Toward a biogeochemical framework of nitrogen fixation in the terrestrial biosphere

Explaining the distribution and abundance of symbiotic N₂ fixing organisms within the terrestrial biosphere has proven challenging. Yet, such explanation is fundamental to the diversity, dynamics, and responses of terrestrial ecosystems to global change. Combining observations, experiments, and modeling, we present a biogeochemical framework toward explaining the variation of N₂ fixers in boreal and temperate forests, and tropical savannas and lowland rainforests. Empirical evidence indicates that putative N₂ fixers have evolved an N rich strategy of P acquisition, thus explaining the high P contents of their leaves in the wild. By collating the data from published literature, we find a temperature optimum of ~26 degrees C in nitrogenase enzyme activity, the enzyme responsible for cleaving the triple N₂ bond. Combining these two perspectives into a resource optimization paradigm of C, N, P interactions, our model simulates high abundance of symbiotic N₂ fixers in tropical forests and savannas (20 to 40 % of community NPP), despite seemingly unfavorable conditions for N₂ fixation (i.e., P limitation).  In contrast, our model simulates little advantage to symbiotic N₂ fixers in the N-limited temperate or boreal forests, owing to the temperature-dependence of nitrogenase activity. These model predictions are consistent with empirical data regarding global patterns of N₂ fixation. We suggest that the ability of N₂ fixers to invest in P acquisition appears critical to their persistence in vast areas of the tropics, whereas modern-day temperatures may constrain the abundance of N₂ fixing organisms in the high-latitudes.