Contrasting influence of exogenous organic matter application on the transfer of trace elements from soil to plants

Trace elements pose significant risks to environmental and human health, particularly when their levels exceed critical thresholds in contaminated ecosystems. In agricultural soils, the addition of exogenous organic matter (EOM) can substantially influence trace element dynamics by modifying soil organic matter (SOM) chemistry, offering a potential lever for mitigating associated risks. This study aimed to: (1) evaluate the influence of three EOM types (cattle manure, green waste compost, and cover crop restitution) on trace element transfer from agricultural soils with contrasting contamination levels to plants; and (2) assess how differences in EOM quality affect these transfers. Greenhouse and field experiments were conducted across a range of contamination contexts in Wallonia, Belgium. EOM application either increased or decreased trace element concentrations in soil solutions and plant tissues compared to the control, indicating that organic matter simultaneously acted as a source of trace elements and a retention compartment. Cattle manure and green waste compost induced the greatest magnitude of variation in trace element concentrations, including both increases and decreases, compared to cover crop restitution. This likely results from their higher carbon inputs and greater proportion of labile organic compounds. Soil properties emerged as key determinants of the direction and magnitude of these effects. Contrasting patterns between greenhouse and field experiments suggested that accelerated organic matter decomposition under greenhouse conditions promoted trace element mobilization, highlighting the need for caution when extrapolating such results to field systems. Comprehensive molecular characterizations (solid-state ¹³C NMR, Rock-Eval, and Py-GC-MS) further showed that organic matter quality (maturity and lability), both of SOM and EOM, plays a central role in shaping trace element behavior in soil solutions. Overall, these findings underscore the importance of considering both soil characteristics and EOM type to improve trace element management in agricultural systems.