Contrasting Land‑Uses Affect Chemical Composition of Organic Matter in Tropical Soils: A Case Study via Os Staining and FTIR

Particulate organic matter (POM) is a significant component of soil organic carbon (SOC), particularly in tropical soils, where it often serves as a primary precursor for the formation of mineral-associated organic matter (MAOM). This study investigates the impact of contrasting land-use practices on composition of POM in tropical soils. Using dual-energy X-ray microtomography approach with osmium tetroxide (OsO₄) staining, we visualized and quantified two types of POM: root-derived and pyrogenic (pyC), in soils of three land-use systems: crop succession, integrated crop-livestock (ICL), and managed pasture. Fourier transform infrared (FTIR) spectroscopy was employed to analyze the chemical composition of the two POM types. Results showed that land-use practices significantly influenced pyC chemical composition and Os staining efficiency, with ICL exhibiting the highest concentration of staining Os, followed by pasture and crop succession. Root-derived POM showed no significant differences in Os staining across land uses. FTIR analysis revealed distinct chemical signatures among land uses, with ICL demonstrating lower aromatic-to-aliphatic carbon ratios in pyC, suggesting enrichment by plant-derived aliphatic materials. These findings can be attributed to intensive decomposition of fresh organic inputs during crop-pasture rotations in ICL system, providing new insights into the mechanisms through which POM of pyrogenic origin may contribute to SOC accrual and protection in tropical soils under different management practices.