盐酸提取的土壤无机磷酸盐中氧同位素分析

【目的】 磷酸盐中的P—O键在自然条件下稳定。磷酸盐中最重和最轻氧同位素 (18O和16O) 的比值，即δ18O-P，可以表征生态系统中磷的转化。由盐酸提取的土壤无机磷 (HCl-Pi) 难以被作物直接吸收利用，但在根系分泌物和土壤微生物的作用下，可转化为有效形态。建立δ18O-P测定方法可以为示踪土壤中HCl-Pi的循环、转化过程提供基础。 【方法】 本文选择潮土和黑土长期定位试验中NK (不施磷肥)、NPK (施用无机磷肥)、NPKM (施用无机磷肥 + 粪肥)、NPKS (NPK + 秸秆还田) 4个施肥处理土样，按照土壤磷化学分级方法逐级浸提，收集1 mol/L HCl浸提液，通过磷钼酸铵沉淀、磷酸铵镁沉淀等过程分离纯化其中磷酸盐，获得磷酸银沉淀，利用元素分析仪稳定性同位素比例质谱仪测定氧同位素组成δ18O-P值。 【结果】 磷酸盐经过分离纯化沉淀3个关键过程，回收率均达到92%以上，损失较少，未出现氧同位素分馏。X射线衍射仪和氧含量测定表明，实验获取磷酸银样品纯度较高。潮土与黑土各处理中HCl-Pi的δ18O-P值分别为17.21‰ 和17.90‰ (NK)、17.25‰ 和17.61‰ (NPK)、17.62‰和17.87‰ (NPKM)、17.95‰ 和17.70‰ (NPKS)，4个施肥处理之间土壤δ18O-P值无显著差异。 【结论】 本研究建立盐酸提取土壤磷酸盐中氧同位素分析测定方法，可为农田土壤中磷的循环转化及其溯源研究提供技术支撑。

The oxygen isotopic composition of phosphates in soils extracted with HCl

【Objectives】 The P—O bond of PO₄3– is a relatively steady structure in nature. The ratio between the heaviest and lightest oxygen isotopes (18O and 16O) in phosphate (Pi), i.e. the oxygen isotope composition of phosphate (δ18O-P) is used for tracing the biological use of P and deciphering P cycle in soils. The fraction of Pi extracted with HCl from soils is considered slowly-released and low bio-available for plant. To establish the determination method of δ18O-P in HCl-Pi will provide a reliable technical support for the estimation of soil P transformation. 【Methods】 In this study, fluvo-aquic soil and black soil were sampled from the plots with four treatments including NK, NPK, NPK+straw, NPK+manure in the long-time fertilization trials located in Henan and Heilongjiang province, respectively. Soil with 1 mol/L of HCl-extractable Pi was isolated by precipitation first as ammonium phosphomolybdate (APM) and later as magnesium ammonium phosphate (MAP). The final purity of the Ag₃PO₄ precipitates was pyrolitically decomposed to carbon monoxide and analyzed for δ18O by elemental analyzer-stable isotope ratio mass spectrometer. 【Results】 The results showed that P recovery rate in the process of three key purification precipitations was more than 92% with very low phosphate loss. There was no isotope fractionation during extraction and purification. The obtained silver phosphate sample had high purity based on X-ray diffraction and oxygen content determination. The δ18O-P values of HCl-Pi from fluvo-aquic soil and black soil were varied from 17.21‰ to 17.95‰. There was no significant difference among the four fertilization treatments. 【Conclusions】 The established method for δ18O-P analysis of phosphates has shown satisfactory recovery and purity of HCl-Pi, and will not be impacted by the soil physical and chemical properties. Therefore, the methods could be used for the estimation of potential P cycle and transformation in the soils.