Soil chemical extractions can alter potassium coordination in agricultural soils: A combined wet chemical and X-ray absorption spectroscopic approach

Abstract

• Potassium XANES spectroscopy from soils is paired with six wet chemical extractions. • Dampening of post-edge XANES oscillations was caused by the Mehlich III extraction. • XANES indicates Mehlich III can dislodge K from inner-sphere siloxane bonds. • Dampening was more significant in the loamy sand soil versus the sandy clay loam soil. • Dampening increased the LCF component of K associated with montmorillonite by 25% Soil wet chemical extractions are used to determine plant available potassium (K); yet problems with K bioavailability still persist in soils. Bioavailability is directly related to how elements are bonded and coordinated to the solid phases of soil. However, there is a critical knowledge gap with respect to identifying the in situ, solid-phase K coordination environments in soils treated with wet chemical extractions. Synchrotron-based X-ray absorption spectroscopy (XAS) measures the molecular coordination environment of elements and has been used here to study K chemistry in two agricultural field soils. The objective of this work is to pair XAS with wet chemical K extractions and analyze changes in the coordination environment of K throughout the soil profile. Soil profiles collected down to 1.2 m of a loamy sand and a sandy clay loam were treated with six different extraction methods: (1) Mehlich III, (2) ammonium acetate, (3 & 4) Haney, Haney, Hossner, and Arnold 1 and 2, (5) citrate dithionite, and (6) water extractable potassium. The residual solid was analyzed using XAS and X-ray diffraction (XRD). Citrate dithionite extracted the largest amount of K (1860 mg kg −1 ), and the water extraction removed the least (30 mg kg −1 ). XAS and XRD indicate K sources in these soils are illite, illite–smectite, smectite, and potassium feldspar. A Linear Combination Fit (LCF) approach indicated changes of interest in the post-edge region of the K, K-edge XANES spectra due to the Mehlich extraction. XANES data indicate that the Mehlich extraction can remove loosely bound K and dislodge a portion of K from interlayer space of illite and illite–smectite. This is significant because the Mehlich extraction is effectively decreasing the number of coordinating O atoms surrounding K and causing a dampening in the oscillations of the XANES spectra in the post-edge region (3,620–3,650 eV). The dislodgement of K due to the Mehlich III extraction indicates that the inner-sphere bonds that K forms to the siloxane surfaces in the interlayer space can be broken, and K can become more hydrated with water molecules also present in the interlayer space. This dampening was dependent on soil texture, was more significant in the coarser textured soil, and would indicate an increased accommodation of K by montmorillonite or an increase in K hydration in illite–smectite. The dampening resulted in an average increase in K intercalated in montmorillonite interlayer spaces by 25% in Mehlich treated samples according to the LCF results. Future studies involving K sorption to silicon and aluminum oxides will further clarify behavior of K on mineral surfaces, particularly during surface precipitation.