Abstract
Diffuse reflectance spectroscopy (DRS) has the potential to predict soil organic carbon (SOC). However, it is still little used as a matter of routine in soil laboratories in Brazil. The objective of this study was to make evaluations as to whether SOC predicted by spectral techniques can replace measurement by routine chemical methods with no loss in quality and be applied in the recommendation of nitrogen fertilizer as well as identifying the best prediction strategies to use. A data set containing 2,471 samples from six soil spectral libraries (SSL) was used to develop spectroscopic models for SOC content prediction, including consideration of sample stratification and preprocessing techniques. The SOC was quantified through the analytical-chemical methods of wet combustion with determination by titration, designated as the reference method (REM), and colorimeter, designated as the routine method (ROM in an independent data set). SOC contents predicted by the spectral analysis method (SAM) were compared to the REM and ROM results, converted to soil organic matter (SOM) and used for N recommendations. The best estimate for SOM content using the SAM was achieved through stratification of the SSL and application of the standard normal variate (SNV) preprocessing. The SOC predicted by spectral techniques proved capable of replacing the SOC measured by routine chemical methods with no loss of quality and supported by an appropriate nitrogen fertilizer recommendation, provided the models met the conditions and possessed the characteristics of the samples to be analyzed.
Highlights
Soil organic carbon (SOC) is fundamental to a profitable and sustainable agricultural system as an indicator of soil quality (Global Soil Partnership, 2017; Bailey et al, 2017)
The soil organic carbon (SOC) predicted by spectral techniques proved capable of replacing the SOC measured by routine chemical methods with no loss of quality and supported by an appropriate nitrogen fertilizer recommendation, provided the models met the conditions and possessed the characteristics of the samples to be analyzed
Most of the samples have between 1 and 4 % SOC (Figure 3A), values commonly found in subtropical soils in the south of Brazil (Boddey et al, 2010) while SOC values above 8 %, characteristic of organic soils, are uncommon
Summary
Soil organic carbon (SOC) is fundamental to a profitable and sustainable agricultural system as an indicator of soil quality (Global Soil Partnership, 2017; Bailey et al, 2017). Among different chemical analytical methods employed by the soil analysis laboratories to determinate SOC/SOM, it is very common to use those based on soil carbon oxidation by a sulfochromic solution, followed by spectrophotometric colorimeter determination (Tedesco et al, 1995). This is based on the original procedure of Walkley and Black (1934), using hazardous chemical reagents, such as the chromium solution, and are relatively laborious and time-consuming to execute. Studies have addressed the DRS technique when estimating the nitrogen content (Li et al, 2015; Padmanabhi and Deshmukh, 2017; Conforti et al, 2018)
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