The weathering stage of tropical soils affects the soil-plant cycle of silicon, but depending on land use

Plants take up silicon (Si) from soil solution, and form biogenic silica bodies (phytoliths) that return to soil with plant debris. Since phytolith dissolution releases plant available Si, the soil-plant Si cycle tremendously influences the global Si cycle. Si plant uptake ranges from 0.7 to 1470 kg ha(-1) year(-1) among different terrestrial ecosystems depending on soil properties and processes, climate, plant species, and management practices. The humid tropics shelter a huge variety of soils. Many of them are strongly weathered and desilicated, and exhausted in plant nutrients. Nevertheless, these soils support evergreen forests with the greatest biodiversity and biomass because of an intense pumping of nutrients. This pumping involves non-essential Si, and further governs the soil-plant Si cycle, which is perturbed after converting forest area into cropland. Here, we used literature data quantifying the Si soil-plant cycle in natural forest areas and croplands established on soils that differ in weathering stage. We particularly focused on comparing forest to Si-accumulating rice crop. We show that the impact of soil weathering stage on the soil-plant Si cycle markedly differs depending on land use in the tropics. In slightly or moderately weathered soils, cultivated plants take up Si in larger amounts than forest trees do, likely because the former are stimulated to pump nutrients and dissolve Si from less soluble lithogenic and pedogenic silicates. With increasing soil weathering and desilication, Si plant uptake increases in natural forest ecosystems while it decreases in cultivated ecosystems. Four factors may explain this discrepancy: (i) since they are more soluble than lithogenic and pedogenic silicates, phytoliths make the pool of plant available Si; (ii) forest litter, which is densely exploited by roots, is a place of intense mineral pumping; (iii) deep rooting of forest trees enhances pumping too; (iv) crop harvesting exports Si out of cultivated ecosystems.