Effects of wood ash amendments on greenhouse gas production and soil solute dynamics in hydromorphic forest soils of Northwestern Ontario

Abstract

Wood ash recycling can return nutrients to forest soils, but responses in hydromorphic settings remain poorly characterized. This study examined how ash type and application rate affect pH and dissolved base cations, whether riparian and upland soils differ in methane (CH₄) concentrations, and how gas responses change during anaerobic incubation. Two boiler ashes (PB3 and PB6) were applied at nominal rates of 0, 5, 10, 20 and 50 t ha⁻¹ to five soil composites from northwestern Ontario: two riparian, two upland and one buffer source. Laboratory microcosms were incubated under N₂-flushed conditions for 60 days. Headspace CH₄ and carbon dioxide (CO₂) were measured on Days 10 and 34, and filtered-water chemistry was assessed on Day 60. Ash-amended pH values exceeded the corresponding untreated controls in all five soils by 0.78–2.53 units, including at the lowest rate, supporting the expectation that ash reduces acidity. Dissolved base-cation responses depended on the element, soil and ash: sodium increased in many amended treatments, while calcium and magnesium varied among soils and rates. The hypothesis of higher riparian CH₄ concentrations was partly supported: both riparian sources exceeded their paired upland sources at Day 10, but Black Spruce upland exceeded Black Spruce riparian at Day 34. Temporal gas responses varied among soils and treatments, consistent with the third hypothesis; CO₂ also increased in untreated jars, and neither gas showed a uniform response to increasing ash rate. Exploratory mercury measurements often showed lower filtered-water total mercury (THg) in amended samples, but methylmercury was not measured. Limited field replication and chemistry coverage, together with departures from gas-model assumptions, constrain inference. The findings identify soil- and ash-specific responses for field testing and water-quality monitoring, without establishing an operational application rate.