黄土高原植物与不同粒级中土壤体系氮同位素组成变化及意义

Implications of Stable Nitrogen Isotopic Compositional Variation of Plant and Soil with Different Size Fractions in Loess Plateau

目前观点认为，自然生态条件下的非固氮植物-土壤体系中，根系分解供给是土壤中氮的主要来源。研究植物-土壤体系中氮素的分布转化规律是评估土壤中氮素有效性的重要基础。笔者选取黄土高原这一典型西北生境中植物-土壤样品，通过测定样品氮含量与同位素自然丰度组成变化，来说明不同粒级土壤中氮素的分解水平与潜力。

The plant-soil system is an important pool of global nitrogen cycle. Based on a study of nitrogen transformation, this paper has evaluated nitrogen availability in soil. Many experts consider that the major source of soil nitrogen in natural ecological conditions depends on the decomposition degree of underground portions of plant roots. The authors investigated several plant-soil samples with different size fractions in a typical ecological environment of loess plateau and measured contents of total nitrogen （TN）, mineral nitrogen and δ^15N values in the samples. Studies show that δ^15N fractionation from 3‰ to 5‰ results most likely from plant decomposition in soil of different size fractions. With the decrease of fraction size, δ^15N continuously increases, and the values tend to be stable in the smallest fraction, decomposable nitrogen becomes infrequent in soil and contents of mineral nitrogen increase distinctly. Such a regularity indicates that nitrogen isotope abundance could serve as a sensitive indicator in evaluating the plant decomposition degree and the total nitrogen level and potential in soil. Further studies are recommended such as the diversities of δ^15N values between NH4^＋ - N and NO3^- - N in different size fractions of soil.