Influence of soil organic matter, fertiliser formulation and season on fertiliser nitrogen use efficiency in temperate pastures.

Intensively grazed dairy systems use high inputs of fertiliser nitrogen (N), and often supplementary irrigation, to ensure adequate pasture production to support milk output and meet the growing food demand. However, the efficiency of N use in these systems can be low and potential environmental impacts high. This study aimed to test the hypothesis that (1) use of two inhibitors, the urease inhibitor N-(n-butyl) thiophosphorictriamide (NBTPT) and the nitrification inhibitor 3,4-Dimethylpyrazole phosphate (DMPP) reduced N loss and improved pasture production compared to conventional N fertiliser (urea) in irrigated temperate perennial ryegrass (Lolium perenne L.) dairy pasture, and (2) their efficiency was affected by soil and environmental parameters. The effect of repeated applications of urea, at different rates, and the inhibitors were studied on pasture production and agronomic apparent fertiliser N use efficiency (NUE) over 2.5 years. The fate of a single application of N was determined through recovery of ¹⁵N-labeled fertiliser applied at 20 and 40 kg N ha⁻¹ was studied in the field for one year. The highest yield and NUE occurred in spring–summer (from August to February) reflecting optimal growing conditions. The highest NUE occurred at low rates of urea application (20 and 40 kg N ha⁻¹). Mineralisation played a key role in supplying N to pasture with 64–82% of total plant N derived from soil organic matter (SOM). Less than 50% of the applied N was recovered in the pasture (37–43%) with a large component retained in the soil (26–43% after one year, 0–40 cm), and slowly released in small amounts (< 2%) to the pasture over time, highlighting the abundant capacity of the native soil N pool to supply pasture N. Loss of N fertiliser (14–31%) was attributed to primarily ammonia (NH₃) volatilisation and nitrate (NO₃⁻) leaching. Use of the inhibitors NBTPT and DMPP did not significantly affect pasture yield or NUE, most likely because fertiliser N saved with the inhibitors only played a minor role in plant nutrition with the majority of the plant nutrition provided by the soil organic matter pool. [ABSTRACT FROM AUTHOR]