Estimating oil-palm Si storage, Si return to soils, and Si losses through harvest in smallholder oil-palm plantations of Sumatra, Indonesia

Abstract

Abstract. Most plant-available Si in strongly desilicated soils is provided through litter decomposition and subsequent phytolith dissolution. The importance of silicon (Si) cycling in tropical soil–plant systems raised the question of whether oil-palm (Elaeis guineensis Jacq.) cultivation alters Si cycling. As oil palms are considered Si hyper-accumulators, we hypothesized that much Si is stored in the above-ground biomass of oil palms with time. Furthermore, the system might lose considerable amounts of Si every year through fruit-bunch harvest. To test these hypotheses, we analysed Si concentrations in fruit-bunch stalks, fruit pulp and kernels, leaflets, rachises, and frond bases of mature oil palms on eight smallholder oil-palm plantations in Sumatra, Indonesia. We estimated Si storage in the total above-ground biomass of oil palms, Si return to soils through decomposing pruned palm fronds, and Si losses from the system through harvest. Leaflets of oil-palm fronds had a mean Si concentration of > 1 wt %. All other analysed plant parts had < 0.5 wt % Si. According to our estimates, a single palm tree stored about 4–5 kg Si in its total above-ground biomass. A smallholder oil-palm plantation stored at least 550 kg Si ha−1 in the palm trees' above-ground biomass. Pruned palm fronds returned 111–131 kg of Si ha−1 to topsoils each year. Fruit-bunch harvest corresponded to an annual Si export of 32–72 kg Si ha−1 in 2015 and 2018. Greater Si losses (of at least 550 kg Si ha−1) would occur from the system if oil-palm stems were removed from plantations prior to replanting. Therefore, it is advisable to leave oil-palm stems on the plantations, e.g. by distributing chipped stem parts across the plantation at the end of a plantation cycle (∼ 25 years).

Effects of turning rainforest into oil-palm plantations on silicon pools in soils within the first 20 years after the transformation

“…If they exist, they probably result from low Si M concentrations in the subsoil horizons of the Acrisols under oil-palm plantations (Figures 1A,G). Both rainforest trees (Lucas et al, 1993;Schaller et al, 2018) and oil palms (Munevar and Romero, 2015;Greenshields et al, 2023b) can take up high amounts of Si from soil solution. Yet they differ in their rooting system (vander Linden and Delvaux, 2019).…”

“…Guillaume et al (2015) pointed out that topsoil erosion led to a loss of soil organic carbon (SOC) stocks in oil-palm monocultures (Alexandre et al, 1997;Schaller et al, 2018;Li et al, 2020), where clearing understory vegetation had been part of management practices. Moreover, an assessment of annual soil erosion (by erosion traps in the same 20-year-old oil-palm plantations on sloping terrain), combined with analysis of Si contents of the eroded soil material, pointed to considerable annual Si losses through soil erosion, if the plantations were kept free of ground vegetation (Greenshields et al, 2023b). Soil erosion stopped completely, where ground vegetation was allowed to grow.…”