Tropical soil carbon sink under threat


Source: European Commission, Environment DG

There are strong concerns that the conversion of tropical forests into land for agriculture or plantations has negative effects on the carbon budget. A new study conducted in South-East Asia reveals that moderate or heavy disturbance of tropical forests can damage fine root structures, which in turn reduce carbon transfer to the soil.

Tropical forests are being converted into agricultural land or tree plantations at a rapid rate in parts of the tropics. Changes in forest structure and management are likely to alter the organic carbon content of the soil. This reduction in the carbon sink has implications for climate change.

Trees transfer carbon from the atmosphere to their roots. This carbon is eventually absorbed by the soil after the root has died. This is one of the most important, but least quantified, forms of carbon transfer at the ground level of an ecosystem. Tropical rainforests are particularly affected by the dynamics of these fine root systems.

According to the study, forest conversion has a profound effect on the transfer of carbon into soil by fine root structures. It found that the greater the level of disturbance, the lower the rate of carbon transfer to soil. However, surprisingly this was not the case for areas planted with cacao, which were the most disturbed forest areas among those studied.

The researchers analysed the role of the fine root system in the soil carbon cycle of tropical forest in Indonesia at five different stages of disturbance. These ranged from old natural forest with virtually no human impact, to planted cacao with no natural forest vegetation. It estimated growth of fine roots and associated carbon fluxes from the fine root system to the soil in all five cases.

The annual growth of fine roots in three natural forest sites decreased continuously with increasing forest disturbance. For example, the figures were substantially lower for an area that had suffered logging compared to an area which was relatively untouched. However, contrary to expectations, those areas where cacao had been planted demonstrated a higher rate of fine root growth than areas that still contained some natural forest but had a large timber extraction. This was especially true if the cacao plantations also contained trees that were planted for shade. Previous studies have suggested that dbh ('diameter breast height' - the diameter of a tree at 1.37 metres above ground) is a good predictor of fine root production. However, the low dbh of cacao trees in this study could not have predicted their high rate of root production.

Similarly, the amount of carbon transferred to the soil through fine root growth was highest in the undisturbed forest and decreased according to the level of timber harvesting. But again the carbon transfer was unexpectedly high in the cacao plantations. The authors suggest this result might be explained by a particularly high fine root production in both the species of cacao and the trees that were planted on the plantations for shade.

It also suggests that the species of the crop trees and those planted for shade is important, as it may partly compensate for reductions in the carbon content of the soil. The researchers also believe that relatively new plantations would have less root competition which could also lead to higher growth rates of roots.

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