2.4 Sequentially extracted phosphorus forms
Given that the soils are derived from the calcareous parent material of
lacustrine sediment, and the development of soils is weak in this study,
we assumed that most of soil P was combined with primary minerals,
existing in the form of “Calcium (Ca)-bound P”. Furthermore, the forms
of Ca-bound P in the calcareous soils generally have a series of calcium
phosphates with the complex transformation as follows: from dicalcium
phosphate (i e. CaHPO4·2H2O,
Ca2-P) to octacalcium phosphate (i e.
Ca8H2(PO4)6·5H2O,
Ca8-P) and apatite (i e.
Ca10(PO4)6F2,
Ca10-P) (Jiang and Gu, 1989). Therefore, a sequential
extraction procedure, provided by Jiang and Gu (1989) and modified by
Adhami et al. (2006), was applied to determine P forms instead of the
sequential extraction schemes according to Hedley et al. (1982) for
distinguishing the series of Ca-bound P. Briefly, the
NaHCO3-soluble,
CH3COONH4-soluble,
NH4F-soluble,
NaOH-Na2CO3-soluble, citrate
dithionite-soluble, and H2SO4-soluble
phosphate was successively extracted with 0.25 mol L−1NaHCO3 (pH = 7.5), 0.5 mol L−1CH3COONH4 (pH = 4.2), 0.5 mol
L−1 NH4F (pH = 8.2), 0.1 mol
L−1 NaOH-Na2CO3, 0.3
mol L−1C6H5Na3O7plus 1.0 g Na2S2O4 plus
1.0 mol L−1 NaOH (citrate dithionite) and 0.5 mol
L−1 H2SO4,
respectively. The above soluble phosphate was mainly composed of the
form of Ca2-P, Ca8-P, aluminium oxide
surfaces adsorbed P (Al-P), iron oxide surfaces adsorbed P (Fe-P),
occluded P in iron oxide minerals (occluded P), and
Ca10-P, respectively (Jiang and Gu, 1989; Adhami et al.,
2006), and was termed as Ca2-P, Ca8-P,
Al-P, Fe-P, occluded P, and Ca10-P respectively. Among
these forms, Ca2-P, Ca10-P and occluded
P are equivalent to NaHCO3-extracted P, diluted
HCl-extracted P and concentrated HCl-extracted P, respectively, in
Hedley et al. (1982) scheme. The Ca8-P, Al-P and Fe-P
are inorganic P non-occluded in iron oxide minerals, and named as
non-occluded P.
According to the results of Jiang and Gu (1989) and Adhami et al.
(2006), the solubility and bioavailability of Ca2-P are
higher than those of non-occluded P and thus Ca2-P and
non-occluded P was named as labile P and moderate labile P,
respectively, whereas occluded P and Ca10-P with a
lowest solubility and bioavailability was termed as
slowly cycling P (recalcitrant P
or stable P). Total organic P was obtained as the difference in P
through 0.5 mol L−1H2SO4 extraction between ignition and
non-ignition soils (Saunders and Williams, 1955). The correlation
coefficient (r = 0.990) between total P determined by
H2SO4-HClO4 digestion
and the sum of various P forms (including organic P) was significant
(p < 0.001), which showed that the values of various P
forms were reliable in our study.