Ecosystem productivity and CO2 exchange response to the interaction of livestock grazing and rainfall manipulation in a Kenyan savanna

Abstract

Savanna ecosystems in Kenya are experiencing altered rainfall amount and increased grazing pressure. These environmental alterations occur simultaneously and impact on productivity and CO2 exchange of the savanna in unclear ways. Rainfall was manipulated and its interaction with livestock grazing on productivity and CO2 exchange within the herbaceous vegetation investigated for two years. Rainfall manipulation plots which received ambient rainfall (100% rainfall), fifty percent more rainfall (150% rainfall) or fifty percent less rainfall (50% rainfall) were set up within grazed and fenced areas respectively. Measurement chambers were used to quantify monthly CO2 exchange. Monthly biomass and soil water content (SWC), bulk density, plant and soil C/N were quantified. Grazing reduced CO2 exchange through reduction in aboveground green biomass. The interaction of grazing and rainfall reduction lowered Gross Primary Productivity (GPP), Net Ecosystem Exchange (NEE) and Ecosystem Respiration (Reco) through the imposition and amplification of drought by grazing and rainfall reduction respectively. The interaction of grazing and rainfall increment led to increased GPP and NEE, confirming the role of SWC in driving CO2 exchange in the grazed savanna, however, Reco was not significantly (P > 0.05) affected by the interaction of grazing and rainfall increment. This shows that the CO2 exchange in this ecosystem do not always respond linearly to rainfall variation. These results demonstrate the importance of the interacting environmental variables in determination of carbon balance of savannas