The nitrogen isotope-pairing method to measure sedimentary denitrification is evaluated by means of a general diagenetic model that describes the depth distribution of 02, NH4+, 14NO3-, and 15NO3-, among others. The model simulations demonstrate that isotope fractionation effects during denitrification can be neglected, but that nonuniform mixing of added 15NO3- with locally produced 14NO3- causes underestimation of in situ rates of denitrification. Enrichment of bottom water with sup>15NO3- concentrations above ambient levels not only results in enhanced rates of denitrification, but may also change in situ rates of coupled nitrification-denitrification. However, the resulting rates usually deviate <10% from in situ values, and the nitrogen isotope-pairing method therefore remains a powerful technique to determine rates of denitrification in sediments. We argue that one should not partition denitrification into bottom-water-supported and nitrification-coupled components. This partitioning causes confusion, is not required to understand sedimentary nitrogen cycling, and does not reflect a physically realizable situation.
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