Phosphorus/iron-doped biochar enabling a synergy for cadmium immobilization and carbon sequestration in fluctuating redox paddy soils

Abstract Biochar addition to soils is a promising strategy for mitigating cadmium (Cd) mobilization and carbon emission, but how biochar-to-soil interaction enabling a synergy between these two goals at redox heterointerface remains unclear. Herein, we conducted three types of paddy soil incubations with phosphorus/iron-doped biochar to explore the underlying factors and processes controlling Cd and carbon transformation under redox conditions. Upon flooding, lower soil redox potential resulted in soluble and extractable Cd transformed into Fe/Mn-bound fraction, coinciding with elevated CO2 and CH4 fluxes. During subsequent drainage, soil pH decrease caused associated Cd transformed back into exchangeable fraction, coupled with cumulative CO2 dropped. Both porewater and sequential extraction results revealed that the remobilization of Cd and carbon during redox fluctuations is largely related to Fe/Mn (hydr)oxide-induced effects. Microscopic and spectroscopic techniques determined that the organo-mineral (e.g., aliphatic C and Fe–O/Si–O groups) interactions are of crucial importance in influencing Cd and carbon distribution patterns on soil microaggregates. Further sequencing and correlation analyses vertified that this biochar facilitated simultaneous Cd and carbon retention via altering soil biogeochemistry, especially redox-controlled abiotic and microbial transformation processes. Overall, these findings shed light on the interactive effects of Cd and carbon mitigation with biochar amendment for redox paddy environments. Graphical Abstract