Decoupling total accumulation from ecological and health risks: speciation stabilization of heavy metal(loid)s in soil-maize systems under 16-year application of various organic fertilizers

Long-term application of organic composts is known to enhance soil heavy metal (loid)s (HMs) accumulation; yet the dynamic speciation-bioaccumulation-health risks affected by various organic composts remain unclear. To address this knowledge gap, a 16-year field experiment assessed the effects of chicken manure (CM), sewage sludge composts (SSC), and domestic waste composts (DWC) on the accumulation, speciation, and bioavailability of eight HMs in a soil-maize system. The results showed that continuous application of CM, SSC, and DWC at 6 t ha^-1 increased soil concentrations of Cd by 20-38 %, Cr by 8-16 %, Cu by 9-21 %, Zn by 17-37 %, and Hg by 82-350 % compared to control or chemical fertilizer treatments, with SSC showing the highest Hg accumulation (306-350 % increase). A moderate ecological risk was observed in SSC-treated plots, while other treatments posed low ecological risks. Organic fertilizer application promoted the HM stabilization: e.g. the proportion of stable fraction for Cd increased from 64 to 65-70 % under organic treatments. Such stabilization reduced bioaccumulation factors of Hg, Cd, and Zn by 42-78 %, resulting in no significant increase in HM concentrations in maize grains or health risks. Safe application durations are first estimated as 117 and 131 years for CM and SSC application (limited by Zn), respectively, and theoretically unlimited for DWC. The study concludes that stabilization of HM speciation and regulated application rates of organic fertilizers can decouple total HM accumulation from ecological and health risks. These findings provide practical guidance for the sustainable recycling of organic waste resources while ensuring environmental and food safety.