Mid-IR Spectroscopic Assessment of Structural Features of Organic Carbon In Particle Size Fractions of a Clay Loam Soil.

Soil organic carbon (SOC) content and stability are found to vary with different particle size fractions. Understanding the structural features of SOC associated with these fractions is of great importance to get a better insight in SOC quality. Chemical properties of SOC are normally investigated using NMR spectroscopy. In this study, soil samples (Brookston clay loam) were collected from 0-10 cm of a long-term no-tillage field in Southwestern Ontario, Canada, and SOC associated with mineral particles was separated into three fractions, including SOC in sand (2000-53 μm), silt (53-2 μm), and clay (< 2 μm) fractions, as well as C in particular organic matter (POM, 2000-53 μm). Structural features and recalcitrance of SOC in those fractions were investigated using Fourier transformed mid-infrared (MIR) spectroscopic technique. Single components of the overlapped bands which were largely masked were identified using curve-fitting technique. The MIR spectra differed noticeably among clay, silt, sand, and POM fractions from the same soil. Functional groups calculation without curve-fitting might be biased because many overlapped bands were left unidentified and not included in the calculation. From results after curve-fitting, recalcitrant C content were highest in clay fraction, followed by POM, silt, and sand fractions, indicating that clay fraction had the highest aromaticity and polymerization. There was also a high intensity of aliphatic C in clay, which explained the noted partially readily decomposable nature of clay-associated SOC. Aliphatic C intensity was highest in sand and lowest in silt fraction, indicating that sand associated SOC was most labile while silt associated SOC was the most recalcitrant among all these fractions. Integral of C connected with oxygen atoms in clay was 1.7, 2.1 and 2.5 times higher than in POM, sand, and silt fractions, respectively, revealing that oxidization decreased in the order of clay > POM > sand > silt.