Isotopically Exchangeable Phosphate in Size Fractionated and Unfractionated Soils

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AbstractThe acquisition of precise information on soil phosphate availability is a prerequisite to its sustainable management in agroecosystems. The objective of this study was to assess the exchangeability of phosphate in the particle‐size fractions (2000–50, 50–2, and <2 µm) of seven soil samples. Samples were completely dispersed with a Nasaturated cation‐exchange resin, and phosphate exchangeability was assessed by the isotopic exchange kinetic method on each particle‐size fraction and on the unfractionated samples. Isotopically exchangeable phosphate was then divided into five pools containing phosphate that is exchangeable within different time frames. In the 2000‐ to 50‐µm fraction, an average of 91.8% of the acid‐extractable (1.0 M H2SO4) inorganic phosphate could not be exchanged within 1 yr while 5.5% of it was exchangeable within 1 min. These results are consistent with the presence in this fraction of slowly exchangeable phosphate in the form of condensed Ca phosphates and rapidly exchangeable phosphate associated with plant debris. In the <2‐µm fraction, a mean of 44.8% of the acid‐extractable phosphate was exchangeable within 1 yr. This confirms that this fraction is the main source of phosphate available to plants, and the main sink for water‐soluble phosphate. A comparison between the sum of the single pools assessed in fractions with the phosphate pools of unfractionated soils showed that, on dispersion, up to 30% of the acid‐extractable phosphate was transferred from slowly to more rapidly exchangeable phosphate pools. This result strongly suggests that unfractionated soils contain phosphate that is physically protected from isotopic exchange, probably within water‐stable aggregates.