Extensive Green Roof Substrates Research Articles

Sewage sludge biochar as a sustainable and water-safe substrate additive for extensive green roofs

Article describes the application of nutrient-rich sewage sludge (SS) biochar as an innovative additive to extensive green roof substrate at application rates of 10 (SB10) and 20% (v/v) (SB20) and its long-term impact on discharge quality. In addition to a routine analysis of pH, EC, TSS, and COD in the discharge, concentrations of nutrients (total nitrogen and total phosphorus) and metals (Cd, Cu, Fe, Mn, Pb, and Zn) were used to monitor leaching from the experimental green plots. SS biochar improves the water-holding capacity and air-filled porosity of the developed substrates SB10 and SB20, where the chemical and physical parameters meet the minimum requirements for multi-layer extensive roof substrate. The addition of biochar didn't substantially increase the concentrations of nutrients in discharge compared to the conventional extensive substrate, even after 30 months. Results suggested that biochar tends to release nutrients gradually when leached out by rain and may therefore serve as a source of P and other micronutrients. Although an increase in the concentrations of metals (Cu, Pb, and Zn) in the SB10 and SB20 substrates was observed, no significant differences in metal leaching were found. Metal concentrations in the aboveground parts of Sedum plants indicated that the pyrolysis of SS transforms water-extractable forms of metals into stable forms, thereby significantly reducing their bioavailability to Sedum species and any direct ecotoxicological risk. Based on our results, we assert that SS biochar could be used as a sustainable, valuable and water-safe component of extensive substrates for green roofs.

Field evaluation of the cotton-strip assay for quantifying decomposition rates in extensive green roof substrates

Extensive green roof (EGR) substrates and vegetation layer are critical components to ensure EGR multifunctionality. However, little is known on EGR substrate biological functioning, especially nutrient cycling that is crucial for supporting life in soils. This work aimed at investigating for the first time the effectiveness of an affordable and easy-to-use standardized method, the cotton-strip assay, to assess decomposition rates in ten-year old EGR. Three metrics were evaluated following 34–110 days of cotton-strip incubation into substrates: tensile strength, weight and surface area losses. Tensile strength loss was the most sensitive index after 34 days while weight loss over 110 days confirmed elevated decomposition rates in EGR substrates compared to gravel rooftop or common urban park soil. This could be related to positive influence of slightly acidic pH and vegetation cover. Finally, semi-intensive but thin green roof substrates may be an alternative to intensive ones to sustain ecological processes supporting long-term vegetation growth, as long as there is no net carbon loss. Moreover, broader assessment of decomposition into different green roof substrates configurations could pave the way for a better understanding of green roof ecosystems as a whole and the design of more resilient green infrastructures.

Using Recycled Construction Waste Materials with Varying Components and Particle Sizes for Extensive Green Roof Substrates: Assessment of Its Effects on Vegetation Development

Using construction waste materials as substrates can reduce the resource consumption of minerals and provide sustainability benefits in green roofs. This research examined the viability of crushed bricks and crushed concrete with varying particle sizes as substrates by conducting a simulated drought experiment and roof experiment. Six composite substrates were prepared, respectively, by mixing peat with small-, medium-, and large-sized crushed bricks and crushed concrete particles (peat-to-inorganic-particle volume ratio of 1:2). The properties of each group were within acceptable parameters, with the crushed brick substrates displaying lower bulk density and higher porosity compared to the crushed concrete substrates. Seldom lineare was selected for the experiments, and the substrate thickness was set at 10 cm. Under the simulated drought conditions, the growth and stress resistance of Seldom lineare in the crushed brick substrates was similar to that in the conventional substrates and poorer in the crushed concrete substrates. Seldom lineare in medium-particle-size brick substrates performed the best, surpassing the traditional group. The growth of Seldom lineare in the small-particle-size concrete substrates was the worst. In the rooftop environment, all groups could support Seldom lineare over 180 days without maintenance, with an overall coverage of more than 60%. The plants in the medium-particle-size brick substrates exhibited the highest cover. In conclusion, the medium-particle-size brick substrate exhibits ideal characteristics in terms of substrate physical properties and plant growth, making it a favorable option.

Use of a Dielectric Sensor for Salinity Determination on an Extensive Green Roof Substrate.

The irrigation of extensive green roofs with recycled or saline water could contribute to the conservation of valuable drinking water supplies. In such cases, the continuous monitoring of substrate electrical conductivity (ECsw) is of immense importance for the sustainable growth of the plants growing on the green roof. The present study aimed to estimate the ECsw (pore water EC) of an extensive green roof substrate in lysimeters with the use of the WET-2 dielectric sensor. Half of the 48 lysimeters that simulated extensive green roofs had a substrate depth of 7.5 cm, while the other half had a 15 cm substrate depth. The warm season turfgrass Paspalum vaginatum 'Platinum TE' was established at the lysimeters, and during the summer period, it was irrigated every two days at a rate of 14 mm with NaCl solutions of various electrical conductivities (ECi): (a) 3 dS m-1, (b) 6 dS m-1, and (c) 12 dS m-1, while potable water of 0.3 dS m-1 ECi served as the control. The relation between bulk electrical conductivity, σb, and bulk dielectric permittivity, εb, of the substrate was observed to be linear for all ECi levels up to σb values of 2-2.5 dS m-1. The ECsw was predicted by employing the salinity index method which was modified to be applied to the particular case of a green roof substrate. Knowing the salinity index and organic portion (%, v/v) for a given green roof substrate, we could calculate the ECsw. It was found that the use of the salinity index method predicts reliably the ECsw up to 10-11 dS m-1, while the method overestimates ECsw at very low levels of electrical conductivity.

Comparison of rainwater management performance of modified extensive green roof substrate layer with different additives in rainstorm events

The distinct improvements of rainwater management performance brought about by different additives depend on their predominant pores occupying the substrate layer.

Phosphorus Leaching Behavior from Extensive Green Roof Substrates

The substrate is the key component of a green roof system that directly influences the system’s stormwater mitigation performance; however, it is also identified as a potentially significant source of contaminants. It is interesting whether substrate phosphorus discharge behavior is predictable and, if so, whether the study of substrate phosphorus leaching behavior is representative of the green roof system performance. The substrates tested were extracted from a 4-year-old extensive green roof and a 1-year-old proprietary substrate from an agricultural (Ag) green roof. The column test for the extensive green roof substrate (comprised of 90% v/v pumice and 10% v/v compost) was designed to simulate 3 years of precipitation in a 3-month period, based on field monitoring of rainfall from 2018 to 2020, and assuming precipitation occurred equally each month. Double the amount of precipitation was applied to the Ag substrate to make up for the shorter duration field exposure before the laboratory experiment. Total phosphorus (TP) concentrations in the permeate from the extensive and the Ag substrates showed exponential decreases with cumulative water flow. The decreasing rates of TP concentrations were similar regardless of the initial phosphorus in the two different substrates. Field monitoring indicated that the TP discharge from the newly built green roof behaved differently compared with its substrate in the laboratory. However, substrate tests could be an extension of field monitoring to estimate the phosphorus discharge for aged green roofs.

Nutrient leaching in extensive green roof substrate layers with different configurations

Due to substrate layers with different substrate configurations, extensive green roofs (EGRs) exhibit different rainfall runoff retention and pollution interception effects. In the rainfall runoff scouring process, nutrient leaching often occurs in the substrate layer, which becomes a pollution source for rainwater runoff. In this study, six EGR devices with different substrate layer configurations were fabricated. Then, the cumulative leaching quantity (CLQ) and total leaching rate (TLR) of NH4+, TN, and TP in the outflow of nine different depth simulated rainfall events under local rainfall characteristics were evaluated and recorded. Furthermore, the impact of different substrate configurations on the pollution interception effects of EGRs for rainfall runoff was studied. Results show that a mixed adsorption substrate in the EGR substrate layer has a more significant rainfall runoff pollution interception capacity than a single adsorption substrate. PVL and PVGL, as EGRs with layered configuration substrate layers, exhibited good NH4+-N interception capacity. The CLQ and TLR of NH4+-N for PVL and PVGL were - 114.613mg and - 63.43%, - 121.364mg, and - 67.16%, respectively. Further, the addition of biochar as a modifier significantly slowed down the substrate layer TP leaching effect and improved the interception effect of NH4+-N and TN. Moreover, although polyacrylamide addition in the substrate layer aggravated the nitrogen leaching phenomenon in the EGRs' outflow, but the granular structure substrate layer constructed by it exhibited a significantly inhibited TP leaching effect.

Improvement in pollutant controlling performance by modified aggregate structure in extensive green roof substrate layer

The addition of 2% PAC to the extensive green roof substrate layer can result in the formation of an excellent aggregate structure and improve the NO3−–N, TN and TP-controlling performance of extensive green roofs during heavy rainfall events.

Change of nutrients, microorganisms, and physical properties of exposed extensive green roof substrate

Green roofs (GR) can be used as a nature-based solution to tackle eco-environmental problems caused by climate change and rapid urbanization. The substrate in the GRs is the growing medium for vegetation, and its properties directly affect the ecosystem services of GRs. To investigate the characteristic changes of an exposed substrate after the removal of vegetation, a one-year field experiment was conducted. Substrate properties were comprehensively compared for areas in GR that were planted with Sedum lineare and those with bare substrate. Results show that vegetation cover not only prevented substrate loss by 5.14% (p < 0.05) but also protected the chemical, microbial, and physical properties of the substrate. Moreover, the structure of the substrate changed, as evidenced by a significant increase in fine sand (p < 0.05). The results highlight that attention should be paid to maintaining vegetation cover during GR management. In addition, extensive GRs may not be suitable for fallowing. Once a GR has been established, it needs regular maintenance. Otherwise, the ecological and economic benefits of the GR may be reduced. The findings of the present study can be used to determine the life-cycle costs. Further research should focus on differences in the substrate loss rates, runoff, and temperatures of the substrates under exposure and vegetation cover. The microbial changes after revegetation should also be studied to clarify the role of vegetation in GR ecosystems. The present study provides a reference for improving GR management and ensuring their sustainability.

Green Roof Enhancement on Buildings of the University of Applied Sciences in Neubrandenburg (Germany) in Times of Climate Change

The reduction in evaporative surfaces in cities is one driver for longer and hotter summers. Greening building surfaces can help to mitigate the loss of vegetated cover. Typical extensive green roof structures, such as sedum-based solutions, survive in dry periods, but how can green roofs be made to be more effective for the longer hot and dry periods to come? The research findings are based on continuous vegetation analytics of typical extensive green roofs over the past 20 years. -Survival of longer dry periods by fully adapted plants species with a focus on the fittest and best adapted species. -Additional technical and treatment solutions to support greater water storage in the media in dry periods and to support greater plant biomass/high biodiversity on the roofs by optimizing growing media with fertilizer to achieve higher evapotranspiration (short: ET) values. The main findings of this research: -The climate benefits of green roofs are associated with the quantity of phytomass. Selecting the right growing media is critical. -Typical extensive green roof substrates have poor nutrition levels. Fertilizer can significantly boost the ecological effects on CO2 fixation. -If the goal of the green roof is a highly biodiverse green roof, micro-structures are the right solution.

Novel application and reused materials for extensive green roof substrates and drainage layers in Japan – Plant growth and moisture uptake implementation –

This study investigates the potential utility of reused materials as an alternative material for commercial substrate and drainage layers. The performances of reused materials were assessed using turf with trays on a roof for two years in Chiba, Japan. Two types of materials, a commercial green roof substrate and cocopeat, were used as the substrate. For the drainage layers, five different materials were used: a commercial green roof drainage layer, bamboo stems and nodes, and polyethylene terephthalate (PET) bottle caps and bottoms. In addition, a sample without any drainage layer was used. The trays were prepared with the combination of these two substrates and six drainage applications. Overall, reused materials were observed to function well as commercial green roof materials. The turf in cocopeat was able to survive better than turf in commercial green roof substrate. In the drainage layers, some reused materials, such as the PET bottle caps and bamboo nodes, showed higher final plant coverage than commercial drainage layers in cocopeat. Our study has addressed the importance of realizing the original substrate and drainage layer using reused materials for developing sustainable and inexpensive green roofs.

Balancing substrate depth and irrigation inputs influence the aesthetic quality ofPaspalum vaginatumwhen grown on adaptive extensive green roof systems

Turfgrass use on extensive green roofs could offer a sustainable solution for increasing urban greening within the existing built space in contemporary cities. The aim of the study was to determine Paspalum vaginatum 'Platinum TER' drought tolerance when grown under extensive green roof conditions and two irrigation regimes. Treatments included: (i) six extensive green roof substrates formulated from mixing locally available materials; (ii) two substrate depths (7.5 or 15 cm) and (iii) two irrigation regimes (60% ETc or 100% ETc). Turfgrass drought tolerance was measured, using digital image analysis, as the number of days until a treatment reached 50% green turf cover (GTC). In both study years (2011 and 2012), for each one of the six substrate types, higher GTC values were reached when deeper profiles (15 cm) were combined with the higher irrigation regime (100% ETc), while the worst drought tolerance turfgrass response was obtained when the swallow substrate (7.5 cm) was combined with the deficit irrigation regime of 60% ETc. It was found that increasing substrate depth resulted in delay of drought stress symptoms expression, while irrigation regime affected the velocity of GTC reduction over time with lower irrigation regime resulting in faster GTC decline compared to higher irrigation regime. It was also concluded that the imposed drought stress to the turfgrass by the reduced irrigation could be compensated by increasing substrate depth. Thus, in buildings with adequate load bearing capacity, it is preferable to select a substrate depth of 15 cm since it will result in 40% reduction on irrigation inputs compared to substrate depth of 7.5 cm.

Performance analysis and experimental study on rainfall water purification with an extensive green roof matrix layer in Shanghai, China.

Current research has validated the purification of rainwater by a substrate layer of green roofs to some extent, though the effects of the substrate layer on rainwater purification have not been adequately quantified. The present study set up nine extensive green roof experiment combinations based on the current conditions of precipitation characteristics observed in Shanghai, China. Different rain with pollutants were simulated, and the orthogonal design L9 (33) test was conducted to measure purification performance. The purification influences of the extensive green roof substrate layer were quantitatively analyzed in Shanghai to optimize the thickness, proportion of substrate, and sodium polyacrylate content. The experimental outcomes resulted in ammonium nitrogen (NH4+-N), lead (Pb), and zinc (Zn) removal of up to 93.87%, 98.81%, and 94.55% in the artificial rainfall, respectively, and NH4+-N, Pb, and Zn event mean concentration (EMC) was depressed to 0.263 mg/L, 0.002 mg/L and 0.018 mg/L, respectively, which were all well below the pollutant concentrations of artificial rainfall. With reference to the rainfall chemical characteristics of Shanghai, a combination of a 200 mm thickness, proportions of 1:1:2 of Loam: Perlite: Cocopeat and 2 g/L sodium polyacrylate content was suggested for the design of an extensive green roof substrate to purify NH4+-N, Pb and Zn.

Extensive Green Roof Substrate Composition Based on Sludge Recycling

The experiment was carried out to study the reasonable composition of stable anaerobic sludge in the extensive green roof growth substrate by a total of 650 mm artificial rainfall event. Combined with the N, P leaching problem of the sludge resources utilization, the quality of the effluent and the change of the nutrient content of the substrate in the process of application were investigated. The results showed that the anaerobic sludge could significantly increase the plant biomass and the average weight gain was 808%. Stable anaerobic sludge caused a large amount of leaching of TP and water treatment residual could effectively control TP leaching and did not affect the absorption of P to the plant. During the experiment, the TP of the effluent was reduced by 68.66%.After stabilization, the effluent TP reached the surface water Ⅴ standard. The main form of leached TN was NO3--N and the leaching of NO3--N could be reduced by rice husk charcoal. In the experiment, the NO3- -N leaching mass was reduced by 28.86% totally. Increasing the amount of rice husk charcoal helped to control NO3--N leaching. SS and COD could be quickly decreased and stabilized. The COD of effluent after stabilization was about 30 mg·L-1, which was better than the standard of surface water Ⅴ. Under the condition of actual precipitation with medium and small rainfall, the anaerobic sludge nutrient retention time should be more than 1 year, which meets the growing needs of plants for a long time.

Waste Silica as a Valuable Component of Extensive Green-Roof Substrates

Waste Silica as a Valuable Component of Extensive Green-Roof Substrates

Determination of the Physical, Chemical, and Hydraulic Characteristics of Locally Available Materials for Formulating Extensive Green Roof Substrates

Several locally available materials were tested to create an optimized growth substrate for arid and semiarid Mediterranean extensive green roofs. The study involved a four-step screening procedure. At the first step, 10 different materials were tested including pumice (Pum), crushed tiles grade 1–2 mm (T1–2), 2–4 mm (T2–4), 5–8 mm (T5–8), 5–16 mm (T5–16), and 4–22 mm (T4–22); crushed bricks of either 2–4 mm (B2–4) or 2–8 mm (B2–8); a thermally treated clay (TC); and zeolite (Zeo). All materials were tested for their particle size distribution, pH, and electrical conductivity (EC). The results were compared for compliance with existing guidelines for extensive green roof construction. From the first step, the most promising materials were shown to include Pum, Zeo, T5–8, T5–16, and TC, which were then used at the second stage to develop mixtures between them. Tests at the second stage included particle size distribution and moisture potential curves. Pumice mixed with TC provided the best compliance with existing guidelines in relation to particle size distribution, and it significantly increased moisture content compared with the mixes of Pum with T5–8 and T5–16. As a result, from the second screening step, the best performing substrate was Pum mixed with TC and Zeo. The third stage involved the selection of the most appropriate organic amendment of the growing substrate. Three composts having different composition and sphagnum peat were analyzed for their chemical and physical characteristics. The composts were a) garden waste compost (GWC), b) olive (Olea europaea L.) mill waste compost (OMWC), and c) grape (Vitis vinifera L.) marc compost (GMC). It was found that the peat-amended substrate retained increased moisture content compared with the compost-amended substrates. The fourth and final stage involved the evaluation of the environmental impact of the final mix with the four different organic amendments based on their first flush nitrate nitrogen (NO3−-N) leaching potential. It was found that GWC and OMWC exhibited increased NO3−-N leaching that initially reached 160 and 92 mg·L−1 of NO3−-N for OMWC and GWC, respectively. By contrast, peat and GMC exhibited minimal NO3−-N leaching that was slightly above the maximum contaminant level of 10 mg·L−1 of NO3−-N (17.3 and 14.6 mg·L−1 of NO3−N for peat and GMC, respectively). The latter was very brief and lasted only for the first 100 and 50 mL of effluent volume for peat and GMC, respectively.

Influence of rainfall on the noise shielding by a green roof

The potential of a green roof to reduce diffracting sound waves towards a shielded side of a building has been assessed before by both in-situ and laboratory experiments, and by numerical simulations. However, like any porous material, the acoustic performance of the green roof's substrate could suffer from the presence of water. A 46-day lasting controlled sound propagation experiment was set up near the edge of a 6-m tall building equipped with an extensive green roof, having a substrate thickness of 7 cm. At selected moments, test signals were emitted, allowing to monitor the attenuation between a reference microphone and a microphone at low height positioned on the green roof (at 15.3 m from the roof's edge). Meteorological parameters and the green roof's substrate moisture content were continuously measured. Sound diffracting over a green roof showed to be sensitive to the substrate moisture content in a specific sound frequency range, more precisely between 250 Hz and 1250 Hz. The difference in noise attenuation between a rather dry state (0.1 m3/m3) of the substrate and the maximum observed volumetric water content (close to saturation, 0.33 m3/m3) could range up to 10 dB. However, calculations show that the impact of the water content in an extensive green roof substrate for the specific case of road traffic noise abatement is expected to be limited.

Manipulating soil microbial communities in extensive green roof substrates

There has been very little investigation into the soil microbial community on green roofs, yet this below ground habitat is vital for ecosystem functioning. Green roofs are often harsh environments that would greatly benefit from having a healthy microbial system, allowing efficient nutrient cycling and a degree of drought tolerance in dry summer months. To test if green roof microbial communities could be manipulated, we added mycorrhizal fungi and a microbial mixture (‘compost tea’) to green roof rootzones, composed mainly of crushed brick or crushed concrete. The study revealed that growing media type and depth play a vital role in the microbial ecology of green roofs. There are complex relationships between depth and type of substrate and the biomass of different microbial groups, with no clear pattern being observed. Following the addition of inoculants, bacterial groups tended to increase in biomass in shallower substrates, whereas fungal biomass change was dependent on depth and type of substrate. Increased fungal biomass was found in shallow plots containing more crushed concrete and deeper plots containing more crushed brick where compost tea (a live mixture of beneficial bacteria) was added, perhaps due to the presence of helper bacteria for arbuscular mycorrhizal fungi (AMF). Often there was not an additive affect of the microbial inoculations but instead an antagonistic interaction between the added AM fungi and the compost tea. This suggests that some species of microbes may not be compatible with others, as competition for limited resources occurs within the various substrates. The overall results suggest that microbial inoculations of green roof habitats are sustainable. They need only be done once for increased biomass to be found in subsequent years, indicating that this is a novel and viable method of enhancing roof community composition.

Performance of Zoysia matrella ‘Zeon’ in Shallow Green Roof Substrates under Moisture Deficit Conditions

The use of turfgrasses might provide an additional solution for establishing green roofs in urban environments. The aim of the present study was to determine Manilagrass [Zoysia matrella (L.) Merr. ‘Zeon’] drought tolerance when grown under green roof conditions and under two different irrigation regimes. Treatments included: 1) two extensive green roof substrates {locally produced substrate [3 sandy loam soil:8 pumice:4 perlite:4 compost:1 zeolite (by volume)] and a commercially available substrate based on crushed tiles}; 2) two substrate depths (7.5 cm or 15 cm); and 3) two irrigation regimes (3 mm or 6 mm of irrigation every 3 days). Substrate characteristics (particle size distribution, saturated and dry bulk density, total porosity, water potential curves, in situ substrate moisture, pH, electrical conductivity, and nutrient analysis), turfgrass growth, and physiological status [green turf cover (GTC), normalized difference vegetation index (NDVI), and leaf relative water content (RWC)] were determined. During moisture deficit periods, GTC, NDVI, and RWC were most affected by substrate depth; moderately affected by irrigation regime; and, to a lesser extent, by substrate type. Turfgrass growth and physiological status were best during moisture deficit conditions in the deeper profile (15 cm) using the higher amount of irrigation (6 mm) and the locally mixed substrate.

Evaluating a Natural Zeolite as an Amendment for Extensive Green Roof Substrate

This research examined soilless green roof substrate blends on an existing modular extensive green roof in Denver, Colorado, USA. Substrate blends evaluated include an extensive green roof substrate, Green Grid® and Green Grid® plus varying percentages of ZeoPro™ H-Plus. Plant taxa used included Sedum acre L., Sedum album L., Sedum spurium Marsch-Bieb. ‘Dragon's Blood’ and S. spurium ‘John Creech’. Substrate blends were evaluated based on plant taxa growth performance. Data collected included digital images to measure plant area covered using digital image analysis (DIA) and substrate volumetric moisture content (VMC). All data were analyzed over two growing seasons using the GLIMMIX procedure in SAS as multiple comparisons of substrate blends for each taxa, DIA data were analyzed from eight dates and VMC data were analysed from seven dates. The addition of zeolite (ZeoPro™) to the typical extensive green roof substrate improved establishment year plant cover for S. acre and S. album but hindered overwintering. Conversely, the two cultivars of S. spurium did not show a benefit of plant cover from the addition of ZeoPro™ in the first year but did the second year. As the percentage of ZeoPro™ in the substrate increased, VMC also increased.