Nutrient subsidies in a West African savanna: Assessing teleconnections through fire and dust contributions

Background/Question/Methods

At regional and continental scales the savannas of Africa can be classified into ‘fertile’ and ‘infertile’ systems, corresponding to soil nutrient status, with cascading implications for plant species composition and herbivory. The distribution of fertile and infertile savannas correlates with gradients in rainfall: nutrient rich systems are concentrated in the arid areas, while nutrient poor systems are concentrated in higher rainfall areas. We have been examining patterns of nutrient emissions, atmospheric transport and deposition to explore the extent to which fires and dust transport induce nutrient flows among spatially remote regions in West Africa. Since February 2011 we have been collecting field measurements of dry deposition (both active and passive nitrogen), wet deposition, PM 2.5 (including biomass burning markers and phosphorous) and meteorological data in a Sudanian savanna ecosystem in Mali, West Africa. The Neguèla site is located approximately 50km northwest of Bamako in a national forest that forms the southern buffer of the Baoulé Biosphere Reserve and is part of an established network of long-term savanna research sites across a rainfall gradient in Mali. In addition to deposition measurements we have collected plant and soil samples across the rainfall/fertility gradient to examine changing stoichiometries. 

Results/Conclusions

With a full annual cycle of measurements analyzed, we are beginning to see patterns emerge. Our field measurements indicate elevated levels of nitrogen and phosphorous deposition in gaseous (N) and particulate (N and P) form during the dry season, from February – May 2011 and again from October 2011 through February 2012. In addition, we found elevated levels of the biomass burning marker, Levoglucosan, in late February 2011 and again from October 2011 through February 2012, during the fire season. Initial atmospheric transport simulations suggest these elevated contributions of phosphorous could originate from desert dust from the drier Sahelian and Saharan regions of West Africa, while elevated levels of nitrogen and the biomass burning marker Levoglucosan indicate contributions from savanna fires.  Leaf samples of both trees and grasses, collected over a dry (Sahelian) to wet (Guinean) savanna transect, exhibit a changing N:P ratio that we hypothesize to be related to net pyrogenic and aeolian balance of inputs.   We will discuss how changing patterns of nutrient inputs may be important in savanna community structure and function.