Nutrient and light limitations on grass productivity along the Kalahari Transect in southern Africa

Despite the ubiquity of sub-tropical savannas throughout the earth, limitations of savanna productivity are understudied relative to other terrestrial systems. In particular, there has been little attention on the role of phosphorus (P) in savanna productivity and structure. This study examined the role of increased nitrogen (N) and P in grass productivity over a dramatic precipitation. We added aqueous forms of N and P to four sites along the Kalahari Transect, an International Geosphere Biosphere Site that traverses a major mean annual precipitation gradient ranging from 280 mm/year in the south to 980 mm/year in the north. During the following two growing seasons we measured grass foliar nutrient concentrations, aboveground grass biomass productivity and photosynthetic response at various levels of incident photosynthetically active radiation to estimate the productivity response. We observed a significant increase in grass foliar [P] in P and N+P treatments. N had a negative effect on foliar [P] although the total aboveground biomass was higher in these treatments, suggesting a dilution of foliar [P]. Conversely, we did not see a significant P effect yet saw various N+P effects between years. This research will help broaden our understanding of the biogeochemical processes that govern savanna productivity. Ultimately, these data can be used to model canopy productivity and ecological succession of savannas under scenarios in which bush encroachment and desertification may alter light and nutrients availability.