Organic Amendments Regulate Heavy Metal Uptake, Greenhouse Gas Emissions and Carbon Sequestration in a Contaminated Rice–Vegetable Rotation System

Organic amendments (biochar, manure, and straw) are widely used as soil-applied functional materials in polluted soils; however, integrated comparative assessments within crop rotation systems remain limited. We established a rice (Oryza sativa L.: Hanyou 3015 and Yangtaiyou 128)–celery (Apium graveolens L.: Queen Celery) rotation system on farmland contaminated with heavy metals (HMs). Biochar (B), manure (M), and straw (S) were applied to assess HM dynamics, greenhouse gas emissions (CO2, CH4 and N2O), and overall system responses. B showed limited but generally stabilizing effects on crop performance and greenhouse gas emissions in the rice–celery rotation system; however, it significantly reduced Cd, Pb, and Zn uptake in celery during the later stages of the rotation. Although M enhanced soil N transformation and microbial biomass C, increased yield by 50%, and improved total system carbon sequestration, the associated risk of heavy metal accumulation during the initial rice phase should be considered in agricultural management. S increased rice and celery yields, reduced Pb and Zn accumulation in edible parts, and enhanced carbon sequestration in both crops and soil during the celery phase. These results highlight amendment-specific functions and associated trade-offs in the management of contaminated soils.