Relationships between various sediment P extractants
Table 4 shows the descriptive statistics of the P extractions. The results indicated that different extraction methods showed large differences in P extractability for the same sediment samples. The average ranking order of extracted-P by single extractants was: Colwell > Mehlich III > NaOH 0.1 M > Olsen > Morgan > AB-DTPA > Bray II. Some sediments had a higher Olsen-extractable P than the threshold value of 20 mg kg-1, which value indicates possible release that may adversely affect the aquatic environments (McDowell et al., 2001; Sims et al., 2002).
In order to assess, the correlation of extractants on the concentrations of different P extraction, additional analyses were made for each extractant and the key results are shown in Table 5. Although there were large variations between the P-extractants, they were significantly correlated with each other with r2 values ranging from 0.68 to 0.95. The strongest correlation is observed between sediment Mehlich III-P and Olsen-P. Considering that the pH of most sediments in this study was less than 7.6, potentially less neutralization of the Mehlich III-P extracting solution resulted from sediment CaCO3 (Kuo, 1996; Wang et al., 2015). In Mehlich III, the extraction with nitric acid, acetic acid, and fluoride, lead to the release of P from sediment by dissolution and the formation of complexes with iron and aluminum and calcium. As fluoride and calcium form precipitates of calcium fluoride, this leads to dissolution of calcium phosphates (Kamprath and Watson, 1980). The weakest relationship was found between sediment Olsen- P and Bray-II-P, likely due to small extraction by the Bray-II P test for sediments with pH >6.8 or sediments with a high degree of base saturation (Sims, 2000). However, the lack of correlation between Mehlich III and Bray II was not expected, because there is a common belief that these two methods extract similar P pools from soils and sediments by cation hydrolysis (Kleinman et al., 2001; Ebeling et al., 2008; Wang et al., 2016). We attribute the poor correlation between these two methods was to the high pH of some sediment samples (pH >7.3) (Sotomayor-Ramírez et al., 2004) and a range of sediment P levels studied with Mehlich III-P <50 mg kg-1. There was a significant positive correlation between all P extractions except with Bray II and Total-P contents. Zhou et al. (2001) reported a good correlation between extracted Total-P with Olsen-P and 0.1 M NaOH. However, regression analysis showed that correlation between NaOH 0.1 M and Total-P in sediments was smaller than for other extractants.