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）可能会限制许多土壤中的作物产量，土壤测试可用于指导肥料建议。Mehlich III（M3）土壤测试在北美广泛使用，随后对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法（ICP）。然而，已经观察到通过这些方法测量的M3P浓度存在差异。使用31 P核磁共振（P-NMR）和质谱（MS），我们表征了M3提取物中的P形式。除了在比色分析过程中会检测到的正磷酸盐外，M3提取物中还存在几种有机P形式，这些形式在比色上是无反应性的，但可以通过ICP进行测量（钼酸盐无反应性P，MUP）。通过P3-NMR和MS在整个土壤和M3萃取后的残留物中的NaOH-乙二胺四乙酸萃取物中，通过M3证实了这些P形式的萃取。M3提取物中最丰富的P形式是六磷酸肌醇（肌醇IHP，植酸盐），如果紧密吸附在土壤中，该化合物可能不会对植物体内的P起作用。肌-IHP和其他有机磷形式的浓度在土壤之间，甚至在同一土壤上的处理地块之间都不同。M3中Myo-IHP的提取似乎与阳离子相关，从经过明矾处理的家禽垫料施肥的土壤中提取的Myo-IHP明显多于未经处理的垫料。这些结果表明，ICP分析可能会高估高MUP浓度样品中的植物有效磷，但是，如果不通过比色法和ICP进行分析，目前无法确定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，并证明了P-NMR和MS等技术是互补的，每种技术均提供了单一技术可能无法提供的附加信息。