System openness governs detectable signals of enhanced rock weathering in soils: a global meta-analysis

Zhaofeng Ouyang, Meitong Song, Xiaodan Tang, Yuyan Zhao, Simon A. T. Redfern 

Substantial heterogeneity in enhanced rock weathering (ERW) outcomes is often attributed to dissolution controls. However, we show that experimental boundary conditions – especially system openness – provide a primary explanation because dissolution products can be redistributed or exported beyond the sampled pool. We synthesise 81 peer-reviewed ERW studies (205 comparisons) using treatment-driven changes in exchangeable Ca and Mg as indicators of retained near-field weathering products. These retained exchangeable signals attenuated from incubations to pots/mesocosms and field trials, indicating constraints from hydrological connectivity and sampling boundary. Meta-regression reveals application rate is the principal controllable lever, whereas particle-size effects in controlled studies are not independently robust once correlated design choices are accounted for. Baseline soil pH is a consistent contextual filter, while hydroclimate sensitivities inferred from controlled systems are not reliably expressed in field datasets. A mass-balance conversion anchored to the 0–20 cm exchangeable pool yielded small apparent retained CO₂-equivalent signals (~0.03–0.06 t CO₂ ha⁻¹ yr⁻¹), best interpreted as near-field retention constraints rather than standalone CDR estimates. We propose a boundary-aware framework matching exchangeable signals to system openness and accounting scope before tuning application rate. Robust attribution requires longer field trials pairing exchangeable pools with deeper soil, export and water chemistry.