How does grinding affect the mid-infrared spectra of soil and their multivariate calibrations to texture and organic carbon?

Abstract

Mid-infrared (mid-IR) diffuse reflectance spectroscopy can be used to effectively analyse soil, but the preparation of soil samples by grinding is time consuming. Soil samples are usually finely ground to a particle size of less than 0.250 mm because the spectrometer’s beam aperture is approximately 1–2 mm in diameter. Larger particles can generate specular reflections and spectra that do not adequately represent the soil sample. Grinding soil to small particle sizes enables the diffuse reflectance of light and more representative sample measurements. Here, we report on research that investigates the effect that grinding to different particle sizes have on soil mid-IR spectra. Our aims were to compare the effect of grinding soil to different particle sizes (2.000 mm, 1.000 mm, 0.500 mm, 0.250 mm and 0.106 mm) on the frequencies of mid-IR spectra, and compare the effect of these particle sizes on the accuracy of spectroscopic calibrations to predict organic carbon, sand, silt and clay contents. Using the Commonwealth Scientific and Industrial Research Organisation’s (CSIRO) National visible–near infrared database, we selected 227 soil samples from the National Soil Archive for our experiments, and designed an experiment whereby each soil sample was ground in triplicate to the different particle sizes. These ground samples were measured using an FT-IR spectrometer with a spectral range of 4000–600 cm–1. Grinding to particle sizes that are ≤2.000 mm reduces subsample variability. Smaller particle sizes produce finer and sharper absorption features, which are related to organic carbon, and clay and sand mineralogies. Generally, better predictions for clay, sand and soil organic carbon contents were achieved using soil that is more finely ground, but there were no statistically significant differences between predictions that use soil ground to 1 mm, 0.5 mm, 0.25 mm. Grinding did not affect predictions of silt content. Recommendations on how much grinding is required for mid-IR analysis should also consider the time, cost and effort needed to prepare the soil samples as well as the purpose of the analysis.

Reduced soil respiration beneath invasive Rhododendron ponticum persists after cutting and is related to substrate quality rather than microbial community

“…This suggestion of substrate limitation is supported by the FTIR data, which is a commonly used technique to investigate shifts in SOM composition in a range of soil types (Elliott et al, 2007;Heller et al, 2015;McAnallen et al, 2017). Our fourth hypothesis which stated that soil FTIR spectra would be altered by clearing R. ponticum proved to be true; soil aromaticity was lower 32 weeks post-clearance wavebands associated with aromatic organic compounds, whilst kaolinite absorbance may overlap with the polysaccharide waveband (1020 cm -1 ) (Le Guillou et al, 2015). Despite this, we consider our approach to be appropriate given the peaty nature of the Hexworthy soil (National Soil Resources Institute, 2019) and the igneous bedrock (British Geological Society, 2019) present on the site.…”

Studying the Physical Protection of Soil Carbon with Quantitative Infrared Spectroscopy

“…23,27 Better prediction of soil properties using mid-IR spectra of finely, rather than coarsely, crushed samples has also been reported. 50 Indeed, whatever the spectral range, fine crushing homogenises the sample and reduces light diffusion, which both allow better predictions. This is particularly true for mid-IR spectroscopy because the aperture of the beam is often around 1-2 mm in diameter.…”

“…This is particularly true for mid-IR spectroscopy because the aperture of the beam is often around 1-2 mm in diameter. 50 Comparisons between predictions of soil properties made using NIR vs mid-IR spectra have led to somewhat contradictory reports in the literature. As mid-IR spectra seem much more informative a priori than NIR spectra due to their numerous peaks relating to fundamental bond vibrations, it has been considered that they should logically yield more accurate predictions, and a number of papers have supported this view, e.g.…”

Proximal sensing for soil carbon accounting

“…Where bulk densities differ between management practices or across time periods, more accurate estimates of the C stock and its change can be derived using measures of cumulative or equivalent soil masses per unit area (Wendt and Hauser, 2013). Various studies have recognized the importance of this approach, which also reduces the effect of depth of sampling errors (Ellert et al, 2001;Gifford and Roderick, 2003;Lee et al, 2009;Vanden-Bygaart and Angers, 2006;Wendt and Hauser, 2013). Both measurement-based methods under the Australian ERF (Table 1; Australian Government, 2014, 2018) use an equivalent soil mass (ESM) approach to quantify soil organic C stock change.…”