Selective extraction of labile phosphorus using dialysis membrane tubes filled with hydrous iron hydroxide

Leaching of phosphorus (P) can be a serious problem in P-enriched sandy soils. Techniques that decrease the P content of such soils have been proposed as possible remediation methods. In this study, we determined the effect of P removal from two P-rich sandy soils on extractability of soil P in a laboratory experiment. We created soil samples in increasing stages of P depletion by using a sink method, which consists of a dialysis membrane tube filled with hydrous Fe-(hydr)oxide (DMT-HFO). Total amounts of P removed were relatively small compared with the high initial ammonium-oxalate extractable P contents. However, amounts of water and CaCl2 extractable P in the depleted soil samples decreased by 57 to 80%, on average, for both soils. On the other hand, the ammonium-oxalate-based P saturation index decreased by only 11%. Apparently, labile P forms were readily removed, which means that depletion by the DMT-HFO was selective. Our results suggest that remediation methods that remove a small but selective amount of P from soil may cause a significant decrease of the soil potential to release dissolved P. We also used our results to evaluate the suitability of the DMT-HFO to act as an infinite sink for P. For that, the desorption results were described with a simple kinetic Langmuir equation. Errors of k(d) (desorption constant) and Q(0) (amount of P initially adsorbed) were calculated. Although the model fit was good for both soils (r(2)=0.98*** and 0.99***), errors in Q(0) and k(d) were large. Therefore, the DMT-HFO method could not be used to determine the desorption constants of our soils. Values of k(d) and Q(0) obtained by this method should not be used in modeling studies.