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.

中文翻译：

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添加 Fe-humic acid 以克服土壤中同位素交换 P 测量中的分析问题


同位素稀释已被广泛用于测量土壤中的同位素可交换磷 （P） （E 值），作为潜在植物可利用磷的量度。然而，在 E 值低和/或强 P 吸附的土壤中，由于溶液浓度非常低和胶体不可交换物质的干扰，测量 E 值可能具有挑战性，从而混淆了最感兴趣的土壤中的测量。添加络合化合物可以增加溶液浓度并减少这些分析问题，就像在金属中发现的那样。因此，我们研究了在同位素交换之前将 Fe-腐植酸 （Fe-HA） 作为 P 复合物化合物添加到土壤悬浮液中。这导致形成 P-Fe-HA 复合物，从而通过溶解不稳定池中的 P 并减少悬浮胶体引起的误差来增加 P 溶液浓度。我们使用这种方法测量了 5 种低 P 状态土壤中的 E 值，没有或添加了载体 P。在五种土壤中，在没有添加载体 P 的情况下，添加 Fe-HA（在 50 或 200 mg Fe-HA/L 平衡溶液中加入 Fe-HA/L）大大降低了测得的 E 值，而添加载体 P 时没有或几乎没有影响。较高的 Fe-HA 速率比较低的速率增加了稳定和放射性 P 的溶液浓度，但两个 Fe-HA 速率之间的测量 E 值没有显着差异。该方法也适用于 15 种低 P 含量的底土。总体而言，我们的结果表明，添加 Fe-HA 提供了一种简单而可靠的方法，可以避免在溶液中 P 浓度低的土壤中测定 E 值时出现分析问题。