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Effects of annual harvesting on plants growth and nutrients removal in surface-flow constructed wetlands in northwestern China
Ecological Engineering 83 (2015) 268–275
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Effects of annual harvesting on plants growth and nutrients removal in surface-flow constructed wetlands in northwestern China Yucong Zhenga , Xiaochang C. Wanga,* , Yuan Gea , Mawuli Dzakpasua,b , Yaqian Zhaob , Jiaqing Xionga a Key Laboratory of Northwest Water Resource, Environment and Ecology, MOE, School of Environmental and Municipal Engineering, Xi’an University of Architecture and Technology, Xi'an 710055, China b UCD Dooge Centre for Water Resources Research, School of Civil, Structural and Environmental Engineering, Newstead Building, University College Dublin, Belfield, Dublin 4, Ireland
A R T I C L E I N F O
A B S T R A C T
Article history: Received 5 January 2015 Received in revised form 12 June 2015 Accepted 28 June 2015 Available online xxx
The importance of harvest management for the growth and development of plants and nutrients removal in constructed wetlands (CWs) is highly controversial. This study evaluated the effects of annual harvesting on the growth and productivity of local Phragmites australis in northwestern China. Growth characteristics such as shoot density, biomass and height were studied using two pilot-scale surface-flow CWs over a two-year operation period. Plants were kept unharvested in one CW for comparative studies with the second CW, which was harvested at the end of the growing season. Each CW of 400 m2 was operated with a hydraulic loading of 34 m3/d for the treatment of water from an urban river polluted with municipal and industrial wastewater. The harvested CW recorded a higher shoot density (175 shoots/m2), biomass (1.4 kg/m2) and peak height (3.4 m) than the unharvested one (130 shoots/m2, 1.2 kg/m2 and 3.2 m). The overall nutrients removals were also slightly higher for the harvested CW (46.0% TN and 38.1% TP) than the unharvested CW (40.6% TN and 29.1% TP). Plants harvesting in the first year improved nutrients removal by plant uptake (41.9 g N/m2 and 3.7 g P/m2 versus 37.3 g N/m2 and 3.2 g P/m2) as well as in the substrate layer (216.9 g N/m2 and 8.0 g P/m2 versus 191.0 g N/m2 and 5.7 g P/m2) during the second year. Nonetheless, the increase in nutrients removal by harvesting was minimal. ã 2015 Published by Elsevier B.V.
Keywords: Plant harvest Biomass Nutrients uptake Constructed wetland Polluted river
1. Introduction Eutrophication of lakes and rivers, which is caused by excessive discharge of nutrients, particularly nitrogen (N) and phosphorus (P) into surface water (Wu et al., 2011), is currently the most widespread environmental water quality problem across the world. Consequently, several appropriate measures are being sought to lower the impacts of nutrient pollution from point sources (Park et al., 1998) and via a variety of pathways (Braskerud, 2002). The use of constructed wetlands (CWs) directly in situ may provide a viable option for improving the water quality of surface waters (Wang et al., 2012). CWs have been widely used to treat various types of wastewater for several decades, due to their simple operation and low implementation costs (Vymazal, 2007). The presence of macrophytes in free water surface flow (FWS) CWs is one of the most noticeable and significant features for distinguishing CWs from other lagoons (Vymazal, 2013). The
* Corresponding author. Tel.: +86 29 82205841; Fax: +86 29 82205841. E-mail address: [email protected] (X.C. Wang). http://dx.doi.org/10.1016/j.ecoleng.2015.06.035 0925-8574/ ã 2015 Published by Elsevier B.V.
macrophytes growing in FWS CWs have several functions in relation to the treatment process such as provision of substrates for the growth of attached bacteria, release of oxygen and exudates, uptake of nutrients, surface insulation and wind velocity reduction (Vymazal, 2013). A large body of evidence indicates that CWs with plants are more efficient compared with unplanted CWs (Borin and Salvato, 2012; Elsaesser et al., 2011; Ibekwe et al., 2007). Nevertheless, the effects of different plant species on CW performance vary considerably (Brisson and Chazarenc, 2009; Iamchaturapatr et al., 2007). On the other hand, the growth and functions of plants can be impacted by several factors such as the climate condition, temperature, plant species and pollutant loadings (Liu et al., 2012). Due to the importance of plants in CWs, several management strategies have been proposed to improve the plant growth and productivity and the performance of CWs. These proposed strategies include annual harvesting (Toet et al., 2005; Kadlec and Wallace, 2009) and multiple harvesting (Jinadasa et al., 2008; Vymazal et al., 2010). However, the real benefit of plants harvesting for the removal of nutrients or the development of plants biomass remains a highly controversial issue. On one hand, several studies have demonstrated
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that the nutrients uptake by plants can be definitively removed from the wetland by harvesting the aboveground parts of the plants before senescence and decay during the cold season (Vymazal et al., 2010). In addition, it has been demonstrated that harvesting can open up dense vegetated areas to promote the photosynthetic periphyton in the system (Wetzel, 2001). According to Wetzel (2000), both mature and standing dead (withered) plants shade the attached microbial communities, thus, reducing the nutrient retentive capacities of those communities, and also, contribute to short-circuiting of the water flow (Groeneveld and French, 1995). On the other hand, arguments against plant harvesting indicate that the regular harvest of CWs is impractical, does little to improve water treatment, and reduces the readily available carbon source necessary for denitrification (Kadlec and Wallace, 2009). Additionally, some previous studies have pointed out that the withered plants within CWs may have a thermoregulatory effect in winter (Kadlec and Wallace, 2009; Smith et al., 1997). However, studies on this thermoregulatory function, and the competition between the regenerated plants and the withered plants left in the wetland are scarce. Thus, the regrowth of plants can exhibit marked differences in different plants management practices. Therefore, for the purpose of maximizing the contribution of plants to nutrients removal, an appropriate method for managing the plants needs to be adopted, and require further research. According to Batty and Younger (2004), nutrients uptake and growth rates are higher in young vegetation stands. More particularly, it is important to determine the management strategy that does not adversely affect the regeneration of plants or their capacity to uptake nutrients. Besides, research on the effect of plant management on the growth and development of plants and nutrients removal in large-scale CWs under the climatic conditions of northwestern China are rare, especially during the startup period of the CW operation. Furthermore, the uncertainty of the performance of FWS CWs with the presence or absence of the withered plants over the winter period, and their thermoregulatory effects, in northwestern China, where temperature varies widely, requires research attention. In this study, the growth characteristics of local Phragmites australis in northwest China were evaluated under an
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annual harvest management scheme. The specific objectives were to: (1) assess the growth characteristics and nutrients accumulation in the local P. australis under the condition of annual harvesting; (2) evaluate the effects of plants harvesting on the overall performance of the wetland; and (3) analyze the impact of standing withered plants on the performance of FWS CW for treating polluted river water during the cold season. 2. Materials and methods 2.1. Description of the pilot wetlands The pilot-scale CW system was constructed on the eastern bank near the confluence of an urban river to the Weihe River (the largest tributary of the Yellow River) in Xi’an, northwestern China (34 220 5400 N, 108 5100500 E). The area has a sub-humid continental monsoon climate, which is cold and lacks rainfall during the winter. The average monthly temperature reaches a maximum of 26.3 C in July and a minimum of 1.3 C in January, with an average annual precipitation of 750 mm. The two FWS CWs used for this study were the second stage of a pilot-scale hybrid CW system, the design of which was described by Zheng et al. (2014). The two CWs were identical (40 m length, 10 m width, 0.6 m height), and were filled with sand to a depth of 0.35 m. The water depth was controlled at 0.4 m. After passing through the first stage of treatment (a subsurface flow CW) the highly polluted river water flowed into the FWS CWs continuously. At this stage, the river water contained low concentrations of organic pollutants, but the concentrations of nutrients (N and P) were still high. The inflow rate to each CW was 34 m3/d, on average, which corresponds to an average HRT of 1.8 d and a surface loading of 0.085 m/d. 2.2. Plants harvesting scheme Young P. australis (common reeds) were obtained from the field near the riverbank. Plants of similar size (20–30 cm in height) were selected and washed with tap water in order to remove soils and dead tissues from their roots. They were then planted in the CWs at
Fig. 1. Overview of the free water surface flow constructed wetlands with different plant harvesting schemes (a) before harvesting, (b) harvesting system, (c) regrowth after harvesting, (d) harvested plants.
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a density of 9 shoots per m2 and a roughly equal depth in September. In the first two weeks after planting, river water was intermittently fed into the wetland bed for the plants to acclimate. After this acclimation period, river water was fed into the CWs at the prescribed flow rate. Plants harvesting were carried out in November, for the two successive years, when the plants began to wither. For the purpose of the discussion in this article, the period from November to October of the next year was chosen as one year. At the end of the first year, the aboveground parts of the plants in one CW (CW1) were harvested by cutting their upper parts at about 20 cm above the CW bed level (Fig. 1b). The standing biomass production and nutrients concentration in the plants tissues were determined from the harvested plant parts. On the other hand, the plants in the second wetland (CW2) were not harvested (Fig. 1b); they were left in the wetland. However, samples within three randomly selected quadrats of 0.25 m2 were collected for analyses. The purpose here was to understand how the annual plants harvesting might affect the growth characteristics and nutrients removal by the CWs in the second year (from November to October). At the end of the second year, harvesting was done, in the same way, as in the first year. 2.3. Plant sampling and analysis During the experimental period, the plant heights in the two CWs were measured monthly in three randomly selected quadrats of 0.25 m2. The number of shoots in the two CWs were counted before the harvesting in November. The selected harvested plants were separated into stems, leaves and flowers and washed with distilled water to remove the adhering water and sediments. Plant parts were then oven-dried at 80 C to a constant weight, and their dry biomass were determined. All dried plant samples were ground separately to pass through a 0.25 mm mesh screen, digested and analyzed for total N and P according to the routine analysis method for soil agro-chemistry (Bao, 2000). The average nutrients concentration in the aboveground biomass was calculated as follows:
Ntotal ¼ ðDMleaves Cleaves Þ þ ðDMstems Cstems Þ þ ðDMflowers Cflowers Þ where DM values represent the total biomass of leaves, stems and flowers, and C represent the average concentrations of N and P in these respective plant parts. N values represent the amount of nutrients uptake by the aboveground biomass of plants. 2.4. Water sampling and analysis During the two-year experimental period, water samples were collected weekly from the influent and effluent of the two CWs. All of the water samples were transported to the laboratory for chemical analyses within 24 h. The parameters measured include total nitrogen (TN), ammonia-nitrogen (NH3–N), nitrate (NO3), nitrite (NO2), total phosphorus (TP) and orthophosphate (PO43). Standard methods (MEPC and WWMAA, 2002) were followed for all the chemical analyses. TN was measured with alkaline potassium persulfate oxidation-ultraviolet spectrophotometry. The Nessler reagent colorimetric method was used for NH3–N analysis. NO3–N was determined by the phenol disulfonic acid ultraviolet spectrophotometric method. NO2–N was determined by N-(1-naphthyl)-ethylenediamine colorimetric method. TP was measured by the potassium persulfate oxidation-ultraviolet spectrophotometry method. PO43–P was determined by the Mo–Sb–Vc spectrophotometric method. Water temperature and dissolved oxygen (DO) were measured on site by using a portable meter (HQ30d53LEDTM, HACH, USA). The removal efficiencies for each wetland were calculated from the difference in concentration between the influent and effluent of the CWs. Significant differences were determined at the a = 0.05 by paired samples t-tests and analysis of variance (ANOVA). 3. Results and discussion 3.1. Plants growth characteristics 3.1.1. Temperature and plants growth
As shown in Fig. 2, during the experimental period, the average ðDMleaves Cleaves Þ þ ðDMstems Cstems Þ þ ðDMflowers Cflowers Þ Ctotal ¼ air temperature in the first year (17.8 C) was slightly lower than ðDMleaves þ DMstems þ DMflowers Þ that in the second year (18.6 C). However, the water temperatures where DM = dry matter of a particular shoot part (g), C = concenof the two CWs were similar. Furthermore, plants in both CWs tration of nutrients in the respective plant parts (% DM). grew well, whereby the growth rates were higher than that The amount of nutrients uptake by the aboveground biomass reported in previous studies (Wu et al., 2011). The heights of plants was calculated according to the following equation: in the two CWs were similar in the first year (Fig. 3a). In the second
Fig. 2. Variations of average air temperature and the water temperature of the two wetlands throughout the experimental period. (CW1 harvested and CW2 unharvested in winter of the 1st year).
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required for the growth of new plants. However, by comparison, the heights of the P. australis in the two CWs were higher than that reported for other CWs in northern climates (Liu et al., 2012). The difference in height was probably due to the field conditions, scale of the CWs and the pollutants loadings. The two wetlands of this study had larger open areas and higher nutrient loadings, which were beneficial for plants growth (Jampeetong and Brix, 2009; Konnerup and Brix, 2010). In addition, the plants in the two CWs recorded their highest growth rates in spring (CW1) and early summer (CW2) (about 1.9 cm/d). They reached their maximum heights of more than 3 m at the beginning of the autumn. The plants bloomed at the end of summer, and a slower growth rate was recorded (about 0.6 cm/d). The growth ceased, and plants withered with the decrease in temperature over the winter period of November through February.
Fig. 3. Peak height of plants (a), average plant density (b) and total mean aboveground biomass (c) for the two CWs at the end of the growing season during the experimental period.
year, however, the heights of the plants in CW1 were slightly higher (0.22 m; Fig. 3a). This could be explained by the fact that the withered plants in CW2, which were not harvested, were not lodging completely. They stood in the initial growing position (Fig. 1c), which blocked the sunlight for sprouting plants in the second year. The fallen withered plants also squeezed the space
3.1.2. Variation of plant density and aerial biomass The average densities of plants in CW1 and CW2 were 163 shoots/m2 and 155 shoots/m2 in the first year, and 175 shoots/m2 and 130 shoots/m2 in the second year, respectively (Fig. 3b). Thus, the densities of the plants in both CWs increased exponentially, at up to 18 and 17 times higher than their initial value (9 shoots/m2) by the end of the first year. These values were found to be higher than those reported in the literature (Liang et al., 2011; Wu et al., 2011). The results indicated that whether the plants were harvested or not, an increase in the density of plants mainly occurred during the first year when there was more space available for the plants growth in the CWs. Furthermore, it appears that the plants growth in the wetlands were not as rapid in the second year as they were in the first year. The higher plant densities in the first year resulted in little space available for new plants to grow in the second year. However, the density of the plants in CW1 was slight increased (by 7.4%) while that of CW2 was decreased in the second year (by 16.1%). The explanation for this phenomenon may be that the shading by the standing withered plants, which remained in the wetland inhibited their germination rates and growth density (Chun and Choi, 2009). Perhaps the open space ratio of the wetland in the second year could be a limiting factor for the plants growth in these wetlands.
Fig. 4. Average nutrient content (a, b) and the nutrient uptake rate by the aboveground biomass of P. australis (c, d) for the two CWs at the end of the growing season during the experimental period.
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As the heights of plants in the two CWs were the same at the initial planting, the amount of plants biomass at harvesting can be regarded as the net increase. In the first year, the aboveground biomass of plants in both of the wetlands increased moderately. The dry weights of plants in CW1 and CW2 were 1.31 kg/m2 and 1.30 kg/m2 in the first year, and 1.44 kg/m2 and 1.20 kg/m2 in the second year, respectively (Fig. 3c). The aboveground biomass was slightly higher compared to what is reported in the literatures (Liang et al., 2011; Wu et al., 2011). Nonetheless, Borin and Salvato (2012) reported that the aerial biomass production was double in the second year when the wetlands were fed with higher concentrations of nitrogen. Similar to the density of plants, in the second year, the plants biomass in CW1 was increased (9.9%) while that of CW2 decreased (7.7%). However, the average weight of plants in CW2 (0.92 g/plant) was higher than that of CW1 (0.82 g/plant). This implies that the withered plants left in the wetland stunted the height and the biomass of the new plants, but promoted the new plants in the wetland to be stocky. The results also indicated that harvesting the withered plants from the wetland at the end of the growing period could accelerate the growth characteristics of plants such height and biomass in the next growing season. Moreover, a phenomenon was observed whereby the aerial biomass in the inflow zone was higher than that in the outflow zone in both wetlands during the experimental period, due to the higher nutrient supply in that inflow zone. 3.1.3. Nutrients contents in plant tissues and plant uptake rate The concentrations of nutrients in the aboveground tissues of plants in the two wetlands were similar in the first year and were both increased slightly in the second year (Fig. 4a,b). This may be because the plants were matured and thus, had a higher ability to uptake nutrients in the second year (Borin and Salvato, 2012). Nevertheless, the nitrogen concentration in the plants tissues in CW1 was lower than that in CW2 in the second year (29.13 mg/g, 31.06 mg/g). The opposite wad found for phosphorus (2.68 mg/g, 2.60 mg/g). As shown in Fig. 4c, the nitrogen uptake by the aboveground parts of plants in CW1 and CW2 were 35.2 g/m2 and 35.9 g/m2 in the first year, and 41.9 g/m2 and 37.3 g/m2 in the second year, respectively. The nitrogen uptake by plants in CW1 was 1.13 times higher than that of CW2 in the second year. Fig. 4d shows that the phosphorus uptake by the aboveground parts of plants in CW1 and CW2 were 3.1 g/m2 and 3.0 g/m2 in first year, and 3.7 g/m2 and 3.2 g/m2 in second year, respectively. The phosphorous removed by plants in CW1 was 1.16 times higher than that of CW2 in the second year, while the nutrients uptake by the plants in the two wetlands were similar in the first year. The above findings indicated a considerable increase of nutrients uptake by plants in the second year in the harvested wetland, while only a slight increase was observed in the unharvested wetland. The amount of TN and TP uptake by plants in this study were in the range of values reported in other studies (Stefanakis and Tsihrintzis, 2012; Zhao et al., 2012).
Overall, plants harvesting had minimal effects on the nutrients concentration in plant tissues, but significantly affected the biomass production. As the nutrients uptake by plants relies on biomass more than concentration of nutrients in the plant tissues, plant harvesting can be considered to affect the efficiency of the plants considerably. 3.2. Overall nutrients removal 3.2.1. Comparison of harvesting schemes During the experimental period, the average influent TN and TP concentrations in the first year (24.30 1.03 mg/L and 1.24 0.09 mg/L) were higher than those of the second year (18.98 1.04 mg/L and 1.04 0.08 mg/L). NH3–N and PO43–P were recorded as the primary constituents of the TN and TP, which accounted for 80.6% and 60.5% in the first year and 75.7% and 63.5% in the second year, respectively. Many species of wetland plants prefer NH3–N and PO43–P as the nutrients source (Jampeetong and Brix, 2009; Konnerup and Brix, 2010), which benefited the removal of these nutrients from the river water in the wetlands. The mean influent and effluent nutrients concentrations in the two wetlands for the two-year study period are presented in Table 1. In the first year, the operation parameters of the two wetlands were exactly the same. The removal rates of TN and NH3–N for both wetlands were approximately 0.61 g/m2/d and 0.65 g/m2/d. During the experimental period, the removals of TN in both wetlands showed strong seasonal variation. NO3–N accumulation mainly occurred during the winter, which indicated a limited denitrification potential during this period while nitrification proceeded. This finding may be related to the DO concentration and temperature. Higher DO concentration and lower temperature, which were >4 mg/L and <3 C, respectively, in both wetlands, have been noted to inhibit the denitrification process and consequently, decrease nitrogen removal in FWS wetlands (Peng et al., 2014). The removals of organic nitrogen in the wetlands were limited. This may be caused by seasonal internal releases (Thorén et al., 2004). The removal rates for TP in the two of wetlands were about 0.02 g/m2/d, which confirmed the limited phosphorus adsorption capacity of the FWS CWs. The main physicochemical processes for phosphorus removal in CWs such as the fixation of phosphate by iron and aluminum in the substrate (Arias et al., 2001) are known to be limited in FWS wetland. In the second year, both wetlands showed higher nutrients removal than the first year. In the second year, the removal efficiencies for TN and TP in CW1 increased by about 15%, compared that in the first year, and that for NH3–N was increased by about 30%. There was a further increase of NO3–N accumulation in the wetland during this period. These results indicated that the wetland provided a better aerobic condition for nitrification in the second year. The reason for this is probably that the increased plant density promoted the release of oxygen and root exudates.
Table 1 Influent and effluent nutrients concentrations for the two CWs during the experimental period. Values are mean S.E., n = 81. Parameter
TN NH3–N NO3–N NO2–N Org–N TP PO4–P
First year
Second year
Influent (mg/L)
CW1 (harvested) (mg/L)
CW2 (unharvested) (mg/L)
Influent (mg/L)
CW1 (mg/L)
CW2 (mg/L)
24.301.03 19.580.92 0.490.04 0.100.01 4.130.68 1.240.09 0.750.08
17.071.13 11.901.22 1.210.25 0.320.06 3.630.56 0.940.07 0.630.09
17.051.18 12.171.24 1.390.28 0.310.04 3.190.50 0.930.08 0.650.09
18.981.04 14.370.81 0.900.14 0.130.02 3.590.57 1.040.08 0.660.06
10.260.94 4.340.67 3.230.48 0.150.03 2.530.36 0.640.08 0.400.05
11.291.16 5.240.96 3.040.42 0.160.04 2.840.40 0.730.08 0.460.06
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Consequently, the microbial diversity and other environmental conditions within the rhizosphere were increased (Jenssen et al., 1993). Nevertheless, due to the different plant harvesting schemes, the performance of the two wetlands showed a slight difference. The removal efficiencies of TN and TP in CW1 were higher than those of CW2 by 5.4% and 8.9%, respectively. However, these differences were not statistically significant (P > 0.05). This difference was because the wetland with plants harvested in the autumn provided better space and light conditions for plants regeneration than the unharvested wetland. Also the standing withered plants competed for space and light resources with the new plants. Relative to N removal, the P removal in CW1 was significantly higher than that in CW2 (P < 0.05), which indicated that the plants harvesting had a greater effect on the removal of phosphorus than nitrogen. However, this difference may relate to the different mechanisms of nitrogen and phosphorus removal in wetlands. The nitrogen removal in FWS CWs is mainly achieved through microbial processes while the primary mechanisms for phosphorus removal are plants uptake and physicochemical processes (Vymazal, 2007). Thus, plant uptake can be more important for phosphorus removal than nitrogen removal in the FWS wetlands. This phenomenon will be further discussed later in this article. 3.2.2. Nutrients removal in the winter The nutrients removal efficiencies of the two wetlands were analyzed from when plants were harvested at the end of the first year to their regrowth in the next spring (December–February). This was to aid the evaluation of the effect of plant harvesting on nutrients removal and the thermoregulatory functions of withered plants in the winter. The average temperature in CW1 was slightly lower than that of CW2 during this period (2.12 and 2.23 C, respectively). Fig. 5 shows that the harvested wetland recorded higher TN, NH3–N, Org–N, TP, PO43–P removal than the unharvested wetland. This was so, probably because the unharvested plants started to die-off and no longer assimilated nutrients (Wu et al., 2011). Moreover, the release of nutrients from the withered plants left in the wetland aggrandized this difference (Cui et al., 2009). It should also, be noted that the unharvested wetland had higher NO3–N removal. This could be explained by the fact that the decaying plant parts provided denitrification bacteria with a carbon source. Nonetheless, the oxygen released from the roots of the plants in the harvested wetland was higher than the unharvested wetland.
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3.3. The significance of plants harvesting for nutrients removal As plants play important direct and indirect roles in nutrients removal in wetlands, it is important to consider the effects of plants harvesting strategies on both of these two roles. Table 2 shows the nutrients mass balance components for each wetland. In the first year, there was no apparent difference between the two wetlands with regard to the amount of nutrients removed and the proportion attributable to plants. In the second year, the amounts of TN and TP removed by the wetlands were 258.84 g N/m2 and 11.70 g P/m2 in CW1, and 228.31 g N/m2 and 8.96 g P/m2 in CW2, respectively (Table 2). The amounts of TN and TP uptake by the plants were 41.94 g N/m2 and 3.74 g P/m2 in CW1, and 37.27 g N/m2 and 3.22 g P/m2 in CW2, respectively (Table 2). Furthermore, the amounts of TN and TP removed by the wetland beds were 216.89 g N/m2 and 7.95 g P/m2 in CW1, and 191.04 g N/m2 and 5.74 g P/m2 in CW2, respectively (Table 2). This possibly occurred because as the aboveground biomass of the plants increased, the belowground biomass was also, increased (Borin and Salvato, 2012) and further stimulated microbial activities in the wetlands (Korboulewsky et al., 2012). This finding indicates that harvesting at the end of the growing season does not only promote the new plants’ uptake of more nutrients. It also increases the belowground biomass and stimulates the microbial activities in the wetlands, both of which enhances the roles of the substrate layer. It should also, be noted that the plants nutrients uptake accounted for a greater proportion of the phosphorus removal (32.02–36.71%) than the nitrogen (16.20–17.04%). The difference in the proportion of nitrogen and phosphorus removal by plants uptake is probably due to the main mechanisms for the removal of these parameters. Nitrogen removal in FWS CWs is achieved through microbial processes while the primary mechanisms for phosphorus removal are plants uptake and physicochemical processes. A similar finding was reported by Greenway and Woolley (2001). However, Wu et al. (2011) reported the opposite. Several studies have suggested that the overall nutrients removal would be higher if a multiple or earlier harvesting scheme is adopted (Batty and Younger, 2004; Jinadasa et al., 2008 and Vymazal et al., 2010). However, multiple or earlier harvesting would be detrimental to the plants because they would not have sufficient opportunity to withdraw nutrients and nonstructural carbohydrates from the shoots to belowground plant parts (van der Linden, 1980, 1986). Besides, frequent aboveground harvesting can slow growth and biomass development (Álvarez and Bécares, 2008). Therefore, the annual harvesting of the plants should be based on the consideration of the economic, climatic and wetland
Fig. 5. Average influent and effluent nutrients concentrations of the two CWs in winter under different harvesting schemes.
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Table 2 The nutrient mass balance and proportion of plant uptake in the two CW at the end of the growing seasons during the experimental period. Period
Parameter
Influent (g/m2)
CW1 (harvested)
CW2 (unharvested)
Effluent (g/m2)
Phragmites (aboveground) (g/m2)
Othersa (g/m2)
Plant uptake (%)
Effluent (g/m2)
Phragmites (aboveground) (g/m2)
Othersa (g/m2)
Plant uptake (%)
First year
TN TP
706.48 35.91
496.18 27.43
35.22 3.11
175.08 5.37
16.75 36.71
495.66 26.90
35.92 3.08
174.91 5.93
17.04 34.19
Second year
TN TP
563.10 30.73
304.26 19.03
41.94 3.74
216.89 7.95
16.20 32.02
334.79 21.77
37.27 3.22
191.04 5.74
16.32 35.93
a
Others involve microbial assimilation and physical sorption and plants roots.
operation conditions. Based on the finding from this study, longterm nutrient removal by annual harvesting of shoots in the P. autralia stands at the beginning of November can beconsidered as a better alternative plant management approach in northwestern China. 3.4. Operation and maintenance costs The annualized cost for operation and maintenance (O&M) was about RMB 3314.96 per each wetland. Major components of O&M included personnel (RMB 1858.07), electric costs associated with pumping water through the system (RMB 1203.52), along with some miscellaneous expenditure (RMB 253.37). We estimate the cost of plant harvesting at 0.75 RMB/m2. The difference in the management of the two wetlands was the plant harvesting. Therefore, the O&M costs of the two wetlands were exactly the same except that the CW1 required an additional 300 RMB/year for plant harvesting. 4. Conclusions Both harvested and unharvested FWS wetlands recorded an increase in nutrients removal efficiencies in the second year, compared to the first year of the wetlands operation. Annual harvesting provided better space and light conditions for plants regeneration. Shoot density (175 shoots/m2), biomass (1.44 kg/m2) and peak height (3.4 m) of P. australis were higher in the harvested CW than the unharvested one (130 shoots/m2, 1.2 kg/m2 and 3.1 m). Overall, plants uptake accounted for a greater proportion of the phosphorus removal (32.0–36.7%) than nitrogen (16.2–17.0%). Plants harvesting at the end of the first year did not only improve nutrients removal by plant uptake in the second year (41.9 g N/m2 and 3.7 g P/m2 versus 37.3 g N/m2 and 3.2 g P/m2) but also, the nutrients removal in the substrate layer (217 g N/m2 and 8.0 g P/m2 versus 191 g N/m2 and 5.7 g P/m2). Consequently, higher TN and TP mass removal rates of 258.8 g N/m2 and 11.7 g P/m2 were recorded in the harvested wetland than the unharvested wetland (228.3 g N/ m2 and 8.9 g P/m2). Nonetheless, the overall increase in nutrients removal by harvesting was minimal and required an additional expense. Thus, annual harvesting of emergent vegetation may not have significantly beneficial effects on plants development and nutrient removal under the climatic conditions of northwest China. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 50838005), National Major Project of China (2013ZX07310-001) and the Program for Innovative Research Team in Shaanxi (No.2013KCT-13).
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Effects of annual harvesting on plants growth and nutrients removal in surface-flow constructed wetlands in northwestern China Yucong Zhenga , Xiaochang C. Wanga,* , Yuan Gea , Mawuli Dzakpasua,b , Yaqian Zhaob , Jiaqing Xionga a Key Laboratory of Northwest Water Resource, Environment and Ecology, MOE, School of Environmental and Municipal Engineering, Xi’an University of Architecture and Technology, Xi'an 710055, China b UCD Dooge Centre for Water Resources Research, School of Civil, Structural and Environmental Engineering, Newstead Building, University College Dublin, Belfield, Dublin 4, Ireland
A R T I C L E I N F O
A B S T R A C T
Article history: Received 5 January 2015 Received in revised form 12 June 2015 Accepted 28 June 2015 Available online xxx
The importance of harvest management for the growth and development of plants and nutrients removal in constructed wetlands (CWs) is highly controversial. This study evaluated the effects of annual harvesting on the growth and productivity of local Phragmites australis in northwestern China. Growth characteristics such as shoot density, biomass and height were studied using two pilot-scale surface-flow CWs over a two-year operation period. Plants were kept unharvested in one CW for comparative studies with the second CW, which was harvested at the end of the growing season. Each CW of 400 m2 was operated with a hydraulic loading of 34 m3/d for the treatment of water from an urban river polluted with municipal and industrial wastewater. The harvested CW recorded a higher shoot density (175 shoots/m2), biomass (1.4 kg/m2) and peak height (3.4 m) than the unharvested one (130 shoots/m2, 1.2 kg/m2 and 3.2 m). The overall nutrients removals were also slightly higher for the harvested CW (46.0% TN and 38.1% TP) than the unharvested CW (40.6% TN and 29.1% TP). Plants harvesting in the first year improved nutrients removal by plant uptake (41.9 g N/m2 and 3.7 g P/m2 versus 37.3 g N/m2 and 3.2 g P/m2) as well as in the substrate layer (216.9 g N/m2 and 8.0 g P/m2 versus 191.0 g N/m2 and 5.7 g P/m2) during the second year. Nonetheless, the increase in nutrients removal by harvesting was minimal. ã 2015 Published by Elsevier B.V.
Keywords: Plant harvest Biomass Nutrients uptake Constructed wetland Polluted river
1. Introduction Eutrophication of lakes and rivers, which is caused by excessive discharge of nutrients, particularly nitrogen (N) and phosphorus (P) into surface water (Wu et al., 2011), is currently the most widespread environmental water quality problem across the world. Consequently, several appropriate measures are being sought to lower the impacts of nutrient pollution from point sources (Park et al., 1998) and via a variety of pathways (Braskerud, 2002). The use of constructed wetlands (CWs) directly in situ may provide a viable option for improving the water quality of surface waters (Wang et al., 2012). CWs have been widely used to treat various types of wastewater for several decades, due to their simple operation and low implementation costs (Vymazal, 2007). The presence of macrophytes in free water surface flow (FWS) CWs is one of the most noticeable and significant features for distinguishing CWs from other lagoons (Vymazal, 2013). The
* Corresponding author. Tel.: +86 29 82205841; Fax: +86 29 82205841. E-mail address: [email protected] (X.C. Wang). http://dx.doi.org/10.1016/j.ecoleng.2015.06.035 0925-8574/ ã 2015 Published by Elsevier B.V.
macrophytes growing in FWS CWs have several functions in relation to the treatment process such as provision of substrates for the growth of attached bacteria, release of oxygen and exudates, uptake of nutrients, surface insulation and wind velocity reduction (Vymazal, 2013). A large body of evidence indicates that CWs with plants are more efficient compared with unplanted CWs (Borin and Salvato, 2012; Elsaesser et al., 2011; Ibekwe et al., 2007). Nevertheless, the effects of different plant species on CW performance vary considerably (Brisson and Chazarenc, 2009; Iamchaturapatr et al., 2007). On the other hand, the growth and functions of plants can be impacted by several factors such as the climate condition, temperature, plant species and pollutant loadings (Liu et al., 2012). Due to the importance of plants in CWs, several management strategies have been proposed to improve the plant growth and productivity and the performance of CWs. These proposed strategies include annual harvesting (Toet et al., 2005; Kadlec and Wallace, 2009) and multiple harvesting (Jinadasa et al., 2008; Vymazal et al., 2010). However, the real benefit of plants harvesting for the removal of nutrients or the development of plants biomass remains a highly controversial issue. On one hand, several studies have demonstrated
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that the nutrients uptake by plants can be definitively removed from the wetland by harvesting the aboveground parts of the plants before senescence and decay during the cold season (Vymazal et al., 2010). In addition, it has been demonstrated that harvesting can open up dense vegetated areas to promote the photosynthetic periphyton in the system (Wetzel, 2001). According to Wetzel (2000), both mature and standing dead (withered) plants shade the attached microbial communities, thus, reducing the nutrient retentive capacities of those communities, and also, contribute to short-circuiting of the water flow (Groeneveld and French, 1995). On the other hand, arguments against plant harvesting indicate that the regular harvest of CWs is impractical, does little to improve water treatment, and reduces the readily available carbon source necessary for denitrification (Kadlec and Wallace, 2009). Additionally, some previous studies have pointed out that the withered plants within CWs may have a thermoregulatory effect in winter (Kadlec and Wallace, 2009; Smith et al., 1997). However, studies on this thermoregulatory function, and the competition between the regenerated plants and the withered plants left in the wetland are scarce. Thus, the regrowth of plants can exhibit marked differences in different plants management practices. Therefore, for the purpose of maximizing the contribution of plants to nutrients removal, an appropriate method for managing the plants needs to be adopted, and require further research. According to Batty and Younger (2004), nutrients uptake and growth rates are higher in young vegetation stands. More particularly, it is important to determine the management strategy that does not adversely affect the regeneration of plants or their capacity to uptake nutrients. Besides, research on the effect of plant management on the growth and development of plants and nutrients removal in large-scale CWs under the climatic conditions of northwestern China are rare, especially during the startup period of the CW operation. Furthermore, the uncertainty of the performance of FWS CWs with the presence or absence of the withered plants over the winter period, and their thermoregulatory effects, in northwestern China, where temperature varies widely, requires research attention. In this study, the growth characteristics of local Phragmites australis in northwest China were evaluated under an
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annual harvest management scheme. The specific objectives were to: (1) assess the growth characteristics and nutrients accumulation in the local P. australis under the condition of annual harvesting; (2) evaluate the effects of plants harvesting on the overall performance of the wetland; and (3) analyze the impact of standing withered plants on the performance of FWS CW for treating polluted river water during the cold season. 2. Materials and methods 2.1. Description of the pilot wetlands The pilot-scale CW system was constructed on the eastern bank near the confluence of an urban river to the Weihe River (the largest tributary of the Yellow River) in Xi’an, northwestern China (34 220 5400 N, 108 5100500 E). The area has a sub-humid continental monsoon climate, which is cold and lacks rainfall during the winter. The average monthly temperature reaches a maximum of 26.3 C in July and a minimum of 1.3 C in January, with an average annual precipitation of 750 mm. The two FWS CWs used for this study were the second stage of a pilot-scale hybrid CW system, the design of which was described by Zheng et al. (2014). The two CWs were identical (40 m length, 10 m width, 0.6 m height), and were filled with sand to a depth of 0.35 m. The water depth was controlled at 0.4 m. After passing through the first stage of treatment (a subsurface flow CW) the highly polluted river water flowed into the FWS CWs continuously. At this stage, the river water contained low concentrations of organic pollutants, but the concentrations of nutrients (N and P) were still high. The inflow rate to each CW was 34 m3/d, on average, which corresponds to an average HRT of 1.8 d and a surface loading of 0.085 m/d. 2.2. Plants harvesting scheme Young P. australis (common reeds) were obtained from the field near the riverbank. Plants of similar size (20–30 cm in height) were selected and washed with tap water in order to remove soils and dead tissues from their roots. They were then planted in the CWs at
Fig. 1. Overview of the free water surface flow constructed wetlands with different plant harvesting schemes (a) before harvesting, (b) harvesting system, (c) regrowth after harvesting, (d) harvested plants.
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a density of 9 shoots per m2 and a roughly equal depth in September. In the first two weeks after planting, river water was intermittently fed into the wetland bed for the plants to acclimate. After this acclimation period, river water was fed into the CWs at the prescribed flow rate. Plants harvesting were carried out in November, for the two successive years, when the plants began to wither. For the purpose of the discussion in this article, the period from November to October of the next year was chosen as one year. At the end of the first year, the aboveground parts of the plants in one CW (CW1) were harvested by cutting their upper parts at about 20 cm above the CW bed level (Fig. 1b). The standing biomass production and nutrients concentration in the plants tissues were determined from the harvested plant parts. On the other hand, the plants in the second wetland (CW2) were not harvested (Fig. 1b); they were left in the wetland. However, samples within three randomly selected quadrats of 0.25 m2 were collected for analyses. The purpose here was to understand how the annual plants harvesting might affect the growth characteristics and nutrients removal by the CWs in the second year (from November to October). At the end of the second year, harvesting was done, in the same way, as in the first year. 2.3. Plant sampling and analysis During the experimental period, the plant heights in the two CWs were measured monthly in three randomly selected quadrats of 0.25 m2. The number of shoots in the two CWs were counted before the harvesting in November. The selected harvested plants were separated into stems, leaves and flowers and washed with distilled water to remove the adhering water and sediments. Plant parts were then oven-dried at 80 C to a constant weight, and their dry biomass were determined. All dried plant samples were ground separately to pass through a 0.25 mm mesh screen, digested and analyzed for total N and P according to the routine analysis method for soil agro-chemistry (Bao, 2000). The average nutrients concentration in the aboveground biomass was calculated as follows:
Ntotal ¼ ðDMleaves Cleaves Þ þ ðDMstems Cstems Þ þ ðDMflowers Cflowers Þ where DM values represent the total biomass of leaves, stems and flowers, and C represent the average concentrations of N and P in these respective plant parts. N values represent the amount of nutrients uptake by the aboveground biomass of plants. 2.4. Water sampling and analysis During the two-year experimental period, water samples were collected weekly from the influent and effluent of the two CWs. All of the water samples were transported to the laboratory for chemical analyses within 24 h. The parameters measured include total nitrogen (TN), ammonia-nitrogen (NH3–N), nitrate (NO3), nitrite (NO2), total phosphorus (TP) and orthophosphate (PO43). Standard methods (MEPC and WWMAA, 2002) were followed for all the chemical analyses. TN was measured with alkaline potassium persulfate oxidation-ultraviolet spectrophotometry. The Nessler reagent colorimetric method was used for NH3–N analysis. NO3–N was determined by the phenol disulfonic acid ultraviolet spectrophotometric method. NO2–N was determined by N-(1-naphthyl)-ethylenediamine colorimetric method. TP was measured by the potassium persulfate oxidation-ultraviolet spectrophotometry method. PO43–P was determined by the Mo–Sb–Vc spectrophotometric method. Water temperature and dissolved oxygen (DO) were measured on site by using a portable meter (HQ30d53LEDTM, HACH, USA). The removal efficiencies for each wetland were calculated from the difference in concentration between the influent and effluent of the CWs. Significant differences were determined at the a = 0.05 by paired samples t-tests and analysis of variance (ANOVA). 3. Results and discussion 3.1. Plants growth characteristics 3.1.1. Temperature and plants growth
As shown in Fig. 2, during the experimental period, the average ðDMleaves Cleaves Þ þ ðDMstems Cstems Þ þ ðDMflowers Cflowers Þ Ctotal ¼ air temperature in the first year (17.8 C) was slightly lower than ðDMleaves þ DMstems þ DMflowers Þ that in the second year (18.6 C). However, the water temperatures where DM = dry matter of a particular shoot part (g), C = concenof the two CWs were similar. Furthermore, plants in both CWs tration of nutrients in the respective plant parts (% DM). grew well, whereby the growth rates were higher than that The amount of nutrients uptake by the aboveground biomass reported in previous studies (Wu et al., 2011). The heights of plants was calculated according to the following equation: in the two CWs were similar in the first year (Fig. 3a). In the second
Fig. 2. Variations of average air temperature and the water temperature of the two wetlands throughout the experimental period. (CW1 harvested and CW2 unharvested in winter of the 1st year).
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required for the growth of new plants. However, by comparison, the heights of the P. australis in the two CWs were higher than that reported for other CWs in northern climates (Liu et al., 2012). The difference in height was probably due to the field conditions, scale of the CWs and the pollutants loadings. The two wetlands of this study had larger open areas and higher nutrient loadings, which were beneficial for plants growth (Jampeetong and Brix, 2009; Konnerup and Brix, 2010). In addition, the plants in the two CWs recorded their highest growth rates in spring (CW1) and early summer (CW2) (about 1.9 cm/d). They reached their maximum heights of more than 3 m at the beginning of the autumn. The plants bloomed at the end of summer, and a slower growth rate was recorded (about 0.6 cm/d). The growth ceased, and plants withered with the decrease in temperature over the winter period of November through February.
Fig. 3. Peak height of plants (a), average plant density (b) and total mean aboveground biomass (c) for the two CWs at the end of the growing season during the experimental period.
year, however, the heights of the plants in CW1 were slightly higher (0.22 m; Fig. 3a). This could be explained by the fact that the withered plants in CW2, which were not harvested, were not lodging completely. They stood in the initial growing position (Fig. 1c), which blocked the sunlight for sprouting plants in the second year. The fallen withered plants also squeezed the space
3.1.2. Variation of plant density and aerial biomass The average densities of plants in CW1 and CW2 were 163 shoots/m2 and 155 shoots/m2 in the first year, and 175 shoots/m2 and 130 shoots/m2 in the second year, respectively (Fig. 3b). Thus, the densities of the plants in both CWs increased exponentially, at up to 18 and 17 times higher than their initial value (9 shoots/m2) by the end of the first year. These values were found to be higher than those reported in the literature (Liang et al., 2011; Wu et al., 2011). The results indicated that whether the plants were harvested or not, an increase in the density of plants mainly occurred during the first year when there was more space available for the plants growth in the CWs. Furthermore, it appears that the plants growth in the wetlands were not as rapid in the second year as they were in the first year. The higher plant densities in the first year resulted in little space available for new plants to grow in the second year. However, the density of the plants in CW1 was slight increased (by 7.4%) while that of CW2 was decreased in the second year (by 16.1%). The explanation for this phenomenon may be that the shading by the standing withered plants, which remained in the wetland inhibited their germination rates and growth density (Chun and Choi, 2009). Perhaps the open space ratio of the wetland in the second year could be a limiting factor for the plants growth in these wetlands.
Fig. 4. Average nutrient content (a, b) and the nutrient uptake rate by the aboveground biomass of P. australis (c, d) for the two CWs at the end of the growing season during the experimental period.
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As the heights of plants in the two CWs were the same at the initial planting, the amount of plants biomass at harvesting can be regarded as the net increase. In the first year, the aboveground biomass of plants in both of the wetlands increased moderately. The dry weights of plants in CW1 and CW2 were 1.31 kg/m2 and 1.30 kg/m2 in the first year, and 1.44 kg/m2 and 1.20 kg/m2 in the second year, respectively (Fig. 3c). The aboveground biomass was slightly higher compared to what is reported in the literatures (Liang et al., 2011; Wu et al., 2011). Nonetheless, Borin and Salvato (2012) reported that the aerial biomass production was double in the second year when the wetlands were fed with higher concentrations of nitrogen. Similar to the density of plants, in the second year, the plants biomass in CW1 was increased (9.9%) while that of CW2 decreased (7.7%). However, the average weight of plants in CW2 (0.92 g/plant) was higher than that of CW1 (0.82 g/plant). This implies that the withered plants left in the wetland stunted the height and the biomass of the new plants, but promoted the new plants in the wetland to be stocky. The results also indicated that harvesting the withered plants from the wetland at the end of the growing period could accelerate the growth characteristics of plants such height and biomass in the next growing season. Moreover, a phenomenon was observed whereby the aerial biomass in the inflow zone was higher than that in the outflow zone in both wetlands during the experimental period, due to the higher nutrient supply in that inflow zone. 3.1.3. Nutrients contents in plant tissues and plant uptake rate The concentrations of nutrients in the aboveground tissues of plants in the two wetlands were similar in the first year and were both increased slightly in the second year (Fig. 4a,b). This may be because the plants were matured and thus, had a higher ability to uptake nutrients in the second year (Borin and Salvato, 2012). Nevertheless, the nitrogen concentration in the plants tissues in CW1 was lower than that in CW2 in the second year (29.13 mg/g, 31.06 mg/g). The opposite wad found for phosphorus (2.68 mg/g, 2.60 mg/g). As shown in Fig. 4c, the nitrogen uptake by the aboveground parts of plants in CW1 and CW2 were 35.2 g/m2 and 35.9 g/m2 in the first year, and 41.9 g/m2 and 37.3 g/m2 in the second year, respectively. The nitrogen uptake by plants in CW1 was 1.13 times higher than that of CW2 in the second year. Fig. 4d shows that the phosphorus uptake by the aboveground parts of plants in CW1 and CW2 were 3.1 g/m2 and 3.0 g/m2 in first year, and 3.7 g/m2 and 3.2 g/m2 in second year, respectively. The phosphorous removed by plants in CW1 was 1.16 times higher than that of CW2 in the second year, while the nutrients uptake by the plants in the two wetlands were similar in the first year. The above findings indicated a considerable increase of nutrients uptake by plants in the second year in the harvested wetland, while only a slight increase was observed in the unharvested wetland. The amount of TN and TP uptake by plants in this study were in the range of values reported in other studies (Stefanakis and Tsihrintzis, 2012; Zhao et al., 2012).
Overall, plants harvesting had minimal effects on the nutrients concentration in plant tissues, but significantly affected the biomass production. As the nutrients uptake by plants relies on biomass more than concentration of nutrients in the plant tissues, plant harvesting can be considered to affect the efficiency of the plants considerably. 3.2. Overall nutrients removal 3.2.1. Comparison of harvesting schemes During the experimental period, the average influent TN and TP concentrations in the first year (24.30 1.03 mg/L and 1.24 0.09 mg/L) were higher than those of the second year (18.98 1.04 mg/L and 1.04 0.08 mg/L). NH3–N and PO43–P were recorded as the primary constituents of the TN and TP, which accounted for 80.6% and 60.5% in the first year and 75.7% and 63.5% in the second year, respectively. Many species of wetland plants prefer NH3–N and PO43–P as the nutrients source (Jampeetong and Brix, 2009; Konnerup and Brix, 2010), which benefited the removal of these nutrients from the river water in the wetlands. The mean influent and effluent nutrients concentrations in the two wetlands for the two-year study period are presented in Table 1. In the first year, the operation parameters of the two wetlands were exactly the same. The removal rates of TN and NH3–N for both wetlands were approximately 0.61 g/m2/d and 0.65 g/m2/d. During the experimental period, the removals of TN in both wetlands showed strong seasonal variation. NO3–N accumulation mainly occurred during the winter, which indicated a limited denitrification potential during this period while nitrification proceeded. This finding may be related to the DO concentration and temperature. Higher DO concentration and lower temperature, which were >4 mg/L and <3 C, respectively, in both wetlands, have been noted to inhibit the denitrification process and consequently, decrease nitrogen removal in FWS wetlands (Peng et al., 2014). The removals of organic nitrogen in the wetlands were limited. This may be caused by seasonal internal releases (Thorén et al., 2004). The removal rates for TP in the two of wetlands were about 0.02 g/m2/d, which confirmed the limited phosphorus adsorption capacity of the FWS CWs. The main physicochemical processes for phosphorus removal in CWs such as the fixation of phosphate by iron and aluminum in the substrate (Arias et al., 2001) are known to be limited in FWS wetland. In the second year, both wetlands showed higher nutrients removal than the first year. In the second year, the removal efficiencies for TN and TP in CW1 increased by about 15%, compared that in the first year, and that for NH3–N was increased by about 30%. There was a further increase of NO3–N accumulation in the wetland during this period. These results indicated that the wetland provided a better aerobic condition for nitrification in the second year. The reason for this is probably that the increased plant density promoted the release of oxygen and root exudates.
Table 1 Influent and effluent nutrients concentrations for the two CWs during the experimental period. Values are mean S.E., n = 81. Parameter
TN NH3–N NO3–N NO2–N Org–N TP PO4–P
First year
Second year
Influent (mg/L)
CW1 (harvested) (mg/L)
CW2 (unharvested) (mg/L)
Influent (mg/L)
CW1 (mg/L)
CW2 (mg/L)
24.301.03 19.580.92 0.490.04 0.100.01 4.130.68 1.240.09 0.750.08
17.071.13 11.901.22 1.210.25 0.320.06 3.630.56 0.940.07 0.630.09
17.051.18 12.171.24 1.390.28 0.310.04 3.190.50 0.930.08 0.650.09
18.981.04 14.370.81 0.900.14 0.130.02 3.590.57 1.040.08 0.660.06
10.260.94 4.340.67 3.230.48 0.150.03 2.530.36 0.640.08 0.400.05
11.291.16 5.240.96 3.040.42 0.160.04 2.840.40 0.730.08 0.460.06
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Consequently, the microbial diversity and other environmental conditions within the rhizosphere were increased (Jenssen et al., 1993). Nevertheless, due to the different plant harvesting schemes, the performance of the two wetlands showed a slight difference. The removal efficiencies of TN and TP in CW1 were higher than those of CW2 by 5.4% and 8.9%, respectively. However, these differences were not statistically significant (P > 0.05). This difference was because the wetland with plants harvested in the autumn provided better space and light conditions for plants regeneration than the unharvested wetland. Also the standing withered plants competed for space and light resources with the new plants. Relative to N removal, the P removal in CW1 was significantly higher than that in CW2 (P < 0.05), which indicated that the plants harvesting had a greater effect on the removal of phosphorus than nitrogen. However, this difference may relate to the different mechanisms of nitrogen and phosphorus removal in wetlands. The nitrogen removal in FWS CWs is mainly achieved through microbial processes while the primary mechanisms for phosphorus removal are plants uptake and physicochemical processes (Vymazal, 2007). Thus, plant uptake can be more important for phosphorus removal than nitrogen removal in the FWS wetlands. This phenomenon will be further discussed later in this article. 3.2.2. Nutrients removal in the winter The nutrients removal efficiencies of the two wetlands were analyzed from when plants were harvested at the end of the first year to their regrowth in the next spring (December–February). This was to aid the evaluation of the effect of plant harvesting on nutrients removal and the thermoregulatory functions of withered plants in the winter. The average temperature in CW1 was slightly lower than that of CW2 during this period (2.12 and 2.23 C, respectively). Fig. 5 shows that the harvested wetland recorded higher TN, NH3–N, Org–N, TP, PO43–P removal than the unharvested wetland. This was so, probably because the unharvested plants started to die-off and no longer assimilated nutrients (Wu et al., 2011). Moreover, the release of nutrients from the withered plants left in the wetland aggrandized this difference (Cui et al., 2009). It should also, be noted that the unharvested wetland had higher NO3–N removal. This could be explained by the fact that the decaying plant parts provided denitrification bacteria with a carbon source. Nonetheless, the oxygen released from the roots of the plants in the harvested wetland was higher than the unharvested wetland.
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3.3. The significance of plants harvesting for nutrients removal As plants play important direct and indirect roles in nutrients removal in wetlands, it is important to consider the effects of plants harvesting strategies on both of these two roles. Table 2 shows the nutrients mass balance components for each wetland. In the first year, there was no apparent difference between the two wetlands with regard to the amount of nutrients removed and the proportion attributable to plants. In the second year, the amounts of TN and TP removed by the wetlands were 258.84 g N/m2 and 11.70 g P/m2 in CW1, and 228.31 g N/m2 and 8.96 g P/m2 in CW2, respectively (Table 2). The amounts of TN and TP uptake by the plants were 41.94 g N/m2 and 3.74 g P/m2 in CW1, and 37.27 g N/m2 and 3.22 g P/m2 in CW2, respectively (Table 2). Furthermore, the amounts of TN and TP removed by the wetland beds were 216.89 g N/m2 and 7.95 g P/m2 in CW1, and 191.04 g N/m2 and 5.74 g P/m2 in CW2, respectively (Table 2). This possibly occurred because as the aboveground biomass of the plants increased, the belowground biomass was also, increased (Borin and Salvato, 2012) and further stimulated microbial activities in the wetlands (Korboulewsky et al., 2012). This finding indicates that harvesting at the end of the growing season does not only promote the new plants’ uptake of more nutrients. It also increases the belowground biomass and stimulates the microbial activities in the wetlands, both of which enhances the roles of the substrate layer. It should also, be noted that the plants nutrients uptake accounted for a greater proportion of the phosphorus removal (32.02–36.71%) than the nitrogen (16.20–17.04%). The difference in the proportion of nitrogen and phosphorus removal by plants uptake is probably due to the main mechanisms for the removal of these parameters. Nitrogen removal in FWS CWs is achieved through microbial processes while the primary mechanisms for phosphorus removal are plants uptake and physicochemical processes. A similar finding was reported by Greenway and Woolley (2001). However, Wu et al. (2011) reported the opposite. Several studies have suggested that the overall nutrients removal would be higher if a multiple or earlier harvesting scheme is adopted (Batty and Younger, 2004; Jinadasa et al., 2008 and Vymazal et al., 2010). However, multiple or earlier harvesting would be detrimental to the plants because they would not have sufficient opportunity to withdraw nutrients and nonstructural carbohydrates from the shoots to belowground plant parts (van der Linden, 1980, 1986). Besides, frequent aboveground harvesting can slow growth and biomass development (Álvarez and Bécares, 2008). Therefore, the annual harvesting of the plants should be based on the consideration of the economic, climatic and wetland
Fig. 5. Average influent and effluent nutrients concentrations of the two CWs in winter under different harvesting schemes.
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Table 2 The nutrient mass balance and proportion of plant uptake in the two CW at the end of the growing seasons during the experimental period. Period
Parameter
Influent (g/m2)
CW1 (harvested)
CW2 (unharvested)
Effluent (g/m2)
Phragmites (aboveground) (g/m2)
Othersa (g/m2)
Plant uptake (%)
Effluent (g/m2)
Phragmites (aboveground) (g/m2)
Othersa (g/m2)
Plant uptake (%)
First year
TN TP
706.48 35.91
496.18 27.43
35.22 3.11
175.08 5.37
16.75 36.71
495.66 26.90
35.92 3.08
174.91 5.93
17.04 34.19
Second year
TN TP
563.10 30.73
304.26 19.03
41.94 3.74
216.89 7.95
16.20 32.02
334.79 21.77
37.27 3.22
191.04 5.74
16.32 35.93
a
Others involve microbial assimilation and physical sorption and plants roots.
operation conditions. Based on the finding from this study, longterm nutrient removal by annual harvesting of shoots in the P. autralia stands at the beginning of November can beconsidered as a better alternative plant management approach in northwestern China. 3.4. Operation and maintenance costs The annualized cost for operation and maintenance (O&M) was about RMB 3314.96 per each wetland. Major components of O&M included personnel (RMB 1858.07), electric costs associated with pumping water through the system (RMB 1203.52), along with some miscellaneous expenditure (RMB 253.37). We estimate the cost of plant harvesting at 0.75 RMB/m2. The difference in the management of the two wetlands was the plant harvesting. Therefore, the O&M costs of the two wetlands were exactly the same except that the CW1 required an additional 300 RMB/year for plant harvesting. 4. Conclusions Both harvested and unharvested FWS wetlands recorded an increase in nutrients removal efficiencies in the second year, compared to the first year of the wetlands operation. Annual harvesting provided better space and light conditions for plants regeneration. Shoot density (175 shoots/m2), biomass (1.44 kg/m2) and peak height (3.4 m) of P. australis were higher in the harvested CW than the unharvested one (130 shoots/m2, 1.2 kg/m2 and 3.1 m). Overall, plants uptake accounted for a greater proportion of the phosphorus removal (32.0–36.7%) than nitrogen (16.2–17.0%). Plants harvesting at the end of the first year did not only improve nutrients removal by plant uptake in the second year (41.9 g N/m2 and 3.7 g P/m2 versus 37.3 g N/m2 and 3.2 g P/m2) but also, the nutrients removal in the substrate layer (217 g N/m2 and 8.0 g P/m2 versus 191 g N/m2 and 5.7 g P/m2). Consequently, higher TN and TP mass removal rates of 258.8 g N/m2 and 11.7 g P/m2 were recorded in the harvested wetland than the unharvested wetland (228.3 g N/ m2 and 8.9 g P/m2). Nonetheless, the overall increase in nutrients removal by harvesting was minimal and required an additional expense. Thus, annual harvesting of emergent vegetation may not have significantly beneficial effects on plants development and nutrient removal under the climatic conditions of northwest China. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 50838005), National Major Project of China (2013ZX07310-001) and the Program for Innovative Research Team in Shaanxi (No.2013KCT-13).
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