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

John Cliff, P.J. Bottomley, R. Haggerty, D.D. Myrold

Research output: Contribution to journalArticlepeer-review

17 Citations (Scopus)


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 (NH4+)-N-15 isotope dilution experiments measuring gross production and consumption of NH4+ across a 24-h period. Equations that described transport and reaction of NH4+ assumed Fickian diffusion, linear, equilibrium adsorption, zero-order production of natural abundance N-15, and either pseudo-first-order or zero-order consumption of NH4+ 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
Original languageEnglish
Pages (from-to)1868-1877
JournalSoil Science Society of America Journal
Publication statusPublished - 2002


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