Performance of field‐scale lab vs in situ visible/near‐ and mid‐infrared spectroscopy for estimation of soil properties

Abstract

Comparison of laboratory versus in situ visible/near‐ (visNIR) and mid‐infrared (MIR) spectroscopy for prediction of various soil properties is required to demonstrate trade‐offs between accuracy and efficiency. Field measurements were made on an arable field in Germany (silt loam Haplic Luvisol) using visNIR (ASD FieldSpec 3 Hi‐Res) and MIR (Agilent Technologies 4300 Handheld FTIR) and material was collected for lab visNIR (Foss XDS Rapid Content Analyzer) and MIR (Bruker‐TENSOR 27) measurements on dried and ground soil and determination of total, labile (>63 μm light), stabilized (>63 μm heavy + <63 μm oxidizable) and resistant organic carbon (OC) content, total nitrogen (Nt), pH and texture. Partial least squares regression models were calculated for five repeated partitions of the dataset (n = 238) into training (75%) and test (25%) sets. Lab spectral models outperformed in situ models for total OC (root mean squared error [RMSE] = 0.24–1.0 g kg−1), Nt (RMSE = 0.026–0.088 g kg−1), pH (RMSE = 0.12–0.28) and texture (RMSE = 0.53–1.5%). For both lab and field spectra, the accuracy of visNIR models was comparable or slightly better than MIR for sand, silt and clay. Spectral estimations for labile (RMSE = 0.34–0.47 g kg−1) and stabilized OC (RMSE = 0.41–0.85 g kg−1) were slightly (lab spectra) to substantially (field spectra) inferior to estimations from multiple linear regressions using total OC, Nt, clay and pH as predictors. Variable importance in the projection scores elucidated differences in spectral prediction mechanisms by spectrometer and OC fraction, and found mineral spectral signatures were highly important. For this field‐scale study with 14% median soil gravimetric water content (GWC), the loss of accuracy from lab to field measurement was lower for visNIR than MIR. Analysis of the driest soils (<9% GWC) found field MIR outperformed field visNIR for OC and Nt estimation and vice versa for the wettest soils (>18%), demonstrating the moisture dependence of performance rankings. Highlights Lab vs field visible/near‐ (visNIRS) and mid‐infrared (MIRS) spectroscopy require comparison for prediction of soil C fractions, N, pH and texture. Lab MIRS prediction of C, N and pH were superior, while texture estimations were comparable or slightly inferior to lab visNIRS. At 14% median soil water content, the loss of accuracy from lab to field measurement was lower for visNIRS than MIRS. The ranking of field visNIRS vs MIRS performance for C and N estimation is moisture dependent.

The cost‐effectiveness of reflectance spectroscopy for estimating soil organic carbon

“…Measurements with portable vis-NIR (vis-NIR p ) and mid-IR (mid-IR p ) spectrometers can be made in the laboratory on air-dried and ground soil samples and on wet soil under field conditions (Dhawale et al, 2015;Greenberg et al, 2021;Hutengs et al, 2019;Ji et al, 2016;Li et al, 2015). Hand-held vis-NIR p and mid-IR p spectrometers are available from several manufacturers, and they are becoming increasingly affordable.…”

Estimating soil chemical properties by diffuse reflectance spectroscopy: Promise versus reality

“…In the last two decades, interest in using diffuse reflectance visible and near-infrared (VNIR) spectroscopy of soils to replace the more laborious standard laboratory methods to determine soil physical and chemical properties has grown as evidenced by the steadily increasing number of published studies on this topic (Ahmadi et al, 2021;Bellon Maurel et al, 2010;de Souza et al, 2020;Gates, 2018;Greenburg et al, 2021;Nocita et al, 2015;Pinheiro et al, 2017;Soriano-Disla et al, 2014;Stenberg et al, 2010). Numerous authors have pointed to the advantages that reflectance spectroscopy presents over standard chemical tests of soils, specifically simplicity of soil sample pretreatment, lack of chemical reagents and chemical waste, rapidity and low cost (Mohamed et al, 2018;Zornoza et al, 2008), implying that conventional soil testing procedures can be replaced by this spectroscopic method.…”

A Method to Evaluate Spectral Analysis by Spectroscopy

“…Obviously, spectrometers play an important role in spectral analysis [ 10 , 14 , 15 , 16 , 17 , 18 , 19 ]. During the past three years, many studies have investigated the influences of spectrometers on the spectral analysis of SOC ( Table 1 ).…”

“…The spectral model derived from FieldSpec 3 had higher R 2 and RPD, and lower RMSE than that from STS, and the difference became smaller outdoors compared with the laboratory ( Table 1 ) [ 15 ]. The results also showed that laboratory spectrometers had a better performance than the field ones due to their technical advantages in terms of signal-to-noise ratio, stability of the measurement conditions, spectral resolution and wide spectral range [ 15 , 17 , 19 ]. For example, as mid-IR spectrometers, Vertex 70 (Bruker Optics, Ettlingen, Germany) and Agilent 4200 (Agilent Technologies, Santa Clara, CA, USA) are bench-top and portable, respectively [ 20 ].…”

“…However, spectrometers in the comparison studies had distinct classifications, belonging to near infrared and mid infrared instruments ( Table 1 ) [ 14 , 17 , 19 ]. As shown in Table 1 , Agilent 4200, Agilent 4300 (Agilent Technologies, Santa Clara, CA, USA) and Bruker are all mid infrared spectrometers (mid-IR); the others are near infrared spectrometers (near-IR).…”