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Table 1 summarizes the concentrations of P obtained by the different extraction methods. On average, the concentrations of P obtained using distilled water extraction under the optimal conditions for the ultrasonic method ranged from 14.0 to 24.8% of total P, versus 13.2 to 20.3% for the conventional method (Table 1). In the 0.1 molL−1 NaOH extraction, P values ranged from 36.4 to 85.3% of TP, versus 38.4 to 77.5% for the conventional method. Our results were in close agreement with those of previous studies which proved percent BAP of TP was 32.0 to 83.0% in runoff (Sharpley 1993; Fabre et al. 1996). In addition, the P concentrations extracted using 0.1 molL−1 NaOH plus ultrasonic treatment were notably (F = 99.0, P < 0.0001) higher than those extracted by distilled water. Even after only 1 min, much of the total P was extracted by both the mechanical shaking and ultrasonic treatment methods (Table 1). There are two likely explanations. First, NaOH is a stronger solvent than distilled water, thus it is able to extracted more P. Second, NaOH extraction may lead to selective extraction of iron-bound P that would not be extracted by water (Holtan et al. 1988). The sediment samples were yellow (Sed2 and Sed3) and yellowish grey (Sed1), possibly due to the presence of iron components. Therefore, the P concentrations obtained by extracting these sediment samples with NaOH would be higher than those obtained from the soil samples which were dark grey and therefore probably contained less iron.

Table 1

Mean values of extracted P concentration obtained using the different methods. Sed1, Sed2, Sed3 were the sediment samples respectively collected on 5 September 2014, 11 September 2014, and 6 October 2014. Soil sample was collected on 26 January 2011

  Water extraction
0.1 molL−1 NaOH extraction
P conc. (mg/ kg)TPShake
17 h
Shake
1 min*
Ultra.
5 min
Ultra.
1 min
Shake
17 h
Shake
1 min*
Ultra.
5 min
Ultra.
1 min
Soil 1,124 ± 155 148 ± 4  108 ± 5 157 ± 5 425 ± 15  416 ± 9 409 ± 21 
sed1 2,819 ± 128 444 ± 19 130 535 ± 28 417 ± 58 1,736 ± 186 1,080 1,536 ± 176 1632 ± 30 
sed2 3,413 ± 44 627 ± 47 140 883 ± 34 680 ± 46 2,435 ± 760 2,010 2,497 ± 878 2,908 ± 123 
sed3 2,580 ± 49 525 ± 52 130 736 ± 10 639 ± 19 2,000 ± 41 1,180 1,748 ± 317 1,803 ± 53 
  Water extraction
0.1 molL−1 NaOH extraction
P conc. (mg/ kg)TPShake
17 h
Shake
1 min*
Ultra.
5 min
Ultra.
1 min
Shake
17 h
Shake
1 min*
Ultra.
5 min
Ultra.
1 min
Soil 1,124 ± 155 148 ± 4  108 ± 5 157 ± 5 425 ± 15  416 ± 9 409 ± 21 
sed1 2,819 ± 128 444 ± 19 130 535 ± 28 417 ± 58 1,736 ± 186 1,080 1,536 ± 176 1632 ± 30 
sed2 3,413 ± 44 627 ± 47 140 883 ± 34 680 ± 46 2,435 ± 760 2,010 2,497 ± 878 2,908 ± 123 
sed3 2,580 ± 49 525 ± 52 130 736 ± 10 639 ± 19 2,000 ± 41 1,180 1,748 ± 317 1,803 ± 53 

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