Wednesday, August 8, 2007 - 1:50 PM

COS 90-2: Toward a biogeochemical framework of nitrogen fixation in the terrestrial biosphere

Benjamin Z. Houlton1, Yingping Wang2, Peter M. Vitousek1, and Christopher B. Field3. (1) Stanford University, (2) CSIRO Marine and Atmospheric Research, (3) Carnegie Institution

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.

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