and then fortnightly until the end of the 12 month sampling period, or following any above average precipitation events. Prior to the slurry application N2O measurements were taken to provide baseline information. This sampling schedule resulted in an annual total of c.40 sampling days starting from the day of the first slurry 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.