3.1. Screening potential plants for use in the CWs
3.1.1. Selection of potential plants based on their P content and biomass
Figure 3.1. The P percentage of studied plants
The P removal capacity of plants depends on not only the P percentage in the plant but also the plant biomass (Greenway, 1997). Thus, this study conducted a comparative study with 5 different plants on both their P content in plant and the biomass growth. The experiment was performed with mature plants grown in the P- rich soil in Chuong My district, Ha Noi City. In this study, P content was determined in the different parts of 5 plants. Based on that, the P percentage in the whole plant was calculated. The obtained results were shown in Figure 3.1 The P percentage in the whole plants of 5 different plants tudy was decreased in the following order Ubon paspalum (0.49%) > Piper lolot (0.39%) > Cymbopogon citratus (0.36%) = Colocasia gigantean (0.36%) > Sauropus androgynous (0.23%). The P content in the investigated plants in this study was in the range of 0.23-0.49%. The results obtained in this study agree well with the finding of Mcjannet et al. (1995) and Bodin (2013),
0 0.2 0.4 0.6
Sauropus androgynus
Cymbopogon citratus
Colocasia gigantean
Piper sarmentosum
Ubon paspalum
P percentage of plants (%)
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who reported that the P content in emergent macrophyte plants varied in the range of 0.13-1.07% or 0.05-0.5% dry weight, respectively. Specifically, the P content in 5 investigated plants (0.23-0.49%) were higher than that in Brophytes, Helophytes, Hydrophytes (0.1-0.3%) (Demars and Edwards, 2008). However, it was found to be equivalent to that in water hyacinth and water lettuce (0.3-0.5%) (Bhole, 2013; Lu et al., 2010). In constrast, it was found to be lower than that in such emergent plants as Eleocharis sphacelata, Baumea aniculata, Typha domingensis, and Cyperus involucratus from 1.9 to 4 times. Especially, in comparison with such floating plant as duckweek, Ceratophyllum, Water hyacinth, Nymphoides indica, Aquatica, Ludwigia peploides, Ceratopteris thalictroides, Marsilea, 5 plants in this study exhibited the significantly lower P content (3.3-7 times) (Greenway, 1997). The lower P content of the investigated plants compared with floating plants can be explained by the fact that the plants in this study were emergent plants, which were reported to be inferior to floating plants in P accumulation (Greenway, 1997). This remark was strongly supported by Green et al. (2003), who reported that the P content in floating plants was higher than that in emergent plants (0.2-0.4%). It was worth noting that two emergent plants, namely Phragmites australis and Typha domingensis, which are the most commonly used in CWs, had lower P content than Ubon paspalum and Cymbopogon citratus in this study.
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Table 3.1. The P content in plants use for phytoremediation or CWs
Plants
Parts of plants*
P content
(mg/g)
P content
(%) Reference
FLOATING PLANTS
Water lettuce W 3 0.3 Lu et al. (2010)
Aquatica W 9 0.9 Greenway (1997)
Ipomoea diamantinensis W 10 1 Greenway (1997)
Ludwigia peploides W 10 1 Greenway (1997)
Ceratopteris thalictroides W 10 1 Greenway (1997)
Marsilea W 10 1 Greenway (1997)
Monochoria cyanea W 13 1.3 Greenway (1997)
Ceratophyllum W 14 1.4 Greenway (1997)
Nymphoides indica W 16 1.6 Greenway (1997)
Duckweek W 18 1.8 Greenway (1997)
EMERGENT
MACROPHYTE PLANTS
Reaumuria soongorica L 0.73 0.073 He et al. (2015)
Brophytes W 1 0.1 Demars and
Edwards (2008)
Helophytes W 1.5 0.15 Demars and
Edwards (2008) Phragmites australis W 2 0.2 Greenway et al.
(2003) Typha domingensis W 2.3 0.23 Greenway et al.
(2003) Schoenoplectus validus W 2.6 0.26 Greenway et al.
(2003) Eleocharis sphacelata W 2.7 0.27 Greenway et al.
(2003) Eleocharis acuta W 3.4 0.34 Greenway et al.
(2003) Cymbopogon citratus W 3.59 0.359 THIS STUDY Baumea articulata W 3.7 0.37 Greenway et al.
(2003) Persicaria orientalis W 4.4 0.44 Greenway et al.
(2003)
Ubon paspalum W 4.9 0.49 THIS STUDY
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The P distribution in plants may affect the harvest strategy of plants from CWs.
The P distribution in five investigated plants was shown in Figure 3.1. It was found that all 5 investigated plants showed higher P content in the aboveground parts (leaf and stem). For Ubon paspalum, the P content in leaf and root was 4.4 and 3.0 mg/g.
For Cymbopogon citratus, the P content in leaf and root was 3.89 and 1.82 mg/g. This can be attributed to high content of P in soil. According to previous study, P tended to be concentrated in the root if the P centration in the soil was low. In other words, in the soil rich in P, higher P percentage could be found in the leaf. This feature favors the harvest strategy to collect aboveground part rather than whole plant. This is especially significant since the investigated plants are mainly perennial. This is in the contraty to the results obtained by Greenway et al. (2003) indicating that for many types of emergent plants, higher P content was detected in the root/rhizome. It is interesting that the list included Cuperus spp., Phragmites australis, and Typha sp., which were the common CWs plants. Thus, the search for CWs plants, which both have high P content and distribution in the aboveground part is necessary.
As mentioned above, the biomass growth is one of main factors influencing the P removal potential fo the plant. The P removal potential of the studied plants were estimated based on P content and biomass growth. The results were shown in Table 3.2 It can be seen that among 5 investigated plants, the Ubon paspalum demonstrated the highest P removal potential (24.65 kg/ha/year), which was followed by Cymbopogon citratus (21.80 kg/ha/year). The P removal potential of 3 remaining plants was 1.4-4.9 times lower than that of Ubon paspalum and Cymbopogon citratus.
This can be explained by the fact that these two plants possessed high P content in the whole plant and fast biomass growth. In comparison with Cymbopogon citratus, although Piper lolot had higher P content, P removal potential was lower because of to the lower biomass growth.
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Table 3.2. The P removal potential of the studied plants
Plant
P content in the whole plants (%
DW)
Biomass growth (ton FW/ha/year)
Biomass growth (ton DW/ha/year)
P removal potential (kg/ha/year)
Ubon paspalum 0.49 35 5.03 24.65
Cymbopogon citratus 0.36 27 6.05 21.80
Sauropus androgynus 0.23 28 7.39 17.00
Piper sarmentosum 0.39 17 3.00 11.69
Colocasia gigantean 0.36 18 1.40 5.04
3.1.2. Selection of potential CWs plants based on other growth characteristics In addition to P content and biomass growth, the applicability of plants in CWs depends on other factors, such as flood tolerance, root system, long lasting, economic values, etc. The information about growth characterists of five investigated plants were summarized in Table 3.3.
Concerning with the flood tolerance, all five investigated plants demonstrated the ability to grow well in the flooding conditions. This is also related to the root characteristics of the plants. Ubon paspalum, Cymbopogon citratus, and Colocasia gigantean were found to have deep and wide root systems, which were as high as 0.5-0.8 m. This feature favors the use of these plants in both horizontal and vertical CWs. In contrast, Piper lolot had the shallow and tiny root system.
Consequently, it was recommended to use this plant for vertical CWs with the flow from the top to the bottom of CWs. The Sauropus androgynous had the strong and long major root. However, the minor roots were not developed well, thus limiting the contact between the plant root and flows and plant uptake of contaminants. In addition, most of them are perennial plants. Thus, plant can be grown for only 1 time but be utilized in CWs for many years. As the result, the seed, labor costs and time can be saved. Moreover, the five investigated plants have economic values.
Therefore, the regular harvest of the aboveground parts of plants may receive a greater interest from farmers.
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Table 3.3. Growth characteristics of potential plants
Plant species Perennial plants
Harvest time
Tolerant to flood
Cropping season First
harvesting
Next harvesting Colocasia
gigantean 3 months 50 days All year round
Piper lolot 2 month 30 days All year round
Sauropus
androgynus 2 months 25-30 days All year round
Cymbopogon
citratus 3 months 45-50 days All year round
Ubon paspalum 1,5 months 21 - 30 days All year round
Cymbopogon citratus Piper lolot Sauropus androgynus
Colocasia gigantean Ubon paspalum
Figure 3.2. Images of the investigated plants in this study