| Abstract | Nitrogen (N) is an essential element to sustain all life on Earth, yet it also wreaks havoc when in excess. The production of synthetic N fertilisers through the Haber-Bosch process began in the early 1900s and initiated the ‘green revolution’, seeing agricultural productivity soar. These productivity gains support roughly half of the current global population. Yet while heavy application of synthetic N fertilisers ensures crop success, it also leads to harmful environmental losses of 50-70% of the N applied. Such losses damage fresh and coastal aquatic ecosystems through eutrophication and biodiversity loss, reduce air quality, accelerate climate change and lead to numerous human health implications. The human race has doubled the cycling of N through the environment leading to a global challenge.
To reduce these environmental nutrient losses, enhanced efficiency fertilisers were developed, including slow- and controlled-release fertilisers and stabilised N fertilisers. These products aim to increase the proportion of N taken up by the crop relative to the amount added, meaning that less fertiliser is required. Slow- and controlled-release products specifically aim to deliver N at a rate to match the crop N uptake curve, while N stabilisers are chemical additives that inhibit urease and nitrification in the soil, reducing leaching of highly mobile nitrate-N and lowering denitrification to gaseous nitrogen oxides (NOx) and nitrous oxide (N2O) - a potent greenhouse gas.
Dicyandiamide (DCD) is a commercial nitrification inhibitor that effectively reduces N losses and can improve crop productivity in temperate climates. However, in tropical soils, microbial metabolism of this molecule results in limited efficacy.
This project aimed to improve the efficacy of DCD for tropical agriculture through encapsulation and controlled-release of this soluble, crystalline agrichemical using biodegradable and environmentally friendly polymers. The principle is to protect the DCD from degradation and extend the duration of effective concentration in the soil. Controlled-release DCD pellets were produced through extrusion processing, as a simple, cost-effective, commercially relevant fabrication technique. The polymers tested include thermo-plastic wheat starch (TPS), the bacterial polyester poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) and synthetic polycaprolactone (PCL) as well as blends of PHBV with PCL. DCD was distributed in these polymers through extrusion melt compounding to produce ~3×3 mm cylindrical pellets. The release kinetics were studied and, importantly, the underlying mechanisms that control release were identified and modelled. Much of the mechanistic understanding was developed through advanced imaging of the materials before and after release, using scanning electron microscopy (SEM), mapping with Raman spectroscopy, and high-resolution X-ray micro-computed tomography (μ-CT). The release kinetics were modelled using empirical and mechanistic models.
From the outcomes of these studies, this thesis builds an understanding of the key material design parameters, including:
(1) Polymer(s) selection. The physical and chemical properties of the polymer determined the time for release, ranging from 1 day (for TPS) to 6+ months (for PHBV), and the mechanisms controlling release. Release from TPS occurred by rapid diffusion through and swelling of the hydrophilic polymer matrix. By contrast, PHBV shows promise for long-term release profiles (6+ months), but diffusion through this polymer is so slow that release occurs via other mechanisms. Initially, the release was rapid via the dissolution of surface exposed DCD crystals, confirmed through SEM, resulting in ~20 wt.% release within the first 5 h. Between 5 h and 8 weeks, a further 25 wt.% of the DCD was mobilized as water accessed connected DCD crystals or entered via micro-cracks in the matrix, as determined through high-resolution μ-CT and Raman mapping. A large portion (~50%) of the agrichemical remained encapsulated until the PHBV matrix degraded in soil environments. To increase the rate of matrix diffusion, blending with a more hydrophilic polymer, PCL, were studied. However, the higher affinity between DCD and PCL counter-intuitively resulted in less interconnected DCD crystals, which lead to slower release with increasing PCL content in the blend.
(2) The DCD loading. This determined the degree of percolation within the matrix, with a threshold at between 200 and 400 g.kg-1 for DCD-PHBV. Below the percolation threshold, this parameter controls the thickness of the polymer between agrichemical crystals.
(3) DCD crystal size. Below the percolation threshold, the fractional release from the surface of the pellet was modulated through the grind size of the agrichemical.
(4) DCD pellet size. As identified through mathematical modelling, this parameter can control the fractional release rate and has important consequences on the distribution of pellets within the soil.
Understanding these key parameters and the mechanisms that control release allows cost-effective, environmentally friendly material design to increase the effectiveness of nitrogen stabilisers in tropical climates and reduce N pollution. Moreover, the knowledge gained here is relevant for the controlled-release of any soluble, crystalline agrichemical and could be applied for the design of controlled-release fertilisers, herbicides and pesticides. |
