Pasture productivity benefits from strategic urease and nitrification inhibitor use are limited in rainfed temperate dairy pastures of southern Australia

Urea is the most commonly used nitrogen (N) fertiliser on Australian dairy pastures, but has low N use efficiency due to high loss potential. Urease and nitrification inhibitors can be used to reduce these losses, however, their efficacy is highly variable and the reported impacts on productivity are inconsistent, with assessments often made using a single inhibitor across all seasons.

We examined the effectiveness of two commonly used inhibitors, the urease inhibitor N-(n-butyl) thiophosphoric triamide (NBTPT) and the nitrification inhibitor 3,4-Dimethylpyrazole phosphate (DMPP) applied in a strategic program, on pasture responses to urea (U) fertiliser, and fertiliser N use efficiency in a temperate dairy pasture in Southern Australia.

Pasture productivity, N uptake, agronomic N use efficiency (NU_AE), ¹⁵N fertiliser recovery and soil mineral N were monitored. The nitrogen was applied in the urea form at rates of 160, 320, 480 and 640 kg N ha⁻¹ yr⁻¹

The majority of the plant N (64%-85%), assessed over autumn and spring growth periods using ¹⁵N, was derived from mineralised soil organic matter (SOM). Neither of the inhibitors significantly altered pasture production or NU_AE, most likely because of the major role of SOM in supplying N to plants, and because much of the applied ¹⁵N (up to 55%) was retained in the soil (0-40 cm depth) due to microbial immobilisation. Uptake of fertiliser N was highest, up to 29% of applied N, immediately following fertiliser application and then was substantially reduced over subsequent months, reflecting low levels of mineralisation of the previously immobilised N. The total loss of applied ¹⁵N was greater following autumn applications of N than spring applications, being 14% - 42% and 2% - 7% respectively and was attributed to volatilisation occurring primarily in autumn, with minimal leaching and denitrification predicted under the experimental conditions.