The impact of banding urea and enhanced efficiency fertilizers on nitrogen transformations and the implications for nitrogen use efficiency in high-risk environments

Abstract

The efficient use of fertilizer-nitrogen (N) is a major global challenge for intensive agricultural systems. A suite of enhanced efficiency fertilizers (EEFs) have been developed in response to poor N use efficiency (NUE) in agriculture, but mechanistic understanding to support their effective utilization is not well developed. In particular, banding N-fertilizer creates a vastly different biochemical environment to broadcast and / or incorporated applications, potentially influencing the efficacy of EEFs. Furthermore, the influence of tropical and subtropical conditions on EEF efficacy is not well characterized. Thus, application of EEFs in cropping systems utilizing banded application and / or in (sub) tropical environments is occurring under conditions for which there is little guidance on effective use strategies. The effects of fertilizer-N from coastal cropping areas (i.e., sugarcane) of northeast Australia on nutrient-sensitive ecosystems of the Great Barrier Reef (GBR) is a prominent example of a high-risk environment in which effective utilization of EEFs may mitigate environmental impacts through improved on-farm NUE.   The objective of this PhD research was to take a mechanistic approach to investigating the efficacy of banded EEF’s in conditions typical of the (sub) tropical environment of the Australian sugarcane industry. The aim was to develop mechanistic understanding that would underpin agronomic advice supporting the effective utilization of banded EEFs in sugarcane and other high-risk agricultural systems.An initial laboratory incubation (Chapter 3) investigated the fertosphere (soil within ca. 1- 2.5 cm of the fertilizer band) impacts of various nitrification inhibitors (NIs) and a polymer-coated urea (PCU) on urea-N release and hydrolysis, and the subsequent biochemical effects on N cycling, in a range of soils with varying physico-chemicals properties and under conditions typical of a tropical climate. Compared to standard urea, limited benefits from NIs were found within the fertosphere irrespective of soil type, as the hostile conditions associated with rapid hydrolysis of highly concentrated urea-N already had an inhibitory effect on nitrification. Within PCU bands, lower-than-expected concentrations of mineral N and the retention of significant portions of urea-N in granules led to a hypothesis that incomplete release of urea-N from banded PCU granules may be a result of the proximity of neighbouring granules causing diminished concentrations gradients.Batch-style diffusion studies were conducted (Chapters 4 – 6) to better explore N distribution and transformation and the biochemical changes induced by banded EEFs beyond the fertosphere. Despite limited movement beyond the fertosphere, it was shown that the urease inhibitor (UI) was highly effective at regulating urea hydrolysis and the associated biochemical changes in both soils, although effects were transient (ca. 9 days, Chapter 5). In contrast, the efficacy of the NI varied between soils, despite similarly limited movement of the inhibitor much beyond the fertosphere. In soils where the zone of fertilizer nitrification occurred close to the fertosphere [e.g. high clay, high organic matter (OM), high cation exchange capacity (CEC)], better co-location of the inhibitor and substrate [i.e., ammonium (NH4-N)] enabled greater impacts of the NI, compared to lighter-textured soils which allowed wider substrate distribution (Chapter 5). Comparison of banded and dispersed PCU demonstrated that banding of PCU granules delayed initial urea-N diffusion into the fertosphere and surrounds. However, soil type and moisture content may have a moderating influence on urea-N release by regulating the rate of water entry into PCU granules, which would allow more rapid increases in internal osmotic pressure that would facilitate increased rates and quantities of urea-N diffusion. The rapid hydrolysis and nitrification of urea-N released from PCU granules was identified as a potentially significant risk factor that could allow substantial N loss via leaching or denitrification in high moisture conditions, if N release from PCU is not closely synchronized to plant demand (Chapter 6).The influence of water flows through fertilizer bands on the three-dimensional distribution of N was considered in a field experiment (Chapter 7), which was subject to ca. 500 mm of combined rainfall and irrigation throughout the trial period. This experiment found that NIs exhibited inhibitory effects for at least 50 days longer than the existing inhibitory effects associated with standard urea (ca. 10 days). This extended period of inhibition was hypothesized to be due to preservation of inhibitors from microbial degradation within the fertosphere, which was characterized by conditions that were hostile to microbial activity. However, increased in-band concentrations of 3,4-dimethylpyrazole phosphate (DMPP) resulting from higher fertilizer application rates may have also played a role in the prolonged preservation of NH4-N  due to increasing saturation of the fertosphere with inhibitor. In contrast, the efficacy of the UI was transient with inhibition of urea hydrolysis for only between 7 – 21 days. However, this preservation of N in urea form enabled greater leaching of N deeper into the profile. It is hypothesized that the movement of soluble urea-N out of the fertosphere by mass flow contributed to the rapid loss-of-function of the UI, as the substrate (urea-N) had moved beyond the zone of inhibitor influence while the UI remained within / close to the fertosphere. Banded PCU provided the expected extended release of urea-N to soil. However, as hypothesized from the diffusion studies, the prolonged release of N and subsequent rapid nitrification represented an extended window in which N released from PCU bands was susceptible to loss via leaching or denitrification, if not matched to plant uptake.Collectively, this thesis has provided the fundamental data that will allow predictions of the benefits of different EEF technologies when applications are made in concentrated bands in (sub) tropical conditions. This information will allow users to make informed decisions about the applicability of different EEF technologies to combinations of soil, climate and application methods, which will contribute to improved fertilizer NUE in crop systems.