We studied the effect of land conversion and erosion intensity on the biogenic silica (BSi) pools in a subtropical soil in the south of Brazil. Our study shows that deforestation will rapidly (< 50 years) deplete (10–53 %) the biogenic alkaline extractable Si (AlkExSi) pool in soils. Higher erosion in steeply sloped sites implies increased deposition of biogenic Si in deposition zones near the bottom of the slope, where rapid burial can cause removal of BSi from biologically active zones.