5 Transport and deposition of pyrogenic nitrogen in Africa: Role in regional biogeochemistry and fertility

Tuesday, August 4, 2009: 2:50 PM
La Cienega, Albuquerque Convention Center
Lara Prihodko , Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, CO
Niall P. Hanan , Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, CO

Fire is a major disturbance in African ecosystems, whether natural or human caused. Approximately 40% of global biomass burning emissions originate in Africa. Fire is important in the biogeochemical cycling of carbon and nutrients in ecosystems as they are emitted or chemically transformed into more available, or more recalcitrant, forms during burning.  Vegetation fires are estimated to emit as much as 85% of the nitrogen stored in leaves, twigs and wood. A fraction of this nitrogen is denitrified to molecular nitrogen, while some is returned locally via rapid deposition.  However, some of the most important nitrogen bearing emissions have sufficiently long atmospheric half-lives to be transported longer distances and eventually deposited in remote locations through wet and/or dry deposition.  Because of the prominent role fire plays in African ecosystems, and because industrial and agricultural sources are limited, fire prone ecosystems may be important net sources of nitrogen, providing the potential that downwind systems may receive nutrient subsidies from the upwind savannas.


We have developed annual mean fire related nitrogen deposition and emission climatologies over continental Africa using an atmospheric transport model (HYSPLIT4; Draxler and Hess, 1998), weather reanalysis data (National Centers for Environmental Prediction FNL and GDAS) and emissions from the Global Fire Emissions Database (GFED; van der Werf, et al, 2006). We conducted simulations over 10 years (1997-2006) for early, middle and late burn seasons and calculated the net pyrogenic nitrogen deposition (mean deposition - mean emission).  We also calculated probability distribution functions, which describe the fate of pyrogenic nitrogen from any burn location.  Preliminary results suggest that while the net pyrogenic nitrogen deposition rates to ecosystems adjacent to the fire-prone regions are much less than deposition rates observed near industrial sources in the U.S. and Europe (e.g. 1-2 Kg ha-1yr-1 in our simulations vs. >20 Kg ha-1 yr-1 in industrial regions), over millennia these steady subsidies may play an important role, for example, in the distribution of the nutrient poor (dystrophic) and nutrient rich (eutrophic) savannas of Southern, East and West Africa.