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The Late Devonian emergence of extensive arborescent terrestrial ecosystems produced changes in the Earth's atmosphere and climate that resulted in substantial changes to the biogeochemistry of marine ecosystems. The biogeochemistry of nitrogen in epeiric seas was particularly susceptible to alteration because of its dependence upon the series of redox-mediated reactions that comprise the marine N cycle. In order to explore the impact of climate change on N biogeochemistry in a Late Devonian epeiric sea, we sampled a core from the Appalachian Basin that spans the interval from the greenhouse climate at the base of the Famennian period to glacial conditions near its conclusion. Based upon stable isotope analysis of N, C, and S as well as elemental analysis of N, C, P, S, and Fe, we contend that two distinct biogeochemical regimes obtained during the contrasting climates. The greenhouse regime featured low seawater O-2 and pH that served to significantly curtail the oxidation of ammonia - i.e. nitrification - resulting in an ammonium-dominated water column and low sediment delta N-15 values. The greenhouse also featured more extensive recycling of C, N, and P. In contrast, the higher seawater O-2 and pH of the icehouse regime permitted nitrification in the water column resulting in a nitrate-dominated system and higher sediment delta N-15 values. We conclude that nitrification was the key component of the N cycle that differentiated the two climate/biogeochemical regimes.