The composition and depth of green roof substrates affect the growth of Silene vulgaris and Lagurus ovatus species and the C and N sequestration under two irrigation conditions

Using crushed waste bricks for urban greening with contrasting grassland mixtures: no negative effects of brick-augmented substrates varying in soil type, moisture and acid pre-treatment

Ecological restoration aims at supporting biodiversity and ecosystem services, and urban greening is a great opportunity to achieve this goal. This is facilitated by species-rich seed mixtures based on local provenances, which are designed for certain nutrient and moisture regimes based on functional plant traits. Such grassland mixtures might be cultivated on crushed waste bricks, which would be a new component of water-holding urban substrates. Thus, we studied the effects of brick quantity and quality, acid pre-treatment of bricks, soil type and moisture on biomass of designed seed mixtures. Three greenhouse experiments were conducted, with substrates consisting of different brick ratios (5% vs. 30%), brick types (clean production waste vs. demolition material), and brick treatments (acid vs. control) tested on three trait-based mixtures and a non-regional commercial standard mixture. The trait-based mixtures included information on specific leaf area, seed mass and grass-to-legume ratio. There were no negative effects of demolition bricks, soil texture and moisture on grassland biomass. Acid-treated clean porous bricks improved biomass production of the standard and intermediate mixtures, while the effect was minimal with demolition bricks. Designed seed mixtures had a biomass similar to the standard mixture under dry conditions but did not benefit from high moisture like the standard mixture. In conclusion, waste bricks are a useful additive for urban restoration substrates to save raw material, and specifically designed regional mixtures can replace commercial grassland types on these substrates.

Investigating green roofs' CO2 sequestration with cold- and drought-tolerant plants (a short- and long-term carbon footprint view)

In recent years, green roofs have become the subject of increasing interest because of their good aesthetic qualities, energy conservation, and ability to reduce thermal island effect and absorb greenhouse gases, especially carbon dioxide (CO₂). Given the typically significant carbon emission of construction activities, adding any extra component to a structure increases the amount of carbon to be released during the execution stage. This also applies to green roofs, which require more materials and more extensive construction activities than traditional roofs. However, plants of green roofs absorb substantial amounts of CO₂ during their lifetime, thus leaving both short- and long-term positive impacts on the building's carbon footprint. This study investigated the short- and long-term effects of green roofs on carbon footprint, as compared to conventional roofs. For this investigation, the CO₂ uptake of eight plant species with suitable drought- and cold-resistant properties was measured by infrared gas analysis (IRGA), and the effect of green roof on the building's carbon footprint was analyzed using the software Design Builder. The results showed that building a green roof instead of a traditional roof increases the carbon emission of the construction process by 4.6 kg/m² of roof area. Investigations showed that, under high light intensities (1500-2000 μmol/m² s), Sedum acre L. has the best performance in compensating the extra carbon emission imposed on the construction process (in 264 days only). Under low light intensities (1000-1500 μmol/m² s), Frankenia laevis showed the best increase in the amount of carbon uptake (2.27 kg/m² year).

Sustainable Urban Drainage Systems in Spain: Analysis of the Research on SUDS Based on Climatology

Sustainable urban drainage systems (SUDS), or urban green infrastructure for stormwater control, emerged for more sustainable management of runoff in cities and provide other benefits such as urban mitigation and adaptation to climate change. Research in Spain began a little over twenty years ago, which was later than in other European countries, and it began in a heterogeneous way, both in the SUDS typology and spatially within the peninsular geography. The main objective of this work has been to know through bibliographic review the state of the art of scientific research of these systems and their relationship with the different types of climates in the country. These structures have a complex and sensitive dependence on the climate, which in the Iberian Peninsula is mostly type B and C (according to the Köppen classification). This means little water availability for the vegetation of some SUDS, which can affect the performance of the technique. To date, for this work, research has focused mainly on green roofs, their capabilities as a sustainable construction tool, and the performance of different plant species used in these systems in arid climates. The next technique with the most real cases analyzed is permeable pavements in temperate climates, proving to be effective in reducing flows and runoff volumes. Other specific investigations have focused on the economic feasibility of installing rainwater harvesting systems for the laundry and the hydraulic performance of retention systems located specifically in the northeast of the Iberian Peninsula. On the contrary, few scientific articles have appeared that describe other SUDS with vegetation such as bioretention systems or green ditches, which are characteristic of sustainable cities, on which the weather can be a very limiting factor for their development.

Whether the carbon emission from green roofs can be effectively mitigated by recycling waste building material as green roof substrate during five-year operation?

Green roof (GF) as an important role of urban ecosystem services is more and more focused on carbon sequestration for the mitigation of climate change, which there is still a gap of longer period of investigation on carbon sequestration on GF. This work aims to quantify the carbon sequestration on green roofs from 2012 to 2017 by measuring and calculating parameter on substrate organic carbon and plant organic carbon, when using waste building material substrate (WBMS) as GF substrate for the recycling of waste solid. Green roof group 2 (waste building material substrate (WBMS) as substrate) and green roof group 1 (local natural soil (LNS) as substrate), planting same three native plants (N. auriculata, L. spicata, and L. vicaryi), were both three substrate depth of 20 cm, 25 cm, and 30 cm, respectively. Results show that both innovative WBMS and LNS were a great capability of carbon sequestration and carbon storage on green roofs. Carbon storage of green roof group 1 and green roof group 2 was 65.6 kg C m^-2 and 72.6 kg C m^-2, respectively. Annual mean carbon sequestration of the WBMS was 1.8 times higher than LNS. The overall average carbon sequestration (12.8 kg C m^-2 year^-1) in green roof group 2 using WBMS was 1.1 times than corresponding in green roof group 1 (11.4 kg C m^-2 year^-1 using LNS). WBMS substrate and L. vicaryi could be considered as the most adaptable green roof configuration, which can be a recommendation to promote the carbon sequestration and the function of green roof for the better urban ecosystem services. Future work may focus on the GF carbon model, water interface, long-term monitoring, environmental impact, water quality and quantity, synthesized effect on GF ecosystem, low impact development (LID), management and simulation, and combination on intelligent urban system, based on LCA.

Environmental factors affecting greenhouse gas fluxes of green roofs in temperate zone

This paper investigates the full seasonal greenhouse gas (GHG) dynamics of fluxes from three green roof systems (lightweight clay aggregate-based green roof - LR; grass roof - GR; sod roof - SR) and natural control site on shallow Leptosol (NC), using closed static chambers in the period April 2014 to December 2015. CO₂, CH₄ and N₂O fluxes are measured and their relationships to meteorological parameters and substrate physicochemical characteristics are quantified. Median CO₂ flux values were 21 (LR), 38 (GR), 62 (SR), and 82 (NC) mg CO₂-C m^-2 h^-1. The results show ecosystem respiration (R_eco) clearly increased until July and then decreased until November. Net ecosystem CO₂ exchange (NEE) was more variable than R_eco, depending on plant growth phase and weather conditions. Median NEE values for study period (from April to November 2015) were -7 (LR), -17 (GR), -136 (SR), and -82 (NC) mg CO₂-C m^-2 h^-1. The percentage of autotrophic respiration (R_a) in R_eco showed clear rise from LR (35%) to NC (62%). CH₄ consumption dominated resulting in median fluxes as follows: -2 (LR), -1 (GR), -15 (SR), and -23 (NC) μg CH₄-C m^-2 h^-1. N₂O flux was low and highly variable in time, with median values varying from -0.07 (GR) to 2.18 (NC) μg N₂O-N m^-2 h^-1. During the maximum vegetation growth, NEE exceeded R_eco value. Green roofs are effective CH₄ sinks, but they do not significantly affect N₂O flux. The main environmental factors determining GHG fluxes in linear models were parameters describing moisture regime, meteorological parameters and soil physical characteristics. These models can be used to predict GHG fluxes from similar green roof systems in analogous climatic conditions. We conclude that green roof technology may be used to mitigate excessive ambient GHG levels in urban areas.

Sedum-dominated green-roofs in a semi-arid region increase CO2 concentrations during the dry season

Green roofs are expected to absorb and store carbon in plants and soils and thereby reduce the high CO₂ concentration levels in big cities. Sedum species, which are succulent perennials, are commonly used in extensive green roofs due to their shallow root system and ability to withstand long water deficiencies. Here we examined CO₂ fixation and emission rates for Mediterranean Sedum sediforme on green-roof experimental plots. During late winter to early spring, we monitored CO₂ concentrations inside transparent tents placed over 1m² plots and followed gas exchange at the leaf level using a portable gas-exchange system. We found high rates of CO₂ emission at daytime, which is when CO₂ concentration in the city is the highest. Both plot- and leaf-scale measurements showed that these CO₂ emissions were not fully compensated by the nighttime uptake. We conclude that although carbon sequestration may only be a secondary benefit of green roofs, for improving this ecosystem service, other plant species than Sedum should also be considered for use in green roofs, especially in Mediterranean and other semi-arid climates.

Towards Providing Solutions to the Air Quality Crisis in the Mexico City Metropolitan Area: Carbon Sequestration by Succulent Species in Green Roofs

INTRODUCTION:

In the first months of 2016, the Mexico City Metropolitan Area experienced the worst air pollution crisis in the last decade, prompting drastic short-term solutions by the Mexico City Government and neighboring States. In order to help further the search for long-term sustainable solutions, we felt obliged to immediately release the results of our research regarding the monitoring of carbon sequestration by green roofs. Large-scale naturation, such as the implementation of green roofs, provides a way to partially mitigate the increased carbon dioxide output in urban areas.

METHODS:

Here, we quantified the carbon sequestration capabilities of two ornamental succulent plant species, Sedum dendroideum and Sedum rubrotinctum, which require low maintenance, and little or no irrigation. To obtain a detailed picture of these plants’ carbon sequestration capabilities, we measured carbon uptake on the Sedum plants by quantifying carbon dioxide exchange and fixation as organic acids, during the day and across the year, on a green roof located in Southern Mexico City.

RESULTS:

The species displayed their typical CAM photosynthetic metabolism. Moreover, our quantification allowed us to conservatively estimate that a newly planted green roof of Sedum sequesters approximately 180,000,000 ppm of carbon dioxide per year in a green roof of 100 square meters in the short term.

DISCUSSION:

The patterns of CAM and carbon dioxide sequestration were highly robust to the fluctuations of temperature and precipitation between seasons, and therefore we speculate that carbon sequestration would be comparable in any given year of a newly planted green roof. Older green roof would require regular trimming to mantain their carbon sink properties, but their carbon sequestration capabilities remain to be quantified. Nevertheless, we propose that Sedum green roofs can be part of the long-term solutions to mitigate the air pollution crisis in the Mexico City Metropolitan area, and other “megacities” with marked seasonal drought.

Multifunctionality is affected by interactions between green roof plant species, substrate depth, and substrate type

Green roofs provide ecosystem services through evapotranspiration and nutrient cycling that depend, among others, on plant species, substrate type, and substrate depth. However, no study has assessed thoroughly how interactions between these factors alter ecosystem functions and multifunctionality of green roofs. We simulated some green roof conditions in a pot experiment. We planted 20 plant species from 10 genera and five families (Asteraceae, Caryophyllaceae, Crassulaceae, Fabaceae, and Poaceae) on two substrate types (natural vs. artificial) and two substrate depths (10 cm vs. 30 cm). As indicators of major ecosystem functions, we measured aboveground and belowground biomasses, foliar nitrogen and carbon content, foliar transpiration, substrate water retention, and dissolved organic carbon and nitrates in leachates. Interactions between substrate type and depth strongly affected ecosystem functions. Biomass production was increased in the artificial substrate and deeper substrates, as was water retention in most cases. In contrast, dissolved organic carbon leaching was higher in the artificial substrates. Except for the Fabaceae species, nitrate leaching was reduced in deep, natural soils. The highest transpiration rates were associated with natural soils. All functions were modulated by plant families or species. Plant effects differed according to the observed function and the type and depth of the substrate. Fabaceae species grown on natural soils had the most noticeable patterns, allowing high biomass production and high water retention but also high nitrate leaching from deep pots. No single combination of factors enhanced simultaneously all studied ecosystem functions, highlighting that soil–plant interactions induce trade‐offs between ecosystem functions. Substrate type and depth interactions are major drivers for green roof multifunctionality.