Gypsum Application Reduces Plant Growth in Copper Mining-contaminated Soils

Researchers worldwide have long sought effective strategies to reduce the bioavailability of metals in contaminated soils. While gypsum (CaSO4 · 2H2O) has shown promise as a soil amendment to mitigate metal phytotoxicity in artificially contaminated (metal-spiked) soils, its effectiveness in soils contaminated by anthropogenic activities remains largely unexplored. This study was designed to bridge this knowledge gap by investigating how gypsum influences plant growth in soils contaminated by copper mining operations in central Chile. We conducted controlled laboratory experiments in which perennial ryegrass was grown in both untreated and gypsum-treated soils. Our study yielded unexpected results. Contrary to our initial expectations, gypsum application had a detrimental effect on plant growth. Furthermore, we observed an increase in copper uptake by plants in the presence of gypsum. This phenomenon is likely due to the displacement of copper ions by calcium ions in the soil exchange complex. However, copper remained available to plants because gypsum did not increase soil pH. An increase in soil pH usually leads to immobilization of metals, either by the formation of new solid phases such as through metal precipitation or co-precipitation, or by metal adsorption on the surfaces of soil organic matter and secondary minerals such as clays and iron oxides. These results not only question the efficacy of gypsum as a means of immobilizing copper in mining-contaminated soils, but also underscore that the use of gypsum as a soil remediation method is inappropriate because it exacerbates rather than mitigates environmental hazards.