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Impacts of transformation of rain-forests into oil-palm plantations on silicon pools in soils

Subject Area Soil Sciences
Term from 2017 to 2021
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 391702217
 
Final Report Year 2023

Final Report Abstract

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 worldwide. This project aimed at quantifying the effect of oil-palm cultivation on soil silicon (Si) pools and Si cycling in the system, because oil palms are Si-accumulating plants and thus, we supposed that their cultivation might have a considerable impact on soil-plant Si cycling. For this purpose, we carried out (1) sequential Si extraction of soils under oil-palm plantations and rainforests, both in well-drained higher terrain and in riparian areas; we sequentially extracted mobile Si (SiM), adsorbed Si (SiAd), Si in soil organic matter (SiOrg), Si occluded in pedogenic oxides and hydroxides (SiOcc), and Si in biogenic and minerogenic amorphous silica (SiBa and SiMa); (2) analysis of Si in amorphous silica (SiAm = SiBa + SiMa) in topsoils and in corresponding eroded topsoil sediments caught in erosion traps; (3) analysis of Si concentrations in fruit bunches, fruits, kernels, and fronds of oil palms. Through this workflow, we tested the following four hypotheses (H1-H4): H1) Oil palms produce larger amounts of amorphous silica than rainforest trees. This silica is returned to the soils only under long frond piles, which, according to common management practices, are created in every second oil-palm interrow (i.e., the space between adjacent oil-palm rows). No silica return from above-ground oil-palm biomass takes place in any other area of the plantations. Indeed, the observed spatial pattern of SiAm in topsoils across the plantations fully confirmed this hypothesis. SiAm concentrations in topsoils of oil-palm plantations without grass cover were about twice as high under frond piles (~4.0 mg g^-1) compared to empty interrows (~1.9 mg g^-1). Plantations with grass cover showed similar topsoil SiAm concentrations below frond piles (~3.0 mg g^-1) and in interrows (~2.7 mg g^-1). This can be explained by the (well-known) considerable Si accumulation of grasses, which then return correspondingly large amounts of SiAm to the soils with their litter. H2) Soils of riparian areas receive dissolved silica from higher landscape positions. They are thus not prone to serious Si depletion. This hypothesis was not verified. Soils under oil-palm plantations in riparian areas tended to have greater SiM, SiAd, SiOrg, and SiBa stocks than soils under oil-palm plantations in well-drained higher terrain. However, these trends were not statistically significant, and they were not at all observed in soils under rainforest. Apparently, the water that is regularly flooding the riparian areas does not significantly replenish soil Si pools. H3) Phytoliths accumulate especially in topsoils. They have a lower density than the soils’ mineral grains. Thus, they are preferably removed with surface runoff, which results in direct SiAm export. An above-average erosion of phytoliths would have been verified, if the SiAm concentrations of the eroded soil material, collected in sediment traps that we installed in the oil-palm plantations, exceeded those of the corresponding topsoils of these plantations. This was, however, not the case. Nevertheless, we identified considerable annual SiAm losses related to topsoil erosion, amounting to 5 – 9 kg ha^-1, due to high erosion rates in combination with considerable SiAm concentrations in the eroded soil sediment. H4) Oil palms also accumulate SiBa in their fruit bunches. Thus, harvesting fruit bunches involves direct SiBa export from the system as well. Indeed, we calculated annual Si losses of 32 – 72 kg ha^-1 through fruit-bunch harvest. Overall, we did not observe a decrease in soil Si stocks under oil-palm plantations within the first generation of oil palms, compared to rainforest on similar soils in the same region. Nevertheless, because of the observed annual Si losses and high erosion rates, and because Si is known to strengthen plants against common biotic and abiotic stress factors, we recommend several management practices to avoid Si depletion on the long-term (i.e., over several oil-palm generations). These include (1) maintaining a dense grass cover between the oil palms to reduce erosion, stimulate Si cycling, and increase SOM contents; (2) returning empty fruit bunches to the plantations to minimise the harvest-related leak in the cycle of Si and nutrients.

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