Phosphorus dynamics in Swedish agricultural soils as influenced by fertilization and mineralogical properties: Insights gained from batch experiments and XANES spectroscopy

Abstract

The soil chemistry of phosphorus (P) is important for understanding the processes governing plant availability as well as the risk of environmental losses of P. The objective of this research was to investigate both the speciation and the pH-dependent solubility patterns of P in clayey agricultural soils in relation to soil mineralogy and fertilization history. The study focused on soil samples from six fields that were subjected to different P fertilization regimes for periods of 45 to 57years. Soil P speciation was analyzed by P K-edge XANES spectroscopy and chemical fractionation, sorption isotherms were constructed, and dissolved P was measured as a function of pH. The XANES fitting results showed that organic P and P adsorbed to Fe and Al (hydr)oxides were common P constituents in all soils. Calcium phosphates were identified in five of six soil samples. The XANES results also indicated an increase in P adsorbed to Al and to a lesser extent Fe (hydr)oxides as a result of fertilization. Moreover, the fluorescence intensity from the P K-edge XANES analysis was most strongly correlated with HCl-digestible P (r=0.81***). Consistent with the XANES analysis, laboratory sorption isotherm models showed that the Freundlich sorption coefficient (KF) was most closely related to oxalate-extractable Al. Greater proportions of Ca phosphate in two of the heavily fertilized soils in combination with enhanced PO4 solubilization upon sample acidification indicated neoformation of Ca-phosphate precipitates. The results for the unfertilized soil samples generally showed a minimum in dissolved PO4 between pH6.5 and 7.5, with increases particularly at lower pH. This behavior can be explained either by the dissolution of Al-hydroxide-type sorbents or Ca phosphates at lower pH. In fertilized soils, there was no consistent trend in pH-dependent solubilization of P, with a complex relationship to solid-phase speciation. To conclude, inorganic P species changed most dynamically in agricultural clay soils over a period of several decades, and the role of pH in the solubilization of P depended mainly on P fertilization history and the content of reactive Ca phosphates.

In situ observation of localized, sub-mm scale changes of phosphorus biogeochemistry in the rhizosphere

“…P solubility in soil Fors showed a minimum at pH 7.8. With a bulk soil pH CaCl2 of 7.8, acidification of the calcareous soil in our study should result in P solubilization if Ca-P minerals are present, which is likely according to published information on P speciation in such high Ca soil (Eriksson et al 2016; Luo et al 2017; McLaren et al 2015; Zhang et al 2014), and in line with the measured increase of P solubility in soil Fors when pH was lowered from 7.8 to 7 (Eriksson et al 2016). …”

“…around pH 4.5 - 5.5, of the reported minimum pH range (Barrow 2017; Eriksson et al 2016). In terms of P load, pH and organic matter, the non-calcareous soil of our study is similar to soil Ekebo in the study of Eriksson et al (2016), which has its minimum P solubility around pH 4.5. The observed pH increase from pH 5.5 (bulk soil, Fig.…”

“…According to published minima of P solubility in soil (Barrow 2017; Eriksson et al 2016; Weng et al 2011) lowering pH from above 7.5 to about 7 should lead to decreased P solubility unless it is controlled by Ca-P minerals (Weng et al 2011; Eriksson et al 2016). In terms of P load, pH and organic matter, the calcareous soil of our study is comparable to soil Fors of Eriksson et al (2016). P solubility in soil Fors showed a minimum at pH 7.8.…”

Phosphorus Pool Composition in Soils and Sediments of Transitional Ecotones under the Influence of Agriculture

“…3f,l). The HCl‐P fraction comprises primary and secondary Ca‐Pi minerals, such as apatite or octacalcium phosphate (Daly et al, 2015; Eriksson et al, 2016). Considering the local geology and water chemistry (Supplemental Table S4), both primary and secondary Ca‐Pi minerals are likely to occur at the study sites.…”

Micro-scale heterogeneity of soil phosphorus depends on soil substrate and depth

“…Moreover, improvements of synchrotron-based X-Ray Fluorescence (μ-XRF) spectroscopy and mapping allow in-situ assessments of element distributions at the micro-scale 10 . In our study, both techniques were combined to overcome individual technique limitations, such as matrix effects for NanoSIMS 14 and the comparably large beam penetration depth for μ-XRF spectroscopy 15 . Additionally, direct P speciation of micro-environments can be obtained by synchrotron-based X-Ray Absorption Near Edge Structure (μ-XANES) spectroscopy 16, 17 .…”