盐酸提取的土壤无机磷酸盐中氧同位素分析

The oxygen isotopic composition of phosphates in soils extracted with HCl

【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(^(18)O和^(16)O)的比值,即δ^(18)O-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δ^(18)O-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δ^(18)O-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δ^(18)O-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δ^(18)O-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。

[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.