Contrasting land-uses affect chemical composition of organic matter in tropical soils: A case study via osmium staining and infrared spectroscopy

Particulate organic matter (POM) is a significant component of soil organic carbon (SOC), particularly in tropical soils where the unique mineralogical compositions and climatic accelerate its transformation into mineral-associated organic matter (MAOM). Osmium tetroxide (OsO₄) staining is a promising technique for analyzing POM in intact soil samples, yet its effectiveness in visualizing POM of varying chemical compositions and origins remains insufficiently understood. This study utilizes Os staining to investigate POM from tropical soils subjected to contrasting land use practices, namely, crop succession, integrated crop-livestock (ICL) management, and managed pasture. Combining dual-energy X-ray microtomography approach with Os staining, we examined two types of POM: root-derived and pyrogenic (pyC), and employed Fourier transform infrared (FTIR) spectroscopy to analyze its chemical composition. 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.