David Henry

Soil water repellency (SWR) is a major problem across the globe and often occurs in sandy soils. SWR inhibits seed germination in crops and pastures, enhances surface runoff and erosion, and accelerates the movement of nutrients and pesticides into groundwater. SWR is caused by the accumulation of hydrophobic organic compounds released from plants and soil microbiota, and in some studies the severity of SWR has been related to soil organic carbon (OC) contents and the presence of particular organic compounds. In addition, factors such as root exudates, contributions from multiple plants, compounds produced during the decomposition of organic matter, and fungal bio-products influence SWR. SWR is also related to soil physical properties such as surface area and chemical properties such as pH. In general, SWR increases with increasing soil OC and decreases with increasing soil surface area; therefore sandy soils are more prone to SWR; however if a soil contains sufficient OC, it will be repellent. This review explores the linkage between SWR and physicochemical properties of soils and suggests research directions to uncover the interplay of the various contributing factors to SWR.

Nanoparticles are seeing increased use within the agricultural sector in areas such as fertilizers, pesticides, biosensors, and soil amendments. There are many advantages to using nanomaterials in agriculture, including the ability to achieve targeted delivery of nutrients or pesticides, which can reduce application rates. This has benefits in terms of efficacy and cost. However, the longer-term effects of nanoparticles in the environment are only just being explored. In this regard, the challenge arises from the diversity of nanoparticles available and the massive variation in their properties that depend not only on their composition but also on their size, shape, porosity, and functionalization. Understanding how nanomaterials interact with plants and soils is imperative to increasing their efficacy and minimizing their environmental impact. Determining the fate of nanoparticles in the agricultural environment must also be considered for their safe and effective use. This review focuses on the application of nanoparticles to crops and horticulture.

Soil water repellency (SWR) is a widespread phenomenon that impacts plant growth, groundwater contamination, surface erosion and runoff in many regions of the world. It is a major constraint to agricultural production in southern Australia, affecting over ten million hectares of arable land and is a feature of natural ecosystems, and is particularly exacerbated by wildfire. Water repellency is caused by hydrophobic organic substances coating soil particle surfaces or within interstitial matter, derived from decomposing organic matter, root exudates and microbial by-products. The incidence of SWR predominantly depends on the interaction of soil organic matter (SOM) and the soil surface area.
Given the strong interest in increasing SOM contents to mitigate climate change, SWR will likely also increase. It is thus important to map soils in terms of both the current degree of SWR and its potential to develop in the future, based on intrinsic soil properties such as soil surface area. This will identify where amelioration techniques such as wetting agents and clay-rich amendments can be better targeted. Conventional SWR measurements are costly, time-consuming and labour-intensive and contain a high uncertainty in their representativeness and applicability in the field. This review will consider how different proximal sensing techniques can be used to assess the current incidence and severity of SWR and the risk of it developing, to provide integrated site-specific management.

Deep soils are located in most continents of the world. Soil carbon measurements are invariably made from the surface horizons, whereas much larger carbon stores occur to depths of many meters, with plant roots providing the main source of carbon. This root biomass persists long after land is deforested for agricultural and other pursuits or forests are killed by pests and fires and may represent a considerable carbon store at the global scale. The impacts on these carbon stores of reforestation or climate change are mostly unknown as the estimation of root biomass and carbon dynamics is challenging in deep soils. This chapter explores deep soil carbon from the perspectives of its definition, source, and persistence; methodologies available to study deep soil carbon; and the effect of land-use change on this carbon store.

The leaf of the lotus plant provides an elegant example of how a natural surface can remain clean even in the dirtiest of environments. The leaf’s fine-scale surface structure combined with its hydrophobic chemistry ensures that water droplets bead off its surface, carrying away contaminant particles [1]. An alternative dirt-shedding surface, also found in nature, is based on hydrophilic surface chemistry. Water droplets that come into contact with hydrophilic surfaces spread, forming a thin film of water and lifting contaminant from the surface. There is significant motivation to pursue both of these types of behaviour for artificial systems because of the wide range of possible applications ranging from self-cleaning paints to dirt-resistant clothing. However, for some applications, pertinent to the system in question, the synthesis of a self-cleaning surface is indeed a difficult task to achieve, and one needs to first elucidate the precise properties required to generate a certain level of resistance to the adhering contaminant. Atomistic simulations provide a useful tool to gain such insight and add value to surfaces where stay-clean properties are highly desirable. Here, using force-field-based molecular mechanics and dynamics, we explore the properties of polymer surfaces in order to develop design aspects for self-cleaning industrial paint-coatings. A polyester surface model is constructed, on the basis of a realistic cured paint coating [2] while various carbon models are selected [3–6], to emulate commonly encountered atmospheric dirt particulates [7–10]. The purpose of this work is to gain a fundamental understanding of the nature of interactions between contaminant and polymer coating in various environments, including aqueous conditions [11], and explore nanoscale modifications of the coating that help reduce the strength of the adhering contaminant. The modifications are based on hydrophobic or alternatively hydrophilic surface treatments. Here we demonstrate that the chemistry, morphology, and stability of the surface play a vital role in resisting adhesion of contaminant particles. Nanoscale surface modification combined with fine-scale roughness reduces adhesion between coating and contaminant by up to 21% [12]. The newly formed surface functional groups comprise “heavy” atoms, which repel the carbon contaminant at a close proximity to the surface via van der Waals interactions. Meanwhile the atomic-scale surface roughness reduces the effective contact area between surface and contaminant [6], in accordance with the Cassie–Baxter construction [13]. However, our modelling suggests that flexible polymer surfaces undergo significant rearrangement [4, 14], even at ambient conditions, in agreement with ageing [15] and hydrophobic recovery studies [16]. A s a result of this, the physical and chemical properties of the coating that initially help shield the surface from the adhering contaminant are diminished with time. E in an aqueous environment, the mobility of the polymer chains plays a vital role in the long-term functionality of the surface [11]. We propose a surface cross-linking procedure, aimed at improving the hardness of the outer region of the coating as a preventative measure against ageing [17]. F or surface cross-linking of the polymer with isophorone di-isocyanate (IPDI) molecules, we observe a much improved stability of the coating’s outer surface, and consequently a ~47% weaker adhesion with our contaminant particle. The rigid outer surface prevents the polymer chains from wrapping around the contaminant, improving the coating’s dirt-resistance capabilities. We anticipate that our modelling studies will be a starting point for the fabrication of a polymer coating which exhibits permanent dirt-shielding qualities. The synthesis of such a coating will require careful control of the chemistry, atomic-scale roughness, and stability of the surface.

Introduction In general, radicals are highly reactive species and can therefore often be difficult to study experimentally. Nevertheless, there is a number of experimental procedures that can he used to determine radical thermochemistry, either directly or indirectly (e.g. through thermochemical cycles. Berkowitz, Ellison and Gutman have reviewed several of these methods and noted their strengths and limitations. Developments in computer technology mean that ab initio molecular orbital theory now provides a viable alternative source of quantitative gas- phase thermochemical information. However, the theoretical treatment of open-shell systems such as radicals presents its own difficulties. Therefore, the accurate prediction of radical thermochemistry with theoretical procedures poses an interesting challenge. In this chapter, we look closely at the performance of several ab initio techniques in the prediction of radical thermochemistry with the aim of demonstrating which procedures are best suited in representative situations. We restrict our attention to several areas in which we have had a recent active interest, namely, the determination of radical heats of formation, bond dissociation energies (BDEs), radical stabilization energies (RSEs), and selected radical reaction barriers end reaction enthalpies. We focus particularly on the results of our recent studies.