The persistence and efficacy of nitrification inhibitors to mitigate nitrous oxide emissions from New Zealand pasture soils amended with urine

Information on the comparative effectiveness of the nitrification inhibitors (NIs), 3,4-dimethylpyrazole phosphate (DMPP) and 2-chloro-6-(trichloromethyl) pyridine (nitrapyrin) on New Zealand (NZ) urine-amended pasture soils is lacking. Therefore, laboratory incubation experiments were conducted to test the half-life and efficacy of these NIs to reduce nitrous oxide (N2O) emissions from two urine-amended pasture soils varying in clay and organic matter content. Two application rates of each NI were tested as follows: 2.5 and 5 mg DMPP kg−1 soil, and 3 and 6 mg nitrapyrin kg−1 soil. Dicyandiamide (DCD) was used as a reference NI, at the recommended application rate (10 mg DCD kg−1 soil). The half-life values (at 15°C) of the NIs varied with clay and organic matter. These were 12–17 days for DMPP and 18–29 days for nitrapyrin compared with 20–33 days for DCD. All the NIs reduced N2O emissions, but their efficacy differed with the amount of NI and soil type. The reductions in N2O emissions were 19–46% with DMPP and 45–59% with nitrapyrin relative to 43–47% with DCD. The higher rate of nitrapyrin achieved significantly (p < 0.05) greater reductions under lower clay and organic matter content. The NIs did not affect the levels of soil microbial biomass carbon and nitrogen (MBC and MBN) during the study period.