Carboxylic Acids Affect Sorption and Micro‐Scale Distribution of Phosphorus in an Acidic Soil

The availability of P fertilizer to plants is decreased in acid soils due to phosphate sorption onto solid‐phase binding sites. A combination of solution analysis, micro‐x‐ray fluorescence (μ‐XRF) and micro‐x‐ray absorption near edge structure (μ‐XANES) spectroscopy were used to investigate the degree to which sorption of fertilizer P is determined by reaction with Al compounds, the extent to which carboxylic acid addition can influence these reactions, and the relative distribution of sorbed P in an acid soil. The addition of carboxylic acids did not change total P sorption, with solid‐phase P binding to metal and organic complexes. No treatment effects on P bonding were evident, as measured by μ‐XRF and μ‐XANES. While μ‐XRF and μ‐XANES did provide information on P interactions, the high spatial heterogeneity within samples and the need to focus on areas of high P concentration diminished our ability to identify clear treatment effects at the micro scale. Nevertheless, this research did highlight the value of utilizing complementary techniques (solution analyses, XRF, μ‐XRF mapping, and XANES) to generate new knowledge about P chemistry in soils.