Can ash from smoldering fires increase peatland soil pH?

Peatland wildfire frequency and severity are increasing globally owing to climate change. The direct risk of elevated greenhouse gas emissions from peat burning receives much attention, yet the risks to vegetation composition or peat decomposition from alkaline ash inputs are poorly understood. We explored whether ash produced during smoldering increases peatland topsoil (∼0–25 cm) pH through field observations and laboratory experiments. We assessed spatial patterns of smoldering and ash presence, and measured soil and ash pH after a peatland wildfire in the Netherlands. Additionally, a peat smoldering experiment was conducted to compare freshly produced ash pH with aged ash pH collected 2 months following the wildfire. Additionally, we assessed the amount of ash needed to increase soil pH. Results showed that ash inputs at the field site were insufficient to increase pH of the acidic peatland after the wildfire. Incubation experiments showed that a ≥3 cm ash layer (ash load 163 t ha–1) would be required to increase soil pH by at least 1 unit. Given that aged ash was slightly acidic and fresh ash was alkaline, leaching and neutralisation of ash after wildfire is likely, suggesting that elevated soil pH from ash input may be transient rather than long term.