Biochar makes green roof substrates lighter and improves water supply to plants

Highlights

• Biochar significantly increased water retention in green roof substrates.

• At 40% biochar an additional 2.3 cm rainfall/cm area could be retained in 10 cm deep substrates.

• Additional water was also plant available; delaying permanent wilting by 2 days (30–40% biochar).

• Biochar (40%) substrates were lighter, meaning an extra 1.5 cm/m² of substrate could be installed.

• Consequently, biochar makes green roof substrates lighter and improves water supply to plants.

Abstract

Green roofs are increasingly being built to manage stormwater runoff in cities. Water-retention additives such as biochar could be a useful way of increasing substrate water holding capacity (WHC) and therefore stormwater retention without increasing substrate weight loading. If this also increases plant available water (PAW), plant selection could be expanded to species with higher water use, further reducing stormwater runoff by drying substrates after rain. We examined the effects of adding one type of green waste biochar (0, 10, 20, 30 and 40%, v/v) to two scoria-based substrates (with or without added organic matter) on WHC, bulk density, PAW and days taken to reach permanent wilting point (PWP). Biochar significantly improved WHC, increasing with greater additions of biochar. Increased water was also plant available, with 30% biochar increasing PAW by 16% (scoria with organic matter) and PWP by 2 days in both substrates. Biochar did not affect plant growth or biomass allocation. Application of 30% biochar was optimal for PAW and delaying PWP. However, as 40% biochar significantly increased WHC, this rate will likely be optimal for stormwater retention, with an additional 2.3 cm rainfall/cm area retained in 10 cm deep substrates. Biochar also significantly reduced bulk density, substrates with 40% biochar could have an additional 1.5 cm/m² of depth compared to the same weight as scoria only, further increasing PAW and rainfall retention. Consequently, in this study, biochar addition makes green roof substrates lighter and improves plant water supply; potentially expanding plant selection in dry climates and improving their stormwater retention.

Introduction

Urban centres have expanded markedly over the last two centuries (Grimm et al., 2008) and by 2050 it is expected that more than two-thirds of the world's population will be living in urban areas (Department of Economic and Social Affairs, 2008). Increasing urbanisation is associated with increased transportation, construction, hard surfaces and energy use, which in turn results in multiple environmental problems including; the urban heat island effect, habitat fragmentation and loss of biodiversity and increased stormwater runoff (Grimm et al., 2008, Pataki et al., 2011). For stormwater runoff, impervious surface cover is particularly important, with 40% of rainfall lost to surface runoff urban centres with about 50% impervious surface cover (Lee and Heaney, 2003). Stormwater runoff degrades urban waterways by altering stream hydrology and polluting aquatic habitats with excess nutrients, urban residue and waste (Walsh et al., 2005). Managing urban stormwater is critical to maintain natural limits of water quality and flows in aquatic ecosystems.

Conventional techniques to mitigate stormwater include constructed wetlands, ponds, basins, open ditches, infiltration sand and settling tanks (Mentens et al., 2006). Recently green roofs have been suggested as a new and novel way of managing urban stormwater without using valuable land at ground level (Getter and Rowe, 2006). For stormwater mitigation, green roofs are often installed as thin engineered soil profiles (substrates <20 cm deep) which support vegetation whilst being lightweight to enable retrofitting on existing buildings and maximise opportunities for installation. In addition to reducing stormwater volumes, green roofs also delay peak runoff and improve stormwater quality (Mentens et al., 2006, Bliss et al., 2009, Berndtsson, 2010).

The capacity of green roofs to retain stormwater is determined by a number of factors, including substrate physical properties and depth (VanWoert et al., 2005), climate and seasonality (Mentens et al., 2006), vegetation water use strategies (Nagase and Dunnett, 2012), and rainfall duration and intensity (Villarreal and Bengtsson, 2005). In strongly seasonal climates, where winter rainfall is dominant, stormwater retention is greatest during summer due to a combination of higher evaporation and plant water use and less frequent rainfall events (Mentens et al., 2006). Generally runoff is greater from low water using succulent species than from other life-forms with higher transpiration rates which dry out substrates between rainfall events (Dunnett et al., 2008). Small rainfall events are generally retained by green roofs (Simmons et al., 2008.) and retention increases with substrate depth (Villarreal and Bengtsson, 2005, Mentens et al., 2006) and greater water holding capacities (WHC) (Berndtsson, 2010). Therefore, manipulating substrate properties to increase WHC and stormwater retention without increasing weight loading is important for optimising the stormwater benefits provided by green roofs.

Substrate properties also determine plant survival and performance on green roofs. As shallow substrates are highly exposed to heat and sunlight and water and nutrients are generally limiting (Rowe et al., 2006), supporting plant growth can be very challenging. High WHC can increase the supply of water to plants but this also needs to be balanced with adequate air filled porosity (AFP) and low bulk density to ensure sufficient oxygen is available to plant roots (Rowe et al., 2006, Thuring et al., 2010). While organic matter content improves substrate nutrition and water holding capacity, decomposition over time can cause shrinkage, altering substrate WHC and AFP over time and dramatically reducing longevity (Bilderback, 2005). Water-retention additives have been suggested as a useful way to improve the physical and chemical properties of substrates without increasing green roof weight loading or compromising substrate drainage and longevity (Kookana et al., 2011, Ramesh and Reddy, 2011).

Also referred to as soil conditioners or amendments, water-retention additives are added to soils, growing media or substrates to improve growing conditions through improved water retention (Le Van Mao et al., 1989, Bres and Weston, 1993), water infiltration (Xiubin and Zhanbin, 2001), aeration and permeability (Oka et al., 1993, Huang and Petrovic, 1994). Where these amendments also reduce bulk density this also leads to reduced weight loading (Nektarios et al., 2003). However, increasing water holding capacity through the addition of soil amendments does not always increase available water to plants. For example, Oka et al. (1993) found that although additions of carbonised rice husks (10 t ha⁻¹) to sandy soils increased WHC by 7%, though plant water available was not improved. Similarly, while hydrophilic acrylic-based polymers (hydrogels) increased water retention in potting media (different combination of peatmoss, perlite, vermiculite and compost), permanent wilting was not delayed for Petunia × hybrida (Petunia) (Jobin, 2004). In green roof substrates, hydrogel and silicate-based amendments have been shown to increase the water holding capacity of green roof substrates by up to 11% but the effects of additives on time to permanent wilting has differed depending on substrate composition and additive type (Farrell et al., 2013a). However, the potential of other water-retention additives, including biochar, to improve plant available water in green roof substrates has yet to be evaluated.

Biochar is the carbon-rich product of high temperature combustion (pyrolysis) of biomass with the source of biomass varying from agricultural and forestry harvest residues to urban green and manufacturing waste (Jha, 2010). Although organic in origin, biochar remains structurally intact and resists decomposition due to its highly aromatic carbon structure (Keiluweit et al., 2010). Biochar has been used as a soil amendment in agriculture (Jha, 2010) and horticulture (Cox et al., 2012), with application rates ranging from 2% to 20% being shown to improve soil properties and plant growth outcomes (Lehmann et al., 2003, Laird et al., 2010). These include increased soil water holding capacity (Masulili et al., 2010, Streubel et al., 2011), nutrient availability (Asai et al., 2009), cation-exchange capacity (Chan et al., 2007) and microbial biomass (Lehmann et al., 2011).

While biochar has shown many benefits in agricultural soils and in containerised horticulture, their use as water-retention additives in green roof substrates is yet to be fully evaluated. Biochar has been shown to improve stormwater retention and quality on a green roof, with 8% biochar (by weight) increasing stormwater retention by about 4% and reducing total nitrogen and phosphorus by 79 and 20% (Beck et al., 2011). However, to the authors’ knowledge there has been no research on how biochar application rate affects water retention in green roof substrates, and whether additional stored water is plant available. For biochar to improve green roof performance and expand plant selection, it is critical to determine whether additional water held in substrates is also available for plant uptake (Farrell et al., 2013a). Also, as biochar has a long half-life, estimated to be 1400 years in soil (Kuzyakov et al., 2009), it may be a useful replacement for less stable organic matter, such as composts and coir in green roof substrates.

The purpose of this study was to address these critical knowledge gaps and assist in designing substrates that maximise water retention, in turn improving stormwater retention and plant performance. Specifically, the aims were to determine how biochar affects: (i) physical and chemical properties; (ii) plant available water (PAW) and days taken to reach permanent wilting point (PWP); and (iii) biomass allocation and growth of Triticum spp. (wheat) under drought stressed conditions in green roof substrates. Although not planted on green roofs, wheat was chosen as an indicator plant due to its well established ability to reflect plant available water (Akhter et al., 2004). The effects of one-type of biochar were compared in two green roof substrates; one with and the other without organic matter to determine whether these amendments could replace the organic component in green roof substrates. Further, we evaluated the effects of biochar application rate on substrate performance and plant growth.