Inhibition of ammonium oxidation by a liquid formulation of 3,4-Dimethylpyrazole phosphate (DMPP) compared with a dicyandiamide (DCD) solution in six new Zealand grazed grassland soils

The oxidation of ammonium (NH (4) (+) ) to nitrate (NO (3) (-) ) in the soil is an important biogeochemical process, which has major environmental implications as it can contribute to NO (3) (-) leaching and nitrous oxide (N2O) emissions. Nitrification inhibitors have been used to slow down this process to reduce NO (3) (-) leaching and N2O emissions from agricultural land. The objective of this study was to determine the effectiveness of a liquid formulation of 3,4-Dimethylpyrazole phosphate (DMPP) compared with a dicyandiamide (DCD) solution in inhibiting the growth of ammonium-oxidizing bacteria (AOB) and ammonium oxidizing archaea (AOA) and slowing down the rate of NH (4) (+) oxidation in soil. An incubation study was conducted to determine the effect of liquid DMPP and a DCD solution on AOB and AOA population growth and on the rate of NH (4) (+) oxidation following the application of an ammonium source in the form of animal urine at the equivalent rate of 1,000 kg N ha(-1) in six grazed grassland soils. The results showed that the growth of the AOB population, as measured by the functional amoA gene copy numbers, was effectively inhibited by both DMPP and DCD. This inhibition by both inhibitors significantly decreased the ammonium oxidation rate constants in all six soils. The half-lives of ammonium in the soils were significantly (P < 0.05) increased from 31 to 58 days in the urine treatments to 109-285 days when DCD or DMPP was applied. There was no significant difference in the NH (4) (+) oxidation rates between the DMPP and DCD treatments across the six soils. These results indicate that both DMPP and DCD have the potential to be used to treat grazed pasture soils to reduce nitrate leaching and nitrous oxide emissions.