Exploring relevant wavelength regions for estimating soil total carbon contents of rice fields in Madagascar from Vis-NIR spectra with sequential application of backward interval PLS

Figures & data

Figure 1. Location of study area and soil sampling points. Source for background images in (a), (b) and (d): Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AeroGRID, IGN, and the GIS User Community

Table 1. Descriptive statistics of soil TC data

Soil parameter	n	Min	Max	Median	Mean	SD	CV
TC (%)	162	0.65	10.15	2.70	3.05	1.72	56.40

Figure 2. (a) Box-and-whiskers plot with outliers and (b) a histogram of soil TC

Figure 3. Raw reflectance spectra (a) and FDR spectra on a log10 scale (b) of the soil samples

Table 2. Selected wavelength regions from dynamic biPLS for estimating the soil TC content using reflectance and FDR data sets and possible soil components

Selected wavelength region (nm)		Previously known wavebands related to soil components
Reflectance	FDR	Waveband (nm)	Soil component	Reference
400–490		420, 427, 480	Goethite	Scheinost et al. (1998), Sherman and Waite (1985)
			Fe3+, ferric oxide	Hunt (1977)
	652–687	570–700	Organic matter	Galvao and Vitorello (1998)
		660	Goethite	Stenberg et al. (2010)
		665	Organic matter, chlorophyll pigment	Ben-Dor et al. (1997)
		682	Hematite	Scheinost et al. (1998)
1402–1440	1322–1443	1367, 1358	OH in water	Ben-Dor et al. (1997)
		1400	Molecular water, first overtone of O–H stretch, Al–OH or Mg–OH	Hunt (1977)
		1400 (1395, 1415)	Kaolinite, overtones of the O–H stretch vibration near 2778 nm, smectite (structural O–H stretching), combination vibrations of water in interlayer lattice	Bishop et al. (1994)
		1415	OH, overtones of O–H stretch vibrations near 2778 nm	Oinuma and Hayashi (1965)
		1449	Carboxylic acid (C=O)	Clark et al. (1990), Clark (1999)
1846–1980	1856–1985	1870	Carbonates	Hunt (1977), White (1971)
		1875	Water	Hunt (1977)
		1900	Smectite, combination vibrations of water in interlayer lattice	Bishop et al. (1994)
		1900, 1906	Molecular H2O	Hunt (1977)
2151–2283		2160	Al-OH, kaolin	Oinuma and Hayashi (1965)
		2169	Starch, lignin, wax, tannins	Ben-Dor et al. (1997)
		2180	Organic matter	Daniel et al. (2003)
		2193	Amid II, protein	Ben-Dor et al. (1997)
		2199, 2206	Illite	Post and Noble (1993)
		2200	Al–OH bend with O–H stretch combinations (kaolinite, montmorillonite, illite, smecite)	Clark et al. (1990), Post and Noble (1993)
		2206	O–H stretch (smecite, illite)	Oinuma and Hayashi (1965)
		2240	protein	Fourty et al. (1996)
		2279	CH2, CH3	Ben-Dor et al. (1997)
		2275	Aliphatics (C–H)	Clark (1999)
	2290–2400	2290	Fe–OH	Clark et al. (1990), Post and Noble (1993)
		2300	Mg–OH	Clark et al. (1990), Post and Noble (1993)
		2300, 2350	C–H stretch fundamentals	Clark et al. (1990)
		2307–2469	Methyls	Clark (1999)
		2347	Aliphatic C–H, cellulose, lignin, glucan	Eyal Ben-Dor et al. (1997)
		2380	Lignin	Fourty et al. (1996)
		2380	Muscovite	Post and Noble (1993)
		2381	Carbohydrates (C–O)	White (1971)

Figure 4. Selected wavelength regions (red bars) from dynamic biPLS for estimating the TC content of paddy soils using reflectance (a) and FDR (b) spectra with the frequencies (count number (N); blue line) of the selected wavebands in dynamic biPLS. Specific absorption bands for the different bonds in soil are specified in the top x-axis (modified by Katuwal et al. (2018))

Table 3. The mean R² and RMSE from the 5-fold cross-validation using the training data sets based on PLS analyses and the R², RMSE and RPIQ based on the model applied to the test data sets

		Training data (n = 120)			Test data (n = 42)
Spectral data type	Regression method	NLV^a	R^2	RMSE	R^2	RMSE	RPIQ	NW^b	NW%^c
Reflectance	FS-PLS	10	0.863	0.606	0.826	0.809	2.344	2001	100.0
	biPLS	12	0.872	0.585	0.877	0.690	2.748	398	19.9
FDR	FS-PLS	8	0.932	0.425	0.933	0.518	3.658	2001	100.0
	biPLS	10	0.947	0.377	0.940	0.494	3.841	399	19.9

^aNumber of latent variable.

^bNumber of wavebands.

^cNumber of wavebands percentage of all available bands.

Figure 5. Observed and predicted soil TC contents from the FS-PLS (blue) and dynamic biPLS (red) models using original reflectance (a) and FDR (b) data

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