High resolution HPLC-MS confirms overestimation of urea in soil by the diacetyl monoxime (DAM) colorimetric method
High resolution HPLC-MS confirms overestimation of urea in soil by the diacetyl monoxime (DAM) colorimetric method
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DOI：
10.1016/j.soilbio.2019.04.015
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发表时间：
2019-08-01
影响因子：
9.7
通讯作者：
Evershed, Richard P.
中科院分区：
文献类型：
--
作者：
Reay, Michaela K.;Yates, Chris A.;Evershed, Richard P.
尿素是有机氮(N)的一种常见形式，在作物（如肥料）和牲畜（如尿液）系统中被大量添加到农业土壤中。此外，土壤中还存在一个与微生物群落代谢功能相关的小的、动态的环境尿素库。双乙酰基单肟（DAM）比色法通常用于定量土壤中的尿素，但由于可能与土壤有机质中常见的结构基团脲醛基（R1NHCONHR2）发生反应，因此缺乏特异性。因此，本研究的目的是严格评价该方法测定土壤中尿素的准确性。使用DAM测定，我们证明了与一系列脲基化合物的显著交叉反应性，其中许多在土壤中普遍存在。因此，我们得出结论，DAM分析很可能高估了环境材料中的尿素浓度。采用高分辨率HPLC-Orbitrap质谱法对草地土壤中尿素进行了标准添加，并与DAM法进行了浓度比较，证实了这种高估。结果表明，DAM量热法对尿素浓度的高估幅度在7.2 ~ 58倍之间。这种对尿素的显著高估强调了需要参照LC-MS分析验证比色法，以确保测量尿素浓度的稳健性。在此基础上，我们建议将DAM比色法的结果报告为“尿素”浓度，并将其报告为“尿素- n”，以识别来自其他化合物的未知和可变贡献。事实上，考虑到定量评估后一种贡献的问题，我们建议在土壤和更广泛环境中尿素浓度的调查中现在避免使用DAM方法。
Urea represents a common form of organic nitrogen (N) which is added to agricultural soils in large quantities in both cropping (e.g. fertiliser) and livestock (e.g. urine) systems. In addition, there is a small, dynamic ambient pool of urea in soil associated with metabolic functioning in the microbial community. The diacetyl monoxime (DAM) colorimetric method is routinely used to quantify urea in soil, however, it lacks specificity due to the potential to react with the ureido group (R1NHCONHR2), a common structural moiety in soil organic matter. The aim of this study was therefore to critically evaluate the accuracy of this method for urea determination in soil. Using the DAM assay, we demonstrated significant cross-reactivity with a range of ureido compounds, many of which are ubiquitous in soil. We conclude therefore that the DAM assay is highly likely to overestimate urea concentrations in environmental materials. Such overestimation was confirmed using high resolution HPLC-Orbitrap MS to quantify urea in grassland soils using standard addition and the concentrations compared with those of the DAM assay. The results obtained show the DAM calorimetric method overestimated urea concentration by between 7.2 and 58 times for the sites studied. This significant overestimation of urea emphasises the need to validate the colorimetric method with reference to the LC-MS assay to ensure the robustness of measured urea concentrations. On this basis we recommend that reporting of the results from the DAM colorimetric method as "urea" concentration be curtailed and reported as "ureido-N" to recognise the contribution of unknown and variable contributions from other compounds. Indeed, given the problems with quantitatively assessing the latter contributions we would recommend the DAM method is now avoided in surveys of urea concentrations in soil and the wider environment.