Modeling the Effects of Diffusion Limitations on Nitrogen-15 Isotope Dilution Experiments with Soil Aggregates
Modeling the Effects of Diffusion Limitations on Nitrogen-15 Isotope Dilution Experiments with Soil Aggregates
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DOI：
10.2136/sssaj2002.1868
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发表时间：
2003-03
影响因子：
2.9
通讯作者：
J. Cliff;P. Bottomley;R. Haggerty;D. Myrold
中科院分区：
文献类型：
--
作者：
J. Cliff;P. Bottomley;R. Haggerty;D. Myrold
同位素稀释方法中固有的假设是标记的均匀分布。然而，由于大多数土壤系统中的传质限制，这种假设可能不成立。采用球形扩散反应模型模拟了15个NH 4 +同位素稀释实验，测定了24 h内NH 4 +的总产量和总消耗量，研究了质量传递限制对半径为0.36 cm的土壤团聚体中同位素稀释的影响。描述NH 4 +迁移和反应的方程假定为Fickian扩散、线性、平衡吸附、天然丰度15 N的零级产生以及NH 4 +的伪一级或零级消耗。在伪一阶消费的情况下，速率计算是敏感的吸附系数（强调需要解释的结果作为表观速率），但不是其他运输参数。在零阶消费的情况下，生产率和消费率总是被低估。误差随着骨料粒径的增大和有效扩散系数的减小而增大。增加消费与生产率的比率增加了生产率估计的误差。允许应用的标签扩散到土壤团聚体为24小时之前，初始时间采样减少误差的一个因素约为3（<-8%相对误差），在最大的团聚体尺寸类。这些模拟再次强调需要优化实验方案时，设计同位素稀释实验在结构化土壤中，并建议平衡期前的初始时间采样将提高准确性的反应速率估计同位素稀释实验。
An assumption inherent in isotope dilution methodologies is that of homogeneous distribution of label. This assumption may not hold, however, because of mass transfer limitations in most soil systems. The effects of mass transfer limitations on isotope dilution in soil aggregates with radii up to 0.36 cm were examined using spherical diffusion-reaction models designed to simulate 15 NH 4 + isotope dilution experiments measuring gross production and consumption of NH 4 + across a 24-h period. Equations that described transport and reaction of NH 4 + assumed Fickian diffusion, linear, equilibrium adsorption, zero-order production of natural abundance 15 N, and either pseudofirst-order or zero-order consumption of NH 4 +. In the case of pseudo-first-order consumption, rate calculations were sensitive to the adsorption coefficient (emphasizing the need to interpret results as apparent rates), but not to other transport parameters. In the case of zero-order consumption, both production and consumption rates were always underestimated. Errors increased as aggregate size increased and as effective diffusivity decreased. Increasing the consumption to production rate ratio increased the error in production rate estimates. Allowing the applied label to diffuse into soil aggregates for 24 h prior to initial time sampling decreased errors by a factor of about three (to <-8% relative error) in the largest aggregate size class. These simulations reemphasize the need to optimize experimental protocol when designing isotope dilution experiments in structured soils and suggest an equilibration period prior to initial time sampling will improve accuracy of reaction rate estimates obtained from isotope dilution experiments.