Fractionation of organic phosphorus
Organic P fractionation was carried out according to Bowman and Cole
(1978) method modified by Ivanoff et al. (1998), and described in Zhang
and Kovar (2009) wherein determination of microbial P was excluded from
the fractionation scheme. Briefly, 1 g of soil was sequentially
extracted with 0.5 M NaHCO3 at pH = 8.5, 1.0 M HCl, and
0.5 M NaOH. The 0.5 M NaHCO3 extract contained labile P
fraction (POlab) and the 1.0 M HCl extract moderately
labile P (POmod). The 0.5 M NaOH extract was acidified
to pH = 1.0 to distinguish P contained in fulvic (Pfulv)
and humic acids (Phum). The sample residue after
sequential extraction was ashed at 550 °C and extracted with
concentrated 1M H2SO4 to determine
residual, non-labile organic P fraction (POres). The
extracts were centrifuged at 4000 rpm for 10 min, filtered and measured
for inorganic P (IP) and total P (TP) by the molybdate colorimetric
method. Total P (TP) in the extracts was determined after digestion with
2.5 M H2SO4 and potassium persulfate.
Organic P in the extracts was calculated as the difference between TP
and IP.