The impact of cultivation on carbon fluxes in woody savannas of Southern Africa

Abstract

The rapid transition from miombo woodland and savanna to maize-based agriculture in Southern Africa results in a near universal loss of total system and biomass carbon. Forests and savannas occupy approximately 3.1 million km² in southern Africa. Two natural ecosystems, a miombo woodland (Zimbabwe) and a broadleafed dry savanna (South Africa), contained 48 and 94 Mg C ha⁻¹, respectively. Clearing of the miombo and establishment of maize-based agriculture on a sandy Alfisol resulted in a decline in total soil organic carbon from 28 to as little as 9 Mg ha⁻¹. This decline is not related to the annual aboveground productivities which, in many cases is greater in the cropping system than in the savanna or forest. Severe declines in total soil organic matter resulting from shifting cultivation were also observed in coastal Mozambique. The CENTURY plant/soil simulation model was used to simulate long-term carbon dynamics a miombo woodland and maize-based cropping system in Marondera, Zimbabwe. The miombo woodland continues to accumulate total system C but shifting cultivation and commercial cultivation of maize result in annual carbon losses of 0.15 and 0.14 Mg ha⁻¹ yr⁻¹. Increases in temperature (2° C) accompanied by 25% increases in photosynthetic efficiency did not effect the decline in total system carbon, however, improved organic matter management within the agroecosystem reduced the losses in total system carbon within the agroecosystem by 57% under the climate change scenario.