Forest conversion impacts on the fine and coarse root system, and soil organic matter in tropical lowlands of Sumatera (Indonesia)

•Forest conversion impacts on root systems, and soil organic matter in tropical lowlands.•Vertical root distribution is affected by land-use change.•Soil organic matter is very low in rubber compared to oil palm and natural forest.

Deforestation and land-use change are occurring on an increasing scale throughout Indonesia with profound effects on ecosystem structure and functions marked by consequences in biogeochemical cycles. This study investigates the influence of forest conversion on soil organic matter as well as the fine and coarse root system. Furthermore, the relationships between carbon (C) and nitrogen (N) stocks in the root biomass were related to the total aboveground tree biomass. Root biomass and fine root morphology were investigated in 150 cm-deep soil pits along a gradient of increasing land-use intensity, i.e. in natural forest, rubber under a natural forest cover ('jungle rubber'), rubber and oil palm monocultures. Total root biomass generally decreased with increasing land-use intensity together with aboveground tree biomass. Subsequently, carbon and nutrient stocks in the root system were over 50% lower in the monoculture plantations compared to the natural forest. Vertical root distribution showed distinct different patterns across the land-use types with a pronounced logarithmic decrease in vertical total root abundance in the natural forest and the jungle rubber plots that was less distinctive in the plantation systems. However, fine root morphology in the jungle rubber system revealed a large specific root area and specific root tip abundance, therefore partly compensating for the reduction in the fine root system after forest conversion. Soil organic matter was particularly low in rubber plantations. In conclusion, the results of our study suggests that conversion of natural forest to agroforestry and monoculture systems has a profound belowground impact reflected in the decrease of root biomass, nutrient stocks in coarse roots, and total soil organic matter.