3.1 Basic physicochemical parameters of the lake water and sediment
The basic physicochemical parameters of the lake water and sediment at the sampling sites are shown in Table 1. As for the lake water observation, the pH and water temperature of the five sampling sites were approximately 8 and 25°C, respectively. The dissolved oxygen of s1, and s2 was higher than that of s3, s4, and s5, but the electrical conductivity was evidently lower than that of s3, s4, and s5. In terms of sediment, the average sediment water content of s1, and s2 was nearly 64.35%, and that of the s3, s4, and s5 was approximately 50.24%. The difference in the sediment water content was not significant. However, the loss on ignition of s3, s4, and s5 was approximately eight times that of s1, and s2 because of the presence of more aquatic plants in the east of Lake Taihu (Ticha et al., 2019).
Table 1
Mean values of basic physicochemical parameters of lake water and sediment (the basic physicochemical parameters of lake water were measured at the sampling sites and those of sediment were determined in laboratory; all samples, n = 3).
Site
|
Lake water
|
Sediment
|
pH
|
Water temperature (°C)
|
Dissolved oxygen (mg·L− 1)
|
Electrical conductivity (ms·cm− 1)
|
Water content (%)
|
Loss on ignition (%)
|
s1
|
7.55
|
24.5
|
8.59
|
0.30
|
61.72
|
1.76
|
s2
|
7.79
|
24.9
|
9.07
|
0.37
|
66.97
|
1.67
|
s3
|
8.91
|
25.6
|
7.39
|
0.59
|
49.30
|
14.88
|
s4
|
8.46
|
25.1
|
6.39
|
0.59
|
58.32
|
10.17
|
s5
|
8.58
|
25.0
|
7.09
|
0.52
|
43.09
|
16.32
|
3.2 Denitrification rates and construction of Arrhenius equation
The potential N2 production and denitrification rates of the five sites at 25°C were determined by 15N isotope incubation experiments with MIMS. The results indicated that the denitrification rate had a decreasing trend in the west to the east direction in Lake Taihu. The potential 29N2 rates were higher than the 30N2 rates at five sites (Fig. 3A). The r29 of site s1 ranked first, r29 of s2 and s3 equally ranked second, and r29 of s5 was only half that of s1. The r30 of s3 was 23.01 µmol·m− 2·h− 1, second only to s2, and r30 of s4 was the smallest (7.54 µmol·m− 2·h− 1). The order of Dtot was the same as that of r29, and the Dtot of s1 was 177.39 µmol·m− 2·h− 1 (Fig. 3B). Except for s4 and s5, the Dwtot of the other sites was more than 100µmol·m− 2·h− 1.
For the purpose of exploring the reason why the total denitrification rate decreases from west to East, the concentrations of NO2−, NO3−, NH4+ and TN in the water at each sampling point of Taihu Lake were detected and analyzed. The concentration of NO3− in Lake Taihu was significantly higher than the concentration of NO2− and NH4+ (Fig. 4), and showed the same distinct regional differences as Dtot, decreasing from west to east.
For further study, the correlation between the total denitrification rate and the concentration of nitrogen compounds in Taihu Lake was analyzed by SPSS statistical software. The statistical results indicated that there was a more significant positive correlation between NO3− and Dtot (Table 2). And the reason for the spatial distribution of NO3− in Taihu Lake is that the western region of Taihu Lake Basin is the main source of pollutants in Taihu Lake, and the eastern region is the drainage area with good water quality (Zhang, 2021).
In addition, the outbreak period of cyanobacteria in Taihu Lake is from May to September, when the water temperature is generally 25 ~ 35°C (Luo et al., 2019). Therefore, the experiments on the effect of water temperature on denitrification were carried out at s1, s2, s3 and s4 points. The results showed that water temperature had a significant effect on denitrification. Specifically, the Dtot and Dwtot increased with an increase in water temperature when the water temperature ranged from 25°C to 30°C (Fig. 5). The findings of this study were consistent with those of Ma et al. (2008) and Appelboom et al. (2010) on the relationship between water temperature and denitrification. This was mainly due to denitrification reaction could completely take place when the water temperature was between 10°C and 30°C (Liao et al., 2018), and the increase of water temperature would reduce the solubility of oxygen in the water under the appropriate water temperature (Veraart et al., 2011; Zhao et al., 2011) (Table 1). In addition, high temperatures also tended to promote respiration instead of photosynthesis (Minuti et al., 2021), which further reduces dissolved oxygen. As an anaerobic reaction, denitrification was enhanced when the concentration of dissolved oxygen in water decreased.
High water temperature not only affects the dissolved oxygen in the water column, but also favors release of nutrients by stimulating microbial mineralization of sediment organic matter, which could increase pore − water nutrient concentrations, or could erode the oxidized microzone at the sediment − water interface by increasing oxygen demand (Holdren and Armstrong, 1980; Jiang et al., 2008; Gebremariam et al., 2021; Wang et al., 2021). Thus, the concentration of the substrate increases and denitrification is promoted, until the nitrate concentration is saturated (Silvennoinen et al., 2008).
On the other hand, temperature has a significant influence on the growth of microorganisms (Perez-Rodriguez et al., 2017). It was obvious from Fig. 5 that when the water temperature exceeded 30°C, the appropriate temperature (Liao et al., 2018), Dtot decreased with the increase of water temperature. Through experiments, Wang et al. (2018) found a response to temperature variation, the microorganisms community presented a little difference after a temperature shock, and the N2 concentration at 34°C was lower than that at 25°C during denitrification. Furthermore, it had been reported that the predominant denitrifying microorganisms belong particularly to the phylum Proteobacteria, when the water temperature was more than 34°C, the activity of the phylum Proteobacteria in Lake Taihu may decrease, resulting in a decrease in the denitrification rates (Xiao et al., 2015; Zhang et al., 2017).
Denitrification rate and water temperature conform to Arrhenius equation (Kaspar, 1982; Chi et al., 2004):
\({D_{tot}}{\text{=}}{D_{tot}}_{{30^\circ C}} \times {\theta ^{(T - 30}}^{)}\), (8)
where
\({D_{tot}}_{{30^\circ C}}\) (µmol·m
− 2·h
− 1) is the total denitrification rate during the incubation experiment at 30°C,
\(\theta\)is the Arrhenius temperature coefficient during denitrification. Based on the experimental results of denitrification rate in Taihu Lake under the condition of sufficient carbon source, the denitrification Arrhenius equation considering the concentration of water temperature can be preliminarily obtained (Table 2). It can provide a scientific basis for determining the parameters of water environment mathematical models such as the Environmental Fluid Dynamics Code (EFDC) model or the MIKE model in water quality prediction.
Table 2
Denitrification Arrhenius equation for at each sampling site in Taihu Lake.
Sampling site
|
Denitrification Arrhenius equation
|
s1
|
\({D_{tot,s1}}{\text{=}}\left\{ {\begin{array}{*{20}{c}} {203.48 \times {{1.0278}^{(T - 30}}^{)},T<30^\circ C} \\ {203.48 \times {{0.9982}^{(T - 30}}^{)},T>30^\circ C} \end{array}} \right.\)
|
s2
|
\({D_{tot,s2}}{\text{=}}\left\{ {\begin{array}{*{20}{c}} {173.44 \times {{1.0418}^{(T - 30}}^{)},T<30^\circ C} \\ {173.44 \times {{0.9706}^{(T - 30}}^{)},T>30^\circ C} \end{array}} \right.\)
|
s3
|
\({D_{tot,s3}}{\text{=}}\left\{ {\begin{array}{*{20}{c}} {176.92 \times {{1.0532}^{(T - 30}}^{)},T<30^\circ C} \\ {176.92 \times {{0.9602}^{(T - 30}}^{)},T>30^\circ C} \end{array}} \right.\)
|
s4
|
\({D_{tot,s4}}{\text{=}}\left\{ {\begin{array}{*{20}{c}} {131.10 \times {{1.0546}^{(T - 30}}^{)},T<30^\circ C} \\ {131.10 \times {{0.9964}^{(T - 30}}^{)},T>30^\circ C} \end{array}} \right.\)
|
3.3 Nitrate source identification of denitrification
As can be seen from Fig. 2 that the total denitrification rate (Dtot) was the sum of coupled denitrification rate rate using nitrate from the water column (Dn) and total uncoupled denitrification rate using nitrate from the sediment (Dwtot) (Svensson et al., 2001). Therefore, the ratio of Dwtot to Dtot (\(\gamma\)) can be expressed as:
\(\gamma =\frac{{{D_{wtot}}}}{{{D_{tot}}}}=\frac{{{D_{wtot}}}}{{{D_{wtot}}+{D_n}}}\), (9)
According to formula (9), if
\(\gamma >0.5\), it meant that
\({D_{wtot}}>{D_n}\), which indicates that the nitrate used for denitrification mainly came from water.
Based on Dwtot and Dtot of each sampling site, the \(\gamma\) was obtained. It was evident that the \(\gamma\) of the five sampling sites were above 0.5, as illustrated in Fig. 6, which showed that the denitrification at the Lake Taihu sediment–water interface primarily occurred at the expense of NO3− from the water column. Besides, when the water temperature rose from 25°C to 30°C, the \(\gamma\) increased slightly with the increase in water temperature, and the increase in s2 and s3 was relatively smaller. When the water temperature approached 35°C, the response \(\gamma\) at each sampling site was inconsistent, but NO3− from the water column was still the main nitrate source of denitrification. In conclusion, the denitrification in the sediment − water interface mainly occurred at the expense of NO3− from the water column, and it was less affected by water temperature.
3.4 Practical implications of denitrification in shallow eutrophic lakes
Due to human activities, such as large-scale synthetic ammonia and large-scale application of chemical fertilizer, a large amount of exogenous nitrogen enters Taihu Lake, resulting in cyanobacteria bloom and threatening human drinking water and ecological security (Li et al., 2013). And denitrification is considered to be the main biological pathway of nitrogen removal, which is of great significance to reduce Lake nitrogen pollution and eutrophication control (Richardson et al., 2004).
Moreover, the algal − bacteria system in the water column can form an important niche that favors denitrification during cyanobacteria blooms (Chen et al., 2016; Chen et al., 2018). In general, the life of algae is primarily divided into growth and decline periods, and then algae subsides to the bottom of the lake. During the growth period, the oxygen produced by photosynthesis of algae provides an aerobic environment for the nitrifying bacteria attached to the cell mass. This can promote the nitrification of nitrifying bacteria to produce NO3−, the substrate for denitrification (Liu et al., 2019). During the decline period, algae are gradually degraded into low molecular weight organic acids (directly available organic carbon) that can be used by denitrifying bacteria (Li et al., 2013).
In general, the concentration of TN in water can characterize the effect of nitrogen removal when the external pollution source is certain. From 2003 to 2006, Lake Taihu maintained a high TN concentration, resulting in cyanobacteria blooms in 2007, following which the Cyanobacteria died and were accumulated as sediments. According to the monitoring data, the TN concentration in Lake Taihu gradually decreased from 2008 to 2018 (Fig. 1B). And this further proves that algae contribute to the denitrification during the decay period.
Therefore, the study of denitrification in various regions of Taihu Lake can deepen the understanding of nitrogen cycle process in Taihu Lake and contribute to the effective development of water environment and eutrophication control in Taihu Lake.