Changing fire regimes in response to climate change are likely to have significant effects on terrestrial ecosystems and biogeochemical cycles. While effects of fire on some nutrient cycles have been quite well-studied, little attention has been paid to the silicon cycle. We used an alkaline continuous extraction to examine changes in the quantity and characteristics of alkaline extractable Si (AlkExSi) after applying two burning treatments (no heating, 350 °C and 550 °C) to three types of organic soil material (from spruce forest, beech forest and a commercial peat). The total AlkExSi measured was 25.1 ± 2.1 mg g− 1 and 15.4 ± 0.9 mg g− 1 for spruce and beech respectively, and 1.2 ± 0.5 mg g− 1 for peat. The alkaline extraction parameters confirm a purely biogenic AlkExSi source in untreated spruce and beech organic soil material samples. Organic soil material of beech forest had two biogenic silica pools, differing in reactivity during alkaline extraction. Burning severely alters the alkaline dissolution parameters suggesting a significant crystallization of biogenic Si (BSi) with increased burning severity. Additionally, dissolution experiments carried out in rain water showed that fire increased the solubility of BSi by a factor of 40 and 20 in the case of the spruce and beech organic soil material respectively. The extent of enhanced Si solubility appears to be a trade-off function between organic matter losses and degree of crystallization. The burned soils could provide a strong and immediate Si source for the environment. In situ ecosystem characteristics that affect the uptake-leaching balance will determine the fate of the dissolved Si. Ecosystems low in BSi, like Sphagnum peatland, will not show drastic alteration in the Si cycle due to fire.