Up on the roof and down in the dirt: Differences in substrate properties (SOM, potassium, phosphorus and pH) and their relationships to each other between sedum and wildflower green roofs.

In urban areas green roofs provide important environmental advantages in regard to biodiversity, storm water runoff, pollution mitigation and the reduction of the urban heat island effect. There is a paucity of literature comparing different types of green roof substrates and their contributions to ecosystem services or their negative effects. This study investigated if there was a difference between sedum and wildflower green roof substrate properties (soil organic matter (SOM), potassium (K) and phosphorus (P) concentrations and pH values) of 12 green roofs in the city of Brighton & Hove. One hundred substrate samples were collected (50 from sedum roof substrates and 50 from wildflower roof substrates) and substrate properties were investigated using standard protocols. Comparisons were made between substrate characteristics on both types of roof substrate with a series of multiple linear regressions. Sedum roofs displayed significantly higher values of SOM, P and pH. There were significant positive relationships between SOM and K concentrations, SOM and P concentrations, pH and K concentrations and pH and P concentrations on sedum roofs. This study concluded that sedum roof substrates are more favourable for plant water use efficiency and also contained a significantly higher percentage of SOM than wildflower roofs. However, higher concentrations of P in sedum roof substrates may have implications in regard to leachates.

Introduction

Green roofs have been posited as a technology that can improve the urban environment in regard to biodiversity, storm n class="Chemical">water runoff, pollutionpan> mitigationpan> and the reductionpan> of the urban heat island effect. Research inpan>to green roofs is relatively recent [1] with onpan>ly a small proportionpan> of studies focussinpan>g onpan> the conpan>tributionpan> of green roofs to urban biodiversity [1-3] as the predominpan>ant focus is onpan> their mitigatinpan>g capabilities. These inpan>clude decreasinpan>g the urban heat island effect [4, 5], pollutionpan> abatement [6, 7], storm n class="Chemical">water retention and flood risk prevention [8-11] as well as reduction in energy bills [12, 13].

Green roof substrates are developed to take into consideration the weight bearing capacity of the roof, its n class="Chemical">water holdinpan>g capacity and its ability to diffuse n class="Chemical">oxygen to plant roots [14]. They need to be lightweight, porous and free draining in order to provide the vegetation with an optimal growing environment whilst addressing the constraints of architecture and stressors. There is a scarcity of research pertaining to green roof vegetation and substrate [15] but some recent studies have investigated certain variables. For example, Bates et al, 2013 [16] examined the interactions between drought and substrate depth and Madre et al, 2013 [2] explored wildflower colonisation on roofs in relation to succession, substrate depth and roof height. Additionally, Gabrych et al, 2016 [15] conducted a study in Finland in regard to substrate depth and roof age.

The complexity and variety of green roof substrates presents the ecologist with a vast number of questions in regard to which substrate qualities are interacting with which. The majority of studies into green roof substrates have investigated substrate depth or substrate composition and their effect on vegetation, storm n class="Chemical">water attenuationpan> and nutrient leachinpan>g [17-21]. For example a recent study examinpan>ed the role of green roof substrates inpan> the preventionpan> of nutrient leachinpan>g by comparinpan>g different compositionpan>s inpan> a laboratory environpan>ment and suggested that n class="Chemical">nitrogen (N) and phosphorus (P) leaching were at their highest soon after installation [22].

Green roof soil organic matter (SOM) concentrations have not been explored but the predominant ecosystem services provided by SOM are sustaining the biodiversity of plant life and n class="Chemical">carbon sequestrationpan> [23]. It has been suggested by van Groenigen et al, 2017 [24] that soils could potentially compensate for the inpan>creases inpan> atmospheric n class="Chemical">CO2 due to climate change but this is dependent on the characteristics of SOM, explicitly the amount of soil organic carbon (SOC). SOM has been seen to increase SOC in many studies as it has been seen to positively correlate with soil carbon storage [25-30].

Continued research into the substrate properties of green roofs may enhance this knowledge and elucidate in which ways green roofs are important in improving urban environments. This current study was a snap shot of 12 green roofs in Brighton and Hove and aimed to expand knowledge of green roof substrates on sedum and wildflower roofs in regard to substrate characteristics. Sedum roofs are planted with succulents and typically have a shallow substrate (2-12cm) and wildflower roofs consist of forbs native to the specific region and have a deeper substrate (11-20cm) [15]. Specifically this study assessed certain substrate properties (SOM, K, P concentrations & pH levels) in both roof types in order to inform better practice for continued green roof development.

Methodology

Study area

The data were gathered in August 2017 on 12 green roofs in Brighton & Hove (see Table 1) and the temperature on sampling days ranged from 18 °–23 °C. The average yearly temperatures in the city range from 1 °–25 °C with an average of 14 °C (Met Office, 2016). The city of Brighton & Hove is located at 50°50’35”N, 0°07’53”E covering an area of 87.5 km2. The roofs surveyed comprised of wildflower roofs and sedum roofs (see Table 1 for details) with an aggregate and green compost based substrate. They were located through a local company (Organic Roofs Ltd, Brighton) and the University of Brighton, which has a number of green roofs located throughout its campuses. All building owners were consulted and permission was obtained from them to removal substrate samples for the purposes of the study.

Table 1

Specifications and properties for green roofs surveyed in Brighton & Hove 2017.

Roof location	Establishment date	Roof area (m2)	Number of samples	Roof type	Substrate type
Varley Hub 150°51’47.58”N, 0°06’27.44”W	2012	64	9	Sedum	No information
Huxley Building 150°50’45.42”N, 0°07’07.86”W	2014	370	15	Sedum	Sedum blanket (includes recycled brick)*
Falmer Sport’s Hall 1 150°51’35.72”N, 0°05’17.87”W	2012	202	14	Sedum	Sedum blanket (includes recycled brick)*
Falmer Sport’s Hall 2 150°51’35.72”N, 0°05’17.87”W	2012	94	12	Sedum	Sedum blanket (includes recycled brick)*
Checkland Roofs (x 6) 150°51’36.43”N, 0°05’12.40”W	2010	42m2 per roof	7 per roof	Wildflower	30-90mm brick based with clay & compost
Organic Roofs Ltd 250°49’47.72”N, 0°12’41.04”W	2015	10	4	Wildflower	Shire Ultralight**
Richardson’s Yard 350°50’00.01”N, 0°08’27.59”W	2013	10	4	Wildflower	Shire Ultralight**

Ownership: 1University of Brighton; 2 Organic Roofs Ltd; 3 Brighton Housing Trust

* Traditionally includes porous aggregate and composted green waste

** Made from secondary aggregates (aircrete and clay) and green waste compost (pH 8.5)

Includes the number of substrate samples taken on each roof. (see S2 Table for full lists of plant species per roof).

Substrate surveys

A stratified random method was utilised to ensure representative sampling. Over the 12 roofs 100 substrate samples were taken, 50 for sedum roofs and 50 for wildflower roofs. The number of samples per roof was established using the method of Gabrych et al, 2016 [15] which was based on roof size (S1 Table). Roof size was measured using Google Earth ruler [31] and verified in the field with a tape measure.

A typical sample was approximately the size of a handful as suggested by Ward (2017, personal communication) and each sample was placed into a sealable plastic bag ensuring most of the air was removed. The substrate samples were then frozen for 3 days and, after freezing, placed in an oven in 250 ml beakers at 40 °C for 5 days to dry [32]. This is preferable to air drying as the shorter time necessary reduces the risk of microbial activity changing the status of the soil, and also lessens costs in relation to laboratory activity [33].

Soil organic matter (SOM)

To measure soil n class="Disease">organic matter the loss on ignpan>ition technpan>ique was used. Samples were placed inpan>to 50 ml beakers after weighinpan>g the empty beaker and tarinpan>g the scale. The samples and beaker were then weighed againpan>. Samples were then placed inpan> a muffle furnpan>ace at 400 °C for 24 hours as suggested by [34] for soils containpan>inpan>g compost. After removinpan>g from the muffle furnpan>ace the resultinpan>g sample mass was then remeasured to determinpan>e the amounpan>t of organic matter that had been lost inpan> the process and this was calculated as the percentage of organic matter inpan> the substrate.

Substrate preparation

Each sample was then disaggregated in a pestle and mortar and half of each sample was retained in the event of future necessity. The substrate was then dry sieved through a 2 mm flat sieve and the homogenised fragments were placed in plastic bags and labelled. The percentages of fraction <2 mm and >2 mm were recorded. Sieving is predominantly used for particle size analysis to determine soil structure [35], but here it was used to prepare the substrate for later analysis techniques.

Potassium and phosphorus (K & P)

K and P measurements were taken using a PerkinElmer Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES). This method has been suggested to be a quick and accurate technique to determine K and P measurements [36, 37]. Samples <2 mm were prepared in a ratio of 0.1:3:7, substrate sample to n class="Chemical">nitric acid (n class="Chemical">HNO3) to deionised water. HNO3 (3 ml) was added to the substrate sample (0.1 g) in a fume cupboard and left to stand for 3 hours before adding 7 ml of deionised water. The analytes were then centrifuged and processed in the ICP-OES. Data for both K and P samples were recorded as mg/L.

Statistical analysis

The substrate characteristics (% SOM, K mg/L, P mg/L and pH) were compared between types of roof substrate samples using Mann-Whitney tests as the data were non-normally distributed. Two outliers were removed from the wildflower % SOM data as they were skewing the results and did not represent the general trend according to Cook’s distance [38, 39]. A principal components analysis (PCA) was used for data reduction purposes for relationships between substrate characteristics. Once PCA results were analysed a series of multiple linear regressions were performed to ascertain the relationships between substrate characteristics and their differences between sedum and wildflower roofs. Spearman’s rank correlation coefficients and their p values were recorded for each regression analysis.

Results

Substrate characteristics were compared for both sedum and wildflower roof types (Table 2) showing that sedum roofs contained a significantly higher amounts of SOM and P and higher pH levels. Sedum roofs had an average of 21% SOM and whilst wildflower roofs had an average of 9.6%. K levels did not significantly vary between the types of roof.

Table 2

Mean differences (± SD) between substrate characteristics in relation to sedum and wildflower green roof type.

Substrate characteristic	Sedum	Wildflower	Mann-Whitney W-score	P value
Soil organic matter (%)	21 ± 11.86	9.6 ± 8.05	3303.0	<0.0001
Phosphorus (mg/L)	0.1383 ± 0.0852	0.0548 ± 0.0206	3120.0	<0.0001
pHPotassium (mg/L)	8.78 ± 0.150.1864 ± 0.0945	8.32 ± 0.10.1761 ± 0.0597	3754.52581.0	<0.0001N/S

Results are shown for differences between sedum and wildflower roofs (n = 50 per roof type).

Relationships between substrate characteristics

An initial exploration of the data using principal components analysis (PCA) was conducted for substrate characteristics. An initial exploration of the data using principal components analysis (PCA) was conducted for substrate characteristics. Percentage organic matter and nutrient loading accounted for the key proportion of variation in the two first components (89.6%). The nutrient variations of the roof substrates were positive correlated for n class="Chemical">phosphorus and negative for n class="Chemical">potassium. Substrate characteristics were then compared against each other and compared between both sedum and wildflower roof substrates. Percentage SOM and P concentrations and SOM and K concentrations were highly correlated on sedum roofs (Figs 1 and 2). pH and K concentrations and pH and P concentrations were also highly correlated on sedum roofs (Figs 3 and 4).

Fig 1

The relationship between percentage soil organic matter and potassium on both sedum and wildflower roofs.

Two outliers have been removed from the wildflower % SOM data (42.42% and 52.9%). Sedum df = 49, r = 0.724, p = <0.0001. Wildflower df = 47, r = -0.020, p = N.S.

Fig 2

The relationship between percentage soil organic matter and phosphorus on both sedum and wildflower roofs.

Two outliers have been removed from % SOM in wildflower roofs (42.42% and 52.9%). Sedum df = 49, r = 0.843, p = <0.0001. Wildflower df = 47, r = -0.079, p = N.S.

Fig 3

The relationship between pH and potassium on both sedum and wildflower roofs.

Sedum df = 49, r = 0.696, p = <0.0001. Wildflower df = 49, r = 0.109, p = N.S.

Fig 4

The relationship between pH and phosphorus on both sedum and wildflower roofs.

Sedum df = 49, r = 0.814, p = <0.0001. Wildflower df = 49, r = 0.148, p = N.S.

Discussion

In the first study of its kind this research has suggested that substrate characteristics significantly differ between sedum and wildflower green roof substrates. SOM, n class="Chemical">phosphorus and pH values were significantly higher onpan> sedum roofs. The relationpan>ships between substrate properties also differed significantly between sedum and wildflower green roofs, with inpan>creasinpan>g SOM correlated to higher n class="Chemical">potassium and phosphorus concentrations on sedum roofs and increasing pH correlated to higher potassium and phosphorus concentrations on sedum roofs. However, given that it was not possible to determine the original substrate characteristics of the roofs and, therefore compare them, results are presented for consideration as a snapshot of substrate characteristics at the time of study.

Soil organic matter

n class="Species">Human activities and inpan>dustrialisationpan> have been shown to have a negative impact onpan> SOM inpan> urban soils [40] hence any mitigationpan> could have a stronpan>g positive impact inpan> regard to n class="Chemical">carbon sequestration in cities. In this study sedum roof substrates displayed a significantly higher SOM and, therefore, higher amount of SOC than wildflower roofs [25, 41] although, as substrates had different compositions, comparisons with initial substrates’ SOM percentages could be of value. Green roofs in general have been shown to sequester carbon [42], however, as roofs age there are inconsistent results in regard to SOM levels as some studies reported increasing levels over time [9] while others have reported decreases [43]. Ground level urban green habitats have been shown to sequester carbon at higher levels [44]. The majority of carbon in cities is sequestered by urban trees and Xu et al, 2018 [45] suggested that, with their loss and the loss of other urban green space, it can take time for the system to recover, so with more green spaces encouraging higher biodiversity and connectivity in a fragmented landscape the more resilient it will be to disturbance. Urban environments also generate ‘black carbon’ from vehicular emissions [46] indicating that increasing carbon sinks (such as with suitable green roofs) will be necessary for climate change remediation considering that black carbon is a major contributor to climate change [47]. As urban areas continue to grow, with their concomitant reduction in ground level green space, green roofs may offer a lifeline in regard to urban carbon sequestration. This study observed a higher amount of SOM on sedum roofs, highlighting the importance of ensuring that green roof substrates address factors such as optimisation of plant growth and percentage of SOC in the green roof substrate.

Simulations have been conducted in relation to the mitigating effects of green roofs on climate change. It has been predicted that the temperature in the United States would rise 1–2 °C by 2100 if no mitigation strategies were adopted in urban areas and suggested green roofs as one of a number of adaptations to attenuate urban warming [48]. Additionally, Alcazar et al, 2016 [49] indicated that green roof systems have the potential to positively impact upon climate change in urban environments. Therefore, studies on the impact of green roof substrates on urban n class="Chemical">carbon sequestrationpan> are imperative as the conpan>sequences of climate change demand an urgent responpan>se and mitigationpan> strategies may be a componpan>ent of the solutionpan>.

Phosphorus

n class="Chemical">Phosphorus levels from sedum roof substrate samples inpan> this study were found to be significantly higher than the wildflower onpan>es, which may have implicationpan>s for nutrient runoff which can affect adjacent n class="Chemical">watercourse quality [50]. Wildflower roofs may have lower phosphorus concentrations than sedum roofs as they are regularly mowed, and mowing regimes have been seen to decrease phosphorus levels in arable grasslands [51].

n class="Chemical">Phosphorus can be stored inpan> soils inpan> inpan>organic and organic soil particles [52] posinpan>g the questionpan> as to whether green roof substrates are sources or sinpan>ks of n class="Chemical">phosphorus. Many green roof substrates are constructed to be nutrient rich and Mitchell et al, 2017 [53] connoted that green roof substrates can, in fact, be a source of phosphorus after initial installation up to a period of 10 or more years. However, they submit that phosphorus depletion may not be only due to runoff but could also be attributed to plant uptake and phosphorus chemical alteration in the substrate. The leaching of phosphorus from green roofs was seen to be significant in some studies [22, 54] which posited that younger green roofs had more phosphorus in their runoff. The initial composition of green roof substrate is paramount in the resultant leaching effects and adjacent habitat pollution as media type has a huge influence on whether green roofs are a source or sink of phosphorus [55]. It has been suggested that there are many factors involved in runoff dynamics on green roofs such as slope, soil moisture, rainfall and seasonal factors, age of the roof and vegetation [56]. However, the majority of studies have indicated that green roofs are generally sources of phosphorus pollution [56, 57] and this present study would suggest that it may be beneficial to investigate ways of mitigating phosphorus levels specifically in sedum roof substrates.

pH

Both sedum and wildflower roofs in this study had relatively high pH values. Sedum species have been seen to thrive in a range of pH values [58] but no studies have suggested values as high as in this study. It has also been suggested that the average optimal pH for Sedum growth was 5.95, with non-optimal levels reducing growth dramatically [58]. Higher pH values have been shown to increase shoot growth in Sedum species during the winter period [59] which is undesirable as it puts extra stress on the plant.

A high amount of species on wildflower roofs were chalk grassland natives which thrive in soils with strong alkalinity. It has been seen that species richness decreases in these species with reduced pH levels [60]. However, Basto et al, 2015 [61] indicated that certain wildflower species’ seed banks decrease with an increase in pH which has implications for the future species richness on wildflower roofs. It has been suggested that species richness in grasslands is reduced when mowing is stopped, which could indicate that the twice yearly mowing regime is important in regulating pH levels by reducing nutrients on wildflower green roofs [62].

The relationships between substrate characteristics

There was a strong correlation between SOM and n class="Chemical">potassium onpan> sedum roof substrates inpan> this study. It has been shown that SOM adsorbs n class="Chemical">potassium at a quicker and much higher rate than mineral constituents [63] in the soil and potassium improves water use efficiency (WUE) in plants [64, 65]. Soil Organic Matter has also been seen to directly affect the dynamic processes that make potassium available to plants [66]. Increased SOM in soils increases the cation exchange capacity and potassium, being of low positive charge, which is then more easily taken up by plants [63]. As well as improving potassium exchangeability into available forms SOM assists potassium in reducing soil acidity [67], perhaps illustrating the high pH values in this study. However, in this study SOM and potassium in wildflower roofs had no significant relationship.

In this study there was also a significant strong relationship between SOM and total n class="Chemical">phosphorus onpan> sedum roof substrates. The amount of SOM inpan> soils has been seen to be correlated with higher available n class="Chemical">phosphorus concentrations [68-70]. However, negatively charged functional groups in SOM such as carboxyl groups can interact with iron oxides in the SOM which can increase phosphorus adsorption thus binding it to soil particles and making it unavailable to plants [71]. Many biogeochemical factors affect the availability of phosphorus in the soil such as soil moisture, SOM and clay content [72] as well as the interaction of humic acids with some metal oxides [73]. Although the relationship between SOM and phosphorus is a complex one, it is generally suggested that higher SOM fractions in soil increase phosphorus availability [74, 75]. As phosphorus is a limiting factor in plant growth the fact that sedum roofs had a high SOM versus phosphorus correlation in this study suggested that sedum roof substrates may be a healthier ecosystem for Sedum survivability.

Soil pH has an effect on both n class="Chemical">potassium and phosphorus availability and, inpan> this study, sedum roofs were seen to have a stronpan>g to very stronpan>g positive relationpan>ship between the two variables. There is a paucity of literature inpan> regard to these factors inpan> soils but nonpan>-peer reviewed data sources have suggested that there is a relationpan>ship between pH and both n class="Chemical">potassium and phosphorus availability to vegetation. Hargreaves, 2015 [76] suggested that, in elevated levels of pH, the dominating ion is calcium (Ca) and the greater amount of Ca ions in the soil will increase the availability of potassium. As stated before, potassium increases the WUE of plants, suggesting that the higher pH on sedum roofs is beneficial in this respect. However, Hargreaves, 2015 [76] also indicated that increased Ca ions actually reduce the availability of phosphorus but this study found that, on sedum roofs, phosphorus concentrations actually increased with increasing pH levels very significantly. This is an interesting result, as Westermann, 1992 [77] and Hopkins & Ellsworth, 2005 [78] also posited that alkaline soils impede the amount of available phosphorus due to the formation of calcium phosphate minerals which have poor solubility.

This present study investigated the presence of total n class="Chemical">phosphorus inpan> green roof substrates rather than available (Olsen) n class="Chemical">phosphorus which may explain these results. Although the substrate on sedum roofs contained a significantly higher amount of total phosphorus than wildflower roofs in relation to pH, many alkaline soils have been seen to have relatively low amounts of available phosphorus in relation to total phosphorus [79]. Some studies have investigated solutions to this problem and have examined various soil additives which may decrease pH in order to increase phosphorus availability. For example, Liu et al, 2013 [80] advanced the addition of potassium sulfate (K2SO4) to substrates to reduce pH levels and microbial interactions within the soil have been seen to increase the amount of available phosphorus due to their ability to solubilise phosphorus [81].

Conclusions

This study indicated that there is a difference between the majority of substrate properties on sedum and wildflower roofs, highlighting the need to consider the complex associations between green roof variables in order to further scientific knowledge and inform industry. Results suggested that sedum roofs provide more desirable roof substrate properties than wildflower roofs for certain ecosystem services such as soil and n class="Chemical">water conpan>servationpan>. The key finpan>dinpan>gs demonpan>strated significantly higher SOM amount onpan> sedum roofs compared to wildflower roofs. Soils higher inpan> SOM are also higher inpan> SOC and so the amount of n class="Chemical">carbon stored may be higher in sedum roofs. However, in this study sedum and wildflower roofs were installed with varying initial substrates and these may well have had an influence on the differences in SOM percentages. Sedum roofs also displayed higher concentrations of phosphorus which can lead to nutrient runoff that can negatively affect both urban and adjacent aquatic habitats, specifically in the earlier stages of the green roof life.

There have been no studies collecting data in regard to relationships between substrate properties on green roofs and this study represents a fresh approach in examining these variables. The positive relationship between SOM and n class="Chemical">potassium onpan> sedum roofs was significant and sinpan>ce SOM can inpan>crease the availability of n class="Chemical">potassium in soils, the results indicate that the increased SOM in sedum roofs aids vegetation in regard to water use efficiency (WUE). Soil Organic Matter can also increase available phosphorus in soils and, considering that phosphorus is a limiting factor in plant growth, this relationship would seem to be beneficial in regard to Sedum health on green roofs. However, dependent on the level of phosphorus values there are issues in regard to phosphorus leachates. The higher pH on sedum roofs would suggest that there would be a higher amount of available potassium for these species, again increasing WUE and survivability of Sedum species. There are many variables to take into consideration when deciding whether to install sedum or wildflower green roofs and this study concludes that green roof choice can be dependent on which ecosystem services are desired whilst also taking into consideration green roof runoff.

Number of samples based on roof size.

(DOCX)

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Plant species lists for individual roofs.

(DOCX)

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18 Jul 2019

n class="Chemical">PONE-D-19-19569

Up on the roof and down in the dirt: differences in soil properties (SOM, n class="Chemical">potassium, n class="Chemical">phosphorus and pH) and their relationship to each other between sedum and wildflower green roofs.

PLOS ONE

Dear Ms McAlister,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

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Additional Editor Comments:

While the topic is of interest, I would like to ask for further details regarding the methods and a critical assessment of the findings. 7 roofs were selected and a varying number of samples (depending on the size of the roof) were sampled. What is the reasoning to consider each sample taken at a given roof as independent (n=50). In my opionion these are pseudo-replicates. Further, did you test for an age effect (time since establishment) and even more critical is the difference between the substrates. It remains unclear whether your questions relate to substrate (e.g. line 163) or plant type (e.g. 172/173). How can substrate and plant type be separated (what is cause and effect)? Another aspect which needs to be addressed before I consider sending the manuscript for review is the difference between the n class="Chemical">carbon conpan>tent at the time of establishment (baselinpan>e - n class="Chemical">carbon emboddied in the substrate) and time of sampling. In my opinion it is more relevant to consider the difference (not the total amount). I am looking forward to your response.

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1 Aug 2019

I am very thankful for your kind comments on the manuscript following the submission for publication.

The manuscript has been revised in line with your comments (formatting and lines 112, 171 and 747), and I am happy to respond to your remarks below:

7 roofs were selected and a varying number of samples (depending on the size of the roof) were sampled. What is the reasoning to consider each sample taken at a given roof as independent (n=50). In my opinion these are pseudo-replicates.

A total of 12 roofs were surveyed, as the Checkland building had 6 individual green roofs of different heights and aspects. I was guided by the methodology of green roof researchers Gabrych M., Kohtze, D.J & Lehvävirta, 2016. Substrate depth and roof age affect plant abundance on sedum-moss and meadow green roofs in Helsinki, Finland. They published their work using 475 sample plots over 51 roofs, basing the sample size per roof on roof size. Ecological in situ studies are naturally imposed with limitations but our results indicate there was soil property variability not just between roofs, but within roofs. This was a site-specific snap shot of the soil properties of 12 green roofs in Brighton and Hove and, although its pseudo-replication has been questioned, I believe it has scientific merit as this is one of the first studies to analyse these particular four properties in green roof soils.

Further, did you test for an age effect (time since establishment) and even more critical is the difference between the substrates. It remains unclear whether your questions relate to substrate (e.g. line 163) or plant type (e.g. 172/173).

The ages of the roofs were recorded but not included in the statistical analysis. As this was a snap shot of the present situation on the green roofs, we did not incorporate this data. The different types of substrates were not analysed independently, only the soil samples (lines 95 – 98).). The question in regard to substrate/plant type has been addressed in line 171 which, we believe, clarifies your enquiry. The questions relate to the difference in soil properties between the two green roof types, sedum and wildflower

Another aspect which needs to be addressed before I consider sending the manuscript for review is the difference between the n class="Chemical">carbon conpan>tent at the time of establishment (baselinpan>e - n class="Chemical">carbon embodied in the substrate) and time of sampling. In my opinion it is more relevant to consider the difference (not the total amount).

I was not able to measure the n class="Chemical">carbon baselinpan>e of the roofs at inpan>stallation and nor were the ownpan>ers of the roofs able to provide us with this inpan>formation. Therefore this study was a scientific depiction of the present state of the roofs, rather than a comparison between their soil properties at inpan>stallation and at the time of surveyinpan>g.

Once more, thank you for your comments and the opportunity to respond. I look forward to hearing from you.

Sincerely

Renée McAlister

Submitted filename: Response to Reviewers.docx

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23 Sep 2019

n class="Chemical">PONE-D-19-19569R1

Up on the roof and down in the dirt: differences in soil properties (SOM, n class="Chemical">potassium, n class="Chemical">phosphorus and pH) and their relationship to each other between sedum and wildflower green roofs.

PLOS ONE

Dear Ms McAlister,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

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This paper provides a good snapshot but further revisions are required.

I agree with reviewer 1. If the material is an engineered green roof it is not appropriate to refer to them as soil. I recommend to use the term "substrate" throughout the manuscript. And replace "edaphic" by substrate characteristics.

The authors have to address this concern ("However, I don’t know how any conclusions can be made that sedum may be better than wildflowers at sequestering n class="Chemical">carbon. Are the differences inpan> all the variables due to the plant type or to the differences inpan> the soil components? The soils on the sedum roofs were composed of recycled bricks while the soils on the wildflower roofs were composed of an assortment of different aggregates and composts. Differences inpan> SOM, pH, and P likely existed before plants were even inpan>stalled. Maybe plant type had no effect.") and provide further clarification. This is a critical limitation of the study and needs to be discussed.

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Kind regards,

Luitgard Schwendenmann

Academic Editor

PLOS ONE

Additional Editor Comments (if provided):

This paper provides a good snapshot but further revisions are required.

I agree with reviewer 1. If the material is an engineered green roof it is not appropriate to refer to them as soil. I recommend to use the term "substrate" throughout the manuscript. And replace "edaphic" by substrate characteristics.

The authors have to address this concern ("However, I don’t know how any conclusions can be made that sedum may be better than wildflowers at sequestering n class="Chemical">carbon. Are the differences inpan> all the variables due to the plant type or to the differences inpan> the soil components? The soils on the sedum roofs were composed of recycled bricks while the soils on the wildflower roofs were composed of an assortment of different aggregates and composts. Differences inpan> SOM, pH, and P likely existed before plants were even inpan>stalled. Maybe plant type had no effect.") and provide further clarification. This is a critical limitation of the study and needs to be discussed.

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Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

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Reviewer #1: (No Response)

Reviewer #2: (No Response)

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Reviewer #1: Yes

Reviewer #2: Partly

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Reviewer #1: Yes

Reviewer #2: I Don't Know

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Reviewer #1: Yes

Reviewer #2: Yes

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Reviewer #1: Yes

Reviewer #2: Yes

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6. Review Comments to the Author

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Reviewer #1: I noticed that you have changed substrate to soil in many places in the manuscript (eg lines 96 , 171 in the revised manuscript with track changes). If the substrates on the green roofs really are engineer green roof media, I am not sure it is appropriate to refer to them as soils. I do understand that this can make the terminology cumbersome, but I think it is an important distinction.

Introduction

Lines 49-51: the list is missing commas.

Results

Lines 197-199: you say the first two components account for 89.6% of variation, which components are these?

Discussion:

Line 293: “However, the caveat that n class="Chemical">phosphorus…” this wordinpan>g is a bit odd. I would suggest replacinpan>g caveat with another word.

Line 376: “connoting” seems like the wrong word to use hear.

Reviewer #2: The paper provides a good snapshot in time of the soil properties present at the time of measurement. However, I don’t know how any conclusions can be made that sedum may be better than wildflowers at sequestering n class="Chemical">carbon. Are the differences inpan> all the variables due to the plant type or to the differences inpan> the soil components? The soils on the sedum roofs were composed of recycled bricks while the soils on the wildflower roofs were composed of an assortment of different aggregates and composts. Differences inpan> SOM, pH, and P likely existed before plants were even inpan>stalled. Maybe plant type had no effect. There needs to be a soil analysis of the originpan>al soils. This is impossible now, but the soil samples could be mixed with the originpan>al inpan>gredients that would give a reasonable estimate of the baselinpan>e properties.

Also, there is little information on plants. What is a sedum roof and what is a wildflower roof? Any grasses? Plant species could have a major impact on soil properties. A list of plant species should be included to describe each roof.

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Reviewer #1: No

Reviewer #2: No

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While revising your submission, please upload your figure files to the Preflight Analysis and Conversion Engine (PACE) digital diagnostic tool, https://pacev2.apexcovantage.com/. PACE helps ensure that figures meet PLOS requirements. To use PACE, you must first register as a user. Registration is free. Then, login and navigate to the UPLOAD tab, where you will find detailed instructions on how to use the tool. If you encounter any issues or have any questions when using PACE, please email us at figures@plos.org. Please note that Supporting Information files do not need this step.

7 Nov 2019

Associate Professor Luitgard Schwendenmann

Academic Editor – PLOS ONE

Dear Professor Schwendenmann

I am very thankful for the kind comments of your reviewers on the manuscript following my initial revisions.

The manuscript has been revised in line with their comments. All references to soil have been changed to substrate, the missing commas (lines 52-53) have been added and the first two components of the PCA (lines 209-216) have been explained. Additionally, the wording has been changed on lines 320 and 400 in accordance with their suggestions (line 320 caveat has been changed to submit and line 404 connoting has been changed to suggesting).

However, I don’t know how any conclusions can be made that sedum may be better than wildflowers at sequestering n class="Chemical">carbon. Are the differences inpan> all the variables due to the plant type or to the differences inpan> the soil components?

I agree with the reviewers in regard to my conclusions about n class="Chemical">carbon sequestration as these conclusions were rather over speculative. I have therefore removed all references to this inpan>ference.

Also, there is little information on plants. What is a sedum roof and what is a wildflower roof? Any grasses? Plant species could have a major impact on soil properties. A list of plant species should be included to describe each roof.

I have included lists of all plant species on all roofs in Supporting Information 2 (signposted in Table 1). Additionally, I have added an explanatory sentence in the introduction (lines 97-100) giving a brief overview of the differences between sedum and wildflower green roofs.

The soils on the sedum roofs were composed of recycled bricks while the soils on the wildflower roofs were composed of an assortment of different aggregates and composts. Differences in SOM, pH, and P likely existed before plants were even installed. Maybe plant type had no effect.

I believe I have addressed this concern in lines 256-260 by qualifying that the research is presented for consideration as a snapshot of the substrate characteristics at the time of study. I have also added information that comparing the initial substrates in regard to SOM would be beneficial (lines 270-272) and included information regarding opposing studies which report either increasing or decreasing levels of SOM over time on green roofs (lines 273-276).

Additionally, Thuring & Dunnett (2014) suggested that green roof substrates do change over time in regard to their characteristics and therefore I believe this snapshot is a good representation of the current substrate characteristics taking aging into consideration. The roofs I studied were between 2 and 7 years of age, and Thuring & Dunnett (2014) indicate that substrate characteristics can indeed alter in a 2 year time frame.

Once again, thank you very much for your comments and the opportunity to respond. I look forward to hearing from you.

Sincerely

Renée McAlister

Thuring, C.E. & Dunnett, N., 2014. Vegetation composition of old extensive green roofs (from 1980s Germany). Ecological Processes. 3 (4) pp.1-11.

Submitted filename: Response to Reviewers v1.docx

Click here for additional data file.

11 Nov 2019

Up on the roof and down in the dirt: differences in substrate properties (SOM, n class="Chemical">potassium, n class="Chemical">phosphorus and pH) and their relationship to each other between sedum and wildflower green roofs.

n class="Chemical">PONE-D-19-19569R2

Dear Dr. McAlister,

We are pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it complies with all outstanding technical requirements.

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With kind regards,

Luitgard Schwendenmann

Academic Editor

PLOS ONE

Additional Editor Comments (optional):

Thank you for replacing "soil" by "substrate". I highly recommend to replace "soil organic matter" by "organic matter" for consistency.

Reviewers' comments:

5 Dec 2019

n class="Chemical">PONE-D-19-19569R2

Up on the roof and down in the dirt: differences in substrate properties (SOM, n class="Chemical">potassium, n class="Chemical">phosphorus and pH) and their relationships to each other between sedum and wildflower green roofs.

Dear Dr. McAlister:

I am pleased to inform you that your manuscript has been deemed suitable for publication in PLOS ONE. Congratulations! Your manuscript is now with our production department.

If your institution or institutions have a press office, please notify them about your upcoming paper at this point, to enable them to help maximize its impact. If they will be preparing press materials for this manuscript, please inform our press team within the next 48 hours. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information please contact onepress@plos.org.

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on behalf of

Dr. Luitgard Schwendenmann

Academic Editor

PLOS ONE