Hobbie Erik A, Macko Stephen A, Shugart Herman H
Department of Environmental Sciences, University of Virginia, Charlottesville, VA 22903, USA, , , , , , US.
Oecologia. 1999 Aug;120(3):405-415. doi: 10.1007/s004420050873.
Nitrogen cycling in forest soils has been intensively studied for many years because nitrogen is often the limiting nutrient for forest growth. Complex interactions between soil, microbes, and plants and the consequent inability to correlate δN changes with biologic processes have limited the use of natural abundances of nitrogen isotopes to study nitrogen (N) dynamics. During an investigation of N dynamics along the 250-year-old successional sequence in Glacier Bay, Alaska, United States, we observed several puzzling isotopic patterns, including a consistent decline in δN of the late successional dominant Picea at older sites, a lack of agreement between mineral N δN and foliar δN, and high isotopic signatures for mycorrhizal fungi. In order to understand the mechanisms creating these patterns, we developed a model of N dynamics and N isotopes (Nitrogen Isotope Fluxes in Terrestrial Ecosystems, NIFTE), which simulated the major transformations of the N cycle and predicted isotopic signatures of different plant species and soil pools. Comparisons with field data from five sites along the successional sequence indicated that NIFTE can duplicate observed patterns in δN of soil, foliage, and mineral N over time. Different scenarios that could account for the observed isotopic patterns were tested in model simulations. Possible mechanisms included increased isotopic fractionation on mineralization, fractionation during the transfer of nitrogen from mycorrhizal fungi to plants, variable fractionation on uptake by mycorrhizal fungi compared to plants, no fractionation on mycorrhizal transfer, and elimination of mycorrhizal fungi as a pool in the model. The model results suggest that fractionation during mineralization must be small (˜2‰), and that no fractionation occurs during plant or mycorrhizal uptake. A net fractionation during mycorrhizal transfer of nitrogen to vegetation provided the best fit to isotopic data on mineral N, plants, soils, and mycorrhizal fungi. The model and field results indicate that the importance of mycorrhizal fungi to N uptake is probably less under conditions of high N availability. Use of this model should encourage a more rigorous assessment of isotopic signatures in ecosystem studies and provide insights into the biologic transformations which affect those signatures. This should lead to an enhanced understanding of some of the fundamental controls on nitrogen dynamics.
多年来，森林土壤中的氮循环一直是深入研究的对象，因为氮通常是森林生长的限制养分。土壤、微生物和植物之间复杂的相互作用，以及由此导致的无法将δN变化与生物过程关联起来，限制了利用氮同位素的自然丰度来研究氮（N）动态。在美国阿拉斯加冰川湾对具有250年历史的演替序列中的氮动态进行调查期间，我们观察到了几种令人困惑的同位素模式，包括在较老地点演替后期占主导地位的云杉的δN持续下降、矿质氮δN与叶片δN之间缺乏一致性，以及菌根真菌的高同位素特征。为了理解产生这些模式的机制，我们开发了一个氮动态和氮同位素模型（陆地生态系统中的氮同位素通量，NIFTE），该模型模拟了氮循环的主要转化过程，并预测了不同植物物种和土壤库的同位素特征。与沿演替序列的五个地点的实地数据进行比较表明，NIFTE可以随时间复制土壤、叶片和矿质氮中观察到的δN模式。在模型模拟中测试了可能解释观察到的同位素模式的不同情景。可能的机制包括矿化过程中同位素分馏增加、氮从菌根真菌转移到植物过程中的分馏、与植物相比菌根真菌吸收过程中的可变分馏、菌根转移过程中无分馏，以及在模型中消除菌根真菌作为一个库。模型结果表明，矿化过程中的分馏必须很小（约2‰），并且在植物或菌根吸收过程中无分馏发生。氮从菌根转移到植被过程中的净分馏最符合矿质氮、植物、土壤和菌根真菌的同位素数据。模型和实地结果表明，在高氮有效性条件下，菌根真菌对氮吸收的重要性可能较低。使用该模型应有助于在生态系统研究中更严格地评估同位素特征，并深入了解影响这些特征的生物转化过程。这应该会增强对氮动态一些基本控制因素的理解。
