Evaluating phosphorus sorption capacity of acidic soils by short-term and long-term equilibration procedures

Estimating soil phosphorus (P) sorption parameters, such as the sorption capacity (PSC) and the degree of phosphorus saturation (DPS), is important in order to evaluate the potential of P loss from soil to water. Generally, PSC is determined by short-term P sorption measurements or, in acidic soils, is taken to be alpha(Al-ox + Fe-ox), where Al-ox and Fe-ox represent the amounts of aluminum (Al) and iron (Fe) extracted by a single extraction of oxalate, and alpha is the scaling factor. Then, DPS is calculated as the percentage of already sorbed P, assumed to be the amount of P extracted from soil with the oxalate solution (P-ox), to PSC. A sorption experiment was carried out over 180 days on twenty-four acidic soils from different countries of Europe, in order to estimate long-term soil P sorption capacity and DPS, and to investigate their relationships with short-term sorption (PSI24 h), and Al-ox and Fe-ox. Soils presented a very wide range in PSC (19.1-284.3 mmol P sorbed kg(-1) soil) and DPS (5.5-63.8%), with P sorption resulting strongly correlated with the sum of oxalate extractable Al and Fe (r = 0.996; p < 0.001). The slope (alpha of the regression equation between PSC and (Al-ox + Fe-ox) was 0.49, very close value to that (0.50) normally used by several researchers to indirectly calculate PSC from Al-ox + Fe-ox measurements. Sorption capacity determined after 180 days of soil-P equilibration was, on average, 2.71 times greater than the 24-h derived P sorption. However, a strong correlation was observed for the relationship between long-term P sorption and PSI24 h (r = 0.950; p < 0.001), which confirms that short-cut P sorption measures can be effectively used to estimate the P sorption capacity of soils with reasonable accuracy.