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The release of nitrogen and phosphorus during the decomposition process of submerged macrophyte (Hydrilla verticillata Royle) with different biomass levels
Ecological Engineering 70 (2014) 268–274
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The release of nitrogen and phosphorus during the decomposition process of submerged macrophyte (Hydrilla verticillata Royle) with different biomass levels Chun-hua Li a , Bo Wang a,b , Chun Ye a,∗ , Yu-xin Ba a a Centre of Lake Engineering & Technology, State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China b Eco-environmental Institute, Liaoning Shihua University, Fushun 113001, China
a r t i c l e
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Article history: Received 26 October 2013 Received in revised form 23 March 2014 Accepted 19 April 2014 Keyword: Decomposition Aquatic plant Submerged macrophytes Water quality Ecological restoration Hydrilla verticillata Royle
a b s t r a c t The purpose of the present study is to elucidate the effects of decomposition of different biomass levels of submerged macrophyte (Hydrilla verticillata Royle) on sediment, overlying water and air. Four biomass levels of submerged macrophyte were used. They are 25 g, 12.5 g, 6.25 g and 3.125 g in 4 L water and 200 g sediment were cultivated in decomposition experiment setting in order to simulate the temperature in Lake Taihu in spring. The 70-day decomposition experimental results showed that the overall decay process of the four biomass levels at 17 ◦ C can be well described by the exponential model, with the average decomposition rate of 0.018 day−1 . The decomposition of H. verticillata increased total nitrogen and phosphorus mass in the whole environment including overlying water, air and sediment. The optimal biomass harvest rate is 75% to 87.5% for a fresh water lake with 6 kg m−3 fresh biomass if the short-term negative effect on water quality is ignored. The present study could provide some scientific bases for aquatic macrophytes management. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Macrophytes are a vital part of lake ecosystem (Qin, 2009; Yu et al., 2010). They convert sunlight energy and chemical element, allowing these to be absorbed by living plants. Fish, waterfowl, insects, mammals, and microscopic animals consume the macrophytes as food. They also replenish the aquatic environment with dissolved oxygen, which is essential to aquatic animals. Submerged macrophytes are rooted in the bottom sediments and grow up through the water. They occupy the whole water column, range from the bottom to the surface of the lake. Besides, they have many important ecological functions, such as improving the self-purification capacity of a lake ecosystem (Moss, 1990; Knight et al., 2003; Wu et al., 2003; Schaller, 2013), and forming a reasonable ecological structure (Carpenter et al., 1998; Mitsch et al., 2005). However, when submerged macrophytes die and
∗ Corresponding author at: Research Center of Lake Ecological Environment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China. Tel.:+86 010 84915191; fax: +86 010 84915190. E-mail addresses: [email protected], [email protected] (C. Ye). http://dx.doi.org/10.1016/j.ecoleng.2014.04.011 0925-8574/© 2014 Elsevier B.V. All rights reserved.
decompose, photosynthetic production of oxygen ceases. The bacteria and fungi which decompose decaying plant material in turn use up the dissolved oxygen for respiration. Therefore, the control and management over submerged macrophytes are very important especially after they were restored in lakes. Among different guidelines for their biomass control and management, the impact of submerged macrophytes decomposition on lake water quality always attracts our attention (Troxler and Richards, 2009). Most studies on the decomposition of aquatic macrophytes focused on weight loss, temperature effect and other changes in the chemical composition of coarse particulate detritus over time (Pagioro and Thomaz, 1999; Carvalho et al., 2005). However, a few studies have examined the effects of the decomposition of aquatic macrophytes on the environment as a whole, including the conditions of sediment, overlying water and air when they were exposed to the decaying submerged macrophytes (Ferreira and Esteves, 1992; Gessner et al., 2001). Furthermore, even less studies have focused on comparing the effects of the different biomass levels of the submerged macrophytes (Best, 1993; Serafy, 1994; Mcclanahan, 1996). Harvesting is an effective way to remove nutrients (Gumbricht, 1993). However, the optimum amounts of harvesting, as well as its possible effects, are not fully understood
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Table 1 Nutrient status of sampling site in East Taihu Lake, China (mean and standard deviation of triplicates are shown; sampling time: September, 2011).
−1
Lake water (mg L ) Sediment (mg g−1 )
Total nitrogen
Total phosphorus
Total organic carbon
Dissolved oxygen
pH
1.3 ± 0.13 1.23 ± 0.28
0.05 ± 0.015 0.51 ± 0.17
1.31 ± 0.17 15.84 ± 1.12
7.21 ± 1.58 –
7.05 ± 0.42 –
(Asaeda and Trung, 2000). Some successful studies have been reported on simulating the harvesting amount by adding different amounts of objective aquatic plant in an experimental system (Carvalho et al., 2005). Additionally, the research on gas emission in nitrogen cycle during decomposition is seriously deficient. Hence, this study aims (i) to track the nitrogen and phosphorus contents in plants, sediment, overlying water and air in the period when they are exposed to the decomposition of submerged macrophytes; (ii) to evaluate the effects of decomposition process of the different biomass levels of submerged macrophytes on the whole environment, and (iii) to provide lake managers or planners information to assess the optimal harvesting amount on submerged macrophytes. 2. Materials and methods 2.1. Experimental submerged macrophyte A perennial submerged macrophyte Hydrilla verticillata Royle was used in this study. It is a native species to China and a member of the Hydrocharitaceae family. H. verticillata is widely used in lake ecological restorations in China because of its extensive growth in fresh water lakes, high adaptability and ability to improve water ecosystem (Yu et al., 2010). H. verticillata was sampled from East Taihu Lake (31◦ 01 50 N, 120◦ 20 18 E), China, in September 2011. The H. verticillata sample was kept in a healthy status and transported to our laboratory as soon as possible. Compared to other water areas in Lake Taihu, East Taihu Lake has better water quality (Table 1), and higher coverage of submerged macrophytes. The average lake water temperature in winter and in spring is 6 ◦ C and 17 ◦ C, respectively. According to in-situ investigation, H. verticillata Royle begins to decay in a large scale from early winter, but it will not reach the rapid decomposition stage until spring, and most plant tissue can be decomposed before summer. Therefore, the collected H. verticillata was kept in a freezer in the temperature of 6 ◦ C for one week to obtain the in-situ plant status in winter. To simulate the decomposition process in spring, the temperature was set at 17 ◦ C. 2.2. Experimental setup 5 L plastic buckets were used as experimental containers. 200 g sediment (Table 1) collected from East Taihu Lake was laid on the bucket bottom and 4 L simulated lake water was added to each bucket. The simulated lake water was made by adding nitrate, ammonia and phosphorus to DI water, in order to simulate total nitrogen (TN) and total phosphorous (TP) concentrations and pH value as the water qualities in East Taihu. According to the survey in East Taihu Lake, the maximum fresh biomass of H. verticillata is about 6 kg/m3 . Therefore, 25 g, 12.5 g, 6.25 g, 3.125 g fresh H. verticillata were used as different biomass levels in the present experiment to reflect the corresponding harvesting rates, i.e., 0%, 12.5%, 25% and 50%, respectively. A control treatment without any plant material and four treatments with 25 g, 12.5 g, 6.25 g and 3.125 g fresh plant materials, respectively, were included. Roots were removed while only the stems and leaves were used. The plant material was rinsed with tap water to remove adhering matter and was cut into many small segments in about 3 cm length.
25 g, 12.5 g, 6.25 g and 3.125 g fresh plant segments were put into micro-nanometer litter bags with pore size of 20 m. These micronanometer litter bags were kept in a freezer in the temperature of 6 ◦ C for one week. They were then transferred to 5 L-experimental buckets. The micro-nanometer litter bags were anchored on the surface of sediment layer. The decomposition experiment was conducted in an artificial climate chamber in the temperature of 17 ◦ C, with a light–dark cycle of 12:12 h and an illumination of 7000 lx (approximately 128 M photons m−2 s−1 of photosynthetically active maintained in radiation). Plants, overlying water and sediment were sampled and analyzed in the interval of 7, 14, 21, 28, 35, 42, 49, 56, 63, 70 d in the decomposition process. Three replicate buckets were scarified in each sampling. A total of 150 buckets were used. During decomposition process, a bit of phytoplankton was found only in last 5–10 days. These small amounts of phytoplankton were taken out from buckets to avoid their influence to light and oxygen condition.
2.3. Experimental analysis On each sampling day, plant material, sediment, overlying water and headspace gas were collected and analyzed. The whole plant material was taken from micro-nanometer litter bag. Its dry weight (DW) was measured by heating the part of plant material at 105 ◦ C for 10 min, then 80 ◦ C for 8 h. The contents of TN and TP in plant material were analyzed by total Kjeldahl nitrogen method and Mo–Sb colorimetric method after acid digestion (Carter and Gregorich, 2007). The concentrations of TN and TP in overlying water were measured according to the Standard Method for the Examination of Water and Wastewater Editorial Board (2002). The sediments were freeze-dried and pre-treated according to the standard methods (Lal et al., 2001). Their concentrations of TN and TP were analyzed by automatic Kjeldahl analyzer (VELP, UDK159), spectrophotometer (LAXCO, Inc. ALPHA1860) and total organic carbon analyzer (SHIMADZU TOC-VCPH), respectively. Gas flux was quantified by the static collection technique (Chen et al., 2010). Each bucket was blown for one minute by common air gas. It was then coiled with two layers of LDPE film for 6 h continuously (3 h in dark and 3 h in light). N2 O gas concentration was determined by Hewlett Packard 6890N GC which is equipped with a 63Ni electron capture detector (ECD) and a RT-QPlot column (RESTEK), and helium gas was used as the carrier gas at a flow rate of 6.5 mL min−1 . The temperature of the injector, column and detector was 100 ◦ C, 70 ◦ C, and 350 ◦ C, respectively. N2 gas concentration was determined directly by N2 detector (HF-800). The nitrogen emitted in gas form was evaluated by adding N2 -N and N2 O-N integral flux. Nitrogen content in plant, sediment and overlying water were calculated by multiplying TN concentration and the dried weight of biomass, respectively.
2.4. Statistical analysis An exponential model was applied to describe mass-loss over time at four biomass levels: Wt = W0 e−kt , where W0 is the initial weight, Wt is the weight at time t; k is the decomposition rate; and t is time in a day.
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100 3.125g 6.25g 12.5g 25g
DW (%)
80
60
40
20
0
0
7
14
21
28
35 42 Time (d)
49
56
63
70
Fig. 1. Change of residual plant biomass (dried weight, DW) during decomposition (mean and standard deviation of triplicates are shown).
The Pearson correlation coefficients (2-tailed test) between decomposition rate and initial plant biomass, the total gas-N mass and initial plant biomass were calculated. ANOVA for repeated measurement was used to test whether TN and TP in plant materials, sediment, overlying water and gas at the five biomass levels (including control treatment with 0 g plant biomass). All statistical analyses were conducted using the software SPSS 16.0 (SPSS Inc., Chicago, IL, USA). 3. Results 3.1. Rate of H. verticillata decomposition Fig. 1 shows the change of plant biomass with the change of decomposition time. The weight of plants in all treatment groups decreased with time, and at the end of experiment (70 d), the ratio of residual dried weights (DW) of plant to their initial DW was 23%, 24%, 31% and 32% for 3.125, 6.25, 12.5 and 25 g treatment groups, respectively. The overall decay process of the four biomass levels at 17 ◦ C was well described by the exponential model, with the average decomposition rate of 0.018 day−1 (R2 range from 0.9315 to 0.9634). No significant correlation was found between decomposition rate and the initial biomass level according to Pearson correlation analysis, although the decomposition rate has the order of 25 g (k = 0.02 day−1 ) >12.5 g (k = 0.019 day−1 ) >6.25 g and 3.125 g (both of them have the same k value, k = 0.016 day−1 ). It indicates that the decomposition rate is almost not influenced by its initial biomass level. 3.2. Change of nitrogen content during decomposition The changes of nitrogen content in plant, overlying water, gas, sediment and summary of them during decomposition are shown in Fig. 2. According to ANOVA analysis, all the treatment groups with different plant biomass levels had significant difference (F, P values for Fig. 2(a)–(e) are (F = 51.184, P = 0.000), (F = 17.526, P = 0.002), (F = 6.313, P = 0.043), (F = 47.622, P = 0.000) and (F = 16.967, P = 0.003), respectively). TN content in plants decreased more quickly in the first 28 days, remained almost unchanged after that. The more of the plant biomass in system, the more TN was released (Fig. 2(a)). For the treatment group with 25 g
plant material, nearly 40 mg TN was released. While for the treatment group with 3.125 g plant material, only 4 mg TN was released. For the overlying water, except the control treatment group, all the treatment groups with plant material had a similar trend that their TN contents climbed to the peak then slipped down gradually and the values on day 70 were lower than their initial values on day 0 (Fig. 2(b)). The order of the maximum value in overlying water was 25 g treatment group > 12.5 g treatment group > 6.25 g treatment group > 3.125 g treatment group > 0 g treatment group. In 70 days experimental period, the TN content in control treatment group (0 g) was decreased slowly from 5.8 mg to 0.03 mg. The increase of TN in overlying water was mainly contributed by the plant decomposition, while the loss of TN was mainly attributed to denitrification and sediment adsorption or absorption. Denitrification could be reflected by the level of N2 and N2 O gas emitted from the system. Fig. 2(c) shows the change of nitrogen content in gas emission during 70 days. With time more and more N2 and N2 O gas was emitted. The total gas-N mass has a positive relationship with the plant biomass in the system (r = 0.997, P < 0.01). As many as 37 mg nitrogen was emitted in the treatment group with 25 g plant material but only around 10 mg nitrogen was emitted in the one with 3.125 g plant. The change of nitrogen content in sediment was more complicated than those of plants, overlying water and gas, as shown in Fig. 2(d). Fluctuations were shown in each treatment group, which indicated nitrogen content in sediment was affected by various factors, such as nitrogen released from plant and water, denitrification from bacterial activity (Zehnder, 1988). Therefore, different trends reflect different driving factors. Sediment-nitrogen content in control treatment group rose slowly from 246 mg on day 0 to 249 mg on day 70. The treatment group with the highest plant biomass (25 g) kept the highest nitrogen content in sediment among all the treatment groups, and increased to 254 mg on day 70. However, the treatment groups with less plant biomass (3.125 g, 6.25 g and 12.5 g) increased slightly in the first 35 days then declined gradually to the level lower than the control treatment group on day 70 or lower than the level they had in their initial value on day 0. This implies that decomposition of certain biomass of plant is beneficial for nitrogen removal from sediment. However, if we observe the total effect of plant decomposition on overlying water, gas and sediment (Fig. 2(e)), we will find that the decomposition process actually increased total nitrogen content in the environment as a whole. The released nitrogen had significant positive correlation with the plant biomass (r = 0.999, P < 0.01). The above results suggested that the decomposition of submerged macrophytes increases total nitrogen mass in the whole environment including overlying water, air and sediment. During 70 days of decomposition, total nitrogen increased 5 mg to 43 mg from low to high plant biomass levels. While the nitrogen in plant materials just decreased about 4 mg to 39 mg from low to high biomass levels. The level of reduced nitrogen in plant materials is slightly lower than the level of increased nitrogen in the environment, including overlying water, gas and sediment. These differences could be attributed to the loss of plant detritus from micro-nanometer litter bags to the surrounding water or sediment. 3.3. Change of phosphorous content during decomposition In the phosphorous cycle of plant decomposition, nearly no gas-phosphorus is released (Boulton and Boon, 1991). Therefore, only TP concentration in plant, sediment and overlying water were measured. The changes of phosphorus content in plant, overlying water, sediment and its summary are shown in Fig. 3. The control treatment group without plant material did not show any significant change during the whole decomposition period. Similar with TN, all the treatment groups with different plant biomass
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Fig. 2. Change of TN contents in plant, overlying water, sediment and gas (mean and standard deviation of triplicates are shown; plant-N: nitrogen content in plant material; overlying water-N: nitrogen content in overlying water; sediment-N: nitrogen content in sediment; gas-N: nitrogen content in headspace gas; overlying water + sediment + gas-N: total nitrogen content in overlying water, sediment and headspace gas).
levels had significant difference in each kind of phosphorus content according to ANOVA analysis (F, P values for Fig. 3(a)–(d) are (F = 18.797, P = 0.002), (F = 15.659, P = 0.003), (F = 9.926, P = 0.012) and (F = 19.134, P = 0.002), respectively). However, the rapid changing stage for TP is shorter than those in TN, 30% to 80% TP in plant materials decreased in the first 7 days and declined slightly until day 28 and then remained in the similar values from day 28 to day 70. TP contents in overlying water of the treatment groups with plant materials reached their maximum values on day 7, after that they decreased slowly and remain
stable from day 35 to day 70. During the whole decomposition period, TP values increased fast in the first 7 days after adding those from sediment and overlying water together and then rose slightly in the treatment groups with higher plant biomass level or remained almost stable for the treatment group with lower biomass level. At the end of experiment, the TP contents of different treatment groups were in increasing order as follows: 25 g treatment group > 12.5 g treatment group > 6.25 g treatments group > 3.125 g treatment group > control treatment group.
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Fig. 3. Change of TP contents in plant, overlying water and sediment (mean and standard deviation of triplicates are shown; plant-P: phosphorous content in plant material; overlying water-P: phosphorous content in overlying water; sediment-P: phosphorous content in sediment; sediment + overlying water-P: total phosphorous content in sediment and overlying water).
4. Discussion 4.1. Decomposition properties and main influencing factors of H. verticillata According to previous studies, the decomposition rate of aquatic plants varies with many factors, such as temperature, aerobic or anaerobic condition, nitrogen, carbon and phosphorus contents in plant and the ratio of rapid and slow decomposable organic matter (Asaeda and Trung, 2000; Carvalho et al., 2005). Among them, temperature, the decomposition rate of slow decomposable material and the fraction of rapid decomposable material are relatively significant (Asaeda and Trung, 2000). For example, Carvalho et al. (2005) reported that the decomposition rate of submerged macrophyte Egeria najas Planchon was 0.014 day−1 and 0.045 day−1 at 17 ◦ C and 27 ◦ C, respectively. The decomposition rate of slow decomposable material and the fraction of rapid decomposable material are varied with species-specific content of structural materials, such as cellulose and lignin (Godshalk and Wetzel, 1977, 1978). The decomposition rate of macrophyte biomass is strongly dependent on the level of original fiber, hemi-cellulose, cellulose, and lignin. For example, Scirpus acutus with 69.7% fiber content (dried weight) has a decomposition rate of 0.011 day−1 , which is three times lower than that of Myriophyllum heterophyllum, which has a decomposition rate of 0.037 day−1 with 32.3% of fiber content (Godshalk and Wetzel, 1978). The plant (H. verticillata) used in
the present experiment has a fiber content of 22.4% (dried weight). Its decomposition rate is much higher than that of S. acutus and M. heterophyllum. Additionally, Fig. 1 shows that the decomposition rates were higher during the first 28 days, about 50% plant biomass was decomposed, mainly because of the presence of the rapid decomposable material, but the rates gradually slowed down afterwards. The decreases in nitrogen level of H. verticillata during decomposition were in the same order of those obtained from other similar aquatic macrophytes and those under similar environmental temperature (De Busk and Dieberg, 1984; Pagioro and Thomaz, 1999; Carvalho et al., 2005). However, the change of TN concentrations in plant materials was different from some other submerged macrophytes, such as Eichhornia azurea and Vallisneria natans (Pagioro and Thomaz, 1999). Our results showed that TN concentrations in plant materials decreased from initial period to the end of decomposition process, while other researchers reported TN concentrations increased during decomposition. These differences are attributed to the different carbon form and C:N ratio. Both E. azurea and V. natans have higher C:N ratio (34.9:1 and 21.3:1, respectively) than H. verticillata has (only 6.57:1), and the former two species have more non-biodegradable carbon materials, such as lignin and cellulose. High carbon ratio and non-biodegradable carbon were considered as two important factors affecting microbial activity and population negatively, especially inhibiting the immobilization of nitrogen by microorganisms (Pagioro and Thomaz, 1999;
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4.2. Some suggestions on aquatic macrophytes management The goal of aquatic management should never be to eliminate aquatic macrophytes from a lake totally, though plant decomposition increases nutrient level in sediment and overlying water. Aquatic macrophytes have many irreplaceable functions, such as providing habitat and shelters for fish and other wildlife, providing food for insects and aquatic animals, producing oxygen and stabilizing shorelines and bottom sediment (Chen et al., 2009; Qin, 2009). However, the function of aquatic macrophytes on enhancing water quality is still questionable. An optimal harvesting biomass is urgently needed in aquatic macrophytes management (Finnegan et al., 2014). The results in present study may provide planners some ideas to come up with measures in assessing effective harvesting policies to enhance water quality. The changes of TN concentrations in overlying water are shown in Figs. 4 and 5, respectively. Although both of them increased in early period, in the first 7 days or 14 days depending on different biomass levels, they began to decline and some of treatments had lower values than their initial TN and TP concentrations afterward. From day 42 to day 70, no significant differences were found on TN concentrations among the control treatment group and the treatment groups with plant biomass of 3.125 g, 6.25 g and 12.5 g While from day 35 to day 70, no significant differences were found on TP concentrations in overlying water among the control
7.2
0g 3.125g 6.25g 12.5g 25g
TN (mg L-1 )
5.4
3.6
1.8
0.0
0
7
14
21
28
35
42
49
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70
Time (d) Fig. 4. Temporal variation of TN concentrations in overlying water (mean and standard deviation of triplicates are shown).
0g 3.125g 6.25g 12.5g 25g
0.8
-1
TP (mg L )
Van Meeteren et al., 2007). Therefore, it is risky to apply a unify decomposition model to all kinds of aquatic plant species, if this model is made only basing on the several plant species only. The change of TP concentrations in H. verticillata had a similar trend as most aquatic macrophytes reported by other researchers (Pagioro and Thomaz, 1999; Carvalho et al., 2005). It decreased fast in early decomposition period. Our experimental results showed that TP concentrations decreased from initial value 3.2 mg g−1 to 1–1.4 mg g−1 on day 14, and then to 1.7–2.2 mg g−1 on day 56, and remained the similar values until day 70. No significant difference was found between different treatment groups with plant materials. The rapid decrease in early decomposition period, according to Ferreira and Esteves (1992) could be attributed to the rapid loss of the cytoplasmic phosphorus from plant cells and its inability to replace it. TP concentrations increased in later period have occurred in many closed chambers (Pagioro and Thomaz, 1999), where part of the phosphorus released to the water became available to the attached microorganisms on the plant materials (Carvalho et al., 2005). Although most aquatic macrophytes share a similar trend in TP concentration change in plants during decomposition, the ratio of final TP concentration to initial TP concentration was quite different, even when sufficient experiment time is allowed to an almost stable TP concentration in the last experiment period. For example, Pagioro and Thomaz (1999) reported TP concentration in E. azurea deceased about 25% in the first 15 days. After that it increased to around 125.9% on day 60, and continuously increased to 175.6% on day 90. However, Ferreira and Esteves (1992) and Neiff and Poi de Neiff (1990), measured that the final TP concentrations were about 60% of initial values after 53 d and 97 d, respectively. Our study showed that the final TP concentrations were about 57% of the initial period. According to Van Huysen’s study on terrestrial plants (Van Huysen, 2009), the initial litter nutrient concentrations, being reflected by C:N:P ratio, are usually species-specific, leading to different decomposition patterns. Comparing the C:N:P ratios of H. verticillata with that of E. azurea, we found that they are quite different, i.e., 69.04:10.50:1 and 264.24:8.183:1, respectively. Therefore, element ratios could also be important predictors of TP transformations during aquatic macrophytes decomposition.
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0.6
0.4
0.2
0.0
0
7
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35 42 Time (d)
49
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70
Fig. 5. Temporal variation of TP concentrations in overlying water (mean and standard deviation of triplicates are shown).
treatment group and the treatment groups with plant biomass of 3.125 g and 6.25 g DO (dissolved oxygen) concentrations had similar trends with TN and TP concentrations in overlying water, and the treatment groups with 3.125 g and 6.25 g plant biomass had no significant differences with control treatment group after day 42. The above results indicated that water quality degraded dramatically in early decomposition period, and the treatment groups with less biomass will obtain the similar water quality as the treatment group without plant material. If the short time negative effect on water quality can be ignored, then the optimal plant biomass range is from 3.125 g to 6.25 g The corresponding optimal harvest biomass range for the lake with fresh biomass as high as 6 kg m−3 , is 75% to 87.5%. Considering the decomposition property of other submerged plant species living in the same area (Zhang et al., 2013), the general harvest ratio is recommended as 80%. 5. Conclusions (1) The overall decay process of the four biomass levels of H. verticillata at 17 ◦ C was well described by the exponential model, with the average decomposition rate of 0.018 day−1 . No significant correlation was found between the rate of decomposition
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and the initial biomass level according to Pearson correlation analysis. (2) The decomposition of H. verticillata increased total nitrogen and phosphorus level in the whole environment including overlying water, air and sediment. The change of TN and TP concentrations in plant material during decomposition shown in the present study is different from the results reported in many previous studies. This could be attributed to their different C:N ratio and carbon forms involved. (3) If the short time of 35–42 days negative effect on water quality is ignored, the optimal biomass harvest range for the lake with fresh biomass is between 75% and 87.5%, can be as high as 6 kg m−3 . Acknowledgments This research was jointly supported by National Water Pollution Control and Management Technology Major Projects of China (No. 2012ZX07101-009), and National Science Foundation of China (No. 40971277). References Asaeda, T., Trung, V.K., 2000. Modeling the effects of macrophyte growth and decomposition on the nutrient budget in Shallow Lakes. Aquat. Bot. 68, 217–237. Best, E., 1993. The impact of mechanical harvesting regimes on the species composition of Dutch ditch vegetation: a quantitative approach. Plant Manage. 31, 148–154. Boulton, A.J., Boon, P.I., 1991. A review of methodology used to measure leaf litter decomposition in lotic environments: time to turn over an old leaf? Aust. J. Mar. Freshwater Res. 42, 1–43. Carpenter, S., van Donk, E., Wetzel, R., 1998. Nutrient-loading gradient in shallow lakes: report of the group discussion. In: Jeppesen, E., Sondergaard, M., Christoffersen, K. (Eds.), The Structuring Role of Submerged Macrophytes in Lakes, 20. Springer, New York, NY, pp. 393–396. Carter, M.R., Gregorich, E.G., 2007. Soil sampling and Methods of Analysis (second edition). CRC press, Boca Raton, FL, USA. Carvalho, P., Thomaz, S.M., Bini, L.M., 2005. Effects of temperature on decomposition of a potential nuisance species: the submerged aquatic macrophyte Egria najas planchon (hydrocharitaceae). Braz. J. Biol. 65, 51–60. Chen, G.C., Tam, N.F.Y., Ye, Y., 2010. Summer fluxes of atmospheric greenhouse gases N2 O, CH4 and CO2 from mangrove soil in South China. Sci. Total Environ. 408, 2761–2767. Chen, K.N., Bao, C.H., Zhou, W.P., 2009. Ecological restoration in eutrophic lake Wuli—a large enclosure experiment. Ecol. Eng. 35, 1646–1655. De Busk, T.A., Dieberg, F.E., 1984. Effect of nitrogen and fiber content on the decomposition of the water hyacinth (Eichhornia crassipes [Mart.] Solms). Hydrobiologia 118, 199–204. Ferreira, C.M.L., Esteves, F.A., 1992. Decomposition of Potamogeton stenostachys K. Schum. and evaluation of its detritus as a potential energy source in a tropical coastal lagoon. Int. J. Ecol. Environ. Sci. 18, 47–54. Finnegan, J., Regan, J.T., O’Connor, M., Wilson, P., Healy, M.G., 2014. Implications of applied best management practice for peatland forest harvesting. Ecol. Eng. 63, 12–26.
Gessner, M.O., 2001. Mass loss, fungal colonization and nutrient dynamics of Phragmites australis leaves during senescence and early aerial decay. Aquat. Bot. 69, 325–339. Godshalk, G.L., Wetzel, R.G., 1977. Decomposition of macrophytes and the metabolism of organic matter in sediments. In: Golterman, H.L. (Ed.), Interactions Between Sediments and Fresh Water, 10. Dr. W. Junk Publishers, The Hague, pp. 258–264. Godshalk, G.L., Wetzel, R.G., 1978. Decomposition of aquatic angiosperms. III. Zostera marina L. and a conceptual model of decomposition. Aquat. Bot. 5, 329–354. Gumbricht, T., 1993. Nutrient removal processes in freshwater submersed macrophyte systems. Ecol. Model 2, 1–30. Knight, R., Gu, B., Clarke, R., Newman, J., 2003. Long-term phosphorus removal in Florida aquatic systems dominated by submerged aquatic vegetation. Ecol. Eng. 20, 45–63. Lal, R., Kimble, J.M., Follett, R.F., Stewart, B.A., 2001. Assessment Methods for Soil Carbon, 10. CRC, Press, Boca Raton, FL, pp. 349–359. Mcclanahan, T., 1996. Harvesting in an uncertain world-impact of resource competition on harvesting dynamics. Ecol. Modell. 80, 21–26. Mitsch, W., Zhang, L., Anderson, C., Altor, A., Hernández, M., 2005. Creating riverine wetlands: ecological succession, nutrient retention, and pulsing effects. Ecol. Eng. 25, 510–527. Moss, B., 1990. Engineering and biological approaches to the restoration from eutrophication of shallow lakes in which aquatic plant communities are important components. Hydrobiologia 200/201, 367–377. Neiff, J.J., Poi de Neiff, A., 1990. Litterfall, leaf decomposition and litter colonization of Tessaria integrifolia (Compositae) in the Paraná River floodplain. Hydrobiologia 203, 45–52. Pagioro, T.A., Thomaz, S.M., 1999. Decomposition of Eichhornia azurea from limnologically different environments of the upper Paraná river floodplain. Hydrobiologia 411, 45–51. Qin, B.Q., 2009. Lake eutrophication: control countermeasures and recycling exploitation. Ecol. Eng. 35, 1569–1573. Schaller, J., 2013. Invertebrate shredder as a factor controlling the fixation potential for metals/metalloids in organic matter during decay. Ecol. Eng. 53, 200–204. Serafy, J., 1994. Mechanical removal of Hydrilla in the Potomac river, Maryland: local impacts on vegetation and associated fishes. Freshwater Ecol. 9, 135–143. Standard Method for the Examination of Water and Wastewater Editorial Board, 2002. Standard Method for the Examination of Water and Wastewater. Environmental Science Press of China, Beijing (In Chinese). Troxler, T., Richards, J., 2009. ␦ 13C, ␦ 15N, carbon, nitrogen and phosphorus as indicators of plant ecophysiology and organic matter pathways in Everglades deep slough, Florida, USA. Aquat. Bot. 91, 157–165. Van Meeteren, M., Tietema, A., Westerveld, J., 2007. Regulation of microbial carbon, nitrogen, and phosphorus transformations by temperature and moisture during decomposition of Calluna vulgaris litter. Biol. Fertil. Soils 44, 103–112. Van Huysen, T.L., 2009. Nitrogen and phosphorus dynamics during decomposition of multiple litter types in temperate coniferous forests. In: Thesis (Ph.D.). Oregon State University. Wu, Z., Qiu, D., He, F., Fu, G., Cheng, S., Ma, J., 2003. Effects of rehabilitation of submerged macrophytes on nutrient level of a eutrophic lake. Ecol. Eng. 35, 1646–1655. Yu, H.C., Ye, C., Song, X.F., Liu, J., 2010. Comparative analysis of growth and physio-biochemical responses of Hydrilla verticillata to different sediments in freshwater microcosms. Ecol. Eng. 36, 1285–1289. Zehnder, A.J.B., 1988. Biology of Anaerobic Microorganisms. John Wiley, New York, NY. Zhang, L.Z., Ye, C., Li, C.H., Song, X.F., Kong, X.L., 2013. The effects of submerged macrophyte (Vallisneria natans) at different biomass levels in one-year life cycle on water quality. China Environ. Sci. 33 (11), 1779–1784 (in Chinese).
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Ecological Engineering journal homepage: www.elsevier.com/locate/ecoleng
The release of nitrogen and phosphorus during the decomposition process of submerged macrophyte (Hydrilla verticillata Royle) with different biomass levels Chun-hua Li a , Bo Wang a,b , Chun Ye a,∗ , Yu-xin Ba a a Centre of Lake Engineering & Technology, State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China b Eco-environmental Institute, Liaoning Shihua University, Fushun 113001, China
a r t i c l e
i n f o
Article history: Received 26 October 2013 Received in revised form 23 March 2014 Accepted 19 April 2014 Keyword: Decomposition Aquatic plant Submerged macrophytes Water quality Ecological restoration Hydrilla verticillata Royle
a b s t r a c t The purpose of the present study is to elucidate the effects of decomposition of different biomass levels of submerged macrophyte (Hydrilla verticillata Royle) on sediment, overlying water and air. Four biomass levels of submerged macrophyte were used. They are 25 g, 12.5 g, 6.25 g and 3.125 g in 4 L water and 200 g sediment were cultivated in decomposition experiment setting in order to simulate the temperature in Lake Taihu in spring. The 70-day decomposition experimental results showed that the overall decay process of the four biomass levels at 17 ◦ C can be well described by the exponential model, with the average decomposition rate of 0.018 day−1 . The decomposition of H. verticillata increased total nitrogen and phosphorus mass in the whole environment including overlying water, air and sediment. The optimal biomass harvest rate is 75% to 87.5% for a fresh water lake with 6 kg m−3 fresh biomass if the short-term negative effect on water quality is ignored. The present study could provide some scientific bases for aquatic macrophytes management. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Macrophytes are a vital part of lake ecosystem (Qin, 2009; Yu et al., 2010). They convert sunlight energy and chemical element, allowing these to be absorbed by living plants. Fish, waterfowl, insects, mammals, and microscopic animals consume the macrophytes as food. They also replenish the aquatic environment with dissolved oxygen, which is essential to aquatic animals. Submerged macrophytes are rooted in the bottom sediments and grow up through the water. They occupy the whole water column, range from the bottom to the surface of the lake. Besides, they have many important ecological functions, such as improving the self-purification capacity of a lake ecosystem (Moss, 1990; Knight et al., 2003; Wu et al., 2003; Schaller, 2013), and forming a reasonable ecological structure (Carpenter et al., 1998; Mitsch et al., 2005). However, when submerged macrophytes die and
∗ Corresponding author at: Research Center of Lake Ecological Environment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China. Tel.:+86 010 84915191; fax: +86 010 84915190. E-mail addresses: [email protected], [email protected] (C. Ye). http://dx.doi.org/10.1016/j.ecoleng.2014.04.011 0925-8574/© 2014 Elsevier B.V. All rights reserved.
decompose, photosynthetic production of oxygen ceases. The bacteria and fungi which decompose decaying plant material in turn use up the dissolved oxygen for respiration. Therefore, the control and management over submerged macrophytes are very important especially after they were restored in lakes. Among different guidelines for their biomass control and management, the impact of submerged macrophytes decomposition on lake water quality always attracts our attention (Troxler and Richards, 2009). Most studies on the decomposition of aquatic macrophytes focused on weight loss, temperature effect and other changes in the chemical composition of coarse particulate detritus over time (Pagioro and Thomaz, 1999; Carvalho et al., 2005). However, a few studies have examined the effects of the decomposition of aquatic macrophytes on the environment as a whole, including the conditions of sediment, overlying water and air when they were exposed to the decaying submerged macrophytes (Ferreira and Esteves, 1992; Gessner et al., 2001). Furthermore, even less studies have focused on comparing the effects of the different biomass levels of the submerged macrophytes (Best, 1993; Serafy, 1994; Mcclanahan, 1996). Harvesting is an effective way to remove nutrients (Gumbricht, 1993). However, the optimum amounts of harvesting, as well as its possible effects, are not fully understood
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Table 1 Nutrient status of sampling site in East Taihu Lake, China (mean and standard deviation of triplicates are shown; sampling time: September, 2011).
−1
Lake water (mg L ) Sediment (mg g−1 )
Total nitrogen
Total phosphorus
Total organic carbon
Dissolved oxygen
pH
1.3 ± 0.13 1.23 ± 0.28
0.05 ± 0.015 0.51 ± 0.17
1.31 ± 0.17 15.84 ± 1.12
7.21 ± 1.58 –
7.05 ± 0.42 –
(Asaeda and Trung, 2000). Some successful studies have been reported on simulating the harvesting amount by adding different amounts of objective aquatic plant in an experimental system (Carvalho et al., 2005). Additionally, the research on gas emission in nitrogen cycle during decomposition is seriously deficient. Hence, this study aims (i) to track the nitrogen and phosphorus contents in plants, sediment, overlying water and air in the period when they are exposed to the decomposition of submerged macrophytes; (ii) to evaluate the effects of decomposition process of the different biomass levels of submerged macrophytes on the whole environment, and (iii) to provide lake managers or planners information to assess the optimal harvesting amount on submerged macrophytes. 2. Materials and methods 2.1. Experimental submerged macrophyte A perennial submerged macrophyte Hydrilla verticillata Royle was used in this study. It is a native species to China and a member of the Hydrocharitaceae family. H. verticillata is widely used in lake ecological restorations in China because of its extensive growth in fresh water lakes, high adaptability and ability to improve water ecosystem (Yu et al., 2010). H. verticillata was sampled from East Taihu Lake (31◦ 01 50 N, 120◦ 20 18 E), China, in September 2011. The H. verticillata sample was kept in a healthy status and transported to our laboratory as soon as possible. Compared to other water areas in Lake Taihu, East Taihu Lake has better water quality (Table 1), and higher coverage of submerged macrophytes. The average lake water temperature in winter and in spring is 6 ◦ C and 17 ◦ C, respectively. According to in-situ investigation, H. verticillata Royle begins to decay in a large scale from early winter, but it will not reach the rapid decomposition stage until spring, and most plant tissue can be decomposed before summer. Therefore, the collected H. verticillata was kept in a freezer in the temperature of 6 ◦ C for one week to obtain the in-situ plant status in winter. To simulate the decomposition process in spring, the temperature was set at 17 ◦ C. 2.2. Experimental setup 5 L plastic buckets were used as experimental containers. 200 g sediment (Table 1) collected from East Taihu Lake was laid on the bucket bottom and 4 L simulated lake water was added to each bucket. The simulated lake water was made by adding nitrate, ammonia and phosphorus to DI water, in order to simulate total nitrogen (TN) and total phosphorous (TP) concentrations and pH value as the water qualities in East Taihu. According to the survey in East Taihu Lake, the maximum fresh biomass of H. verticillata is about 6 kg/m3 . Therefore, 25 g, 12.5 g, 6.25 g, 3.125 g fresh H. verticillata were used as different biomass levels in the present experiment to reflect the corresponding harvesting rates, i.e., 0%, 12.5%, 25% and 50%, respectively. A control treatment without any plant material and four treatments with 25 g, 12.5 g, 6.25 g and 3.125 g fresh plant materials, respectively, were included. Roots were removed while only the stems and leaves were used. The plant material was rinsed with tap water to remove adhering matter and was cut into many small segments in about 3 cm length.
25 g, 12.5 g, 6.25 g and 3.125 g fresh plant segments were put into micro-nanometer litter bags with pore size of 20 m. These micronanometer litter bags were kept in a freezer in the temperature of 6 ◦ C for one week. They were then transferred to 5 L-experimental buckets. The micro-nanometer litter bags were anchored on the surface of sediment layer. The decomposition experiment was conducted in an artificial climate chamber in the temperature of 17 ◦ C, with a light–dark cycle of 12:12 h and an illumination of 7000 lx (approximately 128 M photons m−2 s−1 of photosynthetically active maintained in radiation). Plants, overlying water and sediment were sampled and analyzed in the interval of 7, 14, 21, 28, 35, 42, 49, 56, 63, 70 d in the decomposition process. Three replicate buckets were scarified in each sampling. A total of 150 buckets were used. During decomposition process, a bit of phytoplankton was found only in last 5–10 days. These small amounts of phytoplankton were taken out from buckets to avoid their influence to light and oxygen condition.
2.3. Experimental analysis On each sampling day, plant material, sediment, overlying water and headspace gas were collected and analyzed. The whole plant material was taken from micro-nanometer litter bag. Its dry weight (DW) was measured by heating the part of plant material at 105 ◦ C for 10 min, then 80 ◦ C for 8 h. The contents of TN and TP in plant material were analyzed by total Kjeldahl nitrogen method and Mo–Sb colorimetric method after acid digestion (Carter and Gregorich, 2007). The concentrations of TN and TP in overlying water were measured according to the Standard Method for the Examination of Water and Wastewater Editorial Board (2002). The sediments were freeze-dried and pre-treated according to the standard methods (Lal et al., 2001). Their concentrations of TN and TP were analyzed by automatic Kjeldahl analyzer (VELP, UDK159), spectrophotometer (LAXCO, Inc. ALPHA1860) and total organic carbon analyzer (SHIMADZU TOC-VCPH), respectively. Gas flux was quantified by the static collection technique (Chen et al., 2010). Each bucket was blown for one minute by common air gas. It was then coiled with two layers of LDPE film for 6 h continuously (3 h in dark and 3 h in light). N2 O gas concentration was determined by Hewlett Packard 6890N GC which is equipped with a 63Ni electron capture detector (ECD) and a RT-QPlot column (RESTEK), and helium gas was used as the carrier gas at a flow rate of 6.5 mL min−1 . The temperature of the injector, column and detector was 100 ◦ C, 70 ◦ C, and 350 ◦ C, respectively. N2 gas concentration was determined directly by N2 detector (HF-800). The nitrogen emitted in gas form was evaluated by adding N2 -N and N2 O-N integral flux. Nitrogen content in plant, sediment and overlying water were calculated by multiplying TN concentration and the dried weight of biomass, respectively.
2.4. Statistical analysis An exponential model was applied to describe mass-loss over time at four biomass levels: Wt = W0 e−kt , where W0 is the initial weight, Wt is the weight at time t; k is the decomposition rate; and t is time in a day.
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100 3.125g 6.25g 12.5g 25g
DW (%)
80
60
40
20
0
0
7
14
21
28
35 42 Time (d)
49
56
63
70
Fig. 1. Change of residual plant biomass (dried weight, DW) during decomposition (mean and standard deviation of triplicates are shown).
The Pearson correlation coefficients (2-tailed test) between decomposition rate and initial plant biomass, the total gas-N mass and initial plant biomass were calculated. ANOVA for repeated measurement was used to test whether TN and TP in plant materials, sediment, overlying water and gas at the five biomass levels (including control treatment with 0 g plant biomass). All statistical analyses were conducted using the software SPSS 16.0 (SPSS Inc., Chicago, IL, USA). 3. Results 3.1. Rate of H. verticillata decomposition Fig. 1 shows the change of plant biomass with the change of decomposition time. The weight of plants in all treatment groups decreased with time, and at the end of experiment (70 d), the ratio of residual dried weights (DW) of plant to their initial DW was 23%, 24%, 31% and 32% for 3.125, 6.25, 12.5 and 25 g treatment groups, respectively. The overall decay process of the four biomass levels at 17 ◦ C was well described by the exponential model, with the average decomposition rate of 0.018 day−1 (R2 range from 0.9315 to 0.9634). No significant correlation was found between decomposition rate and the initial biomass level according to Pearson correlation analysis, although the decomposition rate has the order of 25 g (k = 0.02 day−1 ) >12.5 g (k = 0.019 day−1 ) >6.25 g and 3.125 g (both of them have the same k value, k = 0.016 day−1 ). It indicates that the decomposition rate is almost not influenced by its initial biomass level. 3.2. Change of nitrogen content during decomposition The changes of nitrogen content in plant, overlying water, gas, sediment and summary of them during decomposition are shown in Fig. 2. According to ANOVA analysis, all the treatment groups with different plant biomass levels had significant difference (F, P values for Fig. 2(a)–(e) are (F = 51.184, P = 0.000), (F = 17.526, P = 0.002), (F = 6.313, P = 0.043), (F = 47.622, P = 0.000) and (F = 16.967, P = 0.003), respectively). TN content in plants decreased more quickly in the first 28 days, remained almost unchanged after that. The more of the plant biomass in system, the more TN was released (Fig. 2(a)). For the treatment group with 25 g
plant material, nearly 40 mg TN was released. While for the treatment group with 3.125 g plant material, only 4 mg TN was released. For the overlying water, except the control treatment group, all the treatment groups with plant material had a similar trend that their TN contents climbed to the peak then slipped down gradually and the values on day 70 were lower than their initial values on day 0 (Fig. 2(b)). The order of the maximum value in overlying water was 25 g treatment group > 12.5 g treatment group > 6.25 g treatment group > 3.125 g treatment group > 0 g treatment group. In 70 days experimental period, the TN content in control treatment group (0 g) was decreased slowly from 5.8 mg to 0.03 mg. The increase of TN in overlying water was mainly contributed by the plant decomposition, while the loss of TN was mainly attributed to denitrification and sediment adsorption or absorption. Denitrification could be reflected by the level of N2 and N2 O gas emitted from the system. Fig. 2(c) shows the change of nitrogen content in gas emission during 70 days. With time more and more N2 and N2 O gas was emitted. The total gas-N mass has a positive relationship with the plant biomass in the system (r = 0.997, P < 0.01). As many as 37 mg nitrogen was emitted in the treatment group with 25 g plant material but only around 10 mg nitrogen was emitted in the one with 3.125 g plant. The change of nitrogen content in sediment was more complicated than those of plants, overlying water and gas, as shown in Fig. 2(d). Fluctuations were shown in each treatment group, which indicated nitrogen content in sediment was affected by various factors, such as nitrogen released from plant and water, denitrification from bacterial activity (Zehnder, 1988). Therefore, different trends reflect different driving factors. Sediment-nitrogen content in control treatment group rose slowly from 246 mg on day 0 to 249 mg on day 70. The treatment group with the highest plant biomass (25 g) kept the highest nitrogen content in sediment among all the treatment groups, and increased to 254 mg on day 70. However, the treatment groups with less plant biomass (3.125 g, 6.25 g and 12.5 g) increased slightly in the first 35 days then declined gradually to the level lower than the control treatment group on day 70 or lower than the level they had in their initial value on day 0. This implies that decomposition of certain biomass of plant is beneficial for nitrogen removal from sediment. However, if we observe the total effect of plant decomposition on overlying water, gas and sediment (Fig. 2(e)), we will find that the decomposition process actually increased total nitrogen content in the environment as a whole. The released nitrogen had significant positive correlation with the plant biomass (r = 0.999, P < 0.01). The above results suggested that the decomposition of submerged macrophytes increases total nitrogen mass in the whole environment including overlying water, air and sediment. During 70 days of decomposition, total nitrogen increased 5 mg to 43 mg from low to high plant biomass levels. While the nitrogen in plant materials just decreased about 4 mg to 39 mg from low to high biomass levels. The level of reduced nitrogen in plant materials is slightly lower than the level of increased nitrogen in the environment, including overlying water, gas and sediment. These differences could be attributed to the loss of plant detritus from micro-nanometer litter bags to the surrounding water or sediment. 3.3. Change of phosphorous content during decomposition In the phosphorous cycle of plant decomposition, nearly no gas-phosphorus is released (Boulton and Boon, 1991). Therefore, only TP concentration in plant, sediment and overlying water were measured. The changes of phosphorus content in plant, overlying water, sediment and its summary are shown in Fig. 3. The control treatment group without plant material did not show any significant change during the whole decomposition period. Similar with TN, all the treatment groups with different plant biomass
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Fig. 2. Change of TN contents in plant, overlying water, sediment and gas (mean and standard deviation of triplicates are shown; plant-N: nitrogen content in plant material; overlying water-N: nitrogen content in overlying water; sediment-N: nitrogen content in sediment; gas-N: nitrogen content in headspace gas; overlying water + sediment + gas-N: total nitrogen content in overlying water, sediment and headspace gas).
levels had significant difference in each kind of phosphorus content according to ANOVA analysis (F, P values for Fig. 3(a)–(d) are (F = 18.797, P = 0.002), (F = 15.659, P = 0.003), (F = 9.926, P = 0.012) and (F = 19.134, P = 0.002), respectively). However, the rapid changing stage for TP is shorter than those in TN, 30% to 80% TP in plant materials decreased in the first 7 days and declined slightly until day 28 and then remained in the similar values from day 28 to day 70. TP contents in overlying water of the treatment groups with plant materials reached their maximum values on day 7, after that they decreased slowly and remain
stable from day 35 to day 70. During the whole decomposition period, TP values increased fast in the first 7 days after adding those from sediment and overlying water together and then rose slightly in the treatment groups with higher plant biomass level or remained almost stable for the treatment group with lower biomass level. At the end of experiment, the TP contents of different treatment groups were in increasing order as follows: 25 g treatment group > 12.5 g treatment group > 6.25 g treatments group > 3.125 g treatment group > control treatment group.
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Fig. 3. Change of TP contents in plant, overlying water and sediment (mean and standard deviation of triplicates are shown; plant-P: phosphorous content in plant material; overlying water-P: phosphorous content in overlying water; sediment-P: phosphorous content in sediment; sediment + overlying water-P: total phosphorous content in sediment and overlying water).
4. Discussion 4.1. Decomposition properties and main influencing factors of H. verticillata According to previous studies, the decomposition rate of aquatic plants varies with many factors, such as temperature, aerobic or anaerobic condition, nitrogen, carbon and phosphorus contents in plant and the ratio of rapid and slow decomposable organic matter (Asaeda and Trung, 2000; Carvalho et al., 2005). Among them, temperature, the decomposition rate of slow decomposable material and the fraction of rapid decomposable material are relatively significant (Asaeda and Trung, 2000). For example, Carvalho et al. (2005) reported that the decomposition rate of submerged macrophyte Egeria najas Planchon was 0.014 day−1 and 0.045 day−1 at 17 ◦ C and 27 ◦ C, respectively. The decomposition rate of slow decomposable material and the fraction of rapid decomposable material are varied with species-specific content of structural materials, such as cellulose and lignin (Godshalk and Wetzel, 1977, 1978). The decomposition rate of macrophyte biomass is strongly dependent on the level of original fiber, hemi-cellulose, cellulose, and lignin. For example, Scirpus acutus with 69.7% fiber content (dried weight) has a decomposition rate of 0.011 day−1 , which is three times lower than that of Myriophyllum heterophyllum, which has a decomposition rate of 0.037 day−1 with 32.3% of fiber content (Godshalk and Wetzel, 1978). The plant (H. verticillata) used in
the present experiment has a fiber content of 22.4% (dried weight). Its decomposition rate is much higher than that of S. acutus and M. heterophyllum. Additionally, Fig. 1 shows that the decomposition rates were higher during the first 28 days, about 50% plant biomass was decomposed, mainly because of the presence of the rapid decomposable material, but the rates gradually slowed down afterwards. The decreases in nitrogen level of H. verticillata during decomposition were in the same order of those obtained from other similar aquatic macrophytes and those under similar environmental temperature (De Busk and Dieberg, 1984; Pagioro and Thomaz, 1999; Carvalho et al., 2005). However, the change of TN concentrations in plant materials was different from some other submerged macrophytes, such as Eichhornia azurea and Vallisneria natans (Pagioro and Thomaz, 1999). Our results showed that TN concentrations in plant materials decreased from initial period to the end of decomposition process, while other researchers reported TN concentrations increased during decomposition. These differences are attributed to the different carbon form and C:N ratio. Both E. azurea and V. natans have higher C:N ratio (34.9:1 and 21.3:1, respectively) than H. verticillata has (only 6.57:1), and the former two species have more non-biodegradable carbon materials, such as lignin and cellulose. High carbon ratio and non-biodegradable carbon were considered as two important factors affecting microbial activity and population negatively, especially inhibiting the immobilization of nitrogen by microorganisms (Pagioro and Thomaz, 1999;
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4.2. Some suggestions on aquatic macrophytes management The goal of aquatic management should never be to eliminate aquatic macrophytes from a lake totally, though plant decomposition increases nutrient level in sediment and overlying water. Aquatic macrophytes have many irreplaceable functions, such as providing habitat and shelters for fish and other wildlife, providing food for insects and aquatic animals, producing oxygen and stabilizing shorelines and bottom sediment (Chen et al., 2009; Qin, 2009). However, the function of aquatic macrophytes on enhancing water quality is still questionable. An optimal harvesting biomass is urgently needed in aquatic macrophytes management (Finnegan et al., 2014). The results in present study may provide planners some ideas to come up with measures in assessing effective harvesting policies to enhance water quality. The changes of TN concentrations in overlying water are shown in Figs. 4 and 5, respectively. Although both of them increased in early period, in the first 7 days or 14 days depending on different biomass levels, they began to decline and some of treatments had lower values than their initial TN and TP concentrations afterward. From day 42 to day 70, no significant differences were found on TN concentrations among the control treatment group and the treatment groups with plant biomass of 3.125 g, 6.25 g and 12.5 g While from day 35 to day 70, no significant differences were found on TP concentrations in overlying water among the control
7.2
0g 3.125g 6.25g 12.5g 25g
TN (mg L-1 )
5.4
3.6
1.8
0.0
0
7
14
21
28
35
42
49
56
63
70
Time (d) Fig. 4. Temporal variation of TN concentrations in overlying water (mean and standard deviation of triplicates are shown).
0g 3.125g 6.25g 12.5g 25g
0.8
-1
TP (mg L )
Van Meeteren et al., 2007). Therefore, it is risky to apply a unify decomposition model to all kinds of aquatic plant species, if this model is made only basing on the several plant species only. The change of TP concentrations in H. verticillata had a similar trend as most aquatic macrophytes reported by other researchers (Pagioro and Thomaz, 1999; Carvalho et al., 2005). It decreased fast in early decomposition period. Our experimental results showed that TP concentrations decreased from initial value 3.2 mg g−1 to 1–1.4 mg g−1 on day 14, and then to 1.7–2.2 mg g−1 on day 56, and remained the similar values until day 70. No significant difference was found between different treatment groups with plant materials. The rapid decrease in early decomposition period, according to Ferreira and Esteves (1992) could be attributed to the rapid loss of the cytoplasmic phosphorus from plant cells and its inability to replace it. TP concentrations increased in later period have occurred in many closed chambers (Pagioro and Thomaz, 1999), where part of the phosphorus released to the water became available to the attached microorganisms on the plant materials (Carvalho et al., 2005). Although most aquatic macrophytes share a similar trend in TP concentration change in plants during decomposition, the ratio of final TP concentration to initial TP concentration was quite different, even when sufficient experiment time is allowed to an almost stable TP concentration in the last experiment period. For example, Pagioro and Thomaz (1999) reported TP concentration in E. azurea deceased about 25% in the first 15 days. After that it increased to around 125.9% on day 60, and continuously increased to 175.6% on day 90. However, Ferreira and Esteves (1992) and Neiff and Poi de Neiff (1990), measured that the final TP concentrations were about 60% of initial values after 53 d and 97 d, respectively. Our study showed that the final TP concentrations were about 57% of the initial period. According to Van Huysen’s study on terrestrial plants (Van Huysen, 2009), the initial litter nutrient concentrations, being reflected by C:N:P ratio, are usually species-specific, leading to different decomposition patterns. Comparing the C:N:P ratios of H. verticillata with that of E. azurea, we found that they are quite different, i.e., 69.04:10.50:1 and 264.24:8.183:1, respectively. Therefore, element ratios could also be important predictors of TP transformations during aquatic macrophytes decomposition.
273
0.6
0.4
0.2
0.0
0
7
14
21
28
35 42 Time (d)
49
56
63
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Fig. 5. Temporal variation of TP concentrations in overlying water (mean and standard deviation of triplicates are shown).
treatment group and the treatment groups with plant biomass of 3.125 g and 6.25 g DO (dissolved oxygen) concentrations had similar trends with TN and TP concentrations in overlying water, and the treatment groups with 3.125 g and 6.25 g plant biomass had no significant differences with control treatment group after day 42. The above results indicated that water quality degraded dramatically in early decomposition period, and the treatment groups with less biomass will obtain the similar water quality as the treatment group without plant material. If the short time negative effect on water quality can be ignored, then the optimal plant biomass range is from 3.125 g to 6.25 g The corresponding optimal harvest biomass range for the lake with fresh biomass as high as 6 kg m−3 , is 75% to 87.5%. Considering the decomposition property of other submerged plant species living in the same area (Zhang et al., 2013), the general harvest ratio is recommended as 80%. 5. Conclusions (1) The overall decay process of the four biomass levels of H. verticillata at 17 ◦ C was well described by the exponential model, with the average decomposition rate of 0.018 day−1 . No significant correlation was found between the rate of decomposition
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and the initial biomass level according to Pearson correlation analysis. (2) The decomposition of H. verticillata increased total nitrogen and phosphorus level in the whole environment including overlying water, air and sediment. The change of TN and TP concentrations in plant material during decomposition shown in the present study is different from the results reported in many previous studies. This could be attributed to their different C:N ratio and carbon forms involved. (3) If the short time of 35–42 days negative effect on water quality is ignored, the optimal biomass harvest range for the lake with fresh biomass is between 75% and 87.5%, can be as high as 6 kg m−3 . Acknowledgments This research was jointly supported by National Water Pollution Control and Management Technology Major Projects of China (No. 2012ZX07101-009), and National Science Foundation of China (No. 40971277). References Asaeda, T., Trung, V.K., 2000. Modeling the effects of macrophyte growth and decomposition on the nutrient budget in Shallow Lakes. Aquat. Bot. 68, 217–237. Best, E., 1993. The impact of mechanical harvesting regimes on the species composition of Dutch ditch vegetation: a quantitative approach. Plant Manage. 31, 148–154. Boulton, A.J., Boon, P.I., 1991. A review of methodology used to measure leaf litter decomposition in lotic environments: time to turn over an old leaf? Aust. J. Mar. Freshwater Res. 42, 1–43. Carpenter, S., van Donk, E., Wetzel, R., 1998. Nutrient-loading gradient in shallow lakes: report of the group discussion. In: Jeppesen, E., Sondergaard, M., Christoffersen, K. (Eds.), The Structuring Role of Submerged Macrophytes in Lakes, 20. Springer, New York, NY, pp. 393–396. Carter, M.R., Gregorich, E.G., 2007. Soil sampling and Methods of Analysis (second edition). CRC press, Boca Raton, FL, USA. Carvalho, P., Thomaz, S.M., Bini, L.M., 2005. Effects of temperature on decomposition of a potential nuisance species: the submerged aquatic macrophyte Egria najas planchon (hydrocharitaceae). Braz. J. Biol. 65, 51–60. Chen, G.C., Tam, N.F.Y., Ye, Y., 2010. Summer fluxes of atmospheric greenhouse gases N2 O, CH4 and CO2 from mangrove soil in South China. Sci. Total Environ. 408, 2761–2767. Chen, K.N., Bao, C.H., Zhou, W.P., 2009. Ecological restoration in eutrophic lake Wuli—a large enclosure experiment. Ecol. Eng. 35, 1646–1655. De Busk, T.A., Dieberg, F.E., 1984. Effect of nitrogen and fiber content on the decomposition of the water hyacinth (Eichhornia crassipes [Mart.] Solms). Hydrobiologia 118, 199–204. Ferreira, C.M.L., Esteves, F.A., 1992. Decomposition of Potamogeton stenostachys K. Schum. and evaluation of its detritus as a potential energy source in a tropical coastal lagoon. Int. J. Ecol. Environ. Sci. 18, 47–54. Finnegan, J., Regan, J.T., O’Connor, M., Wilson, P., Healy, M.G., 2014. Implications of applied best management practice for peatland forest harvesting. Ecol. Eng. 63, 12–26.
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