The potential of carbon from methane plasmalysis as a soil amendment

Carbon (C)-based materials such as biochar and nanocarbons are widely explored as soil amendments to improve soil health, crop performance, and carbon sequestration. However, their large-scale application is limited by variable composition, high production costs, and uncertain scalability. Carbon from methane plasmalysis (CMP), a solid C co-product of CO₂-free hydrogen production from CH₄, represents a novel and sustainable alternative that has not yet been investigated for agricultural applications. This study presents the first assessment of CMP's physicochemical properties and agronomic potential, benchmarked against biochar. CMP was highly pure (≥98.4 % C), with fine particle size (0.3-7.0 μm) and moderate specific surface area (17.5-28.0 m² g^-1), and in line with biochar certification standards. In greenhouse experiments using three Austrian soils with contrasting pH, KA (pH 4.6), GR (pH 6.5), and AB (pH 7.9), CMP application (w_CMP = 0.1-1 %) improved Zea mays L. biomass, chlorophyll content, and nutrient uptake, particularly under slightly acidic conditions (GR), even at 0.1 %. At higher rates (1 %), CMP and biochar both slightly reduced biomass and nutrient uptake in alkaline soil (AB), indicating comparable pH-dependent behavior. A 29-month field trial at the GR site confirmed CMP's benefits, including increased plant-available phosphorus (up to +60 %) and microbial activity (up to +25 %) and reduced dissolved organic carbon (up to -49 %) compared to control plots without CMP. By introducing CMP as a climate-positive and scalable carbon-based amendment, this work broadens the portfolio of sustainable soil management strategies and establishes a foundation for mechanistic studies on CMP-soil-plant interactions.