Greenhouse gas emissions from savanna (miombo) woodlands: responses to clearing and cropping

Natural vegetation represents an important sink for greenhouse gases (GHGs); however, there is relatively little information available on emissions from southern African savannas. The effects of clearing savanna woodlands for crop production on soil fluxes of N2O, CO2 and CH4 were studied on clay (Chromic luvisol) and loamy sand (Ferric acrisol) soils in Zimbabwe. Maize (Zea mays L.) was the test crop. Gas samples were measured from undisturbed, cleared and cultivated woodlands using the static chamber methodology involving gas chromatography for ample air analysis. Site and climatic variables were particularly important determinants of GHG emissions. Over an average of 154 days emissions of 0.8 – 2.5 kg N2O-N ha-1, 1146 – 2847 kg CO2-C ha-1 and 7.4 – 38.5 kg CH4-C ha-1 were estimated during a season that followed a relatively drier one. Fertiliser-N significantly increased GHG emissions on cropped plots (clay soil). The undisturbed woodland with a relatively higher tree density (loamy sand) was an important GHG source. The high CH4 fluxes from woodlands provide ground based validation of satellite observations of CH4 hotspots in sub-Saharan Africa, and have considerable implications on regional GHG balance.