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

ABSTRACT

Biological N2 fixation sustains the global inventory of nitrogenous nutrients essential for the productivity of terrestrial and marine ecosystems. Like most metabolic processes, rates of biological N2 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 N2 fixation rates and temperature that is similar in both domains despite large taxonomic and environmental differences. Rates of N2 fixation increase gradually to a thermal optimum around ~25°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 N2 fixation rates by ~50% in the tropics while increasing rates by ~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 N2 fixation rates, facilitating cross-fertilization of marine and terrestrial research to assess its response to climate change.