The effect of dicyandiamide addition to cattle slurry on soil gross nitrogen transformations at a grassland site in Northern Ireland

Many studies have shown the efficacy of the nitrification inhibitor dicyandiamide (DCD) in reducing nitrous oxide (N2O) emissions and nitrate (NO3-) leaching. However, there is no information on the effect of DCD on gross soil N transformations under field conditions, which is key information if it is to be used as amitigation strategy to reduce N losses. The current field study was conducted to determine the effect of DCD on ten gross nitrogen (N) transformations in soil following cattle slurry (CS) application to grassland in Northern Ireland on three occasions (June 2010, October 2010 and March 2011). Ammonium (NH4+) oxidation (O-NH4) was the dominant process in total NO3- production (O-NH4+O-Nrec (oxidation of recalcitrant organic N to NO3-)) following CS application, accounting for 0.894-0.949. Dicyandiamide inhibited total NO3- production from CS by 0.781, 0.696 and 0.807 in June 2010, October 2010 and March 2011, respectively. The lower inhibition level in October 2010 was thought to be due to the higher rainfall and soil moisture content in that month compared to the other application times. As DCD strongly inhibited NH4+ oxidation following CS application, it also decreased the rate of total NO3- consumption, since less NO3- was formed. The rates of mineralization from recalcitrant organic-N (M-Nrec) were higher than from labile organic-N (M-Nlab) on all occasions. The DCD significantly increased total mineralization (M-Nrec+M-Nlab) following CS application in June 2010 and March 2011, but had no significant effect in October 2010. In contrast, the rate of immobilization of labile organic-N (INH4-Nlab) was higher than from recalcitrant organic-N (INH4-Nrec) on all occasions, accounting for 0.878-0.976 of total NH4+ immobilization from CS. The DCD significantly increased total immobilization (INH4-Nrec+INH4-Nlab) when CS was applied in June 2010, but had no significant effect at other times of the year. Dicyandiamide was shown to be a highly effective inhibitor of ammonium oxidation at this grassland site. Although there was evidence that it increased both NH4+ mineralization and immobilization following CS application, its effect on these processes was inconsistent. Further work is required to understand the reason for these inconsistent effects: future improvements in N-15 tracer models may help.