IJERD – International Journal of Environmental and Rural Development (2016) 7-1

Research article
erd

An Attempt to Use High Salinity Water for Irrigating
A Green-Roof Garden
SHIGEOKI MORITANI*
Faculty of Agriculture and Life Science, Hirosaki University, Hirosaki, Japan
Email: moritani@hirosaki-u.ac.jp

TAHEI YAMAMOTO
Arid Land Research Center, Tottori University, Tottori, JapanHENINTSOA ANDRY
Arid Land Research Center, Tottori University, Tottori, JapanCHOICHI SASAKI
Faculty of Agriculture and Life Science, Hirosaki University, Hirosaki, JapanCHIHIRO KATO
Faculty of Agriculture and Life Science, Hirosaki University, Hirosaki, JapanHIROTAKA SAITO
Faculty of Agriculture, Tokyo University of Agriculture and Technology, Fuchu, Tokyo, Japan
Received 16 October 2015

Accepted 11 April 2016(*Corresponding Author)

Abstract Though a green roof has been implemented mainly in city area due to the mitigationof heat island effect, it can also offer the benefit of energy saving to a building in rural area. A
reuse of wastewater for irrigation may be an efficient practice for water conservation, especiallyin the water-scarce areas. In this study, we assessed the influence of frequent and intermittent
saline irrigation on evapotranspiration (ET), dry matter yields (DM) and water-use efficiency(WUE) in crassulacean acid metabolism (CAM) plants used in the green roof program. The
CAM plants Sedum kamtschaticum Fischer and Sedum oryzifolium were evaluated with turf
grass, Cynodon dactylon. A sharp reduction in ET with an increase in soil salinity was found in
CAM plants as compared to turf grass; however, the dry matter yield of CAM plants was higher
than that of turf grass at the same amount of cumulative ET. Principle component analysis(PCA) was performed to group the treatments into fewer groups characterized by similarfeatures. CAM plants were categorized by high DM and WUE. These features were expected tomake green roof management sustainable because they have low water requirements withkeeping the high-density vegetation.
Keywords CAM plants, saline irrigation, evapotranspiration, water-use efficiency

INTRODUCTION
As recent climate change has raised the inconsistency of seasonal pattern of rainfall and temperature, it
may cause decreasing precipitation and increasing temperature in some area (Kiem and Austin, 2013).
In case such severe condition for human health continues, an air-conditioning based on the heat pumpwould be spread more rapidly for cooling in residential buildings regardless of their urban or rural
location.
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The growing energy consumption for cooling could adverse effect on the household economy. For
reducing the thermal load into buildings, green roof is being used due to their protection from the solar
radiation (Kumar and Kaushik, 2005). Especially the soil used in this study has a greater effect on thethermal property of insulating because with the large value of for porosity, much of the pore space isfilled with air (Radcliffe and Simunek, 2010). However in dry climate regions, green roofs are notwidely adopted because irrigation water for green roof is scarce. The irrigation water availability could
be enhanced through the use of saline water, the recycling of drainage water and the reuse ofwastewater (FAO, 1992; Rhoades, 1998; Ould-Ahmed et al., 2007). A considerable amount of poorquality water is available in many dryland countries (FAO, 2003). The idea of using poor-quality water
for irrigation to increase food production has been reported successfully for more than five decades;
however, little study has been conducted with poor-quality water with the aim of improving thermal
performance and landscapes, such as developing green roofs (Moritani et al., 2013).
Succulent plants that have a Crassulacean acid metabolism (CAM) have fleshy leaves. CAM
plants open their stomata during night so that CO2 is fixed into organic acids and eventually stored in
the vacuoles. These characteristics prevent water loss from CAM plants and decrease the uptake of salt
into plant bodies (Gravatt and Martin, 1992). In this study, we tested two kinds of CAM plants - Sedum
kamtschaticum and Sedum oryzifolium - both natives of coastal areas with turf grass (Cynodon
dactylon) as the control (Fujita, 2007). The purpose of this study was to examine the possibility ofusing these plants for green roofs with saline water in terms of its dry matter yield and water-use
efficiency (WUE).
METHODOLOGY
Table 1 Physical and chemical properties of the experimental soil
Fraction (%)

Bulk Saturated hydraulic pH
EC
T-C
(H2O)
density
conductivity
Gravel
Sand
Silt
Clay
−1
%
>2.0 mm 2.0–0.02 0.02–0.002 0.002> g cm−3
cm s−1
dS m
48.9

32.1

10.6

8.4

0.48

3.6 × 10−2

6.7

0.70

12.0

T-N

CEC

%

cmol(+) kg-1

0.96

40.4

Description of Soil, Plants and Irrigation Water
The experiment was performed in a glass greenhouse (glasshouse) at Arid Land Research Center(ALRC) of Tottori University in Japan. The soil shown in Table 1 was overlaid at a thickness of 10 cm,
on a gravel layer for drainage, in a plastic pot with a diameter and height of 0.16 m and 0.20 m,
respectively.
In this study, two CAM plants, S. kamtschaticum Fischer and S. oryzifolium, and a turf grass, C.
dactylon, which is one of the primary turf grasses used for green roof in Japan, were used. The
transpiration activity of S. kamtschaticum was relatively higher under normal condition than of S.
oryzifolium because of higher leaf area index. 6 plants of S. kamtschaticum and 25 plants of S.
oryzifolium plants were transplanted into each pot in the middle of June, and seeds of the C. dactylon,
which is a C4-type plant, were sown (Aires et al., 2008). Bare soil packed in same pot was also
prepared for measuring the reference evaporation. All plants were grown for one month by wateringwell before starting the experiment on August 1. The experiment lasted for two months during summer.
The irrigation water was prepared by mixing seawater and tap water shown in Table 2 (Al-Busaidi
et al., 2007). The ratio of the Na ion content to the sum of Ca and Mg ions, expressed as the sodium
adsorption ratio (SAR), is an important index to assess the quality of irrigation water. The definition ofSAR is described in Eq. (1) below (US Salinity Laboratory, 1954):Ⓒ ISERD

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SAR

Na  / (Ca 2   Mg 2  ) / 2(1)

where Na+, Ca2+ and Mg2+ are ion contents (mmeq L-1) of sodium, calcium and magnesium,
respectively. The quality of irrigation water used in this study was averaged to 12 dS m-1 with SAR of
50.4, which was classified as the most severe salinity condition (US Salinity Laboratory. 1954) with
EC higher than 2.3 dS m-1 and SAR higher than 15.
Table 2 Chemical composition of irrigation water
Water typeTap water

pH

8.1
Salineirrigation waterSea water
7.5

EC
dS m-1
0.7

Na +

Ca 2+
ppm

Mg2+

SAR

39.9

95.6

11.2

2.1

12

2752

65

485

50.4

38.5

11211

266

1976

104.0

Experimental Conditions
The humidity and temperature were measured every 15 minutes using a hobo logger (HOBO H8 PRO
SERIES LOGGER). While the range of temperature was generally higher at averaged 32 ºC than thatof an actual green roof setting Takebayashi et al. (2007), there was a slight wind blowing inside theglasshouse through open windows.
Two different irrigation intervals were used: frequent irrigation (FI) and intermittent irrigation (II)with averaged to 2 and 10 days interval, respectively. Evapotranspiration rate (ET) from the vegetation,
evaporation rate from the bare soil (E), and pan evaporation rate were measured by weighing pots withand without plants and a small evaporation pan (internal diameter of 0.35 m). Daily ET ratio (ET r) andE ratio (Er) were calculated by dividing ET and E with pan evaporation rate, respectively. The amountof irrigation water (Vi) applied to each pot was 1.2 times the observed ET during each irrigationinterval so that the excess of water Vd can be used to leach the salt from the soil by drainage(Yamamoto and Cho, 1987a,b ;Corwin et al., 2007). The EC of the drainage water (ECd) was measured
using a portable EC meter (HORIBA Compact Conductivity Meter/B-173). At the end of theexperiment, the dry matter yield (DM) of each plant was observed by weighing the shoots after drying
them at 80 ºC for 3 days. The water use efficiency (WUE) is defined as follows:
WUE

DM
TotalET(2)

where total ET is the total ET during the irrigation experiment, which is measured by weighting pots.
The soil water content and soil EC were calculated from the water and salt balances in pot soil,
described in Moritani et al (2013).
Statistical Analyses
The experimental treatment under saline irrigation which was classified as the most severe salinity
condition was replicated more than 3 times. These samples were subjected to a mean separation
analysis using a one-way ANOVA with a significance level of P <0.05. (PCA) was used. Principal
component analysis (PCA) was carried out under each irrigation method on the standardized data
matrix for reducing the data complexity and extracting the latent patterns. The analysis of PCA cancondense the variations in the treatments by loading in two orthogonal axes that summarize the mainunderlying gradients (Tamene et al., 2006). To analyze the direct impact of the parameters of ECi, DMⒸ ISERD

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and WUE on the gradients, regression methods between the loadings of PC1 or PC2 and thecorrespondents of each parameter were calculated (Moritani et al., 2010; Singh et al., 2005). All ofthese statistical analyses were performed with SPSS software (Ver. 10.0).
RESULTS AND DISCUSSION
Influence of Evapotranspiration on Soil Salinity

3.0

S. kamtschaticum
S. oryzifolium
C. dactylon

ETr

ET for all plants decreased with time as ECs
increased, however ET of C.dactylon wasBare soil
2.0
observed with greatest amount. An average pan
evaporation rate of 4.4 mm day-1 was observed
1.0
during the experiment. Fig. 1 shows the decreasein evapotranspiration ratio (ETr) and evaporation
ratio (Er) with increase in soil water EC (ECs). The
0.0
maximum ETr for S. kamtschaticum, C. dactylon
0
20
40
60
and S. oryzifolium was 2.0, 3.0 and 1.4,
-1
ECs (dS m )respectively. While the Er exhibited a unit value ofapproximately 1.0, a decrease in the ETr wasFig. 1 The relationships between the EC ofobserved for all of the plants caused by salt stress.
soil water (ECs) and the
The ECs value when the ETr became less than theevapotranspiration ratio (ETr) of S.
Er was at 10 dS m-1 for S. kamtschaticum, while at
kamtschaticum, S. oryzifolium and C.
2 dS m-1 for S. oryzifolium. In the case of C.
dactylon and the evaporation ratio
dactylon, the ETr moderately decreased, reaching aof the bare soil (Er)value > 0.8 at an ECs value of 60 dS m-1. The
sharp reduction of the ET, particularly for S. kamtschaticum and S. oryzifolium, was assumed to be theresult of the soil osmotic potential, which decreased the water uptake of the roots. Another reason for
decreasing the ET might be the closure of the stomata during the day (preventing water loss from theplant) under the condition of salinity stress, alternating with the opening of the stomata during the night,
when the evaporative demand was low.
Dry Matter Yields and Water-use Efficiency
The total ET of S. kamtschaticum, S. oryzifolium, and C. dactylon in the control pots were 381, 192 and574 mm, respectively. The 24 % reduction in total ET by soil drought condition under II was observedin S. oryzifolium compared to the control while those in S. kamtschaticum and C. dactylon wereaveraged to 64 %. Table 3 shows the total ET, DM, and WUE for all three types of plants under bothsaline irrigation methods. The total ET of C. dactylon under FI was the highest among others. However,this ET decreased by 48.2% under II that is statistically significant. The DM of all plants under thecontrol showed similar values averaged to 15.7 (±0.8) t ha-1. The DM of S. oryzifolium was the highest,
although the ET was lowest; this led to the highest WUE (Table 3). While the WUE was not affectedsignificantly by the irrigation method, the order of the WUE among these plants was as follows: S.
oryzifolium > S. kamtschaticum > C. dactylon.
As mentioned above, CAM plants consume water efficiently when the stress is given however, theplant biomass decreases. The reduction of biomass is unfortunately not appropriate for improving thethermal performance and landscaping. A combination of high WUE and high DM are thus required forthe high-quality green roof management. The quantification of this combination given as follows wasundertaken as a factor of vegetated condition (FVC):Ⓒ ISERD

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WUE u DMFVC

(3)The FVC in Table 3 shows the relative value against the control in percentage. While thedifference in the FVC between the FI and II conditions in each plant was insignificant (P <0.05), theFVC can be grouped depending significantly on the plant status. The FVC value was highest in S.
oryzifolium, followed by S. kamtschaticum and C. dactylon.
(a )

(b )

S. kamtschaticum PC2
1.5
C. dactylon
18

S. oryzifolium

18

PC2
1.2

18

1.0

ECi
0.8

12 0.5

12

PC1

0.0
- 1.5
6

- 0.5
6
-0.5

0.5

1.5

0

2.5

0
-0.4

-1.0

WUE

0.4

12

0.5

1

PC1

DM

-0.8

0.7
0.7

-1.5

0.7

-2.0

Fig. 3 Results of the principal component analysis under the frequent irrigation method. (a)Loadings: the enclosed lines indicate the group characterized the by underlying
component associated with the factor scores. Numerical values indicate the EC of theirrigation water. (b) Factor scores: ECi, DM and WUE indicate the EC of theirrigation water, dry matter yield and water use efficiency, respectively
Multivariate Analysis
The PCA resulted in two significant principal components (PC), PC1 and PC2, with eigenvalues > 0.8,
which explained approximately 98% of the total variance in the dataset. The PCs provide informationon the most meaningful parameters, which describe a whole dataset, affording a data reduction with aminimal loss of the original information. PC1 and PC2 therefore explain the most significant part ofthe variance within the variables (Pardini et al., 2003). Figs. 3 (a) and (b) show the loadings and factor
scores from the results of the PCA, respectively, under FI. The treatments for each plant tended to be
divided into same group, as shown by the enclosed lines in the figure. The factor scores indicate thatthe higher ECi, for example, decreased the DM but increased the WUE. In the loadings, group of S.
oryzifolium was distributed in the range of a high positive x-axis. From the factor scores, DM and
WUE were gathered in the positive x-axes. This indicates S. oryzifolium meets the requirement of ahigh DM with a high WUE. C. dactylon tended to be in the lower DM and WUE ranges, while S.
kamtschaticum was distributed between these two plants. These tendencies were also observed under II.
One of the different results in PCA between two irrigation methods was that the y-axis of ECi wascloser to the DM and WUE under II. This indicates the influence of the ECi on the DM and WUE
decreased, as compared with that under FI. This is because the drought stress was added under II,
which in turn decreased the influence of the salt stress expressed as ECi, relatively.

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Table 3 Total ET or E, DM, WUE and FVC of S. kamtschaticum, S. oryzifolium and C. dactylon in
the two different of irrigation of frequent (FI) and intermittent (II) with saline irrigation

ET or E (mm)S. kamtschaticum
S. oryzifolium
FI
II
FI
II
157.3 b91.8 a93.3 a93.2 a10.3 a9.0 a17.0 b
15.5 bDM (t ha-1)
WUE (kg m-3) 6.7 bc

9.9 c

FI
366.2 c10.4 aC. dactylon
II189.0 b8.2 a18.3 d

16.9 d

2.9 a

4.4 abFVC (%)
133.7 b151.3 b241.8 cLetters indicate significance level at 0.05

207.7 c62.7 a75.5 aBare soilFIII
184.2 b163.9 b

The higher WUE observed in the CAM plants, with a little reduction of the DM, highlights thebenefit of using them as green roof vegetation, in terms of low water management requirements and ahigh quality of landscape. This study did not account for the leaf fall that is characteristic of S.
oryzifolium during the winter season that may change the WUE and may damage the green roof
landscape. Therefore, a year-round study needs to be performed in future to evaluate the WUE during
the dormant season.
CONCLUSION
In this study, the performance of two CAM plant species, S. kamtschaticum and S. oryzifolium, were
evaluated under frequent and intermittent irrigation conditions with saline water. A prompt decrease in
ET with increasing ECs was found under S. kamtschaticum and S. oryzifolium compared to that of turf
grass, C. dactylon. As a consequence, the dry matter yield showed the following order: S. oryzifolium >S. kamtschaticum > C. dactylon. This is evidence that saline irrigation reduced the dry matter yield but
increased the WUE in CAM plants, resulting in a decrease in the amount of irrigation water required.
In other words, saline irrigation to CAM plants saves water. The paradox is that maximum plant
growth would imply maximum dry matter yield, which would contribute to carbon sequestration, butwould also require maximum irrigation water. However, the higher WUE seen in CAM plants with alittle reduction of DM makes them suitable for green roofs in terms of low water management
requirements with keeping high quality of landscape. This knowledge of CAM plants’ response tosaline irrigation, dry matter yield and WUE could help in selecting suitable plants and in designing
optimal saline irrigation systems and scheduling for green roofs.
ACKNOWLEDGEMENTS
This work was partly supported by JSPS KAKENHI Grant Number 26850150. The authors gratefully
acknowledge Mitsuhiro Inoue and M.C.Saxena for helpful comments for improving this study,
Toyohiro Fujita for providing the CAM plants used in this study, and Tomoki Shimizu for technicalsupport.
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