A new evaluation of biological nitrogen fixation for inland and coastal waters concludes that these habitats are an overlooked but important source of fixation globally. Robinson Fulweiler and colleagues found that despite accounting for less than 10% of the globe’s surface area, inland and coastal aquatic systems create about 15% of the nitrogen fixed on land and in the open ocean. Biological nitrogen fixation is the microbial process that makes inert nitrogen gas available to organisms, fueling primary production and enhancing carbon storage on land and sea. Although researchers have studied this key process extensively in open oceans and on land, much less is known about the process in transitional aquatic habitats. Fulweiler et al. analyzed a global database of nitrogen fixation rates published between 1990 and 2022, containing information from lakes, rivers, streams, freshwater wetlands, salt marshes, mangroves, tidal flats, estuaries and continental shelves. One surprise of the study was the key contribution of fixation in sediments in these areas. The ecological role of this fixed nitrogen is uncertain, the researchers write, but its potential “to impact inland and coast water carbon (and other element) cycling is large and a critical topic for future studies.”