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-RamÃrez 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.