Nutrient limitations on understory grass productivity and carbon assimilation in an African woodland savanna

Despite the ubiquity of sub-tropical savannas throughout the earth, limitations of savanna productivity are understudied relative to other terrestrial systems. The limitations of savanna productivity by precipitation have been documented but the roles of soil nutrients, particularly phosphorus (P), and light are not as clear. This study examined the role of increased P and nitrogen (N) in grass productivity in a woodland savanna in Botswana. We added aqueous forms of N and P to selected individual grasses. During the following growing season, we measured foliar nutrient concentrations, aboveground biomass and carbon assimilation at various light levels to estimate the productivity response. We observed an increase in foliar P concentrations for grasses with increased soil P. There was no increase in foliar N concentration for those with increased soil N. Neither N nor P treatments had higher levels of foliar C. Aboveground biomass increased significantly in the N+P treatment but there was no significant difference between the individual N and P treatments relative to the control. We did not see any effect of nutrients on either net carbon assimilation or stomatal conductance. These results suggest that the aboveground productivity of these woodland savanna grasses is co-limited by N and P.