The effect of cattle slurry in combination with nitrate and the nitrification inhibitor dicyandiamide on in situ nitrous oxide and dinitrogen emissions

Abstract. A field study was conducted to determine the effect of the nitrification inhibitor dicyandiamide (DCD) on N₂O and N₂ emissions after cattle slurry (CS) application in the presence of nitrate (NO₃) fertiliser on seven different occasions (between March 2009 and March 2011). N₂O emissions from CS in the presence of NO₃ fertiliser were very high (0.4–8.7% of applied N) over a 20-day period, under mild moist conditions. Emissions were significantly larger from the CS treatment compared to an NH₄⁺-N source, supplying the same rate of N as in the slurry. This study supports the view that organic fertilisers should not be applied at the same time as nitrate-based fertilisers, as significant increases in N₂O emissions occur. The average N₂O mole fraction (N₂O/(N₂O + N₂)) over all seven application dates was 0.34 for CSNO₃ compared to 0.24 for the NH₄ClNO₃ treatment, indicating the dominance of N₂ emissions. The rate of nitrification in CSNO₃ was slower than in NH₄ClNO₃, and DCD was found to be an effective nitrification inhibitor in both treatments. However, as N₂O emissions were found to be predominantly associated with the NO₃ pool, the effect of DCD in lowering N₂O emissions is limited in the presence of a NO₃ fertiliser. To obtain the maximum cost-benefit of DCD in lowering N₂O emissions, under mild moist conditions, it should not be applied to a nitrate containing fertiliser (e.g. ammonium nitrate or calcium ammonium nitrate), and therefore the application of DCD should be restricted to ammonium-based organic or synthetic fertilisers.