# Characteristics and Release Risk of Phosphorus from Sediments in a Karst Canyon Reservoir, China
## 3 Results  
### Content and Characteristics of the P in Sediments  
The sediment TP content in the Wanfeng Reservoir ranged from 79.37 to 438.04 mg\(\cdot\)kg\({}^{-1}\), with an average of 224.55 mg\(\cdot\)kg\({}^{-1}\). There was a significant spatial heterogeneity in TP distribution, as depicted in Figure 2. The variation in extreme TP values was as much as 5.5 times, indicating substantial differences across sampling locations. Among the sampling points, WF3 exhibited the highest TP content in the surface sediments, with an average of 259.78 mg\(\cdot\)kg\({}^{-1}\). On the other hand, WF2 had the lowest TP content, averaging at 151.90 mg\(\cdot\)kg\({}^{-1}\).  
The TP content in WF1 and WF2 displayed similar patterns, with a relatively gradual change in TP content within the 1-13 cm depth range. However, at the WF1 sampling site, there was an increasing trend in TP content between the 13-19 cm depth range. In contrast, WF3 showed a top-down decreasing trend in TP distribution. Across all sampling sites, there was an abrupt trend in the distribution of sediment TP content.  
IP content in the sediments of the Wanfeng Reservoir ranged from 60.91 to 337.88 mg\(\cdot\)kg\({}^{-1}\), with an average of 174.82 mg\(\cdot\)kg\({}^{-1}\). IP accounted for 73.26% to 78.84% of the TP content. There was a strong positive correlation between IP and TP (correlation coefficient r = 0.994, \(p\) < 0.01, n = 47), and a consistent distribution trend between IP and TP.  
OP content in the sediments of the Wanfeng Reservoir ranged from 1.84 to 13.59 mg\(\cdot\)kg\({}^{-1}\), with an average of 6.55 mg\(\cdot\)kg\({}^{-1}\). OP accounted for 2.92% of the TP content. In terms of the surface sediment, WF2 and WF3 exhibited significantly higher OP content compared to the bottom sediment, resulted has a negative correlation between OP and sediment depth. WF3, being closer to the urban area and more influenced by human activities, is at the  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Phosphorus Forms** & **Methods** \\ \hline H\({}_{2}\)O-P & 0.5 g of dry sediment was taken and mixed with 25 mL of deionized water. The mixture was then oscillated for 2 h. \\ H\({}_{2}\)O-P & Afterward, centrifugation was carried out at a speed of \\  & 4500 r/min for 5 min to obtain the supernatant. The phosphorus \\  & content in the extract solution was then determined. \\ \hline NaHCO\({}_{3}\)-P & To extract phosphorus from the residue of the previous stage, \\  & 25 mL of NaHCO\({}_{3}\) (0.5 mol/L) was added. The mixture was then \\  & oscillated for 16 h. Afterward, centrifugation was conducted at a \\  & speed of 4500 r/min for 5 min to obtain the supernatant. The \\  & phosphorus content in the extract solution was then measured. \\ \hline Fe/Al-P & To extract phosphorus from the residue obtained in the previous stage, 25 mL of NaOH (0.1 mol/L) was added. The mixture was added. The mixture was \\  & then oscillated for 16 h. Afterward, centrifugation was conducted at a \\  & at a speed of 4500 r/min for 5 min to obtain the supernatant. The \\  & phosphorus content in the extract solution was then measured. \\ \hline Res-P & To determine the residual phosphorus (Res-P), 25 mL of NaOH (1 mol/L) was added to the residue from the previous stage. The mixture was then boiled for 2 h. After boiling, centrifugation was performed at a speed of 4500 r/min for 5 min to obtain the \\  & supernatant. The phosphorus content in the extract solution was \\  & measured to determine the Res-P. \\ \hline \hline \end{tabular}
\end{table}
Table 1: Graded extraction method for sediment phosphorus forms.  
confluence of the Mengli and Hongran rivers, which provides favorable conditions for organic matter accumulation. At the WF1 sampling site, sediment OP showed an increasing trend with depth, with the highest value recorded at 13.59 mg\(\cdot\)kg\({}^{-1}\).