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

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.