Phosphorus fractions and release patterns in typical Mediterranean soils

Phosphorus forms dictate the P release potential of soils in agronomic and environmental terms. The main purposes of this work were to study P forms by sequential chemical fractionation in soils typical of Mediterranean areas, establishing correlations between P fractions and soil properties, and identify the relationships of specific P fractions to P release potential as determined using P sinks (anion-exchange resins). To this end, three different fractionation methods were used. A comparison of the results obtained with the three sequential fractionation schemes provides useful information about P forms in representative Mediterranean soils, allowing the distinction of P fractions which include: (i) the more labile P forms (essentially adsorbed P), (ii) most of pedogenic Ca phosphates, (iii) most of low soluble pedogenic Ca phosphates, (iv) lithogenic Ca phosphates, (v) P occluded in poorly crystalline Fe oxides, and (vi) P occluded in crystalline Fe oxides. The ratio of the P fraction, which includes the more labile P forms (essentially adsorbed) to combined non-organic P fractions was negatively correlated with soil pH (r = -0.83, P < 0.001) and positively correlated with the portion of combined Fe fractions related to poorly crystalline oxides (r = 0.84, P < 0.001). The greatest of P desorbable from resins (Q(max), in the Johnson-Mehl equation) was related mainly to the combined P fractions including adsorbed P and precipitated Ca phosphates in the studied soils. The amount of P desorbed at 1 h (Q(1h)) accounted for a sizeable fraction of Q(max) (between 29 and 59 %), the Q(1h)/Q(max) ratio being positively correlated with the portion of combined non-organic P fractions related to adsorbed P (r = 0.66; P < 0.01), and negatively correlated with the portion of combined P fractions related to low soluble pedogenic Ca phosphates (r = -0.67; P < 0.01).