Calcium-induced immobilization of arsenic in soil and plant accumulation: evidence from sunflower and maize

Arsenic (As) is a highly toxic metalloid that accumulates in soils, reduces crop productivity, and threatens food security. Effective As management is therefore essential, particularly in calcareous soils where native lime may be insufficient for As immobilization. This study evaluated the effectiveness of a water-soluble Ca source, calcium nitrate (Ca(NO3)2), in precipitating arsenate and reducing As uptake in maize and sunflower. Three treatments were applied: a control, As, As + Ca(NO3)2 to maize and sunflower plants. Precipitation tests showed that Ca(NO3)2 rapidly immobilized As via formation of a Ca-As solid phase, whereas CaCO3 exhibited negligible precipitation capacity. XRD and FTIR analyses confirmed arsenate incorporation into a poorly crystalline Ca-As phase in Ca(NO3)2 treatments. Arsenic significantly reduced dry weight in both crops; Ca(NO3)2 had no effect on maize biomass but reduced sunflower biomass. In maize, N concentration increased from 9.59 g kg−1 in the control to 11.38 g kg−1 with As and to 13.7 g kg−1 with As + Ca(NO3)2; corresponding values in sunflower were 14.5, 17.9, and 17.2 g kg−1. Calcium concentrations increased significantly with Ca(NO3)2 in both crops. Arsenic accumulation rose to 7.30 mg kg−1 in maize and 47.8 mg kg−1 in sunflower under As stress, compared with 0.27 and 0.18 mg kg−1 in the controls. Calcium nitrate reduced As concentrations to 4.50 and 28.8 mg kg−1, respectively. Calcium nitrate effectively precipitated As and reduced plant As uptake, emphasizing the critical role of Ca solubility in As immobilization strategies for calcareous soils.