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

A conceptual framework is proposed for understanding the relationship between temperature and biological N 2 fixation (BNF) across levels of biological organization from enzymes to ecosystems on land and in the ocean. Global compilations of measured N 2 fixation rates reveal a temperature dependence that is broadly similar in marine and terrestrial systems, including a common thermal optimum at ~25°C, suggesting an overarching enzymatic constraint. Rate–temperature relationships also reveal features unique to marine and terrestrial domains, including greater heat tolerance of terrestrial BNF, suggesting the presence of, and adaptation to, distinct thermal and ecological niches. Climate warming is projected to reduce N 2 fixation in the tropics while increasing it in higher latitudes in terrestrial and marine ecosystems, a spatial shift that would impact productivity and N cycling, unless modulated by other environmental factors and physiological adaptations. 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 ~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 N 2 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 N 2 fixation rates, facilitating cross-fertilization of marine and terrestrial research to assess its response to climate change. [ABSTRACT FROM AUTHOR]