Calcium- and iron-related phosphorus in calcareous and calcareous marsh soils:: Sequential chemical fractionation and 31P 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 chemical fractionation can provide some information about major P forms in soils, and allow one to distinguish iron (Fe)-related phosphorus from-calcium (Ca)-bound P. The P-31 nuclear magnetic resonance (NMR) spectroscopy has been used in the identification of organic P, precipitated Ca-phosphates, and aluminum (Al)-related P in acid soils. Three calcareous soils and four calcareous marsh soils were used in this study. These two types of soils differ 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 related to the proportion of occluded P in low crystalline oxides relative to that of high crystalline oxides. Citrate-bicarbonate extractable P (CB-P) in the fractionation schemes can be ascribed to adsorbed P and high soluble calcium 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 released 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 studied. This is consistent with the fractionation data, where acid-extractable P is the main P fraction. The spectra also provide some information about the amount of total inorganic P and Ca-phosphates in calcareous soils.