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Abstract

Accurate quantification of different soil organic carbon (SOC) fractions is needed to understand their relative importance in the global C cycle. Among the chemical methods of SOC fractionation, oxidative degradation is considered more promising because of its ability to mimic the natural microbial oxidative processes in soil. This study focuses on detailed understanding of changes in structural chemistry and isotopic signatures of SOC upon different oxidative treatments for assessing the ability of these chemicals to selectively isolate a refractory fraction of SOC. Replicated sampling (to similar to 1 m depth) of pedons classified as Typic Fragiudalf was conducted under four land uses (woodlot, grassland, no-till and conventional-till continuous corn [Zea mays L.]) at Wooster, OH. Soil samples (< 2 mm) were treated with three oxidizing agents (hydrogen peroxide (H₂O₂), disodium peroxodisulfate (Na2S2O8) and sodium hypochlorite (NaOCl)). Oxidation resistant residues and the bulk soil from A1/Ap1 horizons of each land use were further analyzed by solid-state C-13 nuclear magnetic resonance (NMR) spectroscopy and accelerator mass spectrometry to determine structural chemistry and C-14 activity, respectively. Results indicated that, oxidation with NaOCl removed significantly less SOC compared to Na2S2O8 and H₂O₂. The NMR spectra revealed that NaOCl oxidation preferentially removed lignin-derived compounds at 56 ppm and at 110-160 ppm. On the other hand, the SOC resistant to Na2S2O8 and H₂O₂ oxidation were enriched with alkyl C groups, which dominate in recalcitrant macromolecules. This finding was corroborated by the C-14 activity of residual material, which ranged from -542 to -259aEuro degrees for Na2S2O8 resistant SOC and -475 to -182aEuro degrees for H₂O₂ resistant SOC as compared to relatively greater C-14 activity of NaOCl resistant residues (-47 to 61aEuro degrees). Additionally, H₂O₂ treatment on soils after light fraction removal was more effective in isolating the oldest (C-14 activity of -725 to -469aEuro degrees) SOC fraction. The Delta C-14 signature of SOC removed by different oxidizing agents, calculated by mass balance, was more or less similar irrespective of the difference in labile SOC removal efficiency. This suggests that SOC isolated by many fractionation methods is still a mixture of much younger and older material and therefore it is very important that the labile SOC should be completely removed before measuring the turnover time of stable and refractory pools of SOC.