EGU General Assembly 2020
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Impact of rainforest transformation into oil-palm plantations on Si pools in soils

Britta Greenshields1, Barbara von der Lühe1, Harold J. Hughes1, Aiyen B. Tjoa2, and Daniela Sauer1
Britta Greenshields et al.
  • 1Institute of Geography, Georg-August-Universität Göttingen, Germany
  • 2Fakultas Pertanian, Universitas Tadulako, Palu, Indonesia

As oil-palm plantations are expanding rapidly in SE Asia, it is essential to ensure that soil functions are sustained after land-use transformation. This includes the maintenance of well-balanced soil nutrient levels to prevent soil degradation as well as understanding soil silicon (Si) dynamics to optimize oil-palm management. However, studies on the influence of oil-palm cultivation on soil Si pools have not yet been undertaken, although it is known that oil palms accumulate Si in their biomass and should thus affect Si pools and cycling. We hypothesized that under oil-palm monocultures, Si losses may exceed Si input into soils, due to (1) erosion of phytolith-enriched topsoils, (2) increased Si uptake by oil palms, (3) harvest and palm-frond management. The aim of this study is to compare Si pools in Acrisols of Sumatra (Indonesia) under rainforest and oil-palm plantations to assess whether these soil Si pools are significantly depleted under oil-palm plantations. We included both well-drained and riparian sites, hypothesizing that riparian sites are less prone to net Si depletion, as they receive additional Si through regular flooding and slope water from higher areas. Soil samples (1 g) from soil profiles (≤ 1 m, n = 4 for each land-use type and topographic position) were subjected to sequential Si extraction to determine mobile Si, adsorbed Si, Si in soil organic matter, Si occluded in pedogenic oxides and hydroxides, and biogenic Si.

Si in soil organic matter (SOM) and biogenic Si represent the largest Si pools in the Acrisols. Our preliminary results suggest that these pools are controlled by land use rather than by topographic position (riparian versus well-drained). Ah horizons under oil-palm plantations have lower contents of Si in SOM (0.052-1.04 mg g-1) than those under rainforest (0.59-1.5 mg g-1). There is no significant difference between well-drained and riparian sites, as Si input by slope water and flooding does not affect Si in SOM. Besides, the concentrations of biogenic Si are lower in soils under oil-palm plantations than under rainforest. The contents of both mobile and adsorbed Si in soils are similar to marginally higher in riparian soils (5-30 µg g-1), compared to well-drained soils (5-20 µg g-1), with no clear difference between land-use types. These Si fractions unlike Si in SOM are most directly influenced by Si input through slope water and flooding.

How to cite: Greenshields, B., von der Lühe, B., Hughes, H. J., Tjoa, A. B., and Sauer, D.: Impact of rainforest transformation into oil-palm plantations on Si pools in soils, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13788,, 2020.


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  • CC1: Comment on EGU2020-13788, Damien Cardinal, 07 May 2020

    Dear Britta and colleagues,

    Thanks for this nice and interesting results. I was wondering how do you define and measure your different Si pools? I was more particularly wondering  on: what type of Si - Organic Matter could exist for Si-org? what would be the chemical bonds for Si-occ if it is not adsorbed onto oxides?  Why Si-org, Si-add are not part of Si-M?



    • AC1: Reply to CC1, Britta Greenshields, 07 May 2020

      Dear Damien,

      Thank you for your comment and questions. :)

      In soils, we define our Si pools as follows (defined by Sauer et al. 2006, Georgiadis et al. 2013):

      Mobile Si (SiM): silicic acid and easily mobilizable Si in soil solution

      Adsorbed Si (SiAd): adsorbed silicic acid that is adsorbed to surfaces of various soil components (e.g. clay minerals, sesquioxides)

      Si bound in organic matter (Siorg): silicic acid that is occluded within or complexed with organic soil material; less accessible pool to the oil palm, but can be released when soil organic matter is e.g. mineralized by microorganisms

      Si occluded in pedogenic oxides (Siocc): silicic acid occluded within Al- and Fe-oxides/hydroxides, that is released, when oxides/hydroxides are destroyed with ammonium oxalate and oxalic acid

      The Siorg and Siocc pool may act as a “storage” or long-term plant-available Si pool for the oi palm. We assume that redox conditions and hydrological processes determine the timing of Si release:

      • Under reducing conditions: Organic Matter decomposes slower (Siorg pool): likely longer storage + slower Si release
      • Under reducing conditions: Fe is reduced and mobilized (Siocc pool): shorter Si storage + faster Si release

      We regard Siorg and Siocc as a separate Si pool from SiM, because Si is enclosed in a complex or concretion and thus cannot directly go into soil solution.

      We measure Si pools as follows:

      SiM, SiAd: Si adsorption with a spectrophotometer

      Siorg, Siocc: diluted; ICP-OES

      You can contact me via e-mail ( and I can provide you with the literature from above.