Studying the Physical Protection of Soil Carbon with Quantitative Infrared Spectroscopy

Abstract

Near infrared (NIR) and mid-infrared (mid-IR) reflectance spectroscopy are time-and cost-effective tools for characterising soil organic carbon (SOC). Here they were used for quantifying (i) carbon (C) dioxide (CO 2 ) emission from soil samples crushed to 2 mm and 0.2 mm, at 18°C and 28°C; (ii) physical C protection, calculated as the difference between CO 2 emissions from 0.2 mm and 2 mm crushed soil at a given temperature; and (iii) the temperature vulnerability of this protection, calculated as the difference between C protection at 18°C and 28°C. This was done for 97 topsoil samples from Tunisia, mostly calcareous, which were incubated for 21 days.Soil CO 2 emission increased with temperature and fine crushing. However, C protection in 0.2-2 mm aggregates had little effect on the temperature vulnerability of CO 2 emission, possibly due to preferential SOC protection in smaller aggregates. In general, NIR spectroscopy, and to a lesser extent mid-IR spectroscopy, yielded accurate predictions of soil CO 2 emission (0.60 ≤ R 2 ≤ 0.91), and acceptable predictions of C protection at the beginning of incubation (0.52 ≤ R 2 ≤ 0.81) but not over the whole 21 day period (R 2 ≤ 0.59). For CO 2 emission, prediction error was the same order of magnitude as, and sometimes similar to, the uncertainty of conventional determination, indicating that a noticeable proportion of the former could be attributed to the latter. The temperature vulnerability of C protection could not be modelled correctly (R 2 ≤ 0.11), due to error propagation. The prediction of SOC was better with NIR spectroscopy and that of soil inorganic C (SIC) was very accurate (R 2 ≥ 0.94), especially with mid-IR spectroscopy. Soil CO 2 emission, C protection and its vulnerability were best predicted with NIR spectra, those of 0.2 mm samples especially. Mid-IR spectroscopy of 2 mm samples yielded the worst predictions in general. NIR spectroscopy prediction models suggested that CO 2 emission and C protection depended (i) on aliphatic compounds (i.e. labile substrates), dominantly at 18°C; (ii) on amides or proteins (i.e. microbial biomass), markedly at 28°C; and (iii) negatively, on organohalogens and aromatic amines (i.e. pesticides). Models using mid-IR spectra showed a negative influence of carbonates on CO 2 emission, suggesting they did not contribute to soil CO 2 emission and might form during incubation. They also suggested that CO 2 emission and C protection related to carboxylic acids, saturated aliphatic ones especially.

Infrared spectroscopy approaches support soil organic carbon estimations to evaluate land degradation

“…Many papers have reported better prediction of SOC concentration using MIR than (V)NIR spectra (Dong et al, 2011;Igne et al, 2010;McCarty et al, 2002;Reeves et al, 2002;Reeves, McCarty, & Reeves, 2001;Viscarra Rossel, Walvoort, McBratney, Janik, & Skjemstad, 2006;Xie, Yang, Drury, Yang, & Zhang, 2011); but it is worth noting that they concerned temperate regions (except Dong et al, 2011), and rather homogeneous soil sample sets in general. The trend was less clear in other studies (Madari et al, 2005;Yang et al, 2012), while some even reported better NIRS than MIRS predictions (Madari et al, 2006;Ludwig, Nitschke, Terhoeven-Urselmans, Michel, & Flessa, 2008;Rabenarivo et al, 2013;Barthès et al, 2016;and also Shao & He, 2011, for N concentration), regarding tropical and subtropical regions (except Ludwig et al, 2008) and more diverse sample sets.…”

“…The supposedly superior performance of MIRS has been attributed to the richer information it provides (cf. Importantly, Barthès et al (2016) reported noticeably poorer MIRS predictions using spectra acquired on 2-than on 0.2-mm samples; thus MIRS requires fine grinding, which is somewhat tedious and contradicts the time-and cost-effectiveness of the approach. In contrast, these authors found little difference in SOC prediction accuracy when NIR spectra were acquired on 2-vs. 0.2-mm samples.…”

“…Figure 1), which explains why the former have long been used for molecule identification, based on visual observation. For instance, the regions around 2,750-2,670 and 1730-1710 cm −1 have been assigned to saturated aliphatic carboxylic acids, and the region around 1,650 cm −1 to amides (Barthès et al, 2016;Socrates, 2001). Such peak recognition is hardly possible with near-infrared spectra (except for water, cf.…”

“…Such peak recognition is hardly possible with near-infrared spectra (except for water, cf. Figure 1), though spectral regions could also be assigned to chemical compounds (Barthès et al, 2016;Workman & Weyer, 2008). In general statistical tools are required for analyzing near-infrared spectra, hence the more recent applications of this spectral range for analytical purposes.…”

Guest Editorial: Near Infrared Spectroscopy for a Better Understanding of Soil

“…While applications of NIR spectroscopy for assessments of soil organic matter date back to earlier work in the 1980s 1 the papers in this issue bring new insights to its potential benefits. For example, Barthès et al 2 report a detailed comparison of the benefits of NIR and mid-infrared spectra to analyse soil organic and inorganic carbon and suggest how temperature and sample preparation influence the protected carbon in a soil sample.…”