Projecting global biological N₂ fixation under climate warming across land and ocean

Biological N-2 fixation sustains the global inventory of nitrogenous nutrients essential for the productivity of terrestrial and marine ecosystems. Like most metabolic processes, rates of biological N-2 fixation vary strongly with temperature, making it sensitive to climate change, but a global projection across land and ocean is lacking. Here we use compilations of field and laboratory measurements to reveal a relationship between N-2 fixation rates and temperature that is similar in both domains despite large taxonomic and environmental differences. Rates of N-2 fixation increase gradually to a thermal optimum around similar to 25 degrees C, and decline more rapidly toward a thermal maximum, which is lower in the ocean than on land. In both realms, the observed temperature sensitivities imply that climate warming this century could decrease N-2 fixation rates by similar to 50% in the tropics while increasing rates by similar to 50% in higher latitudes. We propose a conceptual framework for understanding the physiological and ecological mechanisms that underpin and modulate the observed temperature dependence of global N-2 fixation rates, facilitating cross-fertilization of marine and terrestrial research to assess its response to climate change.