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Modeling the Effects of Diffusion Limitations on Nitrogen-15 Isotope Dilution Experiments with Soil Aggregates

Dep. of Geosciences, Oregon State Univ., Corvallis, OR 97331

View larger version (18K): [in a new window]   Fig. 1. Graphical depiction of a composite spherical soil aggregate which has been labeled with 15NH+4. The total aggregate radius (r) is represented by b. At the moment of labeling, the unlabeled portion of the aggregate exists at 0 < r < a, and the labeled portion of the aggregate exists at a < r < b.

View larger version (16K): [in a new window]   Fig. 2. Effect of adsorption without reaction on concentration profiles of 15NH+4 48 h after labeling an aggregate with a 0.356-cm radius. Numbers on graph represent Kd values.

View larger version (32K): [in a new window]   Fig. 3. Effect of Kd (A), aggregate radius (B), initial incubation time (C), and kcA:kpA ratio (D) on errors in zero-order consumption and production estimates in isotope dilution experiments. Unless noted otherwise, experiments lasting from 24 to 48 h after labeling were simulated and used base model parameters presented in Table 1.

View larger version (15K): [in a new window]   Fig. 4. Effect of zero-order consumption and reaction on concentration profiles of 15NH+4 and 14NH+4 in a 0.356-cm-radius aggregate 24 h after labeling. Base model parameters are presented in Table 1.

View larger version (12K): [in a new window]   Fig. 5. Simulated reaction rate profiles in a 0.356-cm-radius aggregate 48 h after labeling with 99.9 atom % 15NH+4. Reaction rate profiles were calculated from the time period of 24 to 48 h at each spatial node in the zero-order consumption, finite difference model. Dashed line represents simulation input reaction rate of kcA = kpA = 1 µg N cm-3 d-1, Kd = 13 cm3 g-1.