Comparison between predictions of C and N contents in tropical soils using a Vis–NIR spectrometer including a fibre-optic probe versus a NIR spectrometer including a sample transport module

International audience Increasing attention is being paid to near-infrared reflectance (NIR) spectroscopy for the rapid and cost-effective determination of soil carbon (C) and nitrogen (N) contents. The objective of the present paper was to compare the performances of two spectrometers: one covered the visible and NIR ranges (Vis-NIR, 350-2500 nm) and included a fibre-optic probe and the other covered the NIR range only (1100-2500 nm) and included a sample transport module. The comparison was carried out on two sets of clayey (n = 97) and sandy (n = 72) soil samples from tropical Africa and America.On the whole, both technologies provided good calibrations (R²>0.74) and predictions (R²>0.62) of soil C and N contents. The most accurate calibrations were achieved with the NIR spectrometer (R²>0.86), which also yielded the most accurate predictions for the sandy soils (R² = 0.90 and standard error of prediction < 15% of the mean). For the clayey soils, the best predictions of both spectrometers were similar (R² >= 0.68 and standard error of prediction <20% of the mean). Using first or second derivatives of spectra did not affect calibration but had an impact on validation.