Maoz Dor, Tom Regier, Zachary Arthur, Andrey Guber, Alexandra Kravchenko

Soil organic carbon (SOC) plays a crucial role in soil fertility, productivity, and global carbon cycling. However, the mechanisms governing SOC persistence and turnover are not fully understood, hindering effective carbon management strategies. Especially limiting are challenges to visualize and characterize spatial distribution patterns of SOC within the intact soil. This study presents a novel approach to map carbon content and composition in intact soil samples using synchrotron X-ray spectromicroscopy at a 4-100 m resolution. Intact soil cores were collected, fixed with sodium silicate, and polished to create smooth surfaces suitable for soft X-ray analysis. X-ray fluorescence (XRF) maps provided an overview of the total carbon distribution, enabling the identification of carbon-rich regions of interest. Near edge X-ray absorption fine structure (NEXAFS) spectromicroscopy was then employed to obtain spatially resolved carbon speciation data within these regions. This method enabled the analysis of relatively large intact soil cores (16,000 m Ø and 15,000 m height), preserving a variety of root and particulate organic matter fragments as well as pores ranging in size from 35 to 850 mm. Spectral fitting using reference standards revealed distinct spatial patterns of aromatic, aliphatic, and carboxylic carbon compounds associated with different soil structural features. Aromatic carbon was enriched around root fragments and the soil matrix, while carboxylic compounds were concentrated at pore-matrix interfaces, suggesting a correlation between soil pore structure and carbon chemical composition. The proposed novel approach provides opportunities for future unprecedented insights into the interplay between pore architecture and organic molecular diversity, the two key factors governing mechanisms of SOC protection and persistence in the soil.