Mapping Carbon Compounds Relevant for Soil Energy Cycles at the Aggregate Scale

Dynamics of soil organic carbon (SOC) are controlled by microscale heterogeneity in substrate distribution, microbial accessibility, and mineral diversity. Still, how the microscale organization of low molecular weight (LMW) compounds influences microbial accessibility and soil energy cycling remains poorly understood. Mass spectrometry imaging (MSI) techniques, that are widely applied in biomedical research, offer new opportunities to visualize the distribution of molecular diversity in complex environmental matrices at the micrometer scale. Here, we explore the feasibility of desorption electrospray ionization mass spectrometry imaging (DESI-MSI), a surface-based ambient ionization technique, for spatially resolved detection of LMWs in soil. Sample preparation was guided by soil-specific constraints, including controlled water content and short incubation times, to minimize molecular redistribution and microbial alteration of existing metabolites. Using embedded samples of a range of different agricultural soils differing in management, we obtained two-dimensional distribution maps of amino acids, sugars, and fatty acids at a spatial resolution of 50 µm. Our preliminary measurements show localized micrometer-scale enrichments and spatially segregated distributions of detected compounds (sucrose, palmitic acid, glutamic acid, leucine, etc.), suggesting the presence of distinct molecular patterns at the microscale. Certain soil samples notably contain biochar particles, providing a chance to explore the distribution of LMWs across sorptive carbonaceous surface. These results demonstrate the ability to detect spatial patterns of LMWs across soil structures using DESI-MSI. Ongoing work aims to improve spatial resolution and identify molecular co-locations with specific organic and mineral soil components. Mapping metabolites at the microscale with spatial metabolomics provides new insights into how soil energy and carbon dynamics are organized.