Mengis M, Walther U, Bernasconi S M, Wehrli B
Swiss Federal Institute for Environmental Science and Technology (EAWAG), Limnological Research Center, CH-6047 Kastanienbaum, Switzerland.
Environ Sci Technol. 2001 May 1;35(9):1840-4. doi: 10.1021/es0001815.
The stable isotopic composition (delta 15N and delta 18O) of nitrate was analyzed in two lysimeter field experiments in order to identify the conditions under which the dual isotope approach can be applied to identify the main source of nitrate in agricultural soils. The first field experiment involved six lysimeters beneath fields that had been fertilized for 10 yr with the same type of fertilizer (NH4NO3; delta 15N = +1.2@1000, delta 18O = +18.6@1000). The isotope ratios of NO3- in the leachate (delta 15N approximately 0@1000; delta 18O approximately +2@1000) could not be interpreted in a conventional way with either fertilizer or soil organic nitrogen as main sources. These results provided clear evidence for the microbial immobilization and subsequent mineralization and nitrification to NO3- (mineralization-immobilization turnover concept). This process masked the original oxygen isotope ratio of the fertilizer source during the summer when microbial activity was high. A second experiment involving the application of Ca(NO3)2 to three lysimeters during the winter confirmed that the dual isotope approach remains valid for the source identification of nitrate under conditions of low microbial activity. The study reveals the limitation of the dual isotope approach to characterize nitrate sources under biologically active conditions and the ability to quantify microbial processes when the main sources can be controlled.
为了确定在何种条件下可应用双同位素方法来识别农业土壤中硝酸盐的主要来源，在两项蒸渗仪田间试验中对硝酸盐的稳定同位素组成（δ¹⁵N和δ¹⁸O）进行了分析。第一项田间试验涉及六个位于农田下方的蒸渗仪，这些农田已使用同一种肥料（NH₄NO₃；δ¹⁵N = +1.2‰，δ¹⁸O = +18.6‰）施肥10年。渗滤液中NO₃⁻的同位素比率（δ¹⁵N约为0‰；δ¹⁸O约为 +2‰）无法以常规方式解释为主要来源是肥料或土壤有机氮。这些结果为微生物固定以及随后矿化和硝化生成NO₃⁻（矿化 - 固定周转概念）提供了明确证据。在夏季微生物活性较高时，这个过程掩盖了肥料来源的原始氧同位素比率。第二项试验在冬季向三个蒸渗仪施加Ca(NO₃)₂，证实了在微生物活性较低的条件下，双同位素方法对于硝酸盐来源识别仍然有效。该研究揭示了双同位素方法在生物活性条件下表征硝酸盐来源的局限性，以及在主要来源可控时量化微生物过程的能力。
