Drastic Variations in Chemical Composition of Organic Inputs: Implications for Organic Fertilization

Soil amendment with organic inputs is gaining importance with the ongoing shift toward circular economies. While these inputs can fertilize soils with micronutrients such as zinc (Zn), it is crucial to prevent potential contamination stemming from Zn accumulation in soils or crop uptake of toxic elements such as cadmium (Cd). While both organic matter (OM) composition and Zn and Cd speciation are key factors controlling Zn and Cd fate in soil-plant systems, these factors remain largely uncharacterized in many commonly used organic inputs. We unveil substantial differences in water-soluble Zn and Cd speciation and the OM molecular composition in various organic inputs. We found that plant-based organic inputs (e.g., green manures, lignified crop residues) were characterized by lower Zn and Cd concentrations and by enrichment in rapidly degradable OM. In contrast, animal-waste-based inputs (e.g., farmyard manures, composts) were characterized by relatively higher Zn and Cd concentrations and more degraded and resistant OM. Combining size exclusion chromatography coupled to atomic mass spectrometry with geochemical equilibrium modeling (WHAM VII) showed that water-soluble Zn in plant-based inputs was mostly bound to lower-molecular-weight OM while Cd was associated with higher-molecular-weight OM. This suggests potentially higher plant availability of water-soluble Zn compared to Cd. In contrast to plant-based inputs, the majority of animal-waste-based inputs showed water-soluble Zn and Cd primarily in the inorganic aqueous form (e.g., Zn²⁺, ZnOH⁺, Cd²⁺, CdOH⁺) and/or bound to higher-molecular-weight OM. The in-depth characterization of organic inputs provided here establishes a foundation for better understanding of the fate of Zn and Cd in circular agroecosystems.