A review on soil amendments for cadmium sequestration and methane emission reduction in paddy soils

Cadmium (Cd) contamination and methane (CH₄) emission from paddy soil are alarming environmental issues that occur simultaneously but are often studied separately, and thus synergistic treatment studies need to be strengthened. This article reviews the involved mechanisms and influencing factors of currently adopted soil amendments for solving the two issues, pinpointing promising amendment materials and environmental conditions for synergistic remediation. The method of application, performances, and mechanisms of currently used soil amendments, including biochar (BC), iron/manganese materials, lime, sulfur, and silicate materials, for mitigating Cd pollution and CH₄ emission in paddy soils are demonstrated. The amendments reduce the bioavailability of Cd in soil through various mechanisms including adsorption, complexation, and co-precipitation, and simultaneously, they potentially inhibit CH₄ emission by increasing soil redox potential (Eh), enhancing methanotrophy, and reducing the abundance of anaerobic methanogens. Synergistic remediation performance is regulated especially by soil pH and Eh. The most effective pH range for synergistic Cd reduction and CH₄ mitigation is between 6.0 and 7.0, with Eh between −150 and −100 mV. However, achieving this promising environmental condition in flooded paddy fields is challenging. Several promising amendment strategies (BC, high-valent Fe/Mn, lime, sulfate, and silicate) are proposed to achieve this goal, identifying research challenges and future directions for in situ remediation of paddy fields.