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KINETICS OF NUTRIENT UPTAKE BY ECONOMICAL VEGETABLE SPECIES GROWN IN CONSTRUCTED WETLANDS.

Byline: Haq Nawaz Abbasi, Jing Xie, Jan Vymazal and Xiwu Lu

ABSTRACT

Constructed wetlands are used to treat wastewater, however, their performance and usage can be improved by selecting a suitable substrate. Various plants may differ in their capacity to assimilate nutrients in a constructed wetland system. In this research, Michaelis-Menten kinetic (Michaelis-Menten constant (Km) and maximum uptake rate (Imax)) provides a useful tool for the identification of plant nutrient uptake efficiency and selection of plants. Six economical plant species (Phyllostachysheteroclada, Medicago hispidaGaertn, Ipomoea aquatica, Brassica junceavar, Colocasiaesculenta L. Schoot, and A. tubersomrottle ex spreng) were selected to investigate the uptake kinetics. I. aquatica showed a higher maximum uptake capacity for all three nutrients (PO4-P, NO3-N and NH4-N).

The second highest uptake capacity for PO4-P, NO3-N and NH4-N was found for A. tuberosum, B. juncea and M. hispida. I. aquatica has the highest Imax and lowest Km values for all three nutrients and it has good capabilities to adjust in high PO4-P, NO3-N and NH4-N nutrient condition. Imax and Km of P.heteroclada were relatively small and suitable for the low nutrients concentration environment, whereas the Imax and Km of A. tuberosum, M. hispidaand B. juncea were relatively large, which is suitable for treating wastewater with high nutrient concentration.

Keywords: Uptake kinetics; Michaelis-Menten kinetic; Michaelis-Menten constant (Km); Maximum uptake rate (Imax); Constructed wetland.

INTRODUCTION

Adequate sanitation and access to clean water become one of the biggest problems that affect human health, especially for developing nations (Bisung and Elliott, 2014). On the other hand, over-exploitation of freshwater resources to meet water demand is a common practice (Hoekstra, 2014; Schyns and Hoekstra, 2014). These approaches cause negative environmental effects and deterioration of the ecosystems (Lemaire et al., 2014; Peng et al., 2016; Wang et al., 2016a). The gap between the supply and demand for water and food is widening with the increase of world population, reaching alarming levels (Bogardi et al., 2012). Experts around the globe are seeking new means to increase food productivity as well as to conserve water (Cai et al., 2016). In this concern, recycling of wastewater and nutrients through constructed wetland systems involved the growth of economical vegetation is desired (Turcios and Papenbrock, 2014).

Constructed wetland systems are used around the world for treating different type of wastewater (Zhang et al., 2014; Wang et al., 2016b). Plants may affect the efficiency of wastewater treatment through nutrients uptake and by the influence on microbial activity (Li et al., 2013). The nutrients assimilating capacity of plants may differ according to plant species (Wu et al., 2015).

There is an important need to understand the efficiency of economical plants roots to acquire nutrients in constructed wetland system. It has been shown that the plant biomass is an important parameter affecting root zone processes and nutrient dynamics (Vymazal and Kropfelova, 2011).

Michaelis-Menten kinetic provides a useful tool for the identification of plant nutrient uptake efficiency, the elucidation of the mechanism of absorption, the identification, and screening of plants (Bucher, 2007). The kinetics of nutrients uptake have been reported in many plant species such as wheat rice (Youngdahl et al., 1982; Goyal and Huffaker, 1986), barley (Kronzuckeret al., 1999), eucalypts (Garnett et al., 2003), citrus (Cerezo et al., 2007) and dotted duck meat (Landoltiapunctata) (Fang et al., 2007a). However, there is insufficient literature on the pollutants absorption kinetics of constructed wetland plants, and especially economical vegetable plants. The study on the kinetics of nutrient uptake in constructed wetland system can provide scientific basis and support to understand the mechanism of nitrogen and phosphorus removal in water bodies and selection of vegetable type macrophytes with high economic value in the local market regarding daily consumption.

Nitrogen is one of the most important nutrients for plants and microbes exists in the inorganic forms of ammonium (NH4+), ammonia (NH3), nitrate (NO3-), and nitrite (NO2-) (Lambers et al., 2008). Ammonium and nitrate are the two forms, which are available for plant uptake and some plant species showing a strong preference for one ionic form over the other (Kronzuckeret al., 1997; Forde and Clarkson, 1999). Variation in nutrient removal between different plant species is likely to reflect differences in growth phases, the efficiency of nutrient uptake and use (Tanner, 1996; Gusewell and Bollens, 2003). Nitrogen removal mechanism is complicated and redox potential is one of the important parameter that play important role in the oxidation and reduction of nitrogen in constructed wetland (Saeed and Sun, 2012).

The objective of this research to identify the performance of economical vegetation for nutrient uptake in a constructed wetland.P.heteroclada, M. hispida, I. aquatica, B. juncea, C. esculenta, A. tuberosumwere selected on the basis of their economic value in the local market. This approach can make the application of constructed wetland broader as well as efficiently reuse of wastewater and nutrients in it.

MATERIALS AND METHODS

Plant materials: Experiment was conducted at Southeast University, Wuxi campus, China in a controlled environment (75-85% relative humidity, temperature 25 +- 1AdegC, 12-h photoperiod). Six plant species (Table 1) were selected from hybrid constructed wetland of Southeast University. After collection, the plants were rinsed with deionized water and transplanted into vessel contained solution culture system. N and P concentrations in the solution were similar to wastewater that is treated in the constructed wetland in the Wuxi campus of the Southeast University. The solution contained following macronutrients (mmol L-1): 1.25 (1: 1= NH4-N: NO3-N) and P = 0.32 (KH2PO4), K+ = 0.64, Ca2+ = 0.25, Mg2+ =0.21 and SO42-= 0.22, and micronutrients (mmol L-1): Zn2+ = 0.6, Cu2+ = 0.15, Fe2+ = 6.1, Mn2+ = 0.9, BO33-= 24 and MoO42-= 5.

The pH value was adjusted and maintained between 5.0 and 6.0 (most of the plants prefer a slightly acidic growing environment) and the nutrient solutions were replaced every 3 days to avoid substantial changes in pH as well as nutrients. After 2 weeks of growth, the uptake kinetics of PO4-P, NO3-N and NH4-N by plants were measured separately.

Uptake kinetics: A depletion method (Barber, 1995) and serial concentration method were used to measure nutrient uptake kinetics. Plants with uniform size were selected, rinsed with deionized water and placed into nutrient solution for 2 days to achieved starvation induces maximal uptake response. The concentration in nutrient solution was similar as described earlier except absence of N or P concentration. Same controlled environmental conditions were used.

After a starvation period, plants were shifted to 2 L beaker contained 0.8 L nutrient solution with concentration of total phosphorus 2.2 mg / L, nitrate nitrogen 20 mg / L and the ammonium nitrogen 15 mg / L. Each treatment had three replicates. Samples were taken at 0, 0.5, 1, 2, 3, 4, 6, 8 and 10 hours. Immediately after the end of the absorption test, the roots were separated from the rhizomes and rinsed with deionized water and dried to a constant weight at 70AdegC for 5 days in the oven and weighed.

Analytical method: PO4-P (phosphate), NO3-N (nitrate)NH4-N (ammonium), were analyzed according to standard methods (Federation and Association, 2005).

Calculations and statistical analyses: Ion consumption dynamic equation is a quadratic polynomial equation (Mao, 1995):

Y = ax2 + bx + c (1)

Where

Y = is the treated ion concentration; a = initial ion concentration; x = the absorption time; b = maximum rate in concentration change; c = quadratic equation constant For the derivative of Eq. 1, we can get the rate equation of concentration change:

By taking derivative from eq. (1)

Y' = 2ax + b (2)

For equation 2, let x = 0, then Y' = b is the maximum change in concentration, which can be the maximum absorption rate:

Imax= b x V / root dry weight (3)

where

Imax is the maximum absorption rate for ions.

V = the volume of solution added to the absorption test

Y' = 1 / 2b into equation 2 to find x, and the value of Y obtained from equation 1 is Km (the Michaelis-Menten constant, which is the concentration of ions in the solution at half of the maximum rate of the absorbed ions).

Initially, the data was recorded in MS-excel (Office package-16) and then SPSS version-18.0 (SPSS Incorporation Chicago, Illinois, USA) program was used to perform one-way analysis of variance (ANOVA), whereas Duncan Multiple Range test was used to find the mean difference among various treated groups.

RESULTS

Kinetics of PO4-P uptake: The kinetic of phosphorus uptake by the experimental plants are shown in Fig. 1. In all cases the concentration of PO4-P decreases gradually with increasing the time of exposure. The absorption of PO4-P by I. aquatica was the lowest while absorption by B. juncea was the highest. Table 2 shows significant differences in the kinetic characteristic of PO4-P absorption among plants. Imax range was between 3.08-79.73 ug PO4-P/g root DW*h and Km was between 0.1529-1.5948 mg/l for all plant species. Imax and Km patterns for PO4-P uptake were: I. aquatica>A. tuberosum>M. hispida> P.heteroclada> B. juncea>C. esculenta; and C. esculenta>M. hispida>A. tuberosum>B. juncea> P.heteroclada>I. aquatica, respectively. There was no significant difference in Km value between C. esculenta, M. hispidaandA. tuberosum (P> 0.05).

Kinetics of NO3-N uptake: Uptake of NO3 - N gradually decreased as contact time increased (Fig. 2) with B. junceashowing high absorption of nitrate compared to other plants. The values for Imax varied between 27.08 and 713.41 mg/ L while values for Km varied between 15.0501 and 16.8377 ug NO3 - N/(g root DW * h) (Table 2). Imaxfor NO3-N uptake decreased in the order of I. aquatica>A. tuberosum>B. juncea>P.heteroclada>M. hispida>C. esculenta, while Km decreased in the order of A. tuberosum>C. esculenta>P.heteroclada>M. hispida>B. juncea> I. aquatica, respectively (Table 2).

NH4-N kinetics: Fig 3 shows the absorption kinetics of NH4-N for various plants species. There is a great variation in NH4-N uptake among all studied plants. Uptake kinetics for NH4-N uptake, The Km varied between 9.33 and 13.14 mg / L and decreased in the order of I. aquatica>M. hispida>A. tuberosum>B. juncea>P.heteroclada>C. esculenta. Imax ranged between 59.03 and 853.57ug NH4-N / (g root DW * h) and decreased in the order of B. juncea>C. esculenta>M. hispida>A. tuberosum>P.heteroclada> I. aquatica.

Table 1. Selected plants used in the experiments.

###S. no###Common name###Scientific name###Used name

###1###Water bomboo###Phyllostachysheteroclada Oliver###P.heteroclada

###2###Chinese cabbage###Medicago hispidaGaertn###M. hispida

###3###Water spinach###Ipomoea aquatica###I. aquatica

###4###Chinese mustard###Brassica junceavar###B. juncea

###5###Taro###Colocasiaesculenta L. Schoot###C. esculenta

###6###Chinese Leek###A. tubersomrottle ex spreng###A. tuberosum

Table 2. Estimated values of Km and Imax forPO4-P, NO3-N and NH4-N uptake kinetics.

###Nutrients Kinetics

###Ion consumption curve###Km###Imax

No.###Plant Species###Nutrients

###equation###(g/g root DW*h;)###(mg*L-1)

###Mean +- sd**

###PO4-P###0.0066x2 - 0.1421x + 2.0327###1.45 +- 0.100a,b###32.48 +- 1.03b

1###P.heteroclada###NO3 - N###0.0587x2 - 1.0299x + 18.842###15.48 +- 2.00a,b

###262.47 +- 1.98b

###NH4 + -N###0.0719x2 - 1.525x + 16.415###10.31 +- 1.00a,b

###331.54 +- 1.95b

###PO4-P###0.0029x2 - 0.0916x + 2.1367###1.53 +- 0.30a,b###39.57 +- 0.95c

2###M. hispida###NO3 - N###0.0141x2 - 0.5103x + 18.835###15.32 +- 2.00a,b###216.00 +- 2.00c

###NH4 + -N###0.055 x2 - 1.1193x + 15.006###11.37 +- 1.00a,b###600.32 +- 5.03c

###PO4-P###0.001x2 - 0.1186x + 2.7903###0.15 +- 0.05aa

###79.73 +- 1.00f

3###I. aquatica###NO3 - N###0.0926x2 - 1.2663x + 18.297###15.05 +- 3.00a

###712.80 +- 2.55f

###NH4 + -N###0.0597 x2 - 1.3337x + 14.913###9.32 +- 0.32a

###853.52 +- 3.00f

###PO4-P###0.0004x2 - 0.0504x + 2.6505###1.41 +- 0.20a,b

###23.28 +- 3.00d

4###B. juncea###NO3 - N###0.0466x2 - 0.637x + 16.771###15.13 +- 1.00a,b

###366.20 +- 4.01d

###NH4 + -N###0.0552 x2 - 0.9131x + 16.139###13.14 +- 2.00a,b###496.31 +- 4.03d

###PO4-P###0.0022x2 - 0.08x + 2.1403###1.56 +- 0.15b###3.08 +- 0.02a

5###C. esculenta###NO3 - N###0.0177x2 - 0.4479x + 18.602###16.42 +- 2.00b

###27.02 +- 3.00a

###NH4 + -N###0.044 x2 - 1.0729x + 17.741###12.81 +- 4.00b

###59.01 +- 2.00a

###PO4-P###0.006x2 - 0.1423x + 2.1512###1.50 +- 0.20a,b

###73.67 +- 3.4e

6###A. tuberosum###NO3 - N###0.0503x2 - 0.7438x + 18.9###16.81 +- 1.00a,b

###394.02 +- 6.00e

###NH4 + -N###0.0311 x2 - 0.8867x + 15.983###11.21 +- 4.00a,b

###533.11 +- 3.00e

DISCUSSION

The results of nutrients uptake kinetics revealed differences between six economical plants in the uptake of PO4-P, NO3-N and NH4-N (Table 2). The preference of different plant species for particular nutrient source can have important ecological and practical implications (Forde and Clarkson, 1999). I. aquatica showed higher maximum uptake capacity for all three nutrients (PO4-P, NO3-O and NH4-N). The second highest uptake capacity for PO4-O, NO3-N and NH4-N was A. tuberosum, B. juncea and M. hispida, respectively. Previously, it has been reported by many researchers that NH4-N is the preference source of N for most wetland plants (Brix et al., 2002; Tylova-Munzarova et al., 2005; Fang et al., 2007b; Jampeetong and Brix, 2009).

Whereas C. esculenta showed minimum uptake capacity for all three nutrients. Fang et al. (Fanget al., 2007b) studied four wetland plants and observed that two plant species (B. monnieri and Azolla spp.) had preference for NO3-N, but L. repens required both nutrients for N. The absorption kinetics for nutrients by economical vegetables has not been reported, whereas many studies are reported on crop species (Rao et al., 1993; Tylova-Munzarova et al., 2005).

According to Zhenhua (Zhang et al., 2009), the differential preferences for nutrient forms between plant species may because of their relative tolerance and habitat preference to inundation. Habitat preference can also relate to other factors such as pH, aeration, temperature, water and salt stress, composition of nutrients, plants growth stage and symbiosis (Bradley and Morris, 1990; Brix, 1994; DYHR-JENSEN and Brix, 1996). These abiotic and biotic factors directly or indirectly may depend on design and operational conditions of wetland and wastewater characteristics, and nutrient uptake can vary from one wetland to another. (Ayaz and Akca, 2001; Coleman et al., 2001; Luederitz et al., 2001; Meuleman et al., 2003).

According to (Cacco et al., 1980), Vmax and Km are useful parameter to indexes the level of adaptation of the genotype in different nutrient conditions, and can be hypothesized; high Vmax and low Km, which represent the most favorable environment, fit in wide set of nutritional conditions; high Vmax and high Km, making the plants advantage at higher concentration of nutrients; low Vmax and low Km adapted to low nutrient condition; low Vmax and high Km, representing the most unsuitable situation of any nutrient conditions. On the proposed basis by using Imax and Km value to evaluate the adoptability of the plant species in different nutrients conditions. Imax and Km values reflect the nutrients affinity of plants; smaller the value has a strong affinity for a particular nutrient. I. aquatica has the highest Imax and lowest Km values for all three nutrients and it has good capabilities to adjust in high PO4-P, NO3-N and NH4-N nutrient condition.

Imax and Km of P.heteroclada was relatively small and suitable for low nutrients concentration environment, whereas the Imax and Km of A. tuberosum, M. hispida and B. juncea were relatively large, which is suitable for treating wastewater with high nutrient concentration.

Conclusion: The absorption kinetics of PO4-P, NO3-N and NH4-N in selected economical plants species were unlike. I. aquatica showed maximum uptake capacity for all three nutrients (PO4-P, NO3-O and NH4-N), whereas A. tuberosum, B. junceaandM. hispida were suitable for PO4-O, NO3-N and NH4-N, respectively. The absorption kinetics parameters, Imaxand Km are importantindicator for selection of economical plants for constructed wetland system. By using constructed wetland for economical plants can reduce the gap between supply and demand for water and food as well as for economic, environmental and social benefits. This approach can make the application of constructed wetland broader.

Acknowledgements: The authors are thankful to Ministry of Environment, People Republic of China for providing funding for this project. This work was financially supported by the "National 12th Five-Year Major Projects" grant number 2012ZX07101-005.

Author contributions: HN Abbasi, Jing Xie and Xiwu Lu conceived and designed the project, materials and analysis tools. HN Abbasi and Jing Xie performed the experimental works and data analysis whereas HN Abbasi and Jan Vymazal did interpretation of data and report writing, however, Xiwu Lu supervised them during all stages.

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