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

<p> </p> <p><strong>Context:</strong> 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.</p> <p><strong>Aims:</strong> 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.</p> <p><strong>Methods:</strong> 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.</p> <p><strong>Key results:</strong> 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 (<em>P</em> < 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 (<em>P</em> < 0.05) with higher application rates.</p> <p><strong>Conclusions:</strong> Compared with DCD and nitrapyrin, a higher DMPP concentration was required to effectively inhibit urinary N nitrification rates in the pasture soils.</p> <p><strong>Implications:</strong> 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.</p>