Decades of nitrate stable N (d¹⁵N_NO3) and O (d¹⁸O_NO3) isotope research have exposed the complexity of the N cycle. Although d¹⁸O_NO3 studies had been limited to freshwater, new methods allow analysis of d¹⁸O_NO3 from saline waters. New data from marine ecosystems such as low-nutrient surface waters and estuaries shed more light on nitrogen cycling in the ocean and beyond. Specifically, deviations from expected patterns of combined d¹⁵N_NO3 and d¹⁸O_NO3 reflect multiple simultaneous processes at work.

In the ocean productivity is largely controlled by N availability, such that upwelling of N rich deep water fuels primary productivity in coastal surface waters. However, regular measurements of d¹⁵N_NO3 and d¹⁸O_NO3 in Monterey Bay, California, reveal an active role of nutrient regeneration and nitrification, such that up to 30% of surface productivity is supported by N recycling in surface waters. In estuaries, such as Elkhorn Slough, California, variations of d¹⁵N_NO3 and d¹⁸O_NO3 stemming from mixing of two sources are further complicated by interaction with reactive sediments, where both uptake and production of nitrate impart differential isotope effects on the d¹⁵N_NO3 and d¹⁸O_NO3 in the overlying water. Both cases show how multiple N cycling processes are reflected by a ‘decoupling’ of d¹⁵N_NO3 and d¹⁸O_NO3.