*2.4. Chemical Analyses*

Rhizosphere and bulk soils were cleared of roots and all organic debris, and ground to pass through a 0.25-mm sieve before analyses of P fractions, total P, organic carbon (C), and total N. In this study, total P concentrations of soil were determined by the molybdenum-antimony colorimetric method after samples through digestion with 1.84 M H2SO4 [27]. For the determination of extractable P, we employed improved Hedely P fractionation methods to quantify soil P functional fractions [28,29]. Air-dried soil samples were processed to follow the soil P fractionation sequential procedure [15]. The corresponding supernatants sequentially exacted with anion exchange resin (weak base), 0.5 M NaHCO3, 0.1 M NaOH, 0.1 M NaOH with sonication, 1.0 M HCl were collected by centrifuge at 1.7 × 10<sup>4</sup> <sup>m</sup>·s<sup>−</sup><sup>1</sup> (3200 rpm) for five minutes, followed by filtering samples through a 0.45-μm micropore filter. Phosphorus concentration in each supernatant was determined by the phosphomolybdic acid blue color method. The extractable P, including Resin-P, NaHCO3-P, NaOH-P, sonication-P, and HCl-P, are defined as available P, soluble P, slowly released P, occluded P, and weathered mineral P, respectively, based on their functions in soils [15]. The residual P is the difference between total P and extractable P.

Soil pH was determined using a pH Meter with a soil:water ratio of 1:2.5. Soil organic carbon (SOC) was determined by dichromate oxidation and titration with ferrous ammonium sulfate [29]. Total N was measured using the microkjeldahl method after digestion with 1.84 M H2SO4. Plant samples were washed with dematerialized water to remove dust, oven-dried, and ground and screened with a 0.25 mm sieve. Total N and total P concentrations in plant tissues were determined using the micro-Kjeldahl method and molybdenum-antimony colorimetric method, respectively [29].
