Biochar made from low density wood has greater plant available water than biochar made from high density wood
Graphical abstract
Introduction
Biochars are highly porous solids produced by the thermochemical conversion of biomass under oxygen-depleted conditions in a process known as pyrolysis; which has been traditionally used to produce charcoal (Lehmann and Joseph, 2015). They are considered a useful soil amendment for improving soil water retention and plant growth in rainfed systems in agricultural (Jeffery et al., 2011; Obia et al., 2016), forestry (Tryon, 1948) and urban landscapes (Cao et al., 2014). As well as improving water retention in soils (Omondi et al., 2016), biochars have also been shown to increase crop yields by increasing nutrient retention (Major et al., 2010), improving soil structure (Lim et al., 2016; Obia et al., 2016) and microbial activity (Wang et al., 2016) and reducing the bioavailability of heavy metals (O'Connor et al., 2018) whilst adding a stable source of soil carbon for sequestration (Lehmann, 2007). However, the effects of biochar as a soil amendment can be highly variable due to (i) differences in feedstock materials, (ii) their interaction with different soil types and (iii) differences in pyrolysis conditions (Atkinson, 2018; Jeffery et al., 2011; Li et al., 2019; Rasa et al., 2018; Ronsse et al., 2013; Sohi et al., 2010; Weber and Quicker, 2018).
While biochar can be made from a wide range of organic materials including crop residues, biosolids or food waste, the most commonly used feedstocks for biochar production worldwide are wood-based (Jirka and Tomlinson, 2015). When wood feedstocks are pyrolysed, their chemical and physical make up changes, influencing their water retention properties, water uptake behaviour (degrees of hydrophobicity) and longevity. Chemically, wood is mainly composed of hemicellulose, cellulose and lignin (Walker, 2006). During pyrolysis, these compounds decompose, releasing volatile liquid and gaseous fractions and leaving behind solid aromatic carbon compounds (Zeriouh and Belkbir, 1995). This results in wood biochars generally having a >80% (w/w) carbon content when pyrolysed above 500 °C (Weber and Quicker, 2018). The aromatic carbon structure of biochar contributes to its long-term stability. This makes biochar a better soil amendment with sustained improvement for water retention than other organic or synthetic water retention amendments. For instance, compost or hydrogels decompose orders of magnitude faster than biochar (Al-Harbi et al., 1999; Bolan et al., 2012; Singh et al., 2012). Another important biochar property governing its water uptake behaviour is the degree of hydrophobicity. Generally, low pyrolysis temperatures (<500 °C) can result in hydrophobic, water repellent biochar (Zornoza et al., 2016). This is caused by incomplete carbonisation of chemical wood components, resulting in remaining aliphatic functional groups on the biochar surface which repel water (Das and Sarmah, 2015; Kinney et al., 2012). Therefore, greater pyrolysis temperature generally results in reduced hydrophobicity of biochar, enabling greater soil water uptake and increased water holding capacity of biochar.
The cellular wood structure remains largely intact after pyrolysis and forms the macroporous structure of biochar (Baltrėnas et al., 2015; Ehrburger et al., 1982; Gray et al., 2014; Hyväluoma et al., 2018; Wildman and Derbyshire, 1991). Biochar macropores are very important for water retention, as they make up the main pore volume for water storage (Brewer et al., 2014; Gray et al., 2014; Hyväluoma et al., 2018; Lu and Zong, 2018). This has been demonstrated by Zhang and You (2013) who showed a strong positive correlation between macropore size and water holding capacity (WHC) for biochars produced from poplar (Populus davidiana) and pine (Pinus sylvestris var. mongolica) wood. The biochar made from poplar wood (hardwood) had greater macropore diameters (1–40 μm) and greater WHC (69–72%) when compared to the biochar made from pine wood (softwood), with smaller macropore diameters (1–10 μm) having a WHC of 29%. The authors concluded that biochars with greater macropore diameters and therefore greater pore volumes will lead to increases in water retention and are therefore best suited for improving soils in water-limited environments. The wood of hardwood species (generally non-coniferous) is made up of fibre, vessel and parenchyma cells (Wilkes, 1988). Fibre cells can be described as hollow tubes with thickened cell walls (Downes et al., 1997) and as they make up a large proportion of the wood structure, fibre properties greatly influence wood density (Abruzzi et al., 2013; Zbonak et al., 2007). In general, higher density hardwoods have greater fibre wall thickness and smaller fibre diameters than lower density hardwoods (Watson and Dadswell, 1961). Therefore, it is likely that biochar produced from lower density hardwoods will retain more water.
However, water holding capacity (WHC) does not always reflect how much water is accessible for plant growth (Cao et al., 2014; Masiello et al., 2015). A better measure is plant available water (PAW), which accounts for the amount of water that is held in pores at tensions which can be extracted by plants. PAW, also referred to as available water content (AWC), is mainly determined by macropore size, with water stored in pores with diameters between 0.2 and 30 μm being plant available, while water stored in smaller pores is held too tightly for plant uptake, mainly due to capillary and adsorption forces (Atkinson, 2018; Cassel and Nielsen, 1986). The total amount of water in pores with diameters smaller than 0.2 μm is held with a negative pressure >1500 KPa which supersedes most plants ability to extract it. It is commonly referred to as permanent wilting point (PWP), whereas water in pores with diameters >30 μm drains under gravity and is only briefly available for plant uptake after a rain event (Atkinson, 2018). The number of pores in biochar in the relevant diameter range for PAW is largely influenced by feedstock type (Lu and Zong, 2018). Therefore, the number of pores in the size range for maximising PAW in biochar should relate to the cell structure of the woody feedstock material, and biochar produced from lower density hardwood should hold more PAW than biochar made from high density hardwood.
While there is an extensive body of research on how biochar influences water retention in soils (Abel et al., 2013; de Melo Carvalho et al., 2014; Hardie et al., 2013; Tryon, 1948), few studies have investigated how feedstock fibre properties and wood density influences water retention and availability of biochar (Hyväluoma et al., 2018; Rasa et al., 2018). Given this gap, in this study we investigated how feedstock wood density was related to biochar WHC and PAW. We hypothesized that lower density hardwood feedstock would result in biochar with greater overall WHC and more of this water would be plant available (PAW) when produced under the same conditions. Our research objectives were: (i) to investigate if wood density can be used as a low cost and easy to measure proxy for feedstock cell structure (ii) to determine how feedstock cell structure influences WHC and PAW of the resulting biochar and (iii) to assess which proportion of the overall water retention is plant available. The water retention measurements (WHC and PAW) in our study were performed on pure biochars without being incorporated into a soil to avoid complications due to biochar-substrate interactions. Our findings need to be validated in future studies with amended soils.
Section snippets
Materials and methods
To determine how hardwood feedstocks with different wood densities, influence biochar water retention and availability, we compared biochars made from 18 Eucalyptus species under the same pyrolysis conditions. The 18 Eucalyptus species represent a gradient of wood density (Table 1) from low density e.g. E. pauciflora (572 kg m−3) to high density e.g. E. polybraktea (960 kg m−3).
Results and discussion
Overall, our study showed that eucalypt feedstock with lower wood density resulted in biochar with up to 35% greater water holding capacity (WHC) and up to 45% greater plant available water (PAW) than biochar made from higher wood density feedstock. Further, WHC was strongly related to PAW, indicating that the additional water retained in biochar produced from lower density feedstocks is also available for plant growth. As feedstock wood density was also related to feedstock cell structure, we
Conclusions
For the 18 eucalypt species evaluated in this experiment, the WHC and PAW of their resulting biochar was strongly related to their feedstock wood density. Biochar produced from eucalypt species with lower wood density had greater WHC and PAW than biochar produced from higher density feedstocks. Wood density was strongly related to feedstock cell structure. Therefore, we suggest that lower density wood species, with greater fibre diameters and thinner fibre walls, result in biochar with a
Acknowledgements
This research was funded by an Australian Research Council Linkage Grant (LP30100731) supported by Melbourne Water and the Inner Melbourne Action Plan (IMAP) group of local governments. Joerg Werdin was supported by a Research Training Program Scholarship and a Graduate Research Studentship. We thank Robert Evans for his help with the wood analysis, Adrian Morphett and Moana Quiatol from Earth Systems for their help producing the biochar, Richard Conn and Petra Katona for their help with the
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