Abstract
Visible and near-infrared (Vis-NIR) diffuse reflectance spectroscopy with partial least squares (PLS) regression is a quick, cost-effective, and promising technology for predicting soil properties. The advantage of PLS regression is that all available wavebands can be incorporated in the model, while earlier studies indicate that PLS models include redundant wavelengths, and selecting specific wavebands can refine PLS analyses. This study evaluated the performance of PLS regression with waveband selection using Vis-NIR reflectance spectra to estimate the total carbon (TC) and total nitrogen (TN) in soils collected mainly from the surface of upland and lowland rice fields in Madagascar (n = 59; after outliers were removed). We used iterative stepwise elimination-based PLS (ISE-PLS) to estimate soil TC and TN and compared the predictive ability with standard full-spectrum PLS (FS-PLS). The predictive abilities were assessed using the coefficient of determination (R2), the root mean squared error of cross-validation (RMSECV), and the residual predictive deviation (RPD). Overall, ISE-PLS using first derivative reflectance (FDR) showed a better predictive accuracy than ISE-PLS for both TC (R2 = 0.972, RMSECV = 0.194, RPD = 5.995) and TN (R2 = 0.949, RMSECV = 0.019, RPD = 4.416) in the soil of Madagascar. The important wavebands for estimating TC (12.59% of all wavebands) and TN (3.55% of all wavebands) were selected from all 2001 wavebands over the 400–2400 nm range using ISE-PLS. These findings suggest that ISE-PLS based on Vis-NIR diffuse reflectance spectra can be used to estimate soil TC and TN contents in Madagascar with an improved predictive accuracy.
Highlights
Carbon (C) and nitrogen (N) contents in soils are two key parameters for sustaining soil and environmental quality, as well as for improving crop productivity because of their involvement in a number of natural processes related to soil health and fertility [1]
We investigated the performance of waveband selection in the spectral estimation of soil total carbon (TC) and total nitrogen (TN) using Visible and near-infrared (Vis-NIR) reflectance data
The results indicated that soil TC and TN in Madagascar can be more accurately estimated by ISE-partial least squares (PLS) than by standard full-spectrum PLS (FS-PLS) using laboratory Vis-NIR spectroscopy
Summary
Carbon (C) and nitrogen (N) contents in soils are two key parameters for sustaining soil and environmental quality, as well as for improving crop productivity because of their involvement in a number of natural processes related to soil health and fertility [1]. Standard procedures for assessing the state of C and N in soils are costly and time consuming [4,5] and require experienced operators Possible alternatives such as visible (Vis, 400–700 nm) and near-infrared (NIR, 700–2500 nm) spectroscopy are gaining attention; both of these alternatives have been widely accepted as fast and non-destructive methods for estimating soil properties [6,7]. These techniques measure the radiation absorbed by various bonds of O-H, C-H, N-H, C=O, C-N, N-H, or C=C, resulting in bending, twisting, stretching, or scissoring [8,9]. The infra-red PLS method of soil property predictions was shown to be well suited for the characterization of soils [13]
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