『Abstract
There has been growing interest in the use of diffuse infrared
reflectance as a quick, inexpensive tool for soil characterization.
In studies reported to date, calibration and validation samples
have been collected at either a local or regional scale. For this
study, we selected 3768 samples from all 50 U.S. states and two
tropical territories and an additional 416 samples from 36 different
countries in Africa (125), Asia (104), the Americas (75) and Europe
(112). The samples were selected from the National Soil Survey
Center archives in Lincoln, NE, USA, with only one sample per
pedon and a weighted random sampling to maximize compositional
diversity. Applying visible and near-infrared (VNIR) diffuse reflectance
spectroscopy (DRS) to air-dry soil (<2 mm) with auxiliary predictors
including sand content or pH, we obtained validation root mean
squared deviation (RMSD) estimates of 54 g kg-1 for
clay, 7.9 g kg-1 for soil organic C (SOC), 5.6 g kg-1
for inorganic C (IC), 8.9 g kg-1 for dithionate-citrate
extractable Fe (FEd), and 5.5 cmolc kg-1
for cation exchange capacity (CEC) with NH4
at pH = 7. For all of these properties, boosted regression trees
(BRT) outperformed PLS regression, suggesting that this might
be a preferred method for VNIR-DRS soil characterization. Using
BRT, we were also able to predict ordinal clay mineralogy levels
for montmorillonite and kaolinite, with 88% and 96%, respectively,
falling within one ordinal unit of reference X-ray diffraction
(XRD) values (0-5 on ordinal scale). Given the amount of information
obtained in this study with 〜4×103 samples, we anticipate
that calibrations sufficient for many applications might be obtained
with large but obtainable soil-spectral libraries (perhaps 104-105
sample). The use of auxiliary predictors (potentially from complementary
sensors), supplemental local calibration samples and theoretical
spectroscopy all have the potential to improve predictions. Our
findings suggest that VNIR soil characterization has the potential
to replace or augment standard soil characterization techniques
where rapid and inexpensive analysis is required.
Keywords: Diffuse reflectance spectroscopy; VNIR; PLS regression;
Clay mineralogy; Boosted regression trees; Soil characterization
』
1. Introduction
1.1. Fundamentals of VNIR-DRS
1.2. Modeling approaches
1.2.1. Boosted regression trees
1.2.2. PLS regression
1.3. Study objectives
2. Methodology
2.1. Sampling design
2.2. Spectral scanning and processing
2.3. Modeling and fit assessment
2.3.1. Mineralogy
2.3.2. Composite soil properties
2.3.3. Assessment lab accuracy and precision for soil C
3. Results
3.1. Clay mineralogy
3.2. Composite soil properties
3.2.1. Summary of soil properties
3.2.2. General results
3.2.3. Validation results for specific properties
3.3. Important wavelengths
3.4. Laboratory soil C determination
4. Discussion
4.1. BRT vs. PLS regression
4.2. Appropriate target parameters
4.3. Auxiliary predictors
4.4. Calibration requirements
4.5. Future development
5. Conclusions
Acknowledgements
References