Evaluation of structural chemistry and isotopic signatures of refractory soil organic carbon fraction isolated by wet oxidation methods

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

Integrating physical and chemical methods for isolating stable soil organic carbon

“…The SOC fraction resistant to chemical treatment was assumed to be stabilized due to strong chemical interaction with soil minerals. From a comparison of different oxidizing agents, it was found that both H 2 O 2 and Na 2 S 2 O 8 are more efficient for isolating a stable fraction of SOC (Jagadamma et al, 2010). Therefore, in the present study, H 2 O 2 was used as the oxidizing agent in methods 2, 3a and 3b.…”

“…The resistance of some constituents of plant residues to H 2 O 2 oxidation was proved when the light fraction (b1.6 g cm − 3 ) materials separated from the soil samples were exposed to H 2 O 2 oxidation. Among the land uses, the H 2 O 2 oxidation left behind a fraction of bleached plant residues with 30-56% of total OC (Jagadamma et al, 2010). The effect of tillage types on the stable SOC pool also depended on the methods used to isolate the stable SOC fraction.…”

Assessment of Soil Aggradation through Soil Aggregation and Particulate Organic Matter by Riparian Switchgrass Buffers

“…The NMR spectra consisted of between 20,000 and 50,000 acquisitions with a contact time of 3 ms and a pulse repetition time of 4 ms. The NMR spectra were divided into four chemical shift regions according to the chemical types of carbon as follows: 0-46 ppm (alkyl), 46-110 ppm (O-alkyl), 110-164 ppm (aromatic), and 164-190 ppm (carbonyl) [22,38]. The relative intensities of the chemical shift regions were determined by integration of the NMR spectra over given chemical shift ranges after phase and baseline corrections [22,38].…”

“…The NMR spectra were divided into four chemical shift regions according to the chemical types of carbon as follows: 0-46 ppm (alkyl), 46-110 ppm (O-alkyl), 110-164 ppm (aromatic), and 164-190 ppm (carbonyl) [22,38]. The relative intensities of the chemical shift regions were determined by integration of the NMR spectra over given chemical shift ranges after phase and baseline corrections [22,38]. Integration of the NMR spectra was done with OriginPro (Version 8, OriginLab Corporation, Northampton, MA, USA).…”

Characterization of soil organic matter and black carbon in dry tropical forests of Costa Rica

“…Thus, aromaticity of the oxidation residue decreased for Of and tended to increase for Qu. NaOCl treatment preferentially degrades lignin, and HF-treatment of NaOCl-treated subsoil samples removes virtually all lignin-derived phenols (Simpson and Hatcher, 2004b;Mikutta et al, 2005;Mikutta et al, 2006;Jagadamma et al, 2010). In contrast to Of, oxidation slightly decreased the relative contributions of O-alkyl C and carboxyl C in the Qu samples indicating that some oxidizable or labile C components were lost upon oxidation (Simpson and Hatcher, 2004b).…”