3.1 Analysis of accumulation characteristics of TN, TP and OM in Sediment
TN is an important indicator to evaluate the nutrient content of the substrate, and it plays an important role in the eutrophication state of the substrate. When the N in the substrate exceeds the extreme capacity it can accommodate, the "supersaturated" substrate becomes a source of pollution, releasing N and other nutrients to the external water body, intensifying the eutrophication state of the water body, and even destroying the nutrients cycle in lakes and wetlands (José et al., 1999; Zhang et al., 2018;Huang et al., 2021). Figure 3 shows how the content of TN in the substrate varies in SFCWs at all levels, and how its content ranges from 0.65 g/kg to 3.78 g/kg, with an average value of 2.30 ± 0.006 g/kg. The content is higher in the fourth, fifth and sixth level areas, which may be related to the water flow rate factor. Comparing with the studies on the sediment of Baiyangdian, Zhongba River and Yanlong Lake, the sediment of Mata Lake has a higher TN content and the sediment is in a eutrophic state, which may be related to factors such as the presence of aquatic and terrestrial organisms inside the wetlands of Mata Lake, rich in species and a more complex environment (Zhu et al., 2018; Zhu et al., 2019 ;Yang et al., 2021). In the same area, the TN content in the sediment of the first-level SFCWs is decreasing along the direction of water flow, probably because the hydraulic load at the first-level SFCWs of Mata Lake is small, so that the sewage can stay in the SFCWs for a longer time, and the nutrients in the sediment are fully purified at the inlet and the middle, and less accumulated at the outlet. However, excessive N accumulated in the substrate at the northern outlet area of the SFCWs may exacerbate the risk of eutrophication in the SFCWs and have a negative impact on the growth of reeds and other SFCWs plants, which is detrimental to the purification of the overlying water bodies and reduces the operational efficiency of the SFCWs.
TP is not only an important criterion for indicating eutrophication of the substrate, but also important for maintaining the dynamic balance of P interchange between the substrate and aquatic plants (Chen et al., 2020; Gurung et al., 2020). Figure 4 shows that the TP content in the sediment of Mata Lake SFCWs ranged from 0.02 g/kg to 0.46 g/kg, with a mean value of 0.26 ± 0.0006 g/kg, being highest at the SFCWs inlet and in the northern region, showing some variability. The overall TP content was less compared with that in Dongping Lake, Dongting Lake, Baiyang Lake and other lakes, which may be related to the inlet water quality conditions, vegetation cover and other factors which present better results for plant growth, adsorption of pollutants by the substrate, and thus purification and treatment of sewage (Chen et al., 2014; Zhu et al., 2018;Li et al., 2021). Vergeles et al. (2016) studied the nutrients of substrate in an CWs in Ukraine that had been in operation for 8 years, and the better water quality of the influent water may be an important reason for the lower TP content in the substrate.
OM can reflect the organic pollution status in the substrate and also plays an important role in the transport of nutrients such as carbon (C), N and P (Zhao et al., 2019). It has been shown that OM in the substrate releases C, N, P and other nutrients into the water column when mineralized, causing eutrophication in the water column (Yuan et al., 2021). Figure 5 shows that the OM content in the sediment of Mata Lake SFCWs ranged from 4.53 g/kg to 20.79 g/kg, with a mean value of 10.49 ± 0.04 g/kg and a maximum content of 20.79 g/kg, of which the content was higher in the middle of the first-level SFCWs and the sixth-level SFCWs area. The OM was enriched in the surface substrate, resulting in a high OM content. However, compared with Li lake and Nan lake, the overall content of OM in the bottom sediment of Mata Lake is lower. It may be related to factors such as the high oxygen content in the surface sediment of Mata Lake SFCWs, which is conducive to the mineralization of organic carbon, and the strong plant growth stage, which has a strong effect on the absorption and utilization of organic carbon in the sediment. (Guo et al., 2020; Li et al., 2021; Qiao et al., 2021; Wu et al., 2021).
3.2 Evaluation of nutrients Pollution in Sediment
The nutrients pollution status of the substrate at each sampling location was evaluated as shown in Table 3. The Org-index, Org-N index and TP pollution index were used to evaluate the pollution status.
The range of Org-index in the sediment of each sampling point was 0.02–0.41, and the average value was 0.14. The comprehensive evaluation of Org-index was slight pollution, and the pollution status of 17 sampling points were clean, slight pollution and moderate pollution. Of these, 11.8% were in the clean level, 70.6% were in the slight pollution level, and 17.6% were in the moderate pollution level.
The range of Org-N was 0.06–0.36, with the mean value of 0.22. The comprehensive evaluation result of Org-N is heavy pollution, with the mean value of 0.22, which is about 1.65 times of the limit value of 0.133 for heavy pollution, and the evaluation of the pollution condition of Mata Lake is serious. Four medium pollution sampling points exist in the evaluation of Org-N in the sediment of 17 sampling points, which may further develop into heavy pollution.
The evaluation results of TP range from 0.08 to 0.77, with a mean value of 0.43, and the comprehensive evaluation of the pollution index is clean. Among these 17 sampling points, there are four sampling points evaluated as lightly polluted, accounting for 23.5% of all sampling points, and the rest of the pollution conditions are evaluated as clean, indicating that the sediment of Mata Lake is less polluted by P.
Studies have shown that when C/N < 10, the source of OM pollution is usually considered as endogenous. When C/N ≈ 10, it indicates that the endogenous and exogenous sources of OM pollution are in balance, while C/N > 10 means that the source of OM pollution is exogenous (Zhang et al., 2015; Liang et al., 2018). The maximum value of C/N in the wetlands sediment of Mata Lake was 9.18, the minimum value was 0.84, and the average value was 3.26. The C/N values of all sampling sites were less than 10, and their OM pollution thus showed strong signs of endogenous pollution.
Studies have shown that plankton in water will absorb or release N and P elements in a certain proportion during the growth, reproduction and decay stages, and if N/P is close to 16, it indicates that the source of P is endogenous pollution (Li et al., 2017; Li et al., 2021). None of the N/P values in the sediment sampling sites of Mata Lake wetlands were in the range of 14–18, indicating that the source of P in the sediment of Mata Lake wetlands was exogenous pollution.
Table 3
Evaluation results for pollution in sediments
Sample points
|
Org-index evaluation results
|
Org-N evaluation results
|
TP evaluation results
|
|
Level
|
Pollution Degree
|
Org-N/%
|
Level
|
Pollution Degree
|
Pi
|
Level
|
Pollution Degree
|
1
|
0.11
|
Ⅱ
|
Slight Pollution
|
0.14
|
Ⅳ
|
Heavy Pollution
|
0.64
|
Ⅱ
|
Slight Pollution
|
2
|
0.07
|
Ⅱ
|
Slight Pollution
|
0.11
|
Ⅲ
|
Moderate Pollution
|
0.45
|
Ⅰ
|
Clean
|
3
|
0.10
|
Ⅱ
|
Slight Pollution
|
0.10
|
Ⅲ
|
Moderate Pollution
|
0.47
|
Ⅰ
|
Clean
|
4
|
0.03
|
Ⅰ
|
Clean
|
0.10
|
Ⅲ
|
Moderate Pollution
|
0.43
|
Ⅰ
|
Clean
|
5
|
0.02
|
Ⅰ
|
Clean
|
0.06
|
Ⅱ
|
Slight Pollution
|
0.39
|
Ⅰ
|
Clean
|
6
|
0.07
|
Ⅱ
|
Slight Pollution
|
0.12
|
Ⅲ
|
Moderate Pollution
|
0.33
|
Ⅰ
|
Clean
|
7
|
0.07
|
Ⅱ
|
Slight Pollution
|
0.21
|
Ⅳ
|
Heavy Pollution
|
0.34
|
Ⅰ
|
Clean
|
8
|
0.10
|
Ⅱ
|
Slight Pollution
|
0.19
|
Ⅳ
|
Heavy Pollution
|
0.28
|
Ⅰ
|
Clean
|
9
|
0.12
|
Ⅱ
|
Slight Pollution
|
0.27
|
Ⅳ
|
Heavy Pollution
|
0.49
|
Ⅰ
|
Clean
|
10
|
0.18
|
Ⅱ
|
Slight Pollution
|
0.35
|
Ⅳ
|
Heavy Pollution
|
0.44
|
Ⅰ
|
Clean
|
11
|
0.11
|
Ⅱ
|
Slight Pollution
|
0.36
|
Ⅳ
|
Heavy Pollution
|
0.08
|
Ⅰ
|
Clean
|
12
|
0.21
|
Ⅲ
|
Moderate Pollution
|
0.25
|
Ⅳ
|
Heavy Pollution
|
0.47
|
Ⅰ
|
Clean
|
13
|
0.13
|
Ⅱ
|
Slight Pollution
|
0.27
|
Ⅳ
|
Heavy Pollution
|
0.39
|
Ⅰ
|
Clean
|
14
|
0.24
|
Ⅲ
|
Moderate Pollution
|
0.28
|
Ⅳ
|
Heavy Pollution
|
0.51
|
Ⅱ
|
Slight Pollution
|
15
|
0.19
|
Ⅱ
|
Slight Pollution
|
0.32
|
Ⅳ
|
Heavy Pollution
|
0.52
|
Ⅱ
|
Slight Pollution
|
16
|
0.17
|
Ⅱ
|
Slight Pollution
|
0.24
|
Ⅳ
|
Heavy Pollution
|
0.39
|
Ⅰ
|
Clean
|
17
|
0.41
|
Ⅲ
|
Moderate Pollution
|
0.34
|
Ⅳ
|
Heavy Pollution
|
0.77
|
Ⅰ
|
Slight Pollution
|
Average value
|
0.14
|
Ⅱ
|
Slight Pollution
|
0.22
|
Ⅳ
|
Heavy Pollution
|
0.43
|
Ⅰ
|
Clean
|
3.3 Trends in nutrients release
With regard to the release trends of TN, the three column samples A, B, and C were roughly the same with the release which was more stable in the first four days, experiencing a rising state from days 4 to 12, and reaching a peak on day 12 before decreasing to a steady state with a peak of 19.00 mg/L (Fig. 6). The increasing trend of TN content in the water column with time indicates that the bottom sediment acts as the water column pollution "source", which can release N to the overlying water bodies (Yuan et al., 2020). The TN content of the three column samples A, B and C were at low and stable levels in the first four days, indicating that the adsorption of N in sediment by the overlying water is in equilibrium. The TN content in the bottom sediment was higher than that in the overlying water, and the release of N from the bottom sediment to the overlying water led to the increase of TN content in the water, indicating that the release of N from the bottom sediment may be the main reason for the increase of N content in the overlying water. It has been shown that when the equilibrium between the N content of the overlying water column and the substrate is broken, the difference in TN content between the two becomes the driving force for N migration (Chen et al., 2020). So, in 12–16 days, the imbalance of N caused the substrate to adsorb TN from the water column again, resulting in a decrease in the TN content of the overlying water column.And return to a stable TN content in the water at the end of the experiment under the exchange of TN between the overlying water and the substrate.
The TP content in the overlying water column A sample was stable from day 0 to day 1, increasing and then decreasing from day 1 to day 4,then passing a stable period until it gradually increased after day 9 and reached a peak on day 20, then showed a decreasing trend with a peak of 0.145 mg/L. In water column B sample,the TP content increased first and then decreased,and then kept rising steadily after day 4, and continued to increase after reaching a high concentration on day 20. The migration factors of P and N in the bottom sediment are similar. When the TP content in the bottom sediment is higher than the TP content in the overlying water body, the bottom sediment will release P to the overlying water body, and vice versa: if the TP content in the bottom sediment is less than the TP content in the overlying water body, the P in the water body is adsorbed by the substrate. It has been shown that the adsorption capacity of the substrate is related to the OM and calcium content (Zhang et al., 2016), and the TP content of column A and B samples showed a large fluctuation at the first 4 days, which may be attributed to the OM content in the substrate at the inlet and the middle area was higher,compared with the outlet, and to the fact that the mineralization process of OM has a facilitating effect on the release of OM (Cao et al., 2011). Therefore, column A and B samples showed a different trend from column C samples in the first four days. After reaching the maximum release rate on days 16–20, column A, B, and C samples showed a decreasing, increasing, and stabilizing trend, respectively, which may be related to the saturation of TP adsorption by the substrate in the column samples.
COD is an important indicator to evaluate the degree of organic pollution in the water body. The higher COD content in the water body, the more dissolved oxygen needs to be consumed and the more likely it is to cause water body anoxia, which is adverse to the growth and reproduction of aerobic organisms in the water body. Excessive COD content can even cause the death of aerobic and proliferation of anaerobic organisms, and thereby accelerate the deterioration of water quality (Li et al., 2018). On days 0–4, the COD content in the overlying water of the three column samples A, B and C decreased and then fluctuated to different degrees from day 4 to 20, with the content of column A increasing to a peak (47 mg/L) on day 9 and the contents of columns B and C increasing to their peaks (43 and 52 mg/L, respectively) on day 16. In all three columns, the COD content decreased to the lowest point on day 20, with the lowest value being in column B (4 mg/L). After day 20 all three columns showed an increasing trend again.
The maximum release rate of TN from column A appeared on days 4–6, while the maximum release rate of TN from columns B and C was on days 9–12; the maximum release rate of TP from column A appeared on days 1–2, while the maximum release rate of TP from column B was on days 0–1, and the maximum release rate of TP from column C was on days 16–20.
Through the analysis of the static release pattern of nutrients in the bottom sediment of Mata Lake wetlands, it was found that the TN content in the water body was stable on the first 4 days and increased on the 5th day; although the TP content of the columns A and B fluctuated in the first few days(C column held steady), it was reduced to a stable state on the 4th day; the COD content of the water body showed a overall trend of reduction on the first 4 days. In summary, the optimal hydraulic retention time of the SFCWs in Mata Lake should be 4 days.
Table 4
Release rate of nutrients in sediment / mg·(m2·d)−1
Times/d
|
TN
|
TP
|
A
|
B
|
C
|
A
|
B
|
C
|
0–1
|
2.81
|
-20.77
|
-8.67
|
0.14
|
2.22
|
0.16
|
1–2
|
-6.8
|
0.00
|
-4.45
|
3.25
|
-1.46
|
0.08
|
2–4
|
18.28
|
14.37
|
20.91
|
-1.65
|
-0.48
|
-0.10
|
4–6
|
179.20
|
141.28
|
90.73
|
0.26
|
0.25
|
0.29
|
6–9
|
-3.83
|
13.67
|
34.06
|
-0.05
|
-0.05
|
-0.08
|
9–12
|
166.59
|
156.91
|
138.00
|
0.53
|
0.34
|
0.42
|
12–16
|
-157.57
|
-158.45
|
-162.49
|
0.05
|
0.07
|
-0.07
|
16–20
|
-2.95
|
4.31
|
22.48
|
1.06
|
0.84
|
0.90
|
20–26
|
-1.48
|
-1.21
|
-0.20
|
-0.22
|
0.08
|
0.00
|
Negative values indicate that the nutrients in the sediment are adsorbed.
3.4 Enlightenment of eutrophication prevention
By analyzing the pollution of TN, TP and OM in the bottom sediment, the high TN and OM content in the bottom sediment of Mata Lake wetlands has seriously exceeded the standard of eutrophication. Wetlands plants can be planted in order to absorb and utilize nutrients in the substrate, and at the same time, proper harvesting time can also enable to obtain maximum nutrients removal efficiency. Studies have shown that in addition to using wetlands plants to purify wetlands substrate, wetlands organisms such as grass carpenter, earthworms and snails in long-running wetlands also play an important role in substrate purification (Deng.et al., 2018). By matching and combining different species of wetlands animals and plants, maximum purification effect can be achieved.
The OM pollution in the sediment of Mata Lake wetlands is mainly endogenous, and the treatment of OM pollution should be started from the inside of the wetlands by planting plants with strong pollution tolerance, developed root system, and strong environmental adaptability to adsorb and utilize OM in the sediment to reduce the OM content in the sediment. The source of P pollution is mainly exogenous, so controlling the water quality monitoring of the wetland's intake, optimizing the water intake pattern, and reducing the P are the key issues here.
According to the static release simulation experiment, the analysis of the release pattern of nutrients in the bottom mud of Mata Lake wetlands after long-term operation shows that the increase of TN and TP in the overlying water body is mainly due to the imbalance of N and P contents in the bottom mud and the overlying water body, which leads to the release of N and P elements from the bottom mud to the water body, thus increasing its N and P elements content and aggravating the eutrophication of the water body.
According to the analyses of the release patterns of TN, TP and COD contents from the substrate to the overlying water body, it is believed that the releases can be inhibited in certain degree by increasing the hydraulic load in the short term. It was found that submerged plant can not only dissipate the N and P released from the sediment, but also inhibit the release of N and P from the substrate to the water body substrate (Wang et al., 2018;Andrzej et al., 2019; Haziq Jamil et al., 2019). Thus, the release of nutrients from the substrate can be improved by planting. Indeed, the nutrients content of the overlying water body can be improved by planting plants with better inhibitory effect on nutrients release from the substrate; meanwhile, the nutrients content in the substrate should be monitored regularly, and timely analysis,treatment and dredging work can be done.