Phosphorus-loaded magnetic biochar for remediation of cadmium contaminated paddy soil: Efficacy and identification of limiting factors.

Alleviating cadmium (Cd) risk in paddy soils is a global research hotspot. Although biochar reduces Cd mobility, a holistic perspective on the effects of biochar on Cd fraction distribution in rice rhizosphere and its immobilization mechanisms is lacking. Here, we developed a pathway model that links soil physicochemical properties, IP formation, enzyme activity, microbial biomass, porewater nutrients, and soil Cd fractions to fill knowledge gaps. Results revealed that phosphorus-loaded magnetic biochar (PMLB) application increased soil pH, available phosphorus (AP), total phosphorus (TP), microbial biomass, and TP and Fe contents in porewater while inhibiting soil enzyme activities. Compared with the control, 0.2 %-1 % w/w PMLB treatment reduced soil acetic acid-extractable Cd (Aci-Cd) content during the tillering, filling, and maturity periods by 23.71-32.92 %, 25.45-37.33 %, and 7.39-18.40 %, respectively. Cd content in brown rice was reduced by 44.02-47.86 %. Soil pH, AP and urease activity were the primary drivers of soil Aci-Cd reduction. Soil microbial biomass contributed most to reducing Cd content in rice tissues (total path coefficient: -0.48), followed by enzyme activity and IP. Additionally, PMLB promoted IP formation and altered the immobilization methods of Cd by IP, from coprecipitation with iron (hydr)oxides and phosphate to ternary complex formation with phosphate as a bridge to band Cd and iron (hydr)oxides.