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

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. Fruitbunch harvest corresponded to an annual Si export of 3272 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 (similar to 25 years).