Nitrogen Subsidies and Fire Prone Ecosystems: the Case of African Savannas

Savannas are the most frequently burned ecosystems on earth and fires are known to have significant effects on community composition and species interactions across all trophic levels. Fire also plays a significant role in the biogeochemical cycles of carbon and various nutrients in savannas as they are emitted or chemically transformed into more available, or more recalcitrant, forms during fires. In Africa, savanna fires occur more frequently in the unstable (mean annual precipitation (MAP) > 516mm), dystrophic (nutrient poor) savannas than they do in the relatively more stable (MAP < 516mm), eutrophic (nutrient rich) savannas. Are these dystrophic savannas dystrophic because they burn? Are the eutrophic savannas eutrophic because they are subsidized by nutrient inputs from spatially remote fires? Africa is an ideal location to study ecosystem sensitivity to atmospheric inputs of nitrogen from biomass burning for several reasons. A large proportion of the continent is very fire prone and approximately 40% of global emissions from biomass burning originate in Africa. In contrast, emissions from fossil fuel sources are low compared to highly industrialized regions in the Americas, Europe and Asia and agricultural input of nitrogen is limited in Africa. As a result, the ecosystem impacts of pyrogenic loss and gain of nitrogen are likely to be more important in Africa than elsewhere. We combine data from recent analyses of fire frequency (GFED) with an atmospheric transport model (HYSPLIT4) to simulate pyrogenic nitrogen deposition fields and analyze the fate of fire-related nitrogen in the atmosphere, gross and net N-deposition fields and thus the role fires may play in displacing nitrogen and depleting or enriching adjacent regions.