LAUSR

Abstract ‘Stabilised’ nitrogen (N) represents an important category of enhanced efficiency fertilizers (EEFs) in which chemicals added to urea granules inhibit target soil N transformation processes in order to optimize N availability for crop uptake. However the mechanistic understanding of how these inhibitors perform when urea-based fertilizers are banded, causing substantial modifications to the soil chemical environment, is poorly understood. In a 16-day laboratory incubation, the efficacies of the nitrification inhibitor (NI), 3,4-dimethylpyrazole phosphate (DMPP), and the urease inhibitor (UI), N-(n-butyl) thiophosphoric triamide (NBPT), were investigated in two soils by incubating two commercially available urea-based products containing these additives in bands at concentrations equivalent to 150 kg N ha−1 at row spacing of 1.8 m. When compared to a granular urea standard, DMPP did not provide any inhibitory benefits except when the inhibitor was able to diffuse beyond the zone affected by urea-N hydrolysis, within which considerable nitrification inhibition occurred because of the chemical conditions. The benefit of the NI was most significant in the soil with higher clay, organic matter and pH buffering capacity, with these properties confining the ureolytic-induced chemical changes and N transformations inhibitory zone closer to the fertilizer band. In contrast, the UI provided some benefit by slowing the rapid rise in pH, electrical conductivity and aqueous ammonia observed in standard urea bands, although the effects of NBPT were temporary (ca. 9 days) in both soils. The benefit of NI and UI technology relative to urea alone is likely to vary considerably between soils and fertilizer application methods. This study provides a physico-chemical approach to determining under what conditions the benefits of banded ‘stabilising’ EEF technology may be realised.