Sedimentary phosphorus sequential extraction method offers new insight into soil development Earth Critical Zone

Fractionation of soil phosphorus (P) by sequential extraction is one of tools to study the soil development in Earth's critical zone for characterization of weathering pattern and soil evolution. A sequential extraction method originally developed for sediment with advantage of separating CaCO3-bound P from igneous fluorapatite has been used to quantify different forms of P in New England native forest soils, USA. Total soil phosphorus is fractionated into exchangeable, iron-bound, CaCO3-bound, igneous fluorapatite and refractory organic P pools. Seven surface soil samples were collected from coastal, terrestrial, and lacustrine environments. Total P (TP) ranges from 11.4 to 21.5 μmol P g−1. Averaging over all sampling locations, the largest fraction is igneous fluorapatite with an average concentration of 4.9 ± 1.6 μmol P g−1, accounting for 29.2 ± 9.5 % of TP. Refractory organic P is the second largest fraction with an average concentration of 4.4 ± 1.6 μmol P g−1, accounting for 27.0 ± 9.6 % of TP. Iron-bound P is the third largest fraction with an average concentration of 3.3 ± 1.7 μmol P g−1, accounting for 20.3 ± 10.0 % of TP. CaCO3 -bound P is the second smallest fraction with an average concentration of 2.8 ± 1.4 μmol P g−1, accounting for 16.3 ± 7.5 % of TP. As usual, the smallest fraction is exchangeable P with an average concentration of 1.2 ± 0.8 μmol P g−1, accounting for 7.2 ± 4.3 % of TP. Data from this study are well fitted to a linear model based on the previous sequential extraction data compiled from global soils and sediments samples. The sequential extraction method used here offers an improved procedure that quantify the abundance of igneous fluorapatite as primary P mineral, which provides an estimate of the state and degree of weathering and soil development in the region.