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Nitrogen fixation along rainfall and disturbance gradients in Southern Africa and its relation with phosphorus concentration and water availability.
Aranibar, Julieta1, Macko, Stephen1, Dowty, Peter1, Shugart, Herman1, 1
ABSTRACT- Nutrient cycles are coupled at many levels, and affect primary productivity and the possibility of vegetation to act as a sink for increased atmospheric CO2. Nitrogen (N) may limit primary productivity under future climate conditions, and it is a limiting factor in some Southern African ecosystems. The dominant trees of many African savannas are legumes, which are commonly associated with N fixing bacteria of the Rhizobium type. It is reasonable to expect that these plants will fulfill the N requirements to increase productivity under elevated CO2 or precipitation. However, the dominant legumes of many Southern African savannas (such as Burkea africana and Acacia sp) have not been previously found to nodulate. Nitrogen fixation can be limited by low phosphorus (P) and molybdenum availability. 15N is generally more abundant in soils than in atmospheric N2. For this reason, differences in 15N natural abundance (del15N ) of presumed N fixing plants and non-N fixing plants can be used to asses N fixation. In this study, the ability of Southern African legumes to fix N was tested with the del15N method. Leaves from 11 sites along rainfall and fire gradients in Southern Africa were analyzed for del15N and C, N and P concentration, with an Optima isotope ratio mass spectrometer coupled with an elemental analyzer, and an Alpkem autoanalyzer. The contribution of N to each ecosystem was calculated with biomass estimations of N fixing species at each site. It was expected that the drier ends of the transect, dominated by Acacia sp, would present higher rates of N fixation. Leaf P concentration was expected to explain the patterns of N fixation, enhancing it under disturbed conditions that mobilize this nutrient, such as fires. Nitrogen fixing trees were present in the moist end of the transect, but absent in dryer sites, even during an extremely wet year. Only forbs and cyanobacterial soil crusts were able to fix N in arid areas. Our results suggest that long-term water availability determines the pool of species present at each ecosystem and their ability to fix N, which does not change even if short-term water availability increases. Under increased precipitation or atmospheric CO2 given by climate change, there should be a shift in species composition before the N requirements can be fulfilled to increase productivity in these ecosystems.
KEY WORDS: nitrogen , fixation, isotopes, legumes