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.