Reply on CC1

Abstract

Enhanced efficiency fertilizer (EEF) technologies that employ product coatings to delay nitrogen (N) release or are chemically stabilized to inhibit key steps of N transformations in soil, offer potential for improving N use efficiency (NUE) in agricultural systems. However, the dynamics of N release and transformation from single technologies may result in a spatial or temporal mismatch of N supply and demand during a growing season. This may be overcome by use of blends of different technologies, provided the reduction in the concentration of stabilizing products does not reduce effectiveness. Laboratory incubations quantified the N dynamics around fertilizer bands of polymer-coated urea (PCU) and nitrification inhibited (NI) urea and varying blends of these technologies, and referenced this against conventional urea and biodegradable, plant oil-coated urea (POCU) applied at the same rates in two contrasting soils over 60 days. Blends of NI-urea and PCU typically resulted in N concentrations and distribution that were intermediate to that of the constituent products in unblended applications. Changes in the proportions of each product were mirrored by urea-N concentrations around the bands in both soils, while the proportions of NI-urea in each blend were only related to the extent of nitrification inhibition in the Vertisol. A proportion of the POCU granules burst during initial water imbibition, resulting in initially higher mineral N concentrations cf. PCU. However, both CRFs delayed N release and generation of NO3-N relative to granular urea, and mineral N distribution was similar within each soil. Soil type had a significant impact on banded N dynamics. Where there was little effect of N-fertilizer treatment on NO3-N production in the Ferralsol, the greater impedance of solutes in the Vertisol contributed to a significant inhibitory effect of NI-urea on nitrification in both pure and blended DMPP-urea treatments. Using NO3-N production as a benchmark for the risk of environmental loss, the efficacy of fertilizer treatments in this soil was of: DMPP-urea-PCU blends (higher ratio of PCU may offer small but insignificant benefit) > DMPP-urea = PCU > urea. These findings highlight the importance of soil properties in determining the N dynamics from different banded EEF products. Insights into the efficacy of biodegradable alternatives to polymer coatings and the efficacy of blended EEF products can improve the reliability of N supply while reducing environmental impacts, therefore offering greater opportunities to sustainably improve fertilizer NUE in cropping systems.