Recent advances in the biogeochemistry of nitrogen in the ocean

Until fairly recently, study of the marine nitrogen cycle was considered to be important but a bit dull. Important because nitrogen had long been recognized as an essential nutrient that often limits primary production in the ocean but dull because the residence time of fixed nitrogen in the ocean was believed to be long (∼10 000 yr; Emery et al., 1955), and its budget, like those of most other elements, at or very near steady state. Serious efforts to quantify the key terms in the combined nitrogen budget – denitrification and nitrogen fixation – began in the 1970s, and despite varying estimates provided by different investigators, the notion of a steady state began to be challenged in the mid-1980s (Codispoti and Christensen, 1985, and references therein). The discovery of large glacial-interglacial changes in atmospheric carbon dioxide content based on analyses of polar ice cores (Delmas et al., 1980), arguably one of the most important findings in Earth Sciences in recent years, left paleoclimatologists scrambling for an explanation, and one of the earliest hypotheses offered invoked changes in the fixed nitrogen inventory of the oceans (McElroy, 1983), subsequently refined by Altabet et al. (1995) and Ganeshram et al. (1995). However, the extent to which variations in denitrification or nitrogen fixation can drive changes in nitrogen inventory (Falkowski, 1997) that contribute to climatic cycles, and whether or not the nitrogen budget in today’s ocean is balanced, are issues that remain unsettled (Sigman and Boyle, 2000; Codispoti et al., 2001; Gruber, 2004). The interest in the balance (or lack thereof) between the input and loss terms of the oceanic nitrogen budget has led to a broad range of studies including focused biological measurements of specific processes (e.g., N2-fixation and denitrification), basin-scale geochemical measurements, and integrative modeling (Gruber and Sarmiento, 1997; Brandes et