Since the mid-20th century, the expansion of oil-palm plantations has considerably contributed to the decrease of rainforests in Southeast Asia, whereby Indonesia has become the largest oil-palm producer world-wide. Several studies, including current investigations within the CRC 990, have shown that oil-palm cultivation may lead to a decline of soil organic carbon and various nutrients. However, no studies on the influence of oil palms on silicon (Si) pools in soils exist yet.

In this project, we intend to close this knowledge gap, because (i) oil palms are Si-accumulating plants and thus supposed to have a considerable impact on soil-plant Si cycling, (ii) oil-palm plantations are established on tropical soils such as Acrisols and Ferralsols, which are characterized by strong natural desilification. In these soils, soil-plant Si cycling is particularly important for maintaining appropriate soil Si levels on the long term.

Four hypotheses (H1-H4) will be tested in this project. H1) Oil palms produce larger amounts of biogenic silica (BSi) than rainforest trees. This BSi is returned to the soils only under long frond piles, which, according to common management practices, are created in every second oil-palm inter-row. No BSi return from above-ground oil-palm biomass takes place in any other area of the plantations. BSi released from decomposing palm fronds, because of its amorphous character, represents the most soluble relevant Si source in the otherwise Si-depleted soils. BSi dissolution and leaching of the released Si with abundant percolating water under humid-tropical conditions leads to net Si loss from the system. H2) Soils of riparian areas receive dissolved silica (DSi) from higher landscape positions. They are thus not prone to serious Si depletion. H3) Phytoliths accumulate especially in top-soils. They have a lower density than soil-mineral grains. Thus, they are preferably removed with surface run-off, which results in direct BSi export. H4) Oil palms also accumulate BSi in their fruit bunches. Thus, harvesting fruit bunches involves direct BSi export from the system as well.

These hypotheses will be tested by (i) sequential Si extraction, carried out on Acrisols under oil palm plantations and rainforests, both in well-drained areas and riparian areas (testing H1 and H2), (ii) analysis of BSi in top-soils and sediments, caught below erosion plots (testing H3), (iii) analysis of BSi in oil-palm fruit bunches, fruits, kernels, woody parts, fronds (testing H4). The outcomes of this project will provide a scientific base for evaluating the risk of long-term Si depletion in Acrisols under oil-palm plantation and for developing potential measures (e.g. returning empty fruit bunches to the plantations, preferably using riparian areas). The project will be closely linked to the CRC 990. All analyses will be carried out on CRC plots. In this way, the project outcomes can be directly integrated into the overall outcomes of the CRC 990.

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