Testing Os Staining Approach for Visualizing Soil Organic Matter Patterns in Intact Samples via X-ray Dual-Energy Tomography Scanning.

Challenges with in situ visualization of nonparticulate organics in porous materials limit understanding and modeling processes of transport, decomposition, and storage of organic compounds. In particular, it impedes deciphering the mechanisms driving accumulation and protection of soil organic matter (SOM), processes crucial for sustaining soil fertility and mitigating effects of global climate change. A recently proposed method of staining soil organics by OsO4 vapors with subsequent dual-energy X-ray computed microtomography scanning (μCT) offers new opportunities to visualize SOM within intact soil matrix. Our objective was to test the method's performance in staining different organic materials located in media with contrasting pore characteristics: (1) roots of switchgrass (Panicum virgatum L.), either placed within fine and coarse sands or grown within soil microcores, (2) biochar fragments, and (3) soils with relatively low and high C contents. We found that the method was effective in staining organic materials of root origin and the organics associated with fine soil particles, but not the biochar. The estimated percent of total C that reacted with OsO4 vapors ranged from 0.7% in plant roots to 3.2% in sand-free fraction of the high C soil and was only 0.2% in the studied biochar. Total soil C and Os concentrations were strongly linearly related, suggesting a potential for future method development. However, we would recommend caution when interpreting the results in cases when gas diffusion through the soil matrix is limited.