Calcium- and iron-related phosphorus in calcareous and calcareous marsh soils: Sequential chemical fractionation and P-31 nuclear magnetic resonance study

Phosphorus (P) forms in soils determine the amount of P available for crops and the potential for this element to be released to water. Sequential che mical fractionation can provide some information about major P forms in soi ls, and allow one to distinguish iron (Fe)-related phosphorus from-calcium (Ca)-bound P. The P-31 nuclear magnetic resonance (NMR) spectroscopy has be en used in the identification of organic P, precipitated Ca-phosphates, and aluminum (Al)-related P in acid soils. Three calcareous soils and four cal careous marsh soils were used in this study. These two types of soils diffe r in the nature of iron oxides, which are the main P sorbent surfaces. The ratio of low crystalline to high crystalline iron oxides is higher in marsh soils than in calcareous soils as a consequence of the special genesis and conditions of the soil (reduction-oxidation cycles). Such a ratio is relat ed to the proportion of occluded P in low crystalline oxides relative to th at of high crystalline oxides. Citrate-bicarbonate extractable P (CB-P) in the fractionation schemes can be ascribed to adsorbed P and high soluble ca lcium phosphates. CB-P is correlated with the sum of P fractions in all the soils, thus indicating that the amount of the P that can be easily release d is related to the rate of P enrichment of the soil. The P-31 NMR spectral data reveal that hydroxyapatite is the dominant P form in the soils studie d. This is consistent with the fractionation data, where acid-extractable P is the main P fraction. The spectra also provide some information about th e amount of total inorganic P and Ca-phosphates in calcareous soils.