Organic and Inorganic Carbon in Paddy Soil as Evaluated by Mid-Infrared Photoacoustic Spectroscopy

Abstract

Paddy soils are classified as wetlands which play a vital role in climatic change and food production. Soil carbon (C), especially soil organic C (SOC), in paddy soils has been received considerable attention as of recent. However, considerably less attention has been given to soil inorganic carbon (SIC) in paddy soils and the relationship between SOC and SIC at interface between soil and the atmosphere. The objective of this research was to investigate the utility of applying Fourier transform mid-infrared photoacoustic spectroscopy (FTIR-PAS) to explore SOC and SIC present near the surface (0–10 µm) of paddy soils. The FTIR-PAS spectra revealed an unique absorption region in the wavenumber range of 1,350–1,500 cm−1 that was dominated by C-O (carbonate) and C-H bending vibrations (organic materials), and these vibrations were used to represented SIC and SOC, respectively. A circular distribution between SIC and SOC on the surface of paddy soils was determined using principal component analysis (PCA), and the distribution showed no significant relationship with the age of paddy soil. However, SIC and SOC were negatively correlated, and higher SIC content was observed near the soil surface. This relationship suggests that SIC in soil surface plays important roles in the soil C dynamics.

Organic Matter and Mineral Composition of Silicate Soils: FTIR Comparison Study by Photoacoustic, Diffuse Reflectance, and Attenuated Total Reflection Modalities

“…On the shortwave shoulder of the broad OH continuum broadband, two peaks of C-H methylene stretching vibrations at 2930 cm −1 (asymmetric) and 2840 cm −1 (symmetric) [35,70,107,108] are seen in all three modalities; they are medium in ATR-FTIR and strong in FTIR-PAS and DRIFT (Figure 6). Following previous studies [52], these peaks are seen in slow-scan modes of FTIR-PAS and almost disappear in rapid-scan IMF modes at 7.5 kHz or higher.…”

“…The broadband at 1670-1580 cm −1 is soil water [70]. The band is centered at 1620-1615 cm −1 and combines two peaks at 1650-1640 and 1600 cm −1 corresponding to the primary bending vibrations of absorbed water [110] and one of the essential overtone bands in quartz and silica (Table 1), respectively.…”

“…Thus, this modality can be considered an alternative to transmission, DRIFT, or ATR techniques and is less dependent on the particle size. This method is a developed instrument for complex samples [53,[66][67][68][69][70][71][72][73]. It is used for mineral, polymer, and technological samples, modeling the dependence of soil properties on its physical and elemental composition, moisture, porosity, and density [31,37,61,[73][74][75][76][77][78].…”