Abstract
Few studies have evaluated structural features of soil organic carbon (SOC) in different soil particle fractions, especially SOC changes induced by tillage, using Fourier transformed mid-infrared (MIR) spectroscopy. To make a contribution in this context, soil samples of a Brookston clay loam (mesic Typic Argiaquoll) with averaged pH and organic matter concentration at 7.28 and 43.9 g kg−1, respectively, were collected from short-term no-tillage (NT97) and mouldboard plow (CT97) treatments initiated in 1997 and long-term no-tillage (NT83) and mouldboard plow (CT83) treatments initiated in 1983 under a corn-soybean rotation, and were separated into sand, silt, and clay fractions using sonication. Structural features of SOC in these soil fractions were investigated using curve-fitting analysis of mid-infrared (MIR) spectra. Aromatic C content was found to be greater in clay- than in sand-sized fractions, while aliphatic C content was higher in sand- than in silt- and clay-sized particles. With decrease in tillage intensity, the aliphatic C gradually increased in sand- and clay-sized fractions but not in the silt-sized fraction. The aliphatic C content in sand fraction was significantly greater in NT83 than CT83 (P < 0.05). The aromatic C in silt- and clay-sized fractions was greater in NT83 than in both CT soils, whereas aromatic C contents were higher in both CT soils than in NT83 soil. Significantly higher aromatic/aliphatic C ratio in CT83 than NT83 was found in sand-sized fractions, while the opposite trend was found in the silt-sized fraction. These findings were not apparent until the curve-fitting technique was employed, which has the capacity to quantify many overlapped bands in the spectra. This study demonstrates that the curve-fitting of MIR spectra advances the analysis of organic matter in soil samples.
Highlights
Soil organic carbon (SOC) content and quality were found to vary with tillage systems and soil depth[1,2,3], with crop rotation[4], as well as with particle size fractions[5,6]
Other tillage practices and the soil organic carbon (SOC) associated with clay fraction in particular was less humified than associated with other particle size fractions of an Oxisol[16]
Limited work has been done on differentiating the characteristics of SOC in soil particle size fractions using MIR spectroscopy, with soils under different tillage systems
Summary
Soil organic carbon (SOC) content and quality were found to vary with tillage systems and soil depth[1,2,3], with crop rotation[4], as well as with particle size fractions[5,6]. The 13C NMR spectroscopy analysis is a powerful means for studying the SOC characteristics associated with soil particle-size fractions[10,13]. Other techniques have been used to study SOC properties, such as pyrolysis-field ionization mass spectrometry Using this technique, the abundance of carbohydrates, phenols and lignin monomers, alkylaromatics and N-containing compounds was found be vary distinctly among the physical fractions[15]. The objectives of this study were to (i) use curve-fitting to identify functional groups from the MIR spectra of soil samples which have distinct overlapping bands and (ii) assess whether curve-fitting can reveal the impacts of tillage practices on structural features of SOC in light of our current understanding of SOC associated with physical-size fractions in soil
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