Can ash from smoldering fires increase peatland soil pH?

Abstract

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.

Evaluating the Effect of Fire on Cultivated Tropical Peat Properties: Lessons Learned from Observation in East Kutai Peatlands

“…From the soil science perspective, meaningful fire' impact was undergone on the N, P, and base cations, as shown by their huge content/concentrations after being burned compared to unburned peat, in agreement with other reports (Dhandapani & Evers, 2020), except for organic C and N contents. Meanwhile, pH increments in both depths at burned peat were not as high as hypothesized, similarly to the findings in subtropical (Marcotte et al, 2022) and tropical (Sulaeman et al, 2021) peatlands. However, the pH ranges at the burned sites presented in this study were much lower than those reported in burned Malayan peatland, North Selangor, either when peat was sampled in the wet or dry season (Dhandapani & Evers, 2020).…”

“…Therefore, we speculated that their study was conducted in the peat area that experienced a relatively immense magnitude and more severe fire than both Sulaeman et al, 2021 and our sites. In addition, a high temperature was observed during peat combustion (~400 to 600°oC), causing the smoldering process and resulting in ash with higher pH (Quintana et al, 2007;Rein et al, 2008;Úbeda et al, 2009;Marcotte et al, 2022). We believe that the combination of the sampling method with the smoldering effect and higher ash' pH was why more burnedaged surface peat from Dhandapani and Evers (2020)' site had extremely high pH and soil nutrients concentration compared to our results.…”

“…Besides being affected by the above reasons, the attenuation of the effect of fires on peat properties recorded in this study might have occurred due to high precipitation before the sampling campaign; however, the magnitudes were different and distinct in every observed variable. Previous authors reported low pH and nutrients concentration in burned peat during wet season, highlighting the effect of precipitation and dilution on the nutrients ions in peat column after burn (Dhandapani and Evers, 2020;Marcotte et al, 2022). A typical case for the this argument could be observed on the concentration of the macronutrients in the surface of the burned peats, which exhibited no statistical significance and their value were close to those observed in the unburned site (Table 1).…”

“…As the organic materials burn, they emit CO2 and other aerosols particulate, thus leaving their macro-and micronutrient loadings as oxides or carbonates which were dependent on the combustion temperature (Usup et al, 2004;Bodi et al, 2014, Hu et al, 2018Sazawa et al, 2018). These oxides or carbonates are highly dissolvable in water, which allows them to be leached down following the deepening of groundwater level or out from the soil system during high rainfall events (Bodí et al, 2014;Marcotte et al, 2022). Monovalent cations, such as K and Na, were reported to be highly mobile in soil, easily transported following groundwater dynamics.…”

“…However, the entire Dhandapani and Evers (2020)' results, either in dry or wet seasons, were remarkably higher than those reported by Sulaeman et al (2021), who collected peat one month after the fire occurrence. Marcotte et al (2022) also found a lower ability of peat ash to significantly decrease peat acidity, indicating the transient effect of elevated peat pH after burning. Reviewing previous authors' sampling designs dan methodologies, both sampling depth and time, as well as fire magnitude or severity about the mobility difference of the nutrient ions in peat, were likely crucial in investigating the effect of fire on peat properties.…”