Abstract

This study evaluated the effects of biochar and metallic oxide nanoparticles (iron oxide and halloysite nanoclays) used as compost additives on the decomposition and mean residence time (MRT) of soil organic matter (SOM) in tropical grassland soils. The objective was to explore their impact on decomposition rates and the chemical structure of compost-derived SOM to optimize carbon storage and organic fertilization strategies. Organic residues that included cow manure and wheat straw, with a C3 signature, were initially composted adding biochar, metallic oxides, and their combination as additives. The end products (stabilized compost) were applied to tropical soil naturally enriched in 13C due to theC4 vegetation in an incubation experiment, where basal respiration, compost, and SOM decomposition were analyzed, and the mean residence time (MRT) was estimated. Additionally, the preservation of different carbon pools and functional groups during the mineralization of SOM and compost organic matter was assessed and modeled, using a combination of ?13C stable isotope and 13C NMR spectroscopy. The results showed that additives such as biochar and halloysite nanoparticles reduced the decomposition rate of compost, increasing its MRT from 4.5 to 7.6 and 5.4 years, respectively. This could be driven by organo-mineral interactions of organic matter (OM) with metallic nanoparticles, and biochar adsorption of the soluble compost-derived OM. However, the combination of biochar and metallic nanoparticles showed no synergistic effect for compost-derived-OM preservation, but a probable mineralization of biochar-derived C. These findings suggest that additives significantly modulate organic matter decomposition and structural rearrangement, extending its MRT in the studied soil. These additives can be crucial in improving soil carbon storage, presenting a promising avenue for long-term organic fertilization and soil management practices. © The Author(s) under exclusive licence to Sociedad Chilena de la Ciencia del Suelo 2025.