Effect of water temperature on internal nitrogen release from sediments in the Pearl River Delta region, China

doi:10.21203/rs.3.rs-2589596/v1

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Effect of water temperature on internal nitrogen release from sediments in the Pearl River Delta region, China

https://doi.org/10.21203/rs.3.rs-2589596/v1

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Nitrogen (N) has received attention as an indicator of water quality and pollution. However, the N content of rivers, lakes, and reservoirs varies significantly by season, and environmental factors such as temperature affect it. An experiment was conducted with a series of tanks containing sediments from the Pearl River Delta region of China and distilled water to assess the release of N from sediments under controlled conditions; temperatures from 10 to 30℃ were assessed. Results show that the effect of temperature on N in the water column and sediment is nonlinear. Only temperatures ranging from 20 ~ 25℃ affected NO₃-N in shallow sediments. Temperatures lower than 20℃ had more effect on NH₄-N concentration in the water column, while NO₃-N concentration was affected heavily at higher temperatures. NH₄-N typically is stable at a relatively high concentration in the water column at 20℃. While NO₂-N is stable in the water column at higher temperatures, NO₃-N is the dominant form of N in water bodies at temperatures from 25℃ to 30℃. At 20℃, dissolved inorganic nitrogen was released from the sediment to the water column, gradually reaching equilibrium. Temperature could affect the internal release of NH₄-N; this process gradually accelerated as the temperature rose. However, the presence of NH₄-N in a water body can inhibit its release, thus the relationship between the diffusive flux with temperature is not linear. The relationship between N diffusive flux at the sediment-water interface can be described by Polynomial2D and Lorentz2D models (R² > 0.87).

Nitrogen pollution

Sediments

Water temperature

Diffusive flux

Sediment-water interface

Pearl River Delta

Rapid population growth and the expansion of the industrial and agricultural economy of China have led to excessive discharge of nitrogen (N) to receiving waters. This is commonly attributed to the widespread use of fertilizer, and intensive discharge of untreated municipal and industrial wastewater (Zhang et al. 2017; Martínez-Santos et al. 2018; Bai et al. 2018). As a nutrient, nitrogenous compounds are essential to the survival of all aquatic life, however, in excess, they can cause severe ecological and environmental impacts, such as eutrophication, hypoxia of water bodies, and toxicity to benthic organisms (Oviatt et al. 2017; Shayo and Limbu 2018; Zhu et al. 2019b; Shahmohamadloo et al. 2020). The disposal of exogenous N into natural receiving waters has been reduced across China, however, decades of accumulation in river sediments results in their continuous release into surface waters, reducing anticipated improvements in water quality. In recent years, with the gradual warming of the climate environment, nitrogen in water bodies has gradually changed in response to it (Greaver et al. 2016). These changes are weak, but they will produce severe consequences over time (Canfield et al. 2021). To develop a reasonable budget of point and nonpoint sources of N loads for water quality control, it is essential to have information about the rate of N release from the sediment, especially at high water temperatures.

Located in southern China, the Pearl River possesses a typical subtropical lotic ecosystem and is the largest in southern China (Wang et al. 2020a). Rapid commercialization, urbanization, and large-scale industrialization have caused serious water pollution in the Pearl River Delta (PRD), with eutrophication of the river and estuary becoming the dominant environmental concern in this region (Wang et al. 2021), underscored by the black-odor phenomenon that has appeared in many rivers (Zhu et al. 2019a). Thus, endogenous N-release studies have been conducted to investigate the reaction of N and other substances (Zhang et al. 2014a; Huang et al. 2020, 2021), the degradation and mineralization of suspended solids lead to the increase of δ¹⁵N in the substrate. The rate of nitrification and denitrification in the sediment and water column (Fig. 1) becomes faster at higher temperatures, resulting in unique distributions of nitrogenous compounds (Young et al. 2017; Chen et al. 2018). N removal and the effect of temperature on the growth of microorganisms, and the behaviors of N in the soil, lake, river, and basins under different temperatures related to climate change have been examined based on site-specific observations (Liu et al. 2018; Wu et al. 2019; Wang et al. 2020b). For the PRD region, measurements of the rate of N release from the sediment are scarce due to field access constraints. The nutrient load resulting from the N release in each season cannot be quantitatively evaluated using typical field methods.

In this paper, N released from the sediment is studied with a physical model. The temporal change of ammonia, nitrite, and nitrate concentrations, DO and pH was measured at a set of positions of the sediment and the water column in the model. The objectives of this experimental study are (1) to assess the effect of the temperature on N release from the sediment; (2) to evaluate the rate of N release under different temperatures; (3) to provide advice on the control of N content in water bodies at different temperatures.

2.1 Experimental design and installation

A physical model was set up in the laboratory to facilitate the measurements of N releases from the collected sediment to the water column, as shown in Fig. 2. The model consisted mainly of a water tank and a constant temperature control system. The tank was fabricated of plexiglass with 50 cm length, 25 cm width, and 35 cm height, and consisted of three sections separated by two vertical plates. The water temperature was automatically kept constant by a cooling-water machine and thermostatic probe. In addition, to prevent light on the sediment, the sidewalls of the tank were wrapped in aluminum foil. The top of the tank was sealed, and a sampling port was provided for the dissolved oxygen (DO) and pH meter.

The sediment was taken from a site located in the main waterway of the Pearl River normally containing NH₄-N of less than 1 mg·L^− 1 as N and DO of approximately 5 mg·L^− 1, in the southern portion of Guangzhou City (113°16′ E, 23°4′ N). A grab-type sampler was used to collect the sediment from a depth of 2 ~ 3 m below the water surface, and the sediment was saved in a sealed polyethylene plastic barrel and transported back to the lab. After the sediment was sieved to remove large stones, debris, leaves, and benthos, it was well stirred thoroughly. Then the sediment was added into the tank, and the Rhizon pipes (2.5mm, Rhizosphere Research Products, Netherlands) were installed into the sediment to extract pore water has many advantages, such as low cost, minimal disturbance, avoiding sample contact with air, and preserving the chemical components of the sample (Seeberg-Elverfeldt et al. 2005; Shotbolt 2010). The sediment surface was scraped smoothly to guarantee it had a uniform thickness of 10 cm. Previous studies (Bao et al. 2021; Yao et al. 2022) have shown that N transformation is faster in the top 5 cm of the surface sediment and the N levels are relatively stable below a sediment depth of 10 cm, thus a sediments depth of 10 cm was chosen. The distilled water was slowly added into the tank to reduce the suspension of the sediment as much as possible until the water depth was up to 18 cm. One sampling position was located at the sediment-water interface (SWI). There were five sampling positions in the sediment at depths of 1, 2, 3, 5, and 7 cm below the SWI. Three sampling positions were located at a distance of 5, 10, and 15 cm above the SWI. To avoid possible air contact with the pore water in the sediment during sampling, a Rhizon pipe (2.5 mm inner diameter, Rhizosphere Research Products, Wageningen, Netherlands) was used to take pore water samples.

The model controlled the water temperature to either 10, 15, 20, 25, or 30℃ in five tanks; these temperatures represent the normal temperature range in the PRD region. Each test with a specified water temperature proceeded for 15 days with the same initial conditions.

2.2 Measurement of N concentration in the water column and pore water

Three water samples were taken at each predetermined sampling position in the water column and pore water with a vacuum tube at the same time each day during the experiment. Then NH₄-N, NO₂-N, and NO₃-N concentrations of the samples were measured using a microplate spectrophotometer (Epoch 2, BioTek Instruments), which applies the indophenol method, Griess chromogenic reagent (Chr-R) method, and Copper-Cadmium (Cu-Cd) reduction method, respectively (Laskov et al. 2007; Tu et al. 2010; Park et al. 2018).

In addition, the pH and DO of the water at 9 cm depth were measured with a calibrated pH meter (PH-801/902, ADVICS, Changzhou, China) and a YSI ODO meter (model # 605404, YSI Incorporated, Yellow Springs, Ohio, USA).

2.3 Statistical analysis methods

ANOSIM (ANalysis Of Similarities) is a statistical method used for testing hypotheses about spatial differences and temporal changes in assemblages of organisms. ANOSIM is an analog of 1-factor analysis of variance based on multispecies data (Chapman and Underwood 1999), which uses the difference between average ranked values of Bray-Curtis measures of differences in quantity and the types of organisms among replicates between samples (r_b) and within samples (r_w) to provide a statistical estimate.

$$R=\frac{{r}_{b}-{r}_{w}}{0.25\left[n\left(n-1\right)\right]}$$

Where R is scaled with a range from − 1 and + 1, and R = 0 represents the null hypothesis that no difference exists among a set of samples; n is the total number of replicates summed for two samples (Clarke 1993). The water column and pore water were separated, and ten groups of sampling data were collected from 5 tanks. The differences between the five groups of data were calculated. The null hypothesis in this study, H₀: There is no difference exbetween different groups. If the p-value is less than the indicated significance level (0.05), the null hypothesis can be rejected, and the samples are likely distinct.

2.4 Diffusive flux

Because of the nitrogenous concentration difference between the pore water and the water column, diffusion can cause nitrogenous mass flux across the SWI. Applying Fick's first law yields:

$$J=-\phi {D}_{s}\frac{\partial C}{\partial z}$$

Where J (mg·m^− 2·d^− 1) is the N diffusive flux of SWI, φ is porosity, $\frac{\partial C}{\partial z}$ (mg·L^− 1·cm^− 1) is the nutrient concentration gradient at the SWI, and D_s (cm²·s^− 1) is the actual molecular diffusive coefficient affected by sediment bending. It is difficult to measure sediments curvature, so an empirical formula (Ullman and Aller 1982) is applied to evaluate the diffusive coefficient and porosity:

$${D}_{s}=\phi {D}_{0} \phi <0.7$$

$${D}_{s}={\phi }^{2}{D}_{0} \phi >0.7$$

Where D₀ (cm²·s^− 1) is the molecular diffusive coefficient of the nutrient in an ideal state, the standard diffusive coefficient D₀ for NH₄-N, NO₂-N, and NO₃-N are 19.8×10^− 6, 19.1×10^− 6, and 19.0×10^− 6 cm²·s^− 1, respectively, at 25℃, obtained by Yuan-Hui and Gregory (1974). The diffusive coefficient at other temperatures can be estimated by:

$${D}_{0,T}={D}_{\text{0,25}}\times \frac{{\eta }_{\text{0,25}}}{{\eta }_{0,T}}\times \frac{T+273.15}{298.15}$$

Where D_{0, T} (cm²·s^− 1) is the standard diffusive coefficient at T℃, η_{0, T} is the viscosity of water at T℃, and T is the temperature (℃). Longitudinally fitting N content in the pore water for various depths and further differentiating on depth z, we can calculate the concentration gradient for the SWI $\frac{\partial C}{\partial z}$. Note that z cannot be 0 cm here, so it was set to 0.1 cm. The surface sediment serves as the active reaction area, and an average porosity for the sediment φ = 0.62 was used, calculated from the average of 0 ~ 10 cm in sediment depth.

3.1 Difference between N concentration under different temperatures

ANOSIM results of the N differences for different temperatures are shown in Table 1. The data collected at the same temperature and location are considered the same group (e.g., data collected from the water column on different days at 10℃ are group 1). It is readily apparent that the water column results have significant differences (p < 0.001) and the differences among groups are pronounced. There is an obvious difference between the nitrate concentrations of pore water (p < 0.001), but the differences between groups are not as high as in the water column; the difference in NH₄-N is not significant (p > 0.05), and there is no significant difference between groups and within the group (R close to 0), which means temperature doesn’t have an effective influence on NH₄-N content in sediments pore water.

Table 1

ANOSIM results of N
Statistics	Water column			Pore water
Statistics	NH₄-N	NO₂-N	NO₃-N	NH₄-N	NO₂-N	NO₃-N
R	0.8187	0.9917	0.9008	-0.0174	0.7496	0.4286
p-value	0.001	0.001	0.001	0.505	0.001	0.001

*p-value < 0.05 means the H₀ is rejected, i.e., it is likely that the means of the respective groups are different.

To quantitively evaluate the difference in N concentrations, box plots of NH₄-N, NO₂-N, NO₃-N, and Dissolved inorganic Nitrogen (DIN) of the water column are shown in Fig. 3. Even though some extreme values exist, from 25% quantiles, 75% quantiles, the median value, and the mean value the NH₄-N concentration of the water column tends to gradually decrease with increasing temperature, and the decline gradually slows down, except at 20℃. NO₂-N is highest between 20 ~ 25℃, and NO₃-N is highest at 25℃. Combined with the DIN content at 25℃ in Fig. 3d, the nitrification reaction speed is fastest at 25℃.

Figure 3d shows that the DIN release gradually decreases with the increase in temperature, yet there is a sudden change at 20℃ where it increases sharply. Although the diffusion process increases at higher temperatures; the process is bidirectional. At the beginning of the experiment, due to the extremely low N content in the water column, N in the sediments also exists in the form of NH₄-N, the release of which is driven by the concentration difference and temperature, gradually accelerating with increasing temperature. During the experiment, nitrification gradually increases with the increase in temperature, NH₄-N decreases while the NO₃-N increases in the water column. The nitrification rate at 30℃ is not significantly increased as compared to that at 25℃, but the diffusion rate is greater. At this time, the difference in the rate of N diffusion up or down is less than that at 25℃. Therefore, both NH₄-N, NO₃-N, and DIN are relatively low in concentration. Boxes of 25℃ and 30℃ are larger than other temperatures (Fig. 3d) indicating that DIN concentration fluctuates greatly, the release rate is faster with the higher temperature at the beginning of the experiment, gradually stabilizes, and falls back to the average level as the experiment progresses. The relatively abnormal temperature is 20℃, the content of NO₂-N and NO₃-N is at an average level while NH₄-N far exceeds other temperatures, which also leads to a greater concentration of DIN at 20℃. The results demonstrate that the net rate of NH₄-N released from internal sources of sediments is the fastest at 20℃.

3.2 Vertical distributions of N

In addition to the difference in the concentration of N in the water column, temperature also affects the longitudinal distribution of N. Average NH₄-N, NO₂-N, NO₃-N, and DIN concentration distributions are plotted as a function of temperature in Fig. 4, which was developed through interpolation. It is observed that the concentration of NH₄-N in the vicinity of the SWI decreases from the bottom to the top caused by its oxidization by DO. All NH₄-N in the water column is released from the sediment, and its concentration varies nonlinearly with rising temperature. Because the diffusion coefficient is normally proportional to temperature, increases in N content in the water column would occur more at high temperatures than at low temperatures. Yet in the experiment, it is observed that NH₄-N concentration in the water column is greatest, and the corresponding NH₄-N concentration in the pore water is lowest at 20℃.

Moreover, it is found that NO₂-N and NO₃-N exist mainly in the water column. NO₂-N concentration is greatest when the temperature is between 20 ~ 25℃, and NO₃-N is greatest at the temperature of about 25℃. Due to the present concentration difference at the SWI, NO₂-N, and NO₃-N enter the pore water, leading to NO₂-N and NO₃-N concentrations in the shallow sediment greater compared to the deep sediment. Because of the diffusion action, NO₂-N and NO₃-N content at the temperature of 20 ~ 25℃ below the SWI is greater than those in the water column at lower temperatures. When the temperature increases from 15 to 20℃, NO₂-N increases from 14 µg·L^− 1 to 122 µg·L^− 1, indicating that NO₂-N can exist in the water column more stably at higher temperatures.

DIN in the water column comes from the sediment, affected by physical diffusion. Compared to the changing concentrations of NH₄-N, NO₂-N, and NO₃-N related to DO, DIN is mainly constant in the longitudinal direction (Fig. 4d). It is seen that the DIN variation is similar to that of NH₄-N, which is highest at 20℃ and decreases with temperature up to 30℃. From the concentration distribution of DIN in the pore water, the DIN in the water column approximates that of the pore water in the shallow sediments at 20℃, a large amount of DIN is released from the sediment to the water column, and gradually reaches an equilibrium state under this temperature condition. With higher or lower temperatures, there is obvious DIN stratification in the vertical direction (Fig. 4d). The DIN concentration at 1 cm below the SWI is 1.64 and 2.46 times that of the SWI at 10 and 30℃, respectively.

3.3 Diffusive flux at the SWI

Diffusion of N occurs at the SWI due to longitudinal concentration differences, N concentration change in the water column, and diffusive flux at the SWI under different temperatures are shown in Fig. 5. The diffusive flux of NH₄-N becomes larger at higher temperatures. When the concentration difference at the SWI changes, the flux will respond. Although the order of NH₄-N from high to low (20 > 10 > 15 > 30 > 25℃) did not follow the order of temperature increase or decrease, the diffusive flux increased with increasing temperature except at 20℃. The result demonstrates that temperature could affect the internal release of NH₄-N, and this process gradually speeds up at higher temperatures. However, the presence of NH₄-N in the water body can inhibit the release process.

Note that initially there is no NO₂-N and NO₃-N in the water column when the experiment begins, and these nitrogenous compounds then diffuse from the sediment to the water column, and their flux increases at higher temperature (Fig. 5b and Fig. 5c). Under anaerobic conditions in the sediment, both NO₂-N and NO₃-N concentrations are relatively low. When the NH₄-N in the water column is converted to NO₂-N and NO₃-N under aerobic conditions, then their concentrations become greater compared to those in the sediment. These nitrogenous compounds further diffuse from the water column to the sediment. Simultaneously, NO₂-N and NO₃-N in the sediment are consumed by denitrification, which further maintains the concentration gradient near the SWI. Although the concentration of NO₂-N is low, there is still a significant difference between NO₂-N concentration at 10 ℃ and 15℃, and at higher temperatures, NO₂-N releasing and concentration, maintaining an increase from 15 to 20℃.

The main factors driving diffusion are the differences in concentration and temperature, affecting both mass transfer flux and ongoing reactions. Overall, the diffusive flux is not a linear function of the temperature. To obtain the average diffusive flux at the SWI and to quantify the effect of the temperature on the N release from sediment, curve fitting was conducted based on the vertical distribution of N concentrations. A Polynomial2D model was chosen for NH₄-N and NO₃-N, and a Lorentz2D model for NO₂-N. Numerous functional fits were assessed based on their R² value; for brevity, only the best relationships are shown. $\stackrel{-}{J}$ is the average diffusive flux of the first t days at T℃. Other variables in the equations are simple fitting parameters chosen to best match the results. It is seen that most of the best-fit curves have good fit results (R² > 0.83), as shown in Table 2 and Fig. 6. Based on the fitted equations, the rate of diffusion of N from sediment to water column at different temperatures (10 ~ 30℃) and its change trend can be estimated.

Table 2

Parameters of fitted equations
Equation	$\stackrel{-}{J}={J}_{0}+{A}_{1}T+{A}_{2}{T}^{2}+{A}_{3}{T}^{3}+{A}_{4}{T}^{4}+{A}_{5}{T}^{5}+{B}_{1}t+{B}_{2}{t}^{2}+{B}_{3}{t}^{3}+{B}_{4}{t}^{4}+{B}_{5}{t}^{5}$
Parameters	J₀	A₁	A₂	A₃	A₄	A₅
NH₄-N	-0.993	-2.895	0.870	-0.077	0.003	-3.453×10^− 5
NO₃-N	0.935	0.703	-0.181	0.016	-5.699×10^− 4	7.361×10^− 6
Parameters	B₁	B₂	B₃	B₄	B₅	R²
NH₄-N	-1.128	0.246	-0.028	0.002	-3.338×10^− 5	0.966
NO₃-N	-0.589	0.167	-0.023	0.001	-3.375×10^− 5	0.876
Equation	$\stackrel{-}{J}={J}_{0}+A/\left\{\left[1+{\left(\frac{T-{a}_{1}}{{b}_{1}}\right)}^{2}\right]\left[1+{\left(\frac{t-{a}_{2}}{{b}_{2}}\right)}^{2}\right]\right\}$
Parameters	J₀	A	a₁	b₁	a₂	b₂	R²
NO₂-N	4.563	-91.073	26.823	3.629	10.871	5.474	0.890

Table 3 shows that the average diffusive flux of NH₄-N at 10℃ is 2.044 mg·m^− 2·d^− 1, 2.6 times that at 30℃. The NO₂-N flux of NO₂-N increases rapidly from − 0.047 µg·m^− 2·d^− 1 at 10℃ to -40.684 µg·m^− 2·d^− 1 at 20℃, and reach the maximum of -49.862 µg·m^− 2·d^− 1 at 25℃ (positive means release from sediment into the water column). NO₃-N and NO₂-N have similar trends with a maximum value of 25℃. D_s increases with the temperature. Both NO₃-N and NO₂-N rise initially and then decrease, which indicates there is a threshold for diffusive flux between 20 and 30℃. From the daily flux in Fig. 5a, it is seen that NH₄-N can stabilize at a greater level in the water column at 20℃, so the average flux is low.

Table 3

Average diffusive flux for nitrogenous compounds
Temperature (℃)	10	15	20	25	30
NH₄-N (mg·m^− 2·d^− 1)	2.044	3.140	1.684	4.698	5.500
NO₂-N (µg·m^− 2·d^− 1)	-0.047	-3.261	-40.684	-49.862	-32.451
NO₃-N (mg·m^− 2·d^− 1)	-0.167	-0.318	-0.480	-0.983	-0.430

3.4 Correlation between every two indicators

In addition to temperature, environmental indicators affected by temperature also have secondary effects on the presence of N pollutants in the water column and pore water. The correlation between environmental factors and forms of N with depth is shown in Fig. 7. The same index is divided into three parts at depths: water column (WC), shallow sediment (SS, 0 ~ 3 cm below SWI), and deep sediment (DS, 5 ~ 7 cm below SWI). The absolute value of r was categorized into four groups: negligible correlation (r < 0.30), low correlation (0.30 < r < 0.50), moderate correlation (0.50 < r < 0.70), and high correlation (r > 0.70).

Based on the Pearson correlation analysis, the temperature is significantly related to NH₄-N (p < 0.01), NO₂-N (p < 0.01), and NO₃-N (p < 0.05) in the water column, but only shows a low correlation. For the N in sediment, only NH₄-N and DIN relate to temperature, the temperature has little effect on NO₂-N and NO₃-N in sediment due to the lack of oxygen. As the main factor of nitrification, DO is related to NH₄-N and NO₂-N in shallow sediment and water column.

Unlike other environmental factors, pH has a more significant impact on NH₄-N (p < 0.01), NO₂-N (p < 0.05), NO₃-N (p < 0.01), and DIN (p < 0.001) in deep sediment. pH shows a very significant correlation (p < 0.001) with NH₄-N and DIN in the water column, since pH is not controlled in this experiment, and pH ranged from 6.8 to 7.9, it appears that under weak alkaline conditions, the pH value of the water column has some influence on N release from sediment and the maintenance of NH₄-N.

There is also a correlation between forms of N with depth. Normal distributions of N are shown in Fig. 8, they are arranged by depth (vertical) and conversion relationship (horizontal, NH₄-N to NO₂-N to NO₃-N). The correlation between NO₂-N and NO₃-N is high at all depths, and there is also a significant correlation between the vertical distribution of the same indicator. The vertical correlation between the same substance was different, NH₄-N only had a low correlation (p < 0.05, r: 0.2 ~ 0.4). NO₂-N and NO₃-N in the water column and pore water at different depths originating from longitudinal physical diffusion and chemical transformation, and had a relatively stable correlation with each other. NH₄-N, however, is not only just involved in the internal diffusion transformation process of water bodies (including water column and pore water). At the same time, NH₄-N in the sediment dissolved and diffused into the pore water and participate in the nitrogen cycle in water, making it hard to have a strong vertical correlation, and only has low correlations with NO₂-N.

4.1 N transformation under different temperatures

The impact of temperature on N internal release is a broad issue. Physical model simulation is a significant and reliable method for analyzing N diffusion (Xu et al. 2016; Liu et al. 2018). In this study, we collected sediment from in situ and brought it back to the laboratory to simulate after thorough mixing, to maintain the consistency of the sediment under different working conditions as much as possible. Unlike a columnar incubator, a physical model can simulate a more realistic culture environment. In this study, the water environment maintains a constant temperature during the experiment, which can accurately simulate the corresponding conditions and thus obtain valid results.

The Biogeochemical transformation of N is an important factor affecting the N content of a water body. The effect of temperature on N transformation has been extensively studied, and it has been found that benthic DIN concentrations play a key role in the transformation process (Hardison et al. 2015; Wei et al. 2020; Huang et al. 2021). Different forms of N in DIN reveal a transformation process (Table 4). The proportion of NH₄-N is greater at lower temperatures (10 ~ 20℃), it was over 80% in 7 days at 10℃ and 8 days at 20℃, respectively. While this result can indicate that NH₄-N is more likely to exist at lower temperatures, it cannot tell us how strong the denitrification process is. Based on the relationship between NO₂-N and NO₃-N, the ratio of nitrite to nitrate ([NO₂-N]/[NO₃-N]) (NNR) is computed as an indicator of the nitrification process (Zhu et al. 2020). The changing NNR as a function of the test period is shown in Fig. 9. The nitrification process was enhanced with increasing temperature at the beginning of the experiment. With a large increase in NO₃-N, the denitrification rate was also gradually enhanced. As the NO₃-N substrate was reduced, NO₃-N concentration in the sediment or the water column will be an effective environmental factor in the NO₃-N reduction process (Tan et al. 2019). Although the results show that the NO₃-N content was over 88% in 8 days at 25℃ and most of the time exceeded 50% at 30℃, indicating N is predominantly in the form of NO₃-N in water bodies at 25℃ and 30℃. When the temperature is higher, NO₃-N concentrations, diffusion rates, and denitrification rates increase. As an intermediate product of nitrification and denitrification, NO₂-N not only reflects the nitrification process but also does serious harm to water bodies (Wu et al. 2020). Additionally, NO₂-N is less than 1% in this temperature range (10 ~ 15℃). As NO₂-N is produced in a similar way to NO₃-N, it is more active at higher temperatures. NO₂-N has the greatest NNR value at 20℃ and shows the most significant ratio at 25℃. It was concluded by Zhang et al. (2014) that NH₄-N and NO₂-N oxidation rates are highly dependent on temperature. Furthermore, the experimental findings indicate that change rates of NH₄-N and NO₃-N have little effect when temperatures are changed between 24℃ and 28℃, and there is a significant difference between 10℃ and 24℃, which is consistent with the fact that the highest NO₃-N content occurred at 25℃.

Table 4

Average DIN content (in percent) and average DO concentration
Constituent	10℃	15℃	20℃	25℃	30℃
NH₄-N	74.48%	60.12%	67.70%	29.65%	45.57%
NO₂-N	0.13%	0.57%	2.29%	3.47%	3.35%
NO₃-N	25.38%	39.30%	30.00%	66.88%	51.09%
DO	7.99 mg·L^− 1	7.18 mg·L^− 1	4.78 mg·L^− 1	4.37 mg·L^− 1	1.35 mg·L^− 1

Among environmental factors, DO is the main factor controlling N transformation (Yan et al. 2019). DO content decreases in time at all temperatures for all tests. Higher temperatures will increase the rate of various chemical reactions, causing more DO consumption correspondingly (Zhu et al. 2019a). During the experiment, the water tank is airtight. The DO content decreases rapidly, which is below 1.5 mg∙L^− 1 at 30℃, and greater than 3 mg∙L^− 1 at other temperatures. For example, DO > 6.4 mg∙L^− 1 at 10 and 15℃. With the consumption of a large amount of DO, NO₃-N content increases at higher temperatures. Even if DO content is low at 30℃, both nitrification and denitrification processes are underway, which results in an increasing proportion of NO₃-N

In addition to the changes caused by chemical processes, temperature also affected the rate of diffusion, thereby affecting the nitrogen concentration in the water column and the pore water. Diffusion is also affected by changes in concentration differences caused by chemical processes.

4.2 N pollution in the PRD region

Due to its location in China's most densely populated and economically developed region, the PRD area faces a severe pollution threat. Thus, much research has focused on the PRD and its water quality issues (Chen and Chen 2020; Rong et al. 2020; Chen et al. 2022), these are predominately in-situ sampling analyses (Zhang et al. 2014a; Hong et al. 2019; Tan et al. 2019). The effect of temperature on release fluxes was estimated primarily by comparing the vertical distribution of N pollution by season. However, nutrients in water bodies come mainly from upstream sources, especially during summer when runoff is large (Zhang et al. 2013), which made in-situ studies results of temperature-dependent endogenous release inaccurate in the PRD region. It is also difficult to compare the results of this study to the in-situ observations. Since the soil is fairly capable of adsorbing NH₄-N, farmland sources are unlikely to contribute to NH₄-N loading in the water (Rush 1915; Hong et al. 2019). As a result of controlling exogenous pollution, NH₄-N in the PRD area primarily comes from endogenous release from sediments, upstream inflows, and DIN transformation. This study shows that NH₄-N can easily be maintained at high levels at lower temperatures (10 ~ 20℃). It is estimated that the average temperature of Guangzhou, a typical city in PRD, is between 10 and 15℃ during December, January, and February, and between 15 and 20℃ during November, March, and April, and above 25℃ from June to August. Water temperature maintains a good linear correlation with air temperature (Stefan and Preud’homme 1993), and tends to be lower than it. To control NH₄-N in water bodies, measures should be taken from November to the following April. In summer, there is a significant increase in the ratio of NO₂-N in DIN, which must be controlled by increasing DO in the water body or promoting denitrification by other means.

N content and its distribution in the water column and the pore water of the sediment under different temperatures are addressed with comprehensive experiments to better understand N transformations in black-odor rivers. N content in the water column depends on water temperature, while in the pore water nitrate concentration in the shallow sediment is affected only at temperatures in the range of 20 ~ 25℃. NH₄-N concentration in the water column is impacted more significantly at lower temperatures; while NO₃-N concentration is affected heavily at higher temperatures. The effect of temperature on nitrogen in the water column and sediment is nonlinear. NH₄-N content in the water column is highest at 20℃, NO₂-N in the range of 20 ~ 25℃, and NO₃-N around 25℃. NH₄-N in the water column stabilizes at a greater level at 20℃. NO₂-N can exist in the water column more stably at higher temperatures. N in water bodies is mainly in the form of NO₃-N at 25℃ and 30℃. The nitrification reaction rate is most rapid at 25℃. Even though DO is low at 30℃, both nitrification and denitrification take place, thus NO₃-N content is not kept low. NO₂-N and NO₃-N in the water column are converted from NH₄-N and are also released from the sediment.As this release process is continuous, obtaining a solid correlation of NO₂-N with depth is difficult. Higher temperature improves the internal release rate of NH₄-N, yet the presence of NH₄-N in the water column can inhibit the release process. The net rate of NH₄-N released from internal sources within the sediment is the fastest while the diffusive flux is the lowest at 20℃. The diffusive process can be described by Polynomial2D and Lorentz2D models, respectively, by which the experimental results were fitted, with a resulting correlation coefficient above 0.87.

Competing Interests: The authors declare that they have no competing interests.

Data Availability Statements: The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.

Acknowledgment: This work was supported by the Open Research Fund Program of the State Key Laboratory of Hydroscience and Engineering (sklhse-2021-B-02), the Fund of Science and Technology Program of Guangzhou (202102020254), and the National Key R&D Program of China (2019YFD0900302).

Author contributions: Dantong Zhu: Investigation, visualization, writing - original draft preparation, writing - review and editing; Xiangju Cheng: Conceptualization, methodology, writing - review and editing, supervision and funding acquisition; David J. Sample: Writing - review and editing; Qingsong Qiao: Writing - review and editing, visualization；Zhaowei Liu: Writing - review and editing, funding acquisition.

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No competing interests reported.

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Effect of water temperature on internal nitrogen release from sediments in the Pearl River Delta region, China

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Abstract

Figures

1 Introduction

2 Methods And Procedures

2.1 Experimental design and installation

2.2 Measurement of N concentration in the water column and pore water

2.3 Statistical analysis methods

2.4 Diffusive flux

3 Results

3.1 Difference between N concentration under different temperatures

3.2 Vertical distributions of N

3.3 Diffusive flux at the SWI

3.4 Correlation between every two indicators

4 Discussion

4.1 N transformation under different temperatures

4.2 N pollution in the PRD region

5 Conclusion

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