The effects of herbivory and nutrients on plant biomass and carbon storage in Vertisols of an East African savanna

• Effects of herbivory and nutrients on plant productivity and C storage.
• Herbivory enhanced foliar P concentrations but reduced aboveground biomass.
• Nitrogen and P enrichment increased N and P stocks but not aboveground biomass.

Herbivory and nutrients are major ecosystem drivers in African tropical savanna. Although previous studies have determined the influence of herbivory on carbon storage in savanna ecosystems, little is known about the interactive effects of nutrients and herbivory. We determined the effects of long term grazing and short-term factorial nitrogen (N) and phosphorus (P) additions on aboveground biomass, soil organic matter (SOM) content, and plant nutrient storage. Grazing reduced aboveground biomass, foliar P and N stocks by 45%, 38% and 45%, respectively, compared to ungrazed plots, although the foliar P concentration was 20% greater in grazed plots. There was no significant increase in the aboveground biomass after nutrient addition despite increases in foliar N and P concentrations, suggesting that productivity was limited by a different resource (e.g., moisture). There were no significant interactions between nutrient enrichment and grazing. We conclude that grazing reduced aboveground biomass, but improved grass quality through increased foliar P concentration.