The NIRS spectra of the root samples as well as the calibration mixtures were analysed with a FTIR Spectrometer (Shimadzu IR Prestige-21, Kyoto, Japan), equipped with an integrating sphere (NIR IntegratIR, PIKE Technologies, WI, USA). Each spectrum was recorded as the average of 64 scans over the range between 3800 and 10000 cm-1 (i.e. 1000–2600 nm) with a resolution of 8 cm-1. Five spectra were taken from each sample and averaged before further statistical analyses. Calibration mixtures were prepared for single species and different combinations of species with incremental additions of approx. 10 % of the respective species components. Data analyses of the NIRS spectra were performed by partial least squares (PLS) regression using The Unscrambler X (version 10.1, Camo Software AS, Oslo, Norway). Mathematical pre-treatments during data analyses comprised no treatment, 1/x weighting of the original x-data, first and second derivative of Savitzky-Golay smoothing, as well as mean and peak normalisation. The part of the spectrum below 1200 nm was omitted due to background noise. The analyses included multivariate calibration and full cross-validation.

Samples taken in August and September 2011