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

自世纪中期以来，油棕种植园的扩张在很大程度上导致了东南亚热带雨林的减少，印度尼西亚已成为世界上最大的油棕生产国。一些研究，包括目前在CRC 990中进行的调查，表明油棕种植可能导致土壤有机碳和各种营养物质的下降。然而，目前还没有关于油棕对土壤硅库影响的研究，本项目旨在填补这一知识空白，因为（i）油棕是硅积累植物，因此对土壤-植物硅循环具有相当大的影响，（ii）油棕种植园建立在热带土壤上，如Acrisol和Ferralsol，其特征是强烈的自然脱硅作用。在这些土壤中，土壤-植物硅循环对于长期保持适当的土壤硅水平尤为重要。本项目将检验四个假设（H1-H4）。H1）油棕榈比雨林树木产生更多的生物硅（BSi）。这种BSi仅在长叶堆下返回到土壤中，根据常见的管理实践，长叶堆是在每两个油棕榈行间创建的。在种植园的任何其他地区，没有来自地上油棕榈生物量的生物硅返回。BSi从分解的棕榈叶释放，因为它的无定形特性，代表最可溶的相关硅源，否则硅贫化土壤。在湿热条件下，BSi的溶解和释放的Si与丰富的软化水的淋溶导致系统的净Si损失。H2）河岸带土壤从较高的景观位置接受溶解硅（DSi）。因此，它们不容易发生严重的Si耗尽。H3）植硅体主要积累在表层土壤中。它们的密度比土壤矿物颗粒低。因此，它们优选地与表面径流一起被去除，这导致直接的BSi输出。H4）油棕榈也在它们的果串中积累BSi。因此，收获果实串涉及直接BSi输出的系统以及。这些假设将通过（i）连续硅提取，进行了在油棕榈种植园和热带雨林，无论是在排水良好的地区和河岸地区的Acrisols（测试H1和H2），（ii）分析侵蚀地块下方表层土壤和沉积物中的BSi（iii）分析油棕榈果串、果实、仁、木质部分、叶中的BSi（测试H4）。该项目的成果将为评估油棕种植下的Acrisol长期Si耗竭风险和制定潜在措施（例如将空果串返回种植园，最好使用河岸区）提供科学依据。该项目将与CRC 990密切相关。所有分析将在CRC图上进行。通过这种方式，项目成果可以直接纳入CRC 990的总体成果。

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.