Grass productivity and carbon storage in relation to rainfall, soil nutrients, and herbivory in an East African savann

Savannas are considered to be carbon (C) neutral. However, climate variability, land use change and land management practices are likely to alter the key ecosystem drivers which will influence the future of the savanna to either a C source or sink. Water, nutrient availability and disturbance (fire, grazing) are considered to be key ecosystem drivers determining the structure and function of savanna ecosystems. Four studies were conducted at Mpala Research Centre, Kenya to determine the effects of rainfall, soil nutrients and herbivory on plant productivity, C and nutrient storage in savanna with two soil types: red sandy loams (Alfisols) and black cotton soils (Vertisols). All studies emphasized on interactions of ecosystem drivers in determining the potential long-term C and nutrient storage in the savanna.;The plant productivity did not follow the rainfall gradient in savanna dominated by Alfisols, but indicated (nitrogen) N limitation to plant productivity. Below average rainfall limited the plant productivity even under nutrient enrichment. Plant litter decomposition was accelerated by rainfall and N enrichment in Alfisols. Nitrogen and P release during litter decomposition was greater in the dry season and did not match the C release, with higher %P release than %N.;Herbivores reduced the aboveground grass biomass in the Vertisols by -45%, but increased the forage quality (+20% foliar P) and soil organic carbon (+4%). Although the savanna dominated by Vertisols indicated N and P (phosphorus) co-limitation to plant productivity, there was luxury uptake of nutrients on nutrient enrichment without increase in biomass suggesting that the plants are adapted to growing under nutrient limiting conditions. In Laikipia district the selected six herbivores produce 5800 kg C km-2, 210 kg N km-2, and 35 kg P km-2 annually. This influence nutrient cycling, plant productivity and C storage.;Future N increase in the Alfisols will favor higher plant productivity and litter decomposition. But in the Vertisols nutrient enrichment improve plant quality, a trade off to C storage. Long term grazing increased SOC and plant quality a trade off to biomass C storage. Future changes in herbivores populations will affect nutrient cycling, indirectly affecting plant production and C storage.