The effect of the nitrification inhibitor dicyandiamide (DCD) on nitrous oxide and methane emissions after cattle slurry application to Irish grassland

Abstract

Application of cattle slurry to grassland can lead to gaseous losses of nitrogen (N). The dynamics of ammonia (NH3) emissions are well documented, but emissions of nitrous oxide (N2O) and other trace gases from land application of cattle slurry, are not as well understood. Nitrification inhibitors such as dicyandiamide (DCD) help to retain soil N in the ammonium (NH4+) form, which is expected to result in reduced N loss and increased N use efficiency. Emissions of methane (CH4) could potentially be affected by DCD, since the enzyme ammonia monooxygenase (AMO), which DCD is thought to inhibit, can oxidise CH4 as well as NH3. The objective of this study was to investigate the impacts of DCD on N2O and CH4 emissions from grassland soils after slurry application. At one experimental site, Johnstown Castle (JC), County Wexford, Ireland, slurry was applied in March, June and October for two years, at a rate of 33m3ha−1, by either bandspread (BS) or splashplate (SP) application methods, with and without DCD. A second site at Hillsborough (HB), County Down, Ireland, was treated in the same way for one year only, using the SP application method. Emissions of N2O and CH4 were measured using static chambers. Over the entire experiment DCD significantly reduced cumulative N2O emissions by 47 and 70% at both sites. Slurry spreading method had no significant effect on direct N2O emissions and there was no effect of DCD on CH4 emissions throughout the experiment. Using an emission factor for indirect N2O emissions of 1%, modelled losses of NH3 through volatilisation, resulted in an estimated 13 times greater indirect than direct N2O emissions. The results suggest that, under typical Irish field conditions, DCD can be used to decrease direct N2O emissions, without increasing CH4 emissions, and bandspreading can be used as a method of decreasing NH3 volatilisation, without increasing direct N2O or CH4 emissions. Since direct N2O emissions were relatively small, targeting indirect N2O emissions through mitigation of NH3 volatilisation and nitrate (NO3−) leaching could be an effective way of decreasing total N2O emissions from slurry application at these sites.