5 research outputs found
Transformation of Calcium Phosphates in Alkaline Vertisols by Acidified Incubation
Acid-soluble soil phosphorus (P) is a potential resource in P-limited agricultural systems that may become critical as global P sources decrease in the future. The fate of P in three alkaline Vertisols, a major agricultural soil type, after acidic incubation was investigated using synchrotron-based K-edge X-ray absorption near-edge structure (XANES) spectroscopy, geochemical modeling, wet chemistry soil extraction, and a P sorption index. Increases in labile P generally coincided with decreased stability and dissolution of calcium phosphate (CaP) minerals. However, only a minor proportion of the CaP dissolved in each soil was labile. In two moderate-P soils (800 mg P kg–1), XANES indicated that approximately 160 mg kg–1 was repartitioned to sorbed phases at pH 5.1 of one soil and at pH 4.4 of the second; however, only 40 and 28% were labile, respectively. In a high-P soil (8900 mg P kg–1), XANES indicated a decrease in P of 1170 mg kg–1 from CaP minerals at pH 3.8, of which approximately only 33% was labile. Phosphorus mobilized by agricultural practices without concurrent uptake by plants may be repartitioned to sorbed forms that are not as plant-available as prior to acidification
Transformation of calcium phosphates in alkaline vertisols by acidified incubation
Acid-soluble soil phosphorus (P) is a potential resource in P-limited agricultural systems that may become critical as global P sources decrease in the future. The fate of P in three alkaline Vertisols, a major agricultural soil type, after acidic incubation was investigated using synchrotron-based K-edge X-ray absorption near-edge structure (XANES) spectroscopy, geochemical modeling, wet chemistry soil extraction, and a P sorption index. Increases in labile P generally coincided with decreased stability and dissolution of calcium phosphate (CaP) minerals. However, only a minor proportion of the CaP dissolved in each soil was labile. In two moderate-P soils (800 mg P kg-1), XANES indicated that approximately 160 mg kg-1 was repartitioned to sorbed phases at pH 5.1 of one soil and at pH 4.4 of the second; however, only 40 and 28% were labile, respectively. In a high-P soil (8900 mg P kg-1), XANES indicated a decrease in P of 1170 mg kg-1 from CaP minerals at pH 3.8, of which approximately only 33% was labile. Phosphorus mobilized by agricultural practices without concurrent uptake by plants may be repartitioned to sorbed forms that are not as plant-available as prior to acidification
XANES Demonstrates the Release of Calcium Phosphates from Alkaline Vertisols to Moderately Acidified Solution
Calcium
phosphate (CaP) minerals may comprise the main phosphorus
(P) reserve in alkaline soils, with solubility dependent on pH and
the concentration of Ca and/or P in solution. Combining several techniques
in a novel way, we studied these phenomena by progressively depleting
P from suspensions of two soils (low P) using an anion-exchange membrane
(AEM) and from a third soil (high P) with AEM together with a cation-exchange
membrane. Depletions commenced on untreated soil, then continued as
pH was manipulated and maintained at three constant pH levels: the
initial pH (pH<sub>i</sub>) and pH 6.5 and 5.5. Bulk P K-edge X-ray
absorption near-edge structure (XANES) spectroscopy revealed that
the main forms of inorganic P in each soil were apatite, a second
more soluble CaP mineral, and smectite-sorbed P. With moderate depletion
of P at pH<sub>i</sub> or pH 6.5, CaP minerals became more prominent
in the spectra compared to sorbed species. The more soluble CaP minerals
were depleted at pH 6.5, and all CaP minerals were exhausted at pH
5.5, showing that the CaP species present in these alkaline soils
are soluble with decreases of pH in the range achievable by rhizosphere
acidification