PARENT SESSION
Oral Session #42: Carbon and Nitrogen Cycling. Presiding: S. Hobbie.
Wednesday, August 8, 2001. 8:00 AM to 11:30 AM. Hall of Ideas E.
Carbon and nitrogen in the soil-plant system along rainfall and land-use gradients in southern Africa.
Feral, Christie1, Epstein, Howard1, Otter, Luanne2, Aranibar, Julieta1, Shugart, Herman1, Macko, Stephen1, Ramontsho, Jerry3, 1 2 3
ABSTRACT- The nearly homogeneous substrate of the Kalahari Transect allowed examination of changes in nutrient concentrations along a climatic gradient and two land use gradients. We anticipated finding foliar and soil nutrient changes that were consistent with vegetation shifts resulting from decreasing rainfall and increasing land use intensity. Plant tissue samples were taken from four functional types at six sites, three along a rainfall gradient and three under varied use regimes, and were analyzed for C and N content. Total organic C and N concentrations also were measured for soil samples taken near each plant base and in bare soils outside the plant canopy. SOC decreased significantly (p<0.05) as the climate dried, with no significant change in SON. High SON (p<0.001) at two land use sites was associated with animal urea deposition; herbivory uncoupled with urea deposition was linked to low SON (p<0.01) at a third site. Ammonium and nitrate were significantly greater only at the wettest site. Higher NH4+ and NO3- concentrations beneath specific tree canopies suggested nitrogen fixation, but this was contradicted by 
15N-isotope analysis. One heavily used site showed 67% greater SON concentrations beneath plant canopies than outside them, suggesting the formation of "fertility islands." Foliar nitrogen co-varied inversely with SON concentrations (R=-0.83). Inputs from rainfall-driven vegetation shifts appear to govern variations in SOC concentration, while the harvest and transport of plant nutrients (land use) determines SON variability.
KEY WORDS: carbon, nitrogen, rainfall, land-use