3.1 Nitrogen concentrations in overlying water and pore water
In different sediment TN concentrations incubation experiment group, the variation range of NH4+-N concentration in overlying water is relatively consistent (Table 3). Average concentrations in artificial solution as overlying water ranged from 2.571 to 3.674 mg·L−1 and in distilled water from 2.625 to 2.796 mg·L−1. The concentration of NH4+-N in the overlying water was highest in S1 treatment, but there was no significant difference in S2 and S3 treatment. Average NO3− concentrations in artificial solution ranged from 1.218-1.542 mg·L−1 whilst those in distilled water ranged from 1.218 to 1.542 mg·L−1. The concentration of nitrate in overlying water changed relatively obvious in each treatment. Overall, the concentration of ammonia nitrogen is significantly higher than that of nitrate. No significant interactions were found between artificial solution and distilled water treatment. And for pore water, the NH4+-N concentrations were variable. Average concentrations in artificial solution ranged from 2.864 to 7.407 mg·L−1 and in distilled water from 3.926 to 8.065 mg·L−1. When artificial solution as overlying water, C(NH4+-N)S1> C(NH4+-N)S2> C(NH4+-N)S3. In control group (distilled water as overlying water), C(NH4+-N)S3> C(NH4+-N)S2> C(NH4+-N)S1. For NO3− concentrations, there is little difference between overlying water and pore water. Average NO3− concentrations in artificial solution ranged from 1.053-1.116 mg·L−1 whilst those in distilled water ranged from 0.994 to 1.137 mg·L−1.
In different overlying water N concentrations incubation experiment group, average NH4+-N concentration in artificial solution ranged from 2.892 to 2.902 mg·L−1 and in distilled water ranged from 2.549 to 2.800 mg·L−1. According to the concentration ranges of ammonia in L, M, H treatment, no significant difference was found by statistic analysis. However, H treatment indicates additional 1.0 mg·L−1 NH4+-N and 2.5 mg·L−1 NO3−N was added to the overlying solution, M referred to 0.5 mg·L−1 NH4+-N and 1.5 mg·L−1 NO3−N was added and for L treatment no extra nitrogen compound added to overlying water. The NH4+-N and NO3−N solvent added was not sufficient to affect the nitrogen content in the overlying water, but it can inhibit the diffusion of nitrogen from sediment to water. The more nitrogen added to surface water, the smaller the gradient concentration between sediment and overlying water, and the less ammonia and nitrate released from sediment. For pore water, average NH4+-N concentration in L, M, H treatment were 11.726, 6.142, 7.148, respectively. And average NO3−N concentration in L, M, H treatment were 1.529, 2.252, 2.199, respectively. Although the concentrations of NH4+-N and NO3−N in each treatment are different, which are relatively small compared with the pore water concentration, and there was little regular effect on them.
Table 3
A summary of ranges and averages of overlying water NH4+(mg·L−1) and NO3− (mg·L−1) concentrations in different sediment TN concentrations and different overlying water N concentrations incubation experiment groups.
Treatment | Overlying water | NH4+ range | Average | NO3− range | Average |
S1 | Artificial solution | 2.248-4.415 | 3.674 | 0.375-1.437 | 1.304 |
Distilled water | 2.039-3.128 | 2.676 | 0.601-1.485 | 1.374 |
S2 | Artificial solution | 2.063-2.835 | 2.571 | 0.437-1.330 | 1.218 |
Distilled water | 2.065-3.029 | 2.625 | 0.414-1.387 | 1.265 |
S3 | Artificial solution | 2.105-3.047 | 2.702 | 0.496-1.691 | 1.542 |
Distilled water | 2.212-3.704 | 2.769 | 0.566-1.624 | 1.492 |
L | Artificial solution | 2.266-3.443 | 2.902 | 0.801-2.405 | 1.600 |
Distilled water | 2.139-3.623 | 2.800 | 0.892-3.535 | 1.804 |
M | Artificial solution | 2.182-3.841 | 3.076 | 0.896-6.738 | 2.696 |
Distilled water | 1.995-2.914 | 2.549 | 0.788-1.688 | 1.414 |
H | Artificial solution | 2.170-3.506 | 2.892 | 0.768-2.926 | 1.685 |
Distilled water | 2.128-3.215 | 2.679 | 0.826-2.029 | 1.597 |
Table 4
A summary of ranges and averages of pore water NH4+(mg·L−1) and NO3− (mg·L−1) concentrations in different sediment TN concentrations and different overlying water N concentrations incubation experiment groups.
Treatment | Overlying water | NH4+ range | Average | NO3− range | Average |
S1 | Artificial solution | 3.501-9.526 | 7.407 | 0.518-1.636 | 1.053 |
Distilled water | 2.569-5.174 | 3.926 | 0.484-1.635 | 1.061 |
S2 | Artificial solution | 2.436-4.222 | 3.343 | 0.419-1.614 | 1.087 |
Distilled water | 3.979-5.987 | 5.257 | 0.399-1.552 | 0.994 |
S3 | Artificial solution | 2.371-3.269 | 2.864 | 0.539-2.023 | 1.116 |
Distilled water | 2.572-11.655 | 8.065 | 0.516-1.801 | 1.137 |
L | Artificial solution | 7.083-12.750 | 11.061 | 0.907-2.021 | 1.483 |
Distilled water | 11.105-13.477 | 12.391 | 1.007-2.145 | 1.574 |
M | Artificial solution | 3.656-5.801 | 4.789 | 1.179-6.449 | 2.869 |
Distilled water | 5.052-9.479 | 7.494 | 1.146-2.486 | 1.635 |
H | Artificial solution | 8.378-10.738 | 9.716 | 1.147-2.255 | 1.683 |
Distilled water | 3.279-5.884 | 4.580 | 1.140-6.504 | 2.715 |
3.2 Effects of different sediment TN concentrations on diffusion flux of nitrogen at sediment-water interface
Sediment-water interface is the most significant place for material exchange between the bottom of aquatic ecosystem and overlying water. Under the solo or synergistic effects of various physical, chemical and biological processes such as concentration gradients, microbial metabolism and benthic fauna disturbance, dissolved particles can migrate between sediment pore water and overlying water through sediment-water interface. Therefore, sediment usually play a role of “source” or “sink” for nutrients and pollutants migration.
The diffusion fluxes of TN at sediment-water interface from different sampling sites are shown in Fig. 1, the diffusion fluxes of TN gradually increase with incubation time. At the beginning of the experiment the sediment act as the accumulation sinks of nitrogen and then transformed into release source. The average diffusion flux of TN at sampling points S1, S2 and S3 were 72.45, 66.35 and 66.30 mg·m−2·d−1, respectively. During the incubation experiment, TN is released from sediment to the overlying water, and the higher TN concentration in the sediment, the greater the diffusion flux was shown.
Across the entire incubation experiment, both ammonia and nitrate showed deposition and release process, and there was no regular pattern in magnitude and direction amongst the treatments. The diffusion flux of ammonia at sediment-water interface is -52.57~84.57 mg·m−2·d−1, and for nitrate diffusion flux, the changing range during the incubation experiment is -110.13~143.25 mg·m−2·d−1, the results indicated that the diffusion flux of nitrate was slightly larger than ammonia (Fig. 2-3). This can be explained by the aerobic condition in the overlying water, in which the dissolved oxygen concentration is 6.93~10.81 mg·L−1, the NH4+-N was transformed into NO3−-N by nitrification process. Thus, there was an increase in NO3−N concentrations, and its diffusion flux also increased. In the early stage of the incubation experiment, NH4+-N dominated the exchange process at sediment-water interface. While in the late stage of the experiment, the exchange process of NO3−N was relatively significant. The average diffusion flux of ammonia at sediment-water interface in S1, S2 and S3 were 16.19, 15.63 and 12.85 mg·m−2·d−1, respectively, and the nitrate average diffusion flux were 3.08, 7.73 and -1.71 mg·m−2·d−1. The previous analysis showed that the diffusion flux of NO3−N was slightly greater than that of NH4+-N, however, the results of average diffusion flux was just the opposite, the average diffusion flux of NO3−N was less than that of NH4+-N, indicating that the migration direction of NO3−N at sediment-water interface varied frequently, the accumulation process counteracts the release process.
3.3 Effects of different overlying water ammonia and nitrate concentrations on diffusion flux of nitrogen at sediment-water interface
The diffusion process of nutrients at sediment-water interface is determined by the concentration gradients of particles between pore water and surface water. When the nitrogen concentrations (NH4+-N, NO3−-N etc.) in the overlying water changes, it will inevitably lead to the changes of nitrogen concentration difference between sediment and overlying water, which will affect both the magnitude of diffusion and direction of migration.
The diffusion flux of TN at sediment-water interface under different overlying water are shown in Figure 4. The diffusion flux of TN between sediment and overlying water reached equilibrium at 10th day of the incubation experiment. For L treatment (0 mg NO3−-N, 0 mg NH4+-N), the sediment acted as the accumulation sink for TN in the overlying water at first and then transformed into release source. However, for M (0.5 mg NO3−-N, 1.5 mg NH4+-N) and H (1 mg NO3−-N, 2.5 mg NH4+-N) treatment, which were opposite to L treatment, the sediment served as the release source of TN and turned into accumulation sink. The diffusion flux of TN in L, M and H treatment were 16.27, 18.68 and 13.52 mg·m−2·d−1, respectively. The diffusion flux of TN did not express the regular variation with the increase of ammonia and nitrate concentration in the overlying water.
The diffusion flux of ammonia at sediment-water interface decreased first and then increased among all treatments, finally achieving the transformation from source to sink (Fig. 5). The calculated results of NH4+-N diffusion flux in L, M, H treatment were 3.37, -4.94, -3.84 mg·m−2·d−1, respectively. In L treatment, the concentrations of NO3−-N and NH4+-N were relatively low in overlying water, NH4+-N was released from sediment to surface water. When the concentrations of NO3−-N and NH4+-N were higher (M and H treatment), it was absorbed by sediment.
The diffusion flux of nitrate at sediment-water interface was transformed from release source to accumulation sink, and the average diffusion flux in L, M and H treatment were 12.30, 10.39 and 7.11 mg·m−2·d−1, and the NO3−-N was released from the sediment in all treatments. The results showed that the diffusion flux of nitrate gradually decreased with the increase of NO3−-N and NH4+-N concentrations in the overlying water, and finally reached equilibrium between release and accumulation (Fig. 6). On the whole, the diffusion flux of nitrate was numerically greater than that of ammonia, which can be explained by the strong absorption of ammonia by sediment. According to the data of DO concentrations (8.98-11.69 mg·L−1) in the overlying water, it was found that the aquatic system was in an aerobic state during the incubation experiment. The ammonia was further nitrated, thus the nitrate concentration in the overlying water was significantly higher than that of ammonia, resulting in the greater diffusion flux of nitrate.
The diffusion flux of dissolved inorganic nitrogen (DIN) was obtained by summing the fluxes of NH4+-N, NO3−-N and NO2−-N, and the average diffusion flux of DIN in L, M, H treatment were 15.77, 5,52 and 3.39 mg·m−2·d−1, respectively. The release rate dominated the process of nitrogen release from sediment to water (Diamond at al., 1990), and the migration of dissolved nitrogen from pore water to sediment and then into surface water was determined by the concentration gradient between the two medias. Since there was no extra NH4+-N and NO3−-N in the overlying water in L treatment, the higher concentration gradient was the main reason for the significant release of DIN from sediment. Therefore, the concentration gradients of DIN between pore water and surface water were affected by controlling the concentration of inorganic nitrogen in the overlying water, resulting in the change of source-sink relationship and TDN diffusion flux at sediment-water interface.