Amending solid carbon from methane cracking to arable soils: A sustainable approach to increase carbon storage and heavy metal immobilization?

Abstract. Carbon dioxide emissions from burning fossil fuels play a major role in driving global climate change. Reducing these emissions through innovative technologies is critical to achieve climate change mitigation goals. Methane pyrolysis, including catalytic and "plasmalytic" approaches, has attracted attention for its ability to produce so-called turquoise hydrogen alongside solid carbon as a by-product that could be reused as soil amendment. This study investigated the potential of solid carbon materials from catalytic pyrolysis and plasmalysis, alongside reference materials (biochar and graphite), to improve soil hydraulic properties and to reduce heavy metal mobility and whether ecotoxicological reactions affect soil organisms. In experiment 1, two arable soils of contrasting textures (sand and silty loam) were amended with these carbon materials at an application rate of 40 t ha^-¹, followed by assessments of soil physical properties, soil respiration rate, microbial biomass, extractable organic carbon, nitrogen mineralization, the activity of soil macro- (earthworms) and mesofauna (springtails, Folsomia candida). In Experiment 2, we evaluated heavy metal mobility and availability in metal-contaminated soils. In uncontaminated soil, solid carbon from plasmalysis (SC_plas) increased water retention of the silty loam, particularly in the range of pF 1.8–3.0, but had no effect in the sandy soil, likely due to its hydrophobic properties, which limited moisture retention. In the silty loam, SC_plas reduced microbial activity and the abundance of springtails. In the sandy soil, it had a negative effect on soil macrofauna (earthworms). The solid carbon from catalytic pyrolysis (SC_cat) had almost no effect on the biological properties studied. In soils contaminated with heavy metals, SC_plas showed strong immobilisation of heavy metals particularly for Cd and Cu, across several sites, outperforming the reference materials. However, SC_cat increased Cd mobility at some sites, indicating little or even adverse effects on heavy metal mobility. Our results highlight the promise of SC_plas for site-specific soil improvement, while cautioning against its hydrophobic effects in sandy soils. In contrast, SC_cat has more negative effects, especially ecotoxicological than positive ones depending on soil.