Phosphorus (P) can limit crop production in many soils, and soil testing is used to guide fertilizer recommendations. The Mehlich III (M3) soil test is widely used in North America, followed by colorimetric analysis for P, or by inductively coupled plasma-based spectrometry (ICP) for P and cations. However, differences have been observed in M3 P concentrations measured by these methods. Using 31P nuclear magnetic resonance (P-NMR) and mass spectrometry (MS), we characterized P forms in M3 extracts. In addition to the orthophosphate that would be detected during colorimetric analysis, several organic P forms were present in M3 extracts that would be unreactive colorimetrically but measured by ICP (molybdate unreactive P, MUP). Extraction of these P forms by M3 was confirmed by P-NMR and MS in NaOH-ethylenediaminetetraacetic acid extracts of whole soils and residues after M3 extraction. The most abundant P form in M3 extracts was myo-inositol hexaphosphate (myo-IHP, phytate), a compound that may not contribute to plant-available P if tightly sorbed in soil. Concentrations of myo-IHP and other organic P forms varied among soils, and even among treatment plots on the same soil. Extraction of myo-IHP in M3 appeared to be linked to cations, with substantially more myo-IHP extracted from soils fertilized with alum-treated poultry litter than untreated litter. These results suggest that ICP analysis may substantially over-estimate plant-available P in samples with high MUP concentrations, but there is no way at present to determine MUP concentrations without analysis by both colorimetry and ICP. This study also tested procedures that will improve future soil P-NMR studies, such as treatment of acid extracts, and demonstrated that techniques such as P-NMR and MS are complimentary, each yielding additional information that analysis by a single technique may not provide.

中文翻译：

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通过 Mehlich III 土壤测试表征从土壤中提取的磷形态。


磷 (P) 会限制许多土壤中的作物产量，土壤测试可用于指导施肥建议。 Mehlich III (M3) 土壤测试在北美广泛使用，然后对 P 进行比色分析，或对 P 和阳离子进行电感耦合等离子体光谱 (ICP) 分析。然而，通过这些方法测量的 M3 P 浓度存在差异。使用 31P 核磁共振 (P-NMR) 和质谱 (MS)，我们对 M3 提取物中的 P 形式进行了表征。除了在比色分析过程中检测到的正磷酸盐外，M3 提取物中还存在几种有机 P 形式，这些形式在比色中不发生反应，但通过 ICP（钼酸盐非反应性 P，MUP）进行测量。 M3 对整个土壤和残留物的 NaOH-乙二胺四乙酸提取物中的 P-NMR 和 MS 证实了 M3 对这些 P 形式的提取。 M3 提取物中最丰富的磷形式是肌醇六磷酸盐（myo-IHP，植酸盐），这种化合物如果紧密吸附在土壤中，可能不会产生植物可利用的磷。 myo-IHP 和其他有机磷形式的浓度因土壤而异，甚至在同一土壤的处理地之间也不同。 M3 中 myo-IHP 的提取似乎与阳离子有关，从用明矾处理的家禽垫料施肥的土壤中提取的 myo-IHP 明显多于未处理的垫料。这些结果表明，ICP 分析可能大大高估了高 MUP 浓度样品中的植物有效磷，但目前没有办法在不通过比色法和 ICP 分析的情况下确定 MUP 浓度。 这项研究还测试了可改善未来土壤 P-NMR 研究的程序，例如酸提取物的处理，并证明 P-NMR 和 MS 等技术是互补的，每种技术都能产生单一技术分析可能无法提供的额外信息。