Arsenic Phytotoxicity in Ryegrass: The Mitigating Role of Phosphorus, Zinc, and Soil Organic Matter

Abstract: Environmental degradation caused by non-ferrous metal smelting is a major concern in many countries. In the Ural Mountains, historical copper smelting has resulted in extensive soil metal contamination. In this study, perennial ryegrass (Lolium perenne L.) was grown under controlled laboratory conditions in eight soils contaminated by emissions from the former Voskresensky copper smelter (Southern Urals, Republic of Bashkortostan, Russia), which operated during the 18th–19th centuries. Although, copper phytotoxicity was expected, arsenic emerged as the primary determinant of phytotoxicity (R² = 0.83, P = 0.01), whereas shoot contents of other elements, including copper, had no statistically significant effect on plant growth. The 50% effective concentration (EC₅₀) for shoot arsenic content was 0.29 mg/kg dry weight. Shoot arsenic content was inversely proportional to soluble soil phosphorus concentration (R² = 0.83, P = 0.01), consistent with well-established antagonism between phosphate and arsenate ions. Shoot arsenic content was also inversely related to shoot zinc content (R² = 0.52, P = 0.042). Although the mechanism behind this arsenic–zinc antagonism remains unclear, it may involve the formation of ternary surface complexes on iron oxides. Despite a positive correlation between soluble soil arsenic and soil organic matter (R² = 0.53, P = 0.039), shoot arsenic content was inversely proportional to soil organic matter (R² = 0.59, P = 0.027), aligning with previous observations that metal-humic acid complexes are not readily phytoavailable. Collectively, the effects of phosphorus, zinc, and soil organic matter on arsenic uptake explain the absence of a statistically significant correlation between soluble (0.01 M CaCl₂-extractable) soil arsenic and shoot arsenic content. Overall, this study provides new insights into arsenic phytotoxicity and the factors controlling arsenic uptake in contaminated soils. However, due to the modest number of samples analyzed in this study (n = 8), the conclusions should be considered preliminary.