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Biochemical effects of banding limit the benefits of nitrification inhibition and controlled-release technology in the fertosphere of high N-input systems

Janke, Chelsea K., Fujinuma, Ryosuke, Moody, Phil, Bell, Michael J.
Soil research 2019 v.57 no.1 pp. 28-40
ammonia, ammonium, buffering capacity, clay fraction, granules, hydrolysis, nitrification, nitrification inhibitors, nitrogen, pH, polymer-coated urea, polymers, sugarcane, urea
Enhanced efficiency fertilisers (EEFs) may have an important role in improving nitrogen (N) use efficiency in agricultural systems. The performance of EEFs when applied by broadcasting and incorporation is well documented; however, little information is available for sub-surface banded N-fertiliser. This study aimed to determine the effectiveness of EEFs within the fertosphere in several soils. This was determined by: (i) establishing the key chemical effects and N-transformation activity within a urea band, and (ii) contrasting these findings with nitrification inhibitor (NI)-coated urea and a controlled-release polymer-coated urea (PCU). A 112-day incubation experiment was conducted with the EEFs band-applied in three contrasting soils with a history of sugarcane production. In standard urea and NI-urea treated soils, the pH within the fertosphere significantly increased to a maximum of ~pH 9.2–9.3. Alkaline conditions and high ammonium concentrations promoted elevated aqueous ammonia concentrations, resulting in complete nitrification inhibition. The PCU granules released ~40% of total urea content within 14 days, followed by subsequent release at significantly lower rates. The initial rapid urea release was attributed to damaged polymer coats, while close proximity of neighbouring granules within the band may have contributed to the subsequent slower release phase through reduced concentration gradients and restricted diffusion from granules. Variation between soils suggests that soil properties such as clay content and pH buffer capacity may influence urea hydrolysis, but not nitrification. These results suggest that both NI and controlled-release technology may not have the expected impacts on N transformations and availability when applied in a concentrated band.