From the ashes: hydrological and biogeochemical responses to wildfire in temperate peatlands

Peatlands are known for their carbon storage but they also provide valuable ecosystem services such as pollutant sequestration, water quality regulation, and flood control. Climate change-driven increases in drought and wildfire risk threaten these ecosystems, potentially causing erosion, runoff, nutrient and metal contamination, and carbon release. The northward shift of such disturbances highlights the need to better understand interactions between climate change, peatland wildfires, ecohydrology, and geochemical responses in temperate peatlands. This thesis addresses knowledge gaps regarding ash characteristics, legacy metal and nutrient mobility, and hydrological responses in two wildfire-impacted ombrotrophic peatlands in northwestern Europe, informing future research and management. Chapter 2 assessed spatial patterns of smoldering, ash production, ash and soil pH, and the impact of ash pH on underlying peat soils in a Dutch peatland (Deurnese Peel wildfire, April 2020). Smoldering was highly variable, likely influenced by pre-fire conditions such as vegetation and groundwater. Ash sampled two months post-fire (“aged ash”) had an average pH of ~6, while fresh ash was alkaline (pH ~9–10). Soil pH in both smoldered and non-smoldered locations was acidic (pH ~3–4). This suggests that wildfire ash initially had high pH but alkaline components were leached by the time of sampling, indicating the impact of alkaline ash on peaty soils may be limited to 1–2 months post-fire.Chapter 3 investigated drought and wildfire impacts on hydrological and biogeochemical responses in a contaminated UK peatland (Saddleworth Moor wildfire, June 2018). Stream chemistry was monitored over nine months post-fire during five rainstorm events. Nutrient concentrations peaked immediately after the wildfire and declined by spring (~9 months post-fire), showing rapid mobilization. Metal concentrations increased in autumn (~3 months post-fire) and spring, exhibiting delayed mobilization. Metals likely originated from distal headwaters, while nutrients were sourced from downstream areas. Seasonal re-wetting and renewed hydrologic connectivity after drought dominated solute mobilization and transport. At the landscape scale, chapter 4 quantified toxic metals from source (hillslope) to sink (reservoir) after the Saddleworth Moor wildfire. Metal concentrations in ash and peat varied spatially but were generally higher than regional baselines. Eroded hillslope material had the highest metal concentrations in summer and autumn (~3 months post-fire), declining by spring. Extreme burn severity areas had higher metal concentrations. Metals in stream sediment peaked during spring storms. Spatiotemporal heterogeneity in metal-rich materials and transport suggests chronic re-working and transport of contaminants months after wildfire, potentially impacting ecosystems and human health. This research advances understanding of wildfire, ash, metal mobility, and hydrological responses in temperate peatlands. Wildfire burn severity and combustion types are highly spatially heterogeneous, depending on pre-fire conditions, resulting in complex biogeochemical and hydrological impacts. Most pronounced changes occurred within 2–3 months post-wildfire, and chronic metal contamination may persist when burn residuals remain in the catchment. Overall, keeping peatlands wet is a key management strategy to maintain peatland health and protect against climate extremes, carbon and pollutant release, and wildfire impacts.