Addition of Fe-humic acids to overcome analytical issues in measurements of isotopically exchangeable P in soil

Isotopic dilution has been widely used to measure isotopically exchangeable phosphorus (P) in soil (E value), as a measure of potentially plant-available P. However, in soils with low E values and/or strong P sorption, measurement of E values can be challenging due to very low solution concentrations and the interference of colloidal non-exchangeable species, thus confounding the measurements in the soils of most interest. The addition of a complexing compound could increase solution concentrations and reduce these analytical issues, as has been found in the case of metals. Therefore, we investigated the addition of Fe-humic acid (Fe-HA) as a P-complexing compound to the soil suspension prior to isotopic exchange. This results in the formation of P-Fe-HA complexes, thus increasing P solution concentrations by solubilizing P from the labile pool and reducing errors caused by suspended colloids. We used this method to measure E values in five soils with low P status, without or with the addition of carrier P. The addition of Fe-HA (at 50 or 200 mg Fe-HA/L to the equilibration solution) substantially decreased the measured E value without carrier P addition in four of the five soils, while there was no or little effect when carrier P was added. The higher Fe-HA rate increased solution concentrations of stable and radioactive P more than the lower rate, but there was no significant difference in measured E values between the two Fe-HA rates. The method was also applied to 15 subsoils with low P status. Overall, our results indicated that the addition of Fe-HA provides an easy and robust way to avoid analytical issues in the determination of E values in soils with low concentrations of P in solution.