The output and bio-cycling of Si in a tropical rain forest developed on young basalt flows (La Reunion Island)

The importance of plant bio-cycling in the biogeochemical cycle of Si has been pointed out by recent studies. We analysed the litter and wood chemical composition of a lowland rainforest (Marelongue Reserve, Réunion Island, Indian ocean) not impacted by human activities, developed on a recent basalt flow dated ci 500 years. The amount of elements stored in the above ground biomass falls within the range of tropical forests developed on moderately fertile older soils with Ca, K, Mg and Si values respectively at 1458 kg ha−1, 1506 kg ha−1, 246 kg ha−1and 17 kg ha−1. The estimated mineral uptake of the old trees gives a flux equivalent to 5 ± 2% of the elements returned to the soil with litterfall. The four species that contribute the most to the litterfall with 70% of the leaf fall represent only 45% of the annual Si bio cycling. The result demonstrates that species that are not highly productive in a rain forest could be significant for the bio-cycling of parent-rock elements such as Si. A mass balance calculation shows that the dissolved silicon (DSi) issued from the dissolution of basalt is 1.8 times the DSi from litterfall. Here, the biogeochemical cycle of Si is characterized by a limited contribution of the bio-cycling compartment compared to other cases reported for rainforests.

Research Highlights
► The concentration of the major elements of the Marelongue rainforest (La Réunion Island, Indian Ocean) developed on 500 years old basalt flows, falls within the same range as those of the rainforests described in the literature.
► We show that species that are not highly productive could be significant for the bio-cycling of parent-rock elements such as Si.
► Here, the biogeochemical cycle of Si is characterized by a limited contribution of the bio-cycling compartment compared to other cases reported for rainforests.