Improving Infrared Spectroscopy Characterization of Soil Organic Matter with Spectral Subtractions

Summary

SOM underlies many soil functions and processes, but its characterization by FTIR spectroscopy is often challenged by mineral interferences. The described method can increase the utility of SOM analysis by FTIR spectroscopy by subtracting mineral interferences in soil spectra using empirically obtained mineral reference spectra.

Abstract

Soil organic matter (SOM) underlies numerous soil processes and functions. Fourier transform infrared (FTIR) spectroscopy detects infrared-active organic bonds that constitute the organic component of soils. However, the relatively low organic matter content of soils (commonly < 5% by mass) and absorbance overlap of mineral and organic functional groups in the mid-infrared (MIR) region (4,000-400 cm^-1) engenders substantial interference by dominant mineral absorbances, challenging or even preventing interpretation of spectra for SOM characterization. Spectral subtractions, a post-hoc mathematical treatment of spectra, can reduce mineral interference and enhance resolution of spectral regions corresponding to organic functional groups by mathematically removing mineral absorbances. This requires a mineral-enriched reference spectrum, which can be empirically obtained for a given soil sample by removing SOM. The mineral-enriched reference spectrum is subtracted from the original (untreated) spectrum of the soil sample to produce a spectrum representing SOM absorbances. Common SOM removal methods include high-temperature combustion ('ashing') and chemical oxidation. Selection of the SOM removal method carries two considerations: (1) the amount of SOM removed, and (2) absorbance artifacts in the mineral reference spectrum and thus the resulting subtraction spectrum. These potential issues can, and should, be identified and quantified in order to avoid fallacious or biased interpretations of spectra for organic functional group composition of SOM. Following SOM removal, the resulting mineral-enriched sample is used to collect a mineral reference spectrum. Several strategies exist to perform subtractions depending on the experimental goals and sample characteristics, most notably the determination of the subtraction factor. The resulting subtraction spectrum requires careful interpretation based on the aforementioned methodology. For many soil and other environmental samples containing substantial mineral components, subtractions have strong potential to improve FTIR spectroscopic characterization of organic matter composition.

Introduction

Soil organic matter (SOM) is a minor constituent by mass in most soil samples but is implicated in multiple properties and processes underlying soil functions, such as nutrient cycling and carbon sequestration¹. Characterizing the composition of SOM is one of several approaches to link SOM formation and turnover with its role(s) in soil functions^2,3. One method of characterizing SOM composition is Fourier transform infrared (FTIR) spectroscopy, which offers detection of functional groups that constitute organic matter in soils and other environmental samples (e.g., carboxyl C-O....

Protocol

1. Prepare Soil for Non-treated DRIFT Spectroscopy and SOM Removal

1. Sieve the soil to < 2 mm using a stainless-steel mesh (the 'fine-earth fraction').
   NOTE: This demonstration employs two soils of similar texture but a nearly 3-fold difference in total SOM content (Table 1).

2. SOM Removal by Chemical Oxidation: Example of NaOCl

1. Adjust the pH of 6% w/v NaOCl to pH 9.5 by adding 1 M HCl dropwise to the solution while mixing and measuring with a pH meter.
   NOTE: Most commercial bleaches (e.g., Clorox) are suitable in quality and concentration (typically 3-7% NaOC....

Results

The method of SOM removal has practical as well as theoretical implications for the interpretation of subtraction spectra. For example, mineral alterations from high temperature ashing can manifest as losses or appearances of peaks and/or as shifted or broadened peaks in the mineral reference spectrum. These spectral artifacts are prone to occur in regions of overlap with organic bands at 1,600-900 cm^-1,²² compromising interpretation of organic bands. Co.......

Discussion

The method of removing SOM carries two considerations: 1) the amount of SOM removed, and 2) absorbance artifacts in the resulting mineral reference spectrum. It is fortunately possible— and arguably necessary— to identify and quantity these issues in order to avoid biased interpretations of SOM composition from the resulting subtraction spectrum. Ideally, spectral subtractions would employ a mineral-only reference spectrum to yield a spectrum of 'pure' SOM. In reality, the resulting subtraction spectr.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Nicolet iS50 spectrometer	Thermo Fisher Scientific	912A0760	infrared spectrometer used to collect spectra
EasiDiff	Pike Technologies	042-1040	high throughput sample holder
OMNIC	Thermo Fisher Scientific	INQSOF018	software used to perform subtractions
6% v/v sodium hypochlorite	Clorox	n/a	generic store-bought bleach for oxidative removal of soil organic matter
Type 47900 Furnace	VWR International	30609-748	muffle furnace for ashing soils to removal soil organic matter
VWR Gooch Crucibles, Porcelain	VWR International	89038-038	crucibles for ashing
VWR Tube 50 mL Sterile CS500	VWR International	89004-364	for sodium hypochlorite
Forced air oven	VWR International	89511-414	for drying soils after oxidation and water washes
VersaStar pH meter	Fisher Scientific	13 645 573	for measuring pH of oxidation solution