Organic fertilizer inputs enhance soil carbon storage while triggering native carbon losses in tropical grasslands

Organic fertilizers are widely promoted as a sustainable solution to enhance soil C storage and nutrient recycling. Their capacity to increase soil organic matter is well documented, yet little is known regarding their long-term influence on native soil organic matter stability and turnover. This study investigated the fate of organic fertilizer-derived C (OF-C) and its interaction with native C pools after 15 years of continuous bovine manure compost and liquid manure application in two contrasting tropical grassland soils: an Arenosol and an Andosol. Using size fractionation, natural abundance isotopic tracers (δ¹³C and ¹⁴C) and combined C × δ¹³C×¹⁴C equations, we quantified the contribution of organic fertilizers to particulate organic C (POC) and mineral-associated organic C (MAOC), while assessing their effects on native C stocks. Results showed that organic fertilizers increased soil C stocks in the top 20 cm (+0.11 to +0.19 kgC.m⁻².yr⁻¹), mainly through POC accumulation, with some contributions to the MAOC pool. However, this gain appeared to be partially offset by native MAOC destabilization, with estimated losses reaching up to 40% of compost-derived C gains in the Andosol, 10% in the Arenosol, and 25% of slurry-derived C gains in the Arenosol. These losses appeared to be more influenced by soil-climate conditions than by the amendment type. In the Arenosol, the MAOC balance remained close to neutral, with formation offsetting losses. In contrast, net losses were noted the Andosol, alongside a decrease in poorly-crystalline minerals with high affinity for organic matter, thus potentially limiting MAOC rebuilding. These patterns suggest that fertilization strategies should be tailored to soil-climate conditions. In reactive–mineral soils such as Andosols, repeated organic inputs may limit long–term C storage unless carefully managed, while in soils with low-reactive mineral soils, e.g. Arenosols, organic amendments tend to enhance soil C accumulation without jeopardizing the long-term stability of the MAOC pool.