Use of organic wastes to create lightweight green roof substrates with increased plant-available water

Highlights

• Substrate design determines green roof stormwater retention and plant survival.

• Organic wastes may lighten substrates, increase water retention, and improve sustainability.

• We evaluated the effects of organic wastes on substrate properties and plant growth.

• Overall, higher rates of organic wastes made substrates lighter and improved PAW.

• However, although nut waste materials lightened substrates, they inhibited plant growth.

Abstract

Substrate design is important for stormwater retention and plant survival on green roofs. Green roof substrates are mostly inorganic, providing long-term stability, while organic components (<20% v/v) reduce substrate weight and increase water retention, depending on rate and type. Selection of organic components depends on availability and organic waste materials can improve sustainability and reduce costs. We aimed to use organic waste materials to create lightweight substrates with good aeration and increased plant-available water (PAW) to support plant growth on green roofs. We evaluated the effects of five locally available organic waste components (coarse coir, fine coir, composted green waste, almond hull, and pistachio shell) on the physical and chemical properties of a scoria-based substrate when added at 0, 5, 10, 15, and 20% by volume. We also examined their phytotoxic effects on plant growth using lettuce (Lactuca sativa) as bioassay species. Overall, higher rates (15–20 % v/v) of coarse coir, fine coir and composted green waste all reduced substrate weight and improved PAW. Pistachio shells and almond hulls reduced saturated bulk density but adversely affected plant growth, likely due to phenolic compounds. For the other amendments, rates above 10–15 % addition also decreased root and shoot growth in lettuce. This indicates a potential trade-off between water availability and growth in these substrates. Therefore, if a green roof needs to be amended to reduce weight but have improved PAW, coarse coir, fine coir and composted green waste would be the best choices.

Introduction

Green roofs are becoming increasingly popular because they provide many economic and environmental benefits for cities. These benefits include rainfall retention and improved stormwater water quality (Berndtsson, 2010), reduced urban heat island effect (Li et al., 2014), reduced energy use (Sailor, 2008), habitat provision for plants and animals (Brenneisen, 2006) and improved air quality (Yang et al., 2008). Rainfall retention and reduced stormwater runoff is the focus of many green roof policies and incentive schemes worldwide (Carter and Fowler, 2008). However, rainfall retention by green roofs can vary from as little as 27% to as much as 81% (Mentens et al., 2006), and in addition to rainfall frequency and intensity, is influenced by substrate depth and water retention (Berndtsson, 2010; Villarreal and Bengtsson, 2005).

Green roof substrates are often shallow (<20 cm deep on extensive green roofs) and are designed to provide water and nutrients for plants whilst being well drained, lightweight, and stable over time (Farrell et al., 2012; Rowe et al., 2006). Variations in purpose and design as well as climatic differences means that there is not a universal substrate that meets the design criteria for all green roofs. Additionally, the type of organic matter added to green roof substrates varies according to cost, availability, suitability and preference (Ampim et al., 2010). Substrates usually contain 80–90% v/v lightweight inorganic components such as; crushed brick, pumice, heat-expanded shale and scoria, which provide stable structure and provide pore space for drainage and oxygen supply to roots (Dvorak and Volder, 2010). Organic components usually make up the remaining 10–20 % v/v, which improves water retention for stormwater management and plant survival, reduces substrate weight (bulk density), improves cation exchange capacity and provides nutrients for vegetation (Fassman and Simcock, 2011). However, while the presence of organic matter is critical for plant growth and survival (Emilsson and Rolf, 2005), careful consideration of the organic matter type and rate of addition is essential when formulating substrates.

Organic components which have been evaluated for use in green roof substrates include: peat (Emilsson and Rolf, 2005), coconut coir (Farrell et al., 2012; Vijayaraghavan and Raja, 2014), composted green waste (Eksi et al., 2015; Graceson et al., 2014b), composted pine bark (Fassman and Simcock, 2011), mushroom compost (Griffin et al., 2017), olive mill residue and grape marc compost (Ntoulas et al., 2015). However, while many of these organic components are effective at sustaining vegetation and increasing water retention, these materials may not always be locally available. It is important to develop and evaluate substrates locally as this reduces transportation embedded energy and financial costs (Ampim et al., 2010). The use of local waste materials is preferred as this turns low value (or no value) materials into a valued resource, which obviously reduces costs and can help encourage green roof implementation (Emilsson and Rolf, 2005; Molineux et al., 2009; Vijayaraghavan, 2016). There are many waste materials that have been successfully used to improve the environmental sustainability of containerized and soil-less horticultural mixes (Barrett et al., 2016), but have not yet been evaluated for green roofs substrates. These organic waste materials include; rice hulls (Papafotiou et al., 2001), pistachio shells (Karimi et al., 2013) and almond shells and hulls (Urrestarazu et al., 2005; Valverde et al., 2013). However, there is also potential for these waste materials to have adverse effects on plant growth through high salinity or the presence of phytotoxic compounds (Çimen et al., 2007; Karimi et al., 2013; Ortega et al., 1996). Therefore, research is needed to evaluate substrate physical and chemical properties with different types of organic components (Ampim et al., 2010; Nagase and Dunnett, 2011).

The proportion of organic matter also influences green roof substrate performance. High rates of organic matter (>20%) are considered problematic on green roofs as rapid decomposition can lead to compaction, which reduces substrate depth, air supply for roots and water infiltration and retention (Handreck and Black, 2010) and can temporarily increase nutrient loads in stormwater runoff (Berndtsson, 2010). Rapid decomposition is especially problematic for organic matter high in cellulose, as opposed to woody material high in lignin, which resists degradation (Handreck and Black, 2010). German green roof standards, often adopted internationally, specify that organic matter in green roof substrates should not exceed 20% v/v (FLL, 2008). Some authors have shown rates as little as 10% v/v organic matter addition to be optimal for plant growth and water retention (Nagase and Dunnett, 2011). However, there is also a high risk of total decomposition if the application rate is too low, as demonstrated by Emilsson and Rolf (2005), who showed almost complete decomposition of organic matter (peat added at 10% w/w) after one year on a green roof in southern Sweden. The optimal rate of addition is likely to vary with the type of organic component, its cellulose:lignin ratio (Handreck and Black, 2010), as well as its interaction with inorganic components in substrate mixes and climate (Ampim et al., 2010; Graceson et al., 2014a; Nagase and Dunnett, 2011).

The desired ecosystem service of the green roof will also be important when selecting the appropriate rate and type of organic components for the substrate (Young et al., 2014). In hot and dry climates, like Mediterranean Europe and parts of southern Australia, it is important for green roof substrates to not only have good water holding capacity for stormwater retention, but also for much of this water to be plant-available (Cao et al., 2014). While there has been some research on the effects of different organic components and rates of application on green roof substrate water retention, the effects of organic components on plant water availability has attracted less consideration (for exceptions, see Fassman and Simcock, 2011; Ntoulas et al., 2015; Papafotiou et al., 2013). In regions where green roofs are not yet common, it is also important to evaluate the effects of different rates and types of organic matter in improving substrate properties when mixed with locally available mineral components, as the interactive effects are not predictable from the physical properties of individual components (Graceson et al., 2014a).

The objective of our study was to evaluate the effects of five different types of locally available organic components and rates of addition on substrate performance. Our primary aim was to create a lightweight substrate with good aeration and increased plant-available water (PAW) for green roofs in hot and dry climates. Secondly, we aimed to determine whether the different types of organic matter were also chemically suitable for plant growth.