Tree species controls on soil carbon sequestration and carbon stability following 20 years of afforestation in a valley-type savanna

Abstract

In the farmland/forest ecotone of southwestern China, many areas are experiencing afforestation and subsequent shift in the ecosystem carbon (C) stocks. The effects of five tree species on soil organic C (SOC) accumulation and C stability following 20years of afforestation in a valley-type savanna since 1991 were investigated in fractions below 0.25mm. The bulk soils were fractionated with a combination of density fractionation and acid hydrolysis techniques.The results showed that SOC densities in the afforested stands have accumulated since the changes in the land uses. The surface soil C sequestration rates varied substantially among the five stands and ranged from 0.13tCha−1year−1 to 0.47tCha−1year−1 during the two decades of afforestation. The percentage of organic C in the heavy fraction (via density fractionation) relative to the total SOC was 61.9–68.0% under the five stands after 20years of afforestation. The overall biochemical recalcitrant C density accounted for 37.6–49.9% of the total SOC in these stands in 2011.The tree species controls on the soil C sequestration and on the C in the separated fractions following 20years of afforestation in a valley-type savanna. However, the biochemical stability of the physically protected C remained lower than that of the unprotected C, irrespective of the tree species. Among the five studied tree species, Leucaena leucacephala was the most suitable tree species for afforestation in the valley-type savanna for the higher C sequestration rates in the soils and for the moderate stability of SOC, which potentially, at least theoretically, lead to the high biological utilisation (e.g. nutrient release) of soil organic matter when SOC decays.