凯氏蒸馏法和元素分析仪法测定沉积物中全氮含量的异同及其意义

Kjeldahl method and the elemental analyzer method in measurement of total nitrogen in sediments: Comparison and its significance

氮循环是生物地球化学研究领域中最重要的课题之一,全氮含量的测定是农业、生物和环境等多领域研究中的常规测试项目。凯氏蒸馏法和元素分析仪法作为常规方法通用于上述领域的全氮含量测定,但两种方法在实际应用中的差异却常被忽视。通过对中国北方某盐湖沉积物序列近百个样品的全氮含量分析,对两种方法测定结果的异同进行了对比研究。凯氏蒸馏法的分析精密度高于元素分析仪法(前者标准偏差为0.007,后者为0.024),但在样品硝态和亚硝态氮的含量较高时,凯氏蒸馏法所测结果显著低于元素分析仪法的测定结果,此时选择元素分析仪法进行全氮含量的测定更为可靠和准确;对硝态和亚硝态氮含量极低的样品,两种方法的测定结果无显著差异。研究表明,选择全氮含量的测定方法,必须对所测样品的无机氮含量有初步的了解。对于中国北方的多数盐湖和地表环境样品,由于其无机氮的含量较高,选择凯氏蒸馏法进行全氮含量的测定是不适当的。尽管如此,对所研究的湖泊沉积物剖面而言,两种方法的对比却可以提供有价值的气候和环境演变信息。随着沉积物剖面的由深到浅,两种方法的测定结果由一致变化到存在显著差异,表明了湖泊水体从早期的淡水向今天的盐湖演化的过程。因此,凯氏蒸馏法和元素分析仪法测定结果的差异可以作为一种独立的地球化学气候代用指标。

Nitrogen cycle is an important biogeochemical process in the environment. Measurement of total nitrogen （TN） is a routine experiment in researches in agriculture, biology and environmental sciences. Kjeldahl method （KM） and elemental analyzer method （EA） are commonly used in measuring TN. Their difference, however, is often overlooked. A comparative study on the two methods is conducted by analysis of TN in nearly 100 samples from the sediment sequence of Gouchi, a salt lake in North China. KM is better than EA with a precision of 0. 007 vs.0. 024. With the presence of nitrate and/or nitrite nitrogen, however, measurement by KM is considerably lower than that by EA. To analyze TN in this kind of samples, EA is the correct choice. For samples without or with negligible nitrate and/or nitrite nitrogen, both techniques are applicable. It is necessary, therefore, to have a first estimate of the TN in samples before making choice of the methods. For samples from lake sediment sequences or soils in North China, KM is inapplicable for TN analysis because of their usually high content of nitrous salt. Despite the inconsistency, use of both methods to the same samples makes sense in reconstructions of climatic and environmental changes from lake sediments. In lake Gouchi, TN content by the two methods changed generally from the same in the lower part of the sequence to the significant difference in the upper part, suggesting a broad change from a fresh water lake to the present day salt lake. Our data indicate that the difference of the TN by the two methods can serve as an independent climatic proxy.