Phosphate-modified biochar attenuates cadmium availability in contaminated soil and reduces its transfer to tomato fruits

Cadmium (Cd) is a toxic heavy metal for plant growth and development. Biochar has been proposed as an effective approach to increase immobilized cadmium fractions in soil and reduce its availability and accumulation by plants. In the present study, the enrichment of biochar with water-soluble polyphosphate (PLB) was tested to assess its ability to modify cadmium immobilization capacities in soils compared with standard biochar (SB) and orthophosphate enriched biochar (OLB) under Cd stress. In a pot experiment, the impact of these three biochar forms (SB, PLB, and OLB) on soil properties, soil Cd fractions, photosynthesis, tomato plant growth, yield, fruit quality, and nutrient uptake was assessed under two levels of Cd (0, and 5 mg kg⁻¹). The obtained results showed that the enrichment of biochar with both forms of phosphorus fertilizers (orthophosphate and polyphosphate) significantly and positively impacted P and K contents in the final enriched biochar compared to SB. Results demonstrate that applying PLB under cadmium stress significantly increased phosphorus availability in soil (+32 %) and reduced Cd exchangeable fraction compared to the SB and OLB treatments. These findings suggest that, under cadmium stress, the slow-release properties as well as the organometallic chelation and precipitation capacities of the PLB can help in improving phosphorus, calcium, and iron uptake by plants and reducing Cd uptake and translocation to the shoot tissues, which resulted in significant enhancement of plant photosynthesis efficiency (PI_abs: +144 %), shoot dry biomass (+117 %), and fruit yield (308 %) and quality, compared to standard biochar. Therefore, the enrichment of biochar with polyphosphate fertilizers could be proposed as an efficient strategy to enhance plant growth, physiology, and nutrition and mitigate cadmium stress and toxicity in contaminated soils.