Spatial couplings between nitrogen fixation and denitrification in the terrestrial biosphere: An earth system hypothesis

Background/Question/Methods

The amount of CO2 that ecosystems can absorb from the atmosphere, and the sustained strength of this ecological service, is strongly regulated by the amount of N available to organisms. Ultimately, the quantity of ecologically available N is controlled by the dominant ways in which N enters ecosystems, how it leaves, and how it interacts with other major chemical cycles. One of the long-standing challenges of the scientific community has therefore been to quantify N input and loss pathways with such a degree of fidelity that a more thoughtful understanding of global change can be achieved. This task has remained rather vexing, however, as the major transfers of N into and out of ecosystems involve processes that are spatially and temporally heterogeneous, and often involve cycling of N2 gas, which cannot be measured directly in field settings. Using simple mass-balance principles coupled with global C,N,P modeling, here I examine large scale patterns in N inputs and losses across the land surface. The focus is on two key processes that involve the transfer of N2: N fixation and dentrification. The coupled models to address biosphere-scale N cycles were run off-line, and parameterized with global data sets of C, N and P in above and below ground pools, and the stable isotopic composition of N in bulk soil pools globally.

Results/Conclusions

Multi-model analysis points to tropical sectors as the major hotspot of both N fixation and denitrification in the terrestrial biosphere. For N fixation, this agrees with data syntheses and the large number of legumes with capacity for N fixation that thrive in many lowland tropical forests. For denitrification, this agrees with field-measures and known soil conditions – high C, high soil moisture – in tropical biomes. A hypothesis for this global N cycle coupling is proposed: the high energy input from the sun near the equator drives the Hadley cell and the inter-tropical convergence zone, leading to high photosynthetic capacity and N demands among terrestrial plants,  and thus a niche for N fixation and dentrification. I will discuss the implications of this hypothesis with respect to global ecology and change, and draw analogies to the global marine N cycle.