• Nitrate leaching;
  • nitrification inhibitor;
  • ammonia oxidizing bacteria;
  • ammonia oxidizing archaea;
  • grazed grassland


Nitrate (NO3) can contribute to surface water eutrophication and is deemed harmful to human health if present at high concentrations in the drinking water. In grazed grassland, most of the NO3-N leaching occurs from animal urine-N returns. The objective of this study was to determine the effectiveness of a nitrification inhibitor, dicyandiamide (DCD), in decreasing NO3 leaching in three different soils from different regions of New Zealand under two different rainfall conditions (1260 mm and 2145 mm p.a.), and explore the relationships between NO3-N leaching loss and ammonia oxidizing bacteria (AOB) and ammonia oxidizing archaea (AOA). The DCD nitrification inhibitor was found to be highly effective in decreasing NO3-N leaching losses from all three soils under both rainfall conditions. Total NO3-N leaching losses from the urine patch areas were decreased from 67.7–457.0 kg NO3-N/ha to 29.7–257.4 kg NO3-N/ha by the DCD treatment, giving an average decrease of 59%. The total NO3-N leaching losses were not significantly affected by the two different rainfall treatments. The total NO3-N leaching loss was significantly related to the amoA gene copy numbers of the AOB DNA and to nitrification rate in the soil but not to that of the AOA. These results suggest that the DCD nitrification inhibitor is highly effective in decreasing NO3 leaching under these different soil and rainfall conditions and that the amount of NO3-N leached is mainly related to the growth of the AOB population in the nitrogen rich urine patch soils of grazed grassland.