A comparison of the threshold concentrations of DCD, DMPP and nitrapyrin to reduce urinary nitrogen nitrification rates on pasture soils – a laboratory study

Context Using nitrification inhibitors (NIs) for the targeted management of urine patches, to reduce nitrous oxide (N2O) emissions, requires determining the threshold concentrations of the NIs in urine for effective nitrification inhibition. Aims This study comparatively assessed the threshold concentrations of three NIs: dicyandiamide (DCD), 3,4-dimethylpyrazole phosphate (DMPP) and 2-chloro-6-(trichloromethyl) pyridine (nitrapyrin) to reduce urinary nitrogen (N) nitrification rates on two contrasting pasture soils. Methods Four rates of each NI (3–27 mg DCD kg-1 soil, 1–13 mg DMPP kg-1 soil and 1–14 mg nitrapyrin kg-1 soil) were added to urine-amended soils and incubated at laboratory room temperature. The amended soils were sampled periodically to monitor changes in mineral-N concentrations. Key results The threshold concentration of DCD (3 mg kg-1 soil, lowest rate tested) was lower than that of nitrapyrin (5–7 mg kg-1 soil) and DMPP (13 mg kg-1 soil, highest rate tested) on both soils. Greater NI effectiveness corresponded to greater NI persistence, with higher (P < 0.05) half-life values observed for DCD (16 ± 2 days, mean ± s.e.) compared with nitrapyrin (10 ± 2 days) and DMPP (9.2 ± 0.3 days). All three NIs persisted longer (P < 0.05) with higher application rates. Conclusions Compared with DCD and nitrapyrin, a higher DMPP concentration was required to effectively inhibit urinary N nitrification rates in the pasture soils. Implications Choosing the ideal application rate of NIs to inhibit nitrification under field condition, and hence mitigate N2O emissions from urine patches, requires consideration of the factors that affect NI loss.