Impact of Biochar and Water Regimes on Arsenic Transfer and Uptake in Rice: Insights into Transporter Behaviour and Soil–Plant Dynamics

We assessed the effect of biochar and water regimes on arsenic (As) accumulation in popular rice cultivars. These management practises altered the bioavailability of As, Si, P, and S in the soil, which in turn modulate the expression of associated As transporters. This study was conducted under two distinct water regimes—alternate wetting and drying (AWD) and continuous flooding (CF)—with varying levels of biochar using two distinct cultivars, Shatabdi and IR 64. Practice of AWD with biochar (B₄₀AWD) enhanced the competitive interactions between As, and Si, and P for uptake, with a concomitant downregulation of silicon (OsLSi1 and OsLSi2) and phosphate (OsPT1 and OsPT2) co-transporters of As in Shatabdi (34.6, and 83.4% and 66, and 25%, respectively) and IR 64 (83.4, and 99% and 92.5, and 49%, respectively). Enhancing sulfur (S) nutrition by 37.1% under AWD conditions with biochar promotes the biosynthesis of phytochelatins (PCs). This process leads to the upregulation of the OsABCC1 gene by 99.1%, ensuring that more As-PC (arsenic-phytochelatin) complexes are sequestered in the shoots. These alterations of the nutritional balance in the soil–plant system and modulation of the expression of As transporters under AWD-biochar reduced As levels in grains by 56.8–63.1% in popular cultivars, making them safer to grow in As-contaminated areas.