FTIR–Fluorescence Two-Dimensional Correlation Spectroscopy of Water-Extractable Particulate Soil Organic Matter Fractions by Sequential Membrane Filtration

Distribution water-extractable particulate (colloid) organic matter (WEOM) in narrow (nano- to micrometer) size fractions of chernozem soil by sequential filtration on track-etched membranes were studied. Multimodal (IR and fluorescence) two-dimensional correlation (2D-COS) spectroscopy was used. Protocols for attenuated total reflectance (ATR) FTIR of WEOM are proposed. ATR-FTIR 2D-COS provides larger volume of information on characteristic bands compared to traditional FTIR, especially in C–H ranges (3000–2800 and 1450–1300 cm–1). Fluorescence excitation-emission-matrix 2D-COS showed that the indexes and ratios of humic- to protein-like compounds are reproducible, exhibit significant variation among size fractions, with maximum amounts of saturated humic-like compounds in the largest (2–10 μm) and finest factions (0.01–0.03 μm) while medium fractions (0.05–1 μm) are dominated by fulvic acids and fresh organic matter. Heterospectral fluorescence–IR 2D-COS enhanced the accuracy of identification and assessment of WEOM group composition and showed that C–H IR band intensities correlate with tyrosine-like EEM bands and biogenic fluorescence in-dexes, while carboxylic components, with humate-like bands and humification fluo-rescence indexes. Element profiles in WEOM fractions correlate with fluorescence in-dexes; humification indexes, with P, S, Cr, Mg, Ca, Cu, and Zn; biogenic, with Mg, P, Cr, Cd, K, S, and Ca.