Evaluation of the nitrogen isotope-pairing method for measuring benthic denitrification: A simulation analysis

The nitrogen isotope-pairing method to measure sedimentary denitrification is evaluated by means of a general diagenetic model that describes the depth distribution of O-2, NH4+, (NO3-)-N-14, and (NO3-)-N-15, among others. The model simulations demonstrate that isotope fractionation effects during denitrification can be neglected, but that nonuniform mixing of added (NO3-)-N-15 with locally produced (NO3-)-N-14 causes underestimation of in situ rates of denitrification. Enrichment of bottom water with (NO3-)-N-15 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.