Biochar amendment in the green roof substrate affects runoff quality and quantity

Ecological Engineering 88 (2016) 1–9

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Ecological Engineering journal homepage: www.elsevier.com/locate/ecoleng

Biochar amendment in the green roof substrate affects runoff quality and quantity Kirsi Kuoppamäki a,∗ , Marleena Hagner a , Susanna Lehvävirta b , Heikki Setälä a a b

University of Helsinki, Department of Environmental Sciences, Niemenkatu 73, FIN-15140 Lahti, Finland Botany Unit, Finnish Museum of Natural History, University of Helsinki, PO Box 7 (Unioninkatu 44), Helsinki, Finland

a r t i c l e

i n f o

Article history: Received 29 April 2015 Received in revised form 27 October 2015 Accepted 2 December 2015 Keywords: Living roof Stormwater Water quality Nutrients Charcoal Water pollution Soil amendment

a b s t r a c t The utilisation of ecosystem services has been suggested as one solution to manage urban environmental problems, one of which is the excessive quantity or poor quality stormwater. As roofs contribute significantly to the amount of runoff, vegetated, i.e. green roofs have become an increasingly popular way to manage urban water in densely built areas. However, green roofs may introduce a new source of water pollution evidenced as higher concentrations of nutrients, especially phosphorus, in runoff compared to that in precipitation inputs. In two controlled, replicated experiments, one in the field for 7 months and another in the laboratory for 6 weeks, the amendment of biochar to green roof substrates was studied for its potential to mitigate the leaching of nutrients from newly installed pre-grown green roof Sedum and meadow mats. Nutrient concentrations were an order of magnitude higher in runoff from green roofs than in rain water. In the field experiment, biochar reduced the cumulative leaching of nutrients, even though biochar did not significantly reduce nutrient concentrations. These results can be interpreted as a combined impact of biochar on both the quantity and quality of runoff over time, the quantitative effect being apparently stronger than the qualitative. In the laboratory experiment, one type of biochar reduced nutrient concentrations and load in runoff while another type had an opposite effect. As the properties of biochar can vary considerably, careful studies are necessary before large-scale implementation of biochar amendment in green roofs are considered, to avoid unintended consequences. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Land use in urban areas such as the high proportion of impervious surfaces alters the flow of water and pollutants carried by water, causing adverse impacts on receiving waters (reviewed in Kaye et al., 2006). Engineered stormwater sewer systems have a limited capacity to manage intense precipitation events and virtually no capacity to improve the quality of runoff. Indeed, diffuse pollution due to increasing urbanisation is considered a major problem for adjacent surface waters (Novotny, 2002). Furthermore, urban areas, being disproportionately located along waterfronts, are also bound to deteriorate the ecological characteristics of aquatic systems (Grimm et al., 2008). In order to develop more sustainable cities, utilisation of ecosystem services has been suggested as an efficient solution to remediate pollution and manage other environmental problems as well as increase the quality of life

∗ Corresponding author. Tel.: +358 02941 20325. E-mail address: kirsi.kuoppamaki@helsinki.fi (K. Kuoppamäki). http://dx.doi.org/10.1016/j.ecoleng.2015.12.010 0925-8574/© 2015 Elsevier B.V. All rights reserved.

in urban areas (Bolund and Hunhammar, 1999; Oberndorfer et al., 2007). As roofs account for considerable areas in cities, they contribute significantly to the amount of runoff. Thus, green roofs have become an increasingly popular way to alleviate stormwater problems in densely built urban areas around the world (reviewed in Berndtsson, 2010 and in Rowe and Getter, 2010). Numerous studies have shown the capacity of green roofs to retain water as well as to delay and reduce runoff peaks, although this capacity varies depending on weather, season and climate. Further, the properties of a green roof, such as substrate thickness and characteristics, vegetation, slope, age and management actions can determine the functionality of green roofs (e.g. Berndtsson, 2010; Oberndorfer et al., 2007). However, besides these apparent regulating ecosystem services, green roofs can also provide an ecosystem disservice, evidenced as higher concentrations of nutrients, especially phosphorus, in runoff compared to that in precipitation inputs (Berndtsson et al., 2009; Harper et al., 2015; Hathaway et al., 2008; Malcolm et al., 2014), which seems to be a problem typically in newly established green roofs (Berndtsson, 2010; Rowe, 2011). Consequently, Oberndorfer et al. (2007) and

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Rowe (2011) have called for research on green roof growing substrates that minimise the leaching of nutrients. In a recent review by Li and Babcock (2014), leaching of nitrogen and especially phosphorus from green roofs was considered as a major problem, addressing also the need for research to develop mitigation actions. Directing runoff from green roofs through alum-filled pouches has been tested as a way of reducing phosphorus leaching, resulting in a 22% reduction of load (Malcolm et al., 2014). Rowe (2011) suggested the addition of activated charcoal that may filter pollutants. Another solution to improve the efficacy of green roofs might be the amendment of biochar in the substrate. Biochar is a solid material produced by thermochemical conversion, like pyrolysis, of biomass in an oxygen-limited environment (IBI, 2012). When used as a soil amendment, biochar is known to increase soil water retention capacity (Cao et al., 2014; Glaser et al., 2002) and adsorption of both inorganic (Cao et al., 2009) and organic (Beesley et al., 2010; Wang et al., 2010) pollutants. Reduced leaching of nutrients such as nitrogen and phosphorus from soils after biochar addition has also been reported (Sohi et al., 2009). A short-term laboratory test with green roofs receiving artificial rainfall showed that the addition of biochar could be an option to decrease nutrient discharge and also to improve water retention of green roofs (Beck et al., 2011). Biochar addition to green roof substrates has also been shown to delay the onset of wilting by plants due to increased water holding capacity, thereby enhancing plant survival (Cao et al., 2014). In the current study, two controlled, replicated experiments were performed to assess (i) the impacts of birch (Betula sp.) wood biochar on the leaching of nitrogen and phosphorus, (ii) the amount of runoff from pre-grown, readymade vegetation mats, and (iii) to compare the corresponding impacts of two types of biochar produced from birch. It was hypothesized that amending fine-grained biochar (1) improves the water retention capacity of green roofs, and (2) reduces the leaching of total nitrogen and total phosphorus from green roofs. Furthermore, it was expected that (3) amending biochar on the surface of green roofs is less effective than adding it at the bottom of the substrate as an additional layer, where water and nutrients, retained by biochar, are better available to be used by plant roots, and thus enhancing the functioning of a green roof. Finally, (4) the impacts of two types of biochar, both made of birch but by two different producers, on water retention and runoff quality were studied in a laboratory experiment, where biochar was sealed into mesh-bags. These bags were studied for their feasibility in installing biochar into green roofs as an alternative to the amendment of loose, finely crushed biochar.

2. Materials and methods 2.1. Study site and experimental designs Two experiments were performed at the University Campus in Lahti (61◦ 0.355 , 25◦ 39.220 ), Finland. A field experiment aimed at testing the effect of the positioning of the biochar layer: Sedum (trade name Xeroflor Moss-Sedum) and meadow green roofs (trade name Färdig äng, 6703) were amended with biochar either on the surface, simulating a possible measure implemented after the installation of a green roof, or as at the bottom of the pre-grown green roof mats, simulating a measure that could be performed at the time of green roof installation. The experiment was established on the rooftop in November 2011. However, monitoring of runoff was initiated only after the winter, in April 2012. Altogether 26 experimental green roof test modules (0.4 × 0.5 m in size), were established on “beds”, made of plywood and adjusted to a slope of 4◦ . The lowest end corner of each bed had a hole that was connected with a hose to a 10 l canister. Each experimental green roof had a 10 mm thick recycled fabric layer (VT-filt) at the

bottom to retain water during periods of drought. The tested green roofs were pre-cultivated vegetation mats produced by VegTech Ltd. (Sweden), imported by Envire VRJ group. Two types of mats were used: (i) moss-Sedum mats with a 30 mm thick substrate and (ii) meadow mats with a 40 mm thick substrate. The plant species of the green roof mats are listed in Appendix A. Biochar (A), made of birch wood (Betula sp. including bark), had been pyrolysed in a continuous retort at 380–420 ◦ C for a holding time of 2 h. Biochar amendment was 200 and 306 g m−2 for each Sedum and meadow green roofs, respectively. This amount was used also by Beck et al. (2011), i.e. 7% biochar by weight. Dry masses (+70 ◦ C for 24 h) of the Sedum and meadow green roofs were 3.8 and 5.0 kg m−2 , respectively. Biochar was sieved through a 2 mm sieve and the resulting fine powder was added either as an even layer at the bottom of the green roof substrate (hereafter referred to as “buried biochar”) or mixed with water and then spread as evenly as possible on the surfaces of the green roofs (referred to as “surface biochar”). Both biochar treatments had four replicates. Four Sedum- and four meadow green roofs were left without biochar addition (referred to as “control”). The amount of precipitation was measured using three plastic funnels (diameter 24.6 cm) that diverted water into 1000 ml Erlenmeyer flasks. To test whether the quality of biochar has an effect on its performance, a 6 week long laboratory experiment with two different biochar products were tested as a layer in the substrate of Sedum mats that were cut from pre-grown vegetation mats produced in Sweden (product name Nordic Green Roof® Sedum matte), delivered by EG-Trading Ltd. The laboratory experiment was established on 20 March 2014, using 3000 ml plastic boxes (18 cm × 18 cm, height 10 cm) with a hole (diameter 0.5 cm) at the bottom corner. The three treatments, each with five replicates, consisted of green roofs without biochar as the control and the two biochar types. Biochar was pyrolysed from birch wood (including bark): biochar A had the same origin as that used in the roof-top experiment (see above), while biochar B was produced by another company in a batch retort at 450 ◦ C for a holding time of 23 h. Specific features of biochars A and B are listed in Table 1. Each box had a 1 cm thick recycled fabric layer (VT-filt from VegTech Ltd.) at the bottom. Above this fabric, a 2 cm thick layer of substrate mixture was added. This substrate consisted of recycled, crushed brick (85% of fresh volume), compost (5%), crushed bark (5%) and sphagnum moss (5%). Three dl of biochar was sealed into mesh-bags (18 cm × 18 cm) made of plastic polyethylene terephthalate (PET) net (mesh size 150–200 ␮m). The mesh-bags were studied for their feasibility in installing biochar into green roofs as an alternative to the amendment of loose, finely crushed biochar. Particle size of the biochar was <15 mm. These mesh-bags were installed into the boxes right above the 2 cm substrate and covered with another 2 cm layer of the substrate. The control treatment included mesh bags without biochar. Finally, a 3 cm thick Sedum mat was added on top of each box. Prior to the start of the experiment, the Sedum mats had been stored on the rooftop of the University Campus building since autumn 2011. The 15 boxes were installed below plant growing lights (240–370 ␮mol photos m−2 s−1 ) with a light:dark cycle of 10:14 h and temperature of 22 ◦ C. Each green roof box was Table 1 Characteristics of biochar A (used both in the field and the laboratory) and biochar B (used in the laboratory).

Dry matter (%) Organic matter (% dw) pH BET surface area (m2 g−1 ) Water holding capacity (%) Bulk density (g l−1 )

A

B

96 97 7.6 7 77 389

84 94 9.2 140 163 245

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irrigated every third day with 200 ml water to keep the substrate moist. For irrigation, snow was collected from the rooftop of the campus building at the beginning of the experiment, melted, and frozen and stored in plastic bags in a freezer (−18 ◦ C), until used in irrigation. 2.2. Measurements, sampling and analyses Canisters were placed below the green roofs of the field experiment to collect runoff. Monitoring of runoff was initiated on the 27th of April 2012, when enough precipitation induced discharge from the green roofs. The canisters were emptied and the quantity of water measured following rain events until 23 November 2012, when the temperature permanently declined below 0 ◦ C until the next spring. Altogether 38 precipitation events were monitored during the 7 months. Runoff samples for pH and total nutrient (N and P) analyses were collected using two 100 ml plastic bottles four times: on 18 May, 18 June, 10 September and 5 November. Moisture (as volumetric water content, +3% VWC) and temperature of the substrates were continuously measured at 1 h intervals using Decagon 5TE sensors that were placed at 2 cm depth in the centre of Sedum and meadow green roofs. Two replicates per control and surface biochar treatments were randomly selected for moisture and temperature measurements. Decagon Em50 data loggers were used to store the measurement data. In the laboratory experiment, 1, 2, 4 and 7 weeks after establishment, the green roofs were irrigated with the melted snow (300–500 ml) so that enough runoff (minimum 200 ml) was obtained for the analysis of pH and total nutrients (N, P). Electrical conductivity of the runoff leachates was measured at the last sampling event. Due to the lack of melting water at the last irrigation event, the green roofs were irrigated using runoff collected from a nearby tin roof. Water samples were measured for pH using Mettler Delta 340 pH meter and for conductivity using WTW Cond 330i meter. Samples for total nutrients were first oxidised in an autoclave at 120 ◦ C for 30 min. Total phosphorus (hereafter TP) was measured spectrophotometrically after the addition of molybdate reagent (ISO 6878:2004). High Performance Liquid Chromatography (HPLC) instrument was used to determine total nitrogen (hereafter TN) with 0.04 M sodium chloride (NaCl) as an eluent according to ISO 29441:2010. 2.3. Data analyses To estimate nutrient load between the sampled rain events in the field experiment, runoff volumes that occurred in between the sampled events were divided into two equal intervals. The volume in each interval was then multiplied by the nutrient concentration of the closest sampled event to interpolate runoff concentrations throughout the experiment. Total runoff and nutrient loads were calculated separately for spring and summer (May–August) and autumn (September–November). In order to obtain the load of nutrients in the laboratory experiment per unit area, the concentration of nutrients (mg l−1 ) was multiplied by the amount of percolated water (l) and divided by the area of the experimental green roofs. In both experiments, repeated measures General linear model, GLM (IBM SPSS Statistics 21) was used to analyse the impact of biochar amendment and/or the type of vegetation (factors) on nutrient concentrations and runoff volumes. In case of a statistically significant interaction between the factors and time, the difference between the treatments was determined separately at each sampling event. GLM was used to analyse the differences in cumulative mass loads and runoff volumes between the treatments at the end of each study period. Tukey’s test was used to detect differences between the two biochar treatments. If the assumptions

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for parametric tests were not met, log transformation was used to homogenize variances or to normalise the distribution.

3. Results 3.1. Field experiment The cumulative amounts of precipitation were equal during the two studied seasons, but accumulated much slower in the summer than in the autumn (Fig. 1), while runoff from green roofs varied between 20–30% and 60–70%, respectively, of that of precipitation. There was a significant interaction between biochar and time on the cumulative runoff. During summer, biochar reduced runoff significantly (repeated measures GLM; F = 4.753, p = 0.022), the difference being between the control and the buried biochar treatment (Tukey; p = 0.018; see Tukey’s test for statistical significance in Section 2). Surface biochar improved the retention capacity of both vegetation types by ca. 5% (75% of rainfall was retained) and buried biochar by an additional 5% (80% retained). During the rainy autumn, buried biochar retained 64 and 61% of rainfall in Sedum and meadow plots, respectively, which was marginally higher (F = 2.93, p = 0.079) than the 50% retention by the control and surface biochar treatments in both vegetation types. Negligible differences in the performance of the two vegetation types may be explained by the poor survival of plants in the meadow mats. Substrate moisture was always lower in the surface biochar treatment than in the control, suggesting that biochar interferes with the measurement of the Decagon sensors, which have been calibrated to measure the moisture of soils, not charcoal. Substrate moisture declined quickly following rain events in summer, reaching lower values in Sedum than in meadow green roofs (Fig. 1). In the autumn, moisture remained constantly close to its saturation level, reflecting the rainy season with declining temperatures (Fig. 1). Rainfall depths of the individual precipitation events varied from <2 mm to 40 mm and the corresponding runoff from green roofs varied from 0 mm to 38 mm. Thus, the overall retention capacity of the green roofs ranged between 100% and 5% across the dataset (Fig. 2). In summer, green roofs retained most of the small and many of the high rain events better than in autumn when the retention capacity declined more steeply with an increase in precipitation (Fig. 2). The biggest summer storm (24–25 June), after a 5-day rainless period, amounted to 40 mm of which 50% and 60–70% was retained in control and biochar treatments, respectively. Vegetation type did not affect water retention capacity (Figs. 1 and 2). Nutrient concentrations were an order of magnitude higher in runoff from green roofs than in rain water (Fig. 3). However, cumulative nutrient loads during the whole study period were lower in treatments with biochar, although biochar did not have significant effects on nutrient concentrations. The concentrations of TP in runoff from meadow mats were higher than those from Sedum mats (F = 6.58, p = 0.025), but vegetation type did not affect TN concentrations. The concentrations of both nutrients were lower in autumn than in summer. During the whole study period, biochar reduced the cumulative nutrient loads of TN (F = 4.97, p = 0.019) significantly, and marginally of TP (F = 3.16, p = 0.066) (Fig. 4). In the summer, biochar significantly reduced the load of TN (F = 5.36, p = 0.015), which was on average 45% lower in buried biochar than the control (p = 0.014; Tukey’s test). During the whole study period, surface biochar reduced TP load by 10% in both green roof types, while buried biochar reduced TP load by 25% and 35% in Sedum and meadow roofs, respectively (Fig. 4). Vegetation type and time, i.e. season, had a significant interaction on the leaching of both TN and TP (F = 14.72, p = 0.001 and F = 28.11, p < 0.001, respectively), which were highest from meadow roofs during autumn (Fig. 4).

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Precipitation, mm

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300

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0

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Fig. 1. Cumulative precipitation and average (+SE) runoff from Sedum and meadow green roofs with the different biochar treatments during summer (May–August 2012) and autumn (September–November 2012). The average moisture in Sedum and meadow substrates in control and surface biochar treatments is shown below the runoff plots. Arrows indicate the time of water sampling for nutrient analyses. Average substrate temperature during the sampling period is shown in the bottom plots. Moisture sensors do not operate at temperatures below zero and the data during these periods were omitted from the figure. Moisture data from Sedum beds from September onwards are missing due to technical failure of the sensors.

K. Kuoppamäki et al. / Ecological Engineering 88 (2016) 1–9

5

surface biochar

AUTUMN SUMMER

buried biochar

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100

surface biochar

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0.08 0.12 0.06

60

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Retention %

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R2 values:

40

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40

0.19

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20 30 Precipitation, mm

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0.06 0.14 0.10

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0.39

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20 30 Precipitation, mm

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40

Fig. 2. Average water retention of Sedum (left) and meadow (right) green roof beds with the different biochar treatments during summer (May–August 2012; top) and autumn (September–November 2012; bottom). The trend lines illustrate the relationship between the amount of precipitation and the water retention of green roofs: the lower the R2 values the better the retention capability.

12

0.8

meadow Sedum

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6 4

TP concentration, mg/l

TN concentration, mg/l

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0.6

rain water 0.4

0.2

2

18.5.

18.6.

10.9.

5.11.

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10.9.

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rain

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5.11.

Fig. 3. Average (+SE) concentrations of total nitrogen (TN, left) and total phosphorus (TP, right) in runoff water from meadow and Sedum beds with the different biochar treatments, as well as in rain water of the four sampling events in 2012.

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Total nitrogen 140

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Fig. 4. Average (+SE) loads of total nitrogen (top) and total phosphorus (bottom) per month during summer (May–August, left) and autumn (September–November, middle) as well as the total load over the study period (May–November, right) in runoff from Sedum and meadow green roofs without biochar (control) and with biochar amended either on the surface or buried at the bottom of the substrate.

3.2. Laboratory experiment

4. Discussion

In the laboratory experiment, the bagged biochar performed well, but the two biochar types showed a contrasting impact on the concentrations of TN and TP in the runoff from green roofs: compared to runoff from the control, biochar A decreased, while biochar B increased the concentrations (Fig. 5, top). However, only the impact on TP concentrations was significant (repeated measures GLM; F = 8.14, p = 0.006) and the difference was between the two biochar types (Tukey p = 0.004). The effect of biochar type on TN concentrations was marginally significant (F = 3.02, p = 0.086). A similar pattern was evident in the cumulative mass load of nutrients (Fig. 5, middle): again the impact of biochar on the TP load was significant (F = 4.06, p = 0.045), the difference being between the two biochar types (Tukey p = 0.036). When comparing the total load of nutrients between the biochar treatments and the control green roofs at the end of the experiment, biochar A reduced TN and TP load by 24% and 27%, respectively. In contrast, biochar B increased TN and TP load by 5% and 21%, respectively. The rain water used during the last irrigation event had a different origin and higher nutrient concentration, especially that of nitrogen, compared to rain water used during the three first irrigations (Fig. 5, the two panels at the. Biochar did not affect the pH of the leachates, but tended to increase the electrical conductivity of leachates (F = 7.780, p = 0.07), the difference being between the control and biochar B (Tukey p = 0.005). Neither of the biochar types affected the water retention capacity of the green roofs (Fig. 5, the lowermost panel).

4.1. Water retention of green roofs Biochar improved the water retention capacity of both Sedum and meadow green roofs, irrespective of the intensity of the rain event in our roof top experiment. This result is in agreement with our hypothesis and supports previous findings about the positive impacts of biochar on water holding capacity of green roofs (Beck et al., 2011; Cao et al., 2014). However, the effect of biochar seems to depend on prevailing weather conditions, as the water retention capacity was improved by biochar especially during the summer with infrequent precipitation events but was less effective during the autumn with frequent precipitation. During the autumn, the substrate remained continuously moist due to the rainy weather, declining temperatures and, consequently, low transpiration by plants and low evaporation from substrate, resulting in a much lower retention capacity compared to the summer season. These factors probably lessened the impacts of biochar. As hypothesized, amending biochar on the surface of green roofs was less effective in reducing runoff than adding it at the bottom of the substrate. It is likely that buried biochar affected the performance of plants: in soil, biochar retains water (Cao et al., 2014) and nutrients (Taghizadeh-Toosi et al., 2012; Hammer et al., 2014), which are therefore available for plant uptake. The results of our field study support the findings by Cao et al. (2014), who reported that biochar may enable green roofs to retain their viability and functionality

K. Kuoppamäki et al. / Ecological Engineering 88 (2016) 1–9

9

2

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Total phosphorus concentration

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Fig. 5. Average (+SE) concentrations (top) and cumulative loads (middle) of total nitrogen (TN, left) and total phosphorus (TP, right) in runoff as well as the cumulative discharge (down) from the different biochar treatments over the course of the laboratory experiment in 2014. For comparison, the upper panels also show the concentrations of TN and TP in rain water that was used for irrigation (see Section 2 for a more detailed description).

longer than green roofs without biochar under fluctuating temperature and moisture conditions. On the other hand, although the roof top experiment showed improved water retention by biochar, no such results were obtained in the laboratory experiment. This may be due to the artificial irrigation regime, where water was sprinkled from a dish on the green roof plots. It is also possible that the experimental setup affected our results: in the laboratory study the biochar closed in mesh-bags may not have imposed the same soil and plant effect as when biochar is in close contact with the soil and plant roots. This effect should be studied further in larger scale, experiments with contrasting ways of adding biochar layers on green roofs. In general, the widely documented superior water retention capability of green roofs during summer compared to colder

seasons (Mentens et al., 2006; Berndtsson, 2010) was evident also in our study. Water retention is strongly dependent on rain event sizes (Teemusk and Mander, 2011). This was most evident during the autumn season of our study, while in the summer the relationship between, e.g. rainfall intensity and water retention capacity was less clear. During warmer weather, with temperature of the green roofs reaching over 40 ◦ C between rain events, the thin substrate dried rapidly, allowing high storage capacity for the next rainfall event. Although grasses have been shown to be more effective in reducing runoff than Sedum (Dunnett et al., 2008), our study did not reveal an effect of vegetation type (Sedum vs. meadow) in terms of water retention. This is probably explained by the weak contribution of plants in the meadow mats with poor survival of grasses

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and especially herbs. On the other hand, it has been shown that substrate depth rather than vegetation type can determine retention capacity (Monterusso et al., 2004). For example, during summertime, every additional 1 cm increase in substrate thickness can result in 2.5 mm less runoff (Mentens et al., 2006). Although our meadow mats were ca. 1 cm thicker than Sedum mats, no such consistent increase in water retention was detected. Dunnett et al. (2008) found inconsistent impacts of the relative contribution of soil and vegetation on runoff values, with bare soil resulting in both the highest and the lowest runoff reduction compared to vegetated platforms. Substrate characteristics obviously greatly affect water retention, as also discussed by Dunnett et al. (2008). Our field study showed that substrate amended with biochar can increase water retention. 4.2. Nutrient leaching from green roofs Nutrient concentrations in the runoff from green roofs were 5–10 times higher than those in the precipitation, thus supporting previous findings of green roofs being a source rather than a sink of nutrient contaminants (Hathaway et al., 2008; Harper et al., 2015; Malcolm et al., 2014). Other studies have also found higher total nutrient concentrations in runoff from green roofs than in rain water or melted snow, but lower concentrations compared to nonvegetated control roofs (Teemusk and Mander, 2007; Gregoire and Clausen, 2011). In general, the nutrient concentrations measured in our study are well within the wide range reported in previous studies, i.e. 0.4–5.4 mg TN l−1 and 0.008–1.5 mg TP l−1 (Berndtsson et al., 2009; Gong et al., 2014; Gregoire and Clausen, 2011; Hathaway et al., 2008; Malcolm et al., 2014; Teemusk and Mander, 2011). The high variation in the leaching of nutrients between different studies can be explained by, e.g. substrate composition and depth on the green roofs, plant species, precipitation properties and maintenance protocols (reviewed by Li and Babcock, 2014). In addition, nutrient concentrations in leachates from green roofs can decrease over time, when fertilisers added during the production process are gradually washed away and if new fertilizers are not added (Berndtsson, 2010; Gong et al., 2014). This is corroborated by our field and laboratory experiment results showing that nutrient concentrations decreased with time. On the other hand, Harper et al. (2015) found sustained loading of total nitrogen, which they hypothesized to originate from nitrogen transforming to labile form through the slow breakdown of the substrate and subsequent flushing. Thus, as various biological, chemical and physical changes occur in green roofs with time and because soil particles and dissolved nutrients get washed off (cf. Berndtsson et al., 2010), long-term monitoring of the performance of green roofs is necessary. Furthermore, given the generally high variation in green roof runoff quality, our study agrees with Harper et al. (2015), who emphasised that care should be taken in generalising the performance of green roofs. Avoiding fertilisation and carefully selecting substrate material, including special amendments like biochar, are means by which nutrient leaching from green roofs could potentially be manipulated. In our study, meadow mats leached more nitrogen and phosphorus than Sedum mats. This may be due to the poor survival of grasses and herbs in the meadow mats used here. Previously, unhealthy plants on green roofs have been shown to cause, e.g. nitrogen leaching into runoff (Aitkenhead-Peterson et al., 2011). Our hypothesis concerning biochar’s ability to reduce the leaching of total nitrogen and total phosphorus was partly supported. In the field experiment, biochar reduced the cumulative leaching of nutrients from green roofs, especially when applied at the bottom the substrate, even though biochar did not significantly reduce nutrient concentrations. These results can be interpreted as a combined impact of biochar on both the quantity and quality of runoff

over time, the quantitative effect being apparently stronger than the qualitative. By retaining water, biochar has been shown to affect the quantity and quality of water (Yu et al., 2013). Similar results may also be achieved through the impacts of biochar on soil microbial properties and plant growth (Jeffery et al., 2011). Our study suggests that to improve retention of water and nutrients, biochar amendment at the bottom of the substrate at the time of green roof installation is clearly more beneficial compared to the application of biochar on the green roof after installation. Pyrolysis conditions and feedstock materials have strong effects on the characteristics of biochar (Downie et al., 2009). Specific properties of biochar, such as surface area, pore size and cation exchange capacity, affect the ability of biochar to adsorb water and nutrients (Scott et al., 2014). This was also seen in our laboratory experiment, where one biochar type reduced nutrient concentrations and load in runoff while another biochar type had an opposite impact. Thus, our results are less straightforward than those by Beck et al. (2011), who found that the addition of biochar to green roof soil consistently reduced the concentrations of both nitrogen and phosphorus in runoff and also improved water retention. Our experiments revealed that biochar might also have an increasing impact on the concentration of especially TP in the water leachates (albeit not statistically significant). Biochar used by Beck et al. (2011) was mainly of agricultural origin, derived from rice hulls, nut shells and coconut shells but also car tires, while biochar used in our study was produced only from birch. Contrasting results can thus depend on the quality of biochar: both its origin and pyrolysis conditions. For example, BET- surface area or water holding capacity (WHC) of the biochar did not explain variation in the reduction of nitrogen and phosphorus. In fact, the biochar with a smaller surface area and WHC retained more nutrients. The essential question here is whether biochar is a plausible means to control nutrient leaching from green roofs. The answer likely depends on the properties of biochar, warranting careful studies before large scale amendment in green roofs to avoid unintended consequences. However, biochar bagged into mesh-bags performed well and, thus, it could be considered as a product that is done already at the site where the biochar is being pyrolysed, making the amendment of biochar on rooftops easier compared to the use of loose biochar. 5. Conclusions Nutrient concentrations were an order of magnitude higher in runoff from green roofs than in rain water. In the field experiment, biochar reduced the cumulative leaching of nutrients from green roofs, especially when applied at the bottom of the substrate, even though biochar did not significantly reduce nutrient concentrations in the runoff. These results can be interpreted as a combined impact of biochar on both the quantity and quality of runoff over time, the quantitative effect being apparently stronger than the qualitative. In a laboratory experiment, the same biochar that was used in the field experiment reduced nutrient concentrations and load in runoff. However, another biochar type in this laboratory study had the opposite impact, i.e. it increased the leaching of nutrients, emphasising the variable properties of biochar and warranting careful studies before the large scale application of biochar to green roofs to prevent unintended consequences. Acknowledgements This study was done in the “Fifth Dimension—Green Roofs in Urban Areas” research group as part of the project ENSURE, Enhancing Sustainable Urban Development through Ecosystem Services, funded by Helsinki University Centre for Environment, HENVI. The study was also financially supported by the

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Helsinki-Uusimaa Region, Kone Foundation as well as Maj and Tor Nessling Foundation. Students in the Master’s Degree Programme in Multidisciplinary Studies on Urban Environmental Issues (MURE) are acknowledged for their invaluable help in the field experiment during autumn of 2012. Appendix A. The list of plant species in the two green roof mats used in the field experiment, according to the catalogue number 6703 by VegTech (2011), supplemented with information provided by the “Fifth Dimension—Green Roofs in Urban Areas” research group in the University of Helsinki. Sedum green roof (trade name: Xeroflor Moss-Sedum)

Meadow green roof (trade name: Färdig äng, 6703)

Sedum species: Hylotelephium ewersii Sedum acre Sedum album Sedum sexangulare Sedum spurium Moss species: Bryum argenteum Ceratodon purpureus Syntrichia ruralis

Forbs: Achillea millefolium Arthemis tinctoria Campanula persicifolia Campanula rotundifolia Centaurea jacea Centaurea scabiosa Cichorium intybus Echium vulgare Filipendula vulgaris Galium verum Hypericum perforatum Hypochoeris maculate Leontodon hispidus Leuchantemum vulgare Lotus corniculatus Malva moschata Plantago media Potentilla tabernaemontanii Primula veris Rhinanthus minor Saxifraga granulata Scabiosa columbaria Senecio jacobea Silene nutans Grasses: Arrhenatherum pratense Briza media Festuca ovina Festuca rubra Phleum phleoides Phleum pratense

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