DOI: 10.6084/m9.figshare.23273243.v1
title: raw data for manuscript: Phosphorus sorption by purple soils in relation to their properties: investigation, characterization, and explanation.xlsx

description: The sampling sites were distributed in a significant agricultural belt with a latitude range of 29°58ʹN‒30°52ʹN and a longitude range of 102°59ʹE‒107°37ʹE within the Sichuan Basin in Sichuan Province. The sites were oriented west-to-east through the hilly region of west and central Sichuan, the Chengdu Plain, and the parallel ridge and valley area of east Sichuan. Soil samples were collected from the cities of Ya’an (YA), Qionglai (QL), Chongzhou (CZ), Chengdu (CD), Ziyang (ZY), Suining (SN), Nanchong (NC), and Dazhou (DZ). Surface soils within a depth of 20 cm were collected with a stainless soil auger and sealed in separate sampling bags. The samples were air-dried for 21 days, then ground by a ceramic mortar and pestle, sieved through 2 mm sieves, and stored in sample bags before testing. The collected soil samples were tested for grain size composition, total phosphorus (TP), total nitrogen (TN), amorphous iron oxides (Fe[amo]), amorphous aluminum oxides (Al[amo]), soil organic matter (SOM), and pH. Grain size distribution was determined using a Mastersizer 3000 laser diffraction particle size analyzer (Malvern Panalytical, UK), and the soil texture was determined according to the soil taxonomy of USDA (1988). Five g of the sieved sample was mixed with 25 mL of deionized water, shaken at 160 times min-1 using a reciprocating, constant-temperature water-bath shaker (SHA-B, Yoke Instrument, China), and set aside for 30 min. The pH of the supernatant was measured using a pHS-3C pH meter (Rex, China). Soil TP was measured following the perchloric acid digestion procedure with minor revision (Eslamian et al., 2020; Sommers and Nelson, 1972). For each sample, 0.2 g of the ground and sieved soil sample were placed in a digestion tube and added 3 mL of 70% HClO4. The tube was placed in an aluminum digestion block and the sample digested at 200 °C for 75 min; a funnel was placed atop the tube throughout the digestion to ensure refluxing of HClO4. Following digestion, the digest was allowed to cool and then diluted with distilled water to 50 mL. The tube was stoppered, inverted several times to mix the contents, and allowed to stand overnight; the residue was removed from the extract by centrifugation at 5000 rpm for 15 min, thereby permitting P analysis. The digestion procedure was also used to determine the soil TN (Wang et al., 2009). The P and N contents of the digested soils were measured by colorimetry using a Proxima continuous flow analyzer (AMS Alliance, France). For Fe[amo] and Al[amo], extraction was performed using an oxalic acid-ammonium oxalate buffer solution (He et al., 2019), and the Fe[amo] and Al[amo] concentrations were measured by inductively coupled plasma-optical emission spectrometry using the 720-ES ICP-OES system (Agilent Technologies, Inc., USA). The SOM content was measured using the Walkley-Black method (Mikhailova et al., 2003). <br> For each soil, there were two sets of measurements of phosphate sorption; in one, sorption was measured after 24 hours for a range of initial concentrations, and in the other, sorption was measured after different periods at one initial concentration. The protocol was adapted from Barrow and Debnath (2014). For both sets, 2 g of the ground and sieved soil sample were weighed, dispersed in 0.01 M KCl solution, and made up to a volume of 30 mL with appropriate volumes of KH2PO4. For the first set, the concentrations were 5, 10, 20, 30, 40, 60, and 80 mg·L-1. For the second set, an initial concentration of 40 mg·L-1 was used for most soils, but 60 mg·L-1 was used for soil YA because of its higher sorption; the periods of mixing were 0.25, 0.5, 1, 2, 5, 10, 24, 32, and 48 hours. The pH of each solution was adjusted to 7 using a 0.01 M HCl solution and 0.01 M NaOH solution, and one drop of 0.1% chloroform solution was added to eliminate the influence of microbial activity on the experiments. After the soil suspension had been shaken with a reciprocating shaker at an amplitude of 6 cm and a frequency of 150 oscillations per minute at 25±1 °C for 24 hours, centrifugation was performed at 5000 rpm for 15 min. The resultant liquid was passed through a 0.45 μm membrane filter and analyzed with the Proxima continuous flow analyzer to measure P concentration in the supernatant. Three replicate samples were tested for each soil type, and the average value was used as the final measurement result of the amount of P sorbed. Consequently, we considered that measurements after 24 hours of sorption were adequate to characterize differences between soils.