Ligand-Driven Secondary Speciation of Silver Sulfide Nanoparticles Governs Fertilizer Nitrogen Fate in Paddy Soils.

Although silver sulfide nanoparticles (Ag2S-NPs) are presumed stable, their interactions with soil ligands remain poorly understood. Combining 15N tracing, synchrotron-based spectroscopy, and microbial profiling, we investigated how chloride (Cl-) and natural organic matter (NOM) regulate Ag speciation and nitrogen (N) cycling in paddy soils. We found that Cl- promoted the transformation of Ag2S-NPs toward labile Ag-Cl species, increasing Ag bioavailability by approximately 3-fold. This Ag mobilization was associated with enhanced denitrification potential, a greater unaccounted 15N fraction (79.0%), and lower rice 15N recovery (14.1%). Conversely, NOM decreased Ag bioavailability through organic matter-mediated Ag stabilization and increased plant 15N recovery to 41.1%. Microbial network analysis suggested greater bacterial network stability but higher fungal sensitivity under Ag-associated stress. Overall, soil ligand composition governs the balance between Ag mobilization and fertilizer N retention, highlighting the importance of considering salinity conditions in sustainable sludge management.