Improving tropical soil properties in the Republic of Congo using domestic waste compost: Is there a minimum threshold application rate?

Improving soil health in a sustainable manner is a strategic priority, particularly for highly weathered tropical soils. This study evaluated the effects of compost derived from solid household biowaste on soil organic matter, microbial communities, and associated physicochemical and biological properties. A randomized block field experiment was conducted in Dolisie (Republic of Congo) using increasing compost application rates (0, 30, 40, 50, and 60 t ha⁻¹). Soil analyses included physicochemical properties, elemental and molecular composition, and microbial abundance, community structure, and activity. Increasing compost application rates significantly enhanced soil organic carbon, total nitrogen, and plant-available phosphorus contents, as well as water retention capacity, without disrupting overall cationic balance. Marked changes in soil elemental and molecular composition were observed from application rates ≥ 50 t ha⁻¹ , indicating substantial accumulation and transformation of OM and associated nutrients. Bacterial and fungal abundances increased significantly with compost dose, accompanied by pronounced shifts in microbial community structure, particularly at higher application rates. Microbial respiration and metabolic efficiency increased, while functional diversity became more specialized, reflecting microbial adaptation to increased inputs of labile organic substrates. Overall, the results demonstrate that the incorporation of solid household biowaste compost into tropical agricultural systems is an effective agroecological practice that improves soil physical, chemical, and biological properties. This approach supports sustainable soil management and contributes to the achievement of broader environmental and development objectives in tropical regions.