Forest soils under alternatives to slash-and-burn agriculture in Sumatra, Indonesia

Abstract

A global project on ‘Alternatives to Slash and Burn’ agriculture was initiated by a consortium of international and national research institutes to facilitate intensification of the use of converted forest land, in order to help alleviate poverty and protect the remaining forest areas for their biodiversity values and their role in mitigating greenhouse gas emissions.

Data for the Indonesian benchmark areas in the lowland peneplain, piedmont and mountain zone of Sumatra are presented. A significant amount of forest land, especially in the lowland peneplain, has been converted in the last ten years into agricultural use, usually following logging concessions. Soils on the peneplain are poor (oxi- and ultisols) and current intensive crop based production systems are not sustainable. In the piedmont zone on better soils (inceptisols), rubber agroforests (still) characterize the area. Agroforests have emerged during the 20’th century as the major alternative to slash-and-burn agriculture, based on a shift of emphasis from food crops to cash-earning tree crops. Emphasis on food crops, however, continues in government resettlement schemes.

Differences in organic C content of the topsoil between forests and crop land are about 0.5% C, with agroforests and tree crop plantations in an intermediate position. A new size-density fractionation scheme for soil organic matter demonstrated larger changes in light and intermediate fractions. Forest soils can be significant sinks for methane and thus partly compensate for the methane emissions in lowland rice production.

Overall, the Sumatra benchmark areas demonstrate the need to combine intensification of land use at the field/household level with effective protection of remaining forest areas at the community level and reducing other driving forces of deforestation at the national level.