Characterization of the sediment
The sediments comprise a wide range in P bioavailability and P-retention
capacity. In preparation for the algal assay, sediments were dried at
25ºC and passed through a 2-mm sieve. Particle size distributions of the
sediments were determined by hydrometer method and organic carbon
content followed the dichromate oxidation method (Rowell, 1994). Soil pH
and electrical conductivity (EC) were measured in a sediment-solution
ratio of 1:5 with a pH and conductivity meter, respectively. Calcium
carbonate equivalents (CCE) were determined by acid-neutralization
method (Rowell, 1994) and the amount of active calcium carbonate
equivalence (ACCE) by reaction with neutral ammonium oxalate (Loeppert
and Suarez, 1996).
Sediment chemical and physical properties are shown in Table 1. The data
was normally distributed according to the Kolmogorov-Smirnov test. The
texture of sediments was often loamy sand and sandy loam. Sand (CV =
58.8%) and clay (CV = 54.3%) contents of sediments showed large
variations in comparison with silt (CV = 39.3%). Electrical
conductivity was low suggesting that the water was fresh (non-saline).
Overall, sediment pH was alkaline (7.26-8.02), which might be related to
higher calcium carbonate equivalents (CCE) in carbonate-rich sediments
in the basin, and pH was quite similar in these samples. The average CCE
and ACCE of sediments were 15.03 (2.50-23.75) and 2.08 (0.54-3.86),
respectively.
Sediments were extracted using several single extraction methods that
are summarized in Table 2. Olsen-P and Colwell-P extraction method have
been designed for calcareous soils. Mehlich III-P and Bary II-P have
been designed for acidic soil, and both contain HCl acid and ammonium
fluoride for dissolution of Fe/Al-P minerals such as Strengite,
(FePO4·2H2O) and Variscite,
(AlPO4·2H2O). AB-DTPA extraction method
is chelating agent for Fe, Al, Ca, and Mg and has been shown to be a
good extraction method for acidic and calcareous soils. Morgan-P was
designed for soil with a high association of Ca-P pools and usually
dissolve soluble calcium phosphate (Ahmad and Jones, 1967). 0.1 M NaOH
is for soil and sediment with high Fe/Al-P pools. Chemical analysis of P
was done colorimetrically (Murphy and Riley, 1962).