Distribution and chemical speciation of nitrogen from band-applied enhanced-efficiency fertilisers

Enhanced-efficiency fertilisers (EEFs) are promoted as tools for improving nitrogen (N) use efficiency (NUE) in agricultural systems, but more information is needed on their behaviour when they are band applied in tropical environments. Incubation studies using banded, urea- based EEFs with nitrification inhibitor (NI), urease inhibitor (UI) and polymer-coating (PCU) technologies have identified how soil physico-chemical characteristics impact on urea dissolution, hydrolysis, volatilisation and microbial nitrification processes that determine the effectiveness of these products. The extent of the affected zone was dependent on the soil type; mineral N species typically diffused further than the inhibitors, resulting in key N transformations often occurring outside the zone of inhibitor influence. The effects of banded UI were transient (ca. 5–9 days) in the two studied soils. More consistent co-location of the NI and nitrifying activity occurred in a clay-textured soil where diffusion was limited by soil properties, and some benefit of the NI was observed late in the incubation. In contrast, PCU slowed the rate of release of N from granules to the surrounding soil solution, and delayed N release from granules in concentrated bands was consistent with poorly developed concentration gradients due to the close proximity of PCU granules to each other. Understanding the N dynamics and subsequent transformations of EEFs in soils with different physico-chemical characteristics, and with different fertiliser application methods, will be the key to developing management practices that allow EEFs to have the greatest impact on fertiliser NUE. Key words Nitrogen, nitrogen-use efficiency, fertilisers, nitrification inhibitors, urease inhibitors, polymer- coated urea