Short-term transport of soluble ions from sludge-based biochar in soil under environmental stress: effects of...

Sludge-based biochar is increasingly considered for land application, yet the short-term mobility of its soluble-ion pool under environmental stress remains poorly understood. In this study, soil column experiments were used to compare two placement modes, mixed incorporation and stratified placement, under dry-wet cycling, freeze-thaw cycling, simulated acid rain, and a control condition. Na⁺, Cl^-, Ca²⁺, and SO₄^2- were monitored in soil layers and leachate. Soil-only columns were included to constrain background contributions. Mixed placement was generally associated with lower divergence among ion-specific release trajectories and earlier stabilization, whereas stratified placement produced clearer staged redistribution. In the stratified columns, Na⁺ and Cl^- stabilized earlier than Ca²⁺ and SO₄^2-, indicating clear ion-specific differences in mobility and retention. Among the tested stressors, dry-wet cycling showed the highest cumulative leaching of all examined ions, especially Ca²⁺ and SO₄^2-, suggesting that water content fluctuation was the strongest driver of short-term salt mobilization in this system. Freeze-thaw cycles and simulated acid rain also altered ion transport, but their effects depended on ion type and application mode. Because the experiment used a high biochar loading (20%, w/w), the results should be interpreted as comparative evidence under intensified conditions rather than as direct field-scale prediction. These findings suggest that pre-application screening of soluble ions and careful selection of placement mode are important when sludge-based biochar is used in salt-sensitive soils.