Efficiency of Wet Oxidation Methods for Isolating Stable Soil Organic Carbon.

Soil organic carbon (SOC) is the third largest global C pool and, transferring atmospheric CO₂ into SOC pool is an important option to mitigate global warming. However, the accurate measurement of the SOC quantity at landscape scale and its permanence are important issues to be addressed. These issues are confounded by the heterogeneous nature of several C fractions with varying cycling rates and stabilization mechanisms. Among the SOC fractions, the passive/stable fraction is responsible for the long-term sequestration of C in soil. An array of fractionation (physical and chemical) protocols is available for isolating the stable C fraction. Among the chemical methods, oxidative degradation is commonly used because it is assumed to mimic the natural microbial oxidative processes in soil. However, the relative efficiency of different oxidants in isolating stable C is poorly understood and is the focus of the present study. Horizon wise replicated sampling (to 1 m depth) of pedons classified as Typic Fragiudalf was conducted under four land uses (pasture, forest, no-till and plow-till continuous corn [Zea mays L.] at Wooster, OH. Soil samples (<2 mm) were treated with oxidants such as hydrogen peroxide (H₂O₂), disodium peroxodisulfate (Na₂S₂O₈) and sodium hypochlorite (NaOCl) using standard oxidation protocols. Among the land uses, the proportion of total SOC resistant to oxidative treatments ranged from 41 to 79 % for NaOCl, 6 to 43 % for Na₂S₂O₈ and 3 to 46 % for H₂O₂.The structural chemistry of stable C as revealed by ¹³C nuclear magnetic resonance (NMR) spectroscopy, however, clearly showed that H₂O₂ preferentially removed most of the O-alkyl C and is therefore the most efficient oxidant in isolating stable C. Accurate quantification of stable C fraction is essential in modeling SOC dynamics precisely, as well as in estimating long-term effects of land use and soil management on global C cycle.