Fundamental soil characteristics Soil samples

2.4.4.2. Diffuse Reflectance Spectroscopy
A constant volume of fine ground GSK-3 Inhibitor IX sample was used for DRS analysis. The use of correct settings for acquisition of diffuse reflectance spectra of solids (integrating sphere) guaranteed obtaining robust and reproducible results. Diffuse Reflectance Spectroscopy in the near IR–vis–UV region was measured using a Perkin Elmer Lambda 35 spectrometer with an integrating sphere (Labsphere), a scanning step of 0.5 nm, and a slit width of 2 nm. Standard tungsten and deuterium lamps were used as the source of vis and UV light, and BaSO4 powder was used as a white reference. Reflectance R∞ (%) was recalculated to absorbance A in Kubelka–Munk units using the formula: A = [1 − (R∞ / 100)]2 / (2 ∗ R∞ / 100).
2.5. Statistical analyses
Soil properties (pH, Cox, clay — texture < 0.002 mm, CEC and iron forms— Feex, Feo, Fed, Fec) which contribute significantly to explanation of the variation of individual Fe forms were identified using a general linear model. Significant soil properties together with microsite (pit, mound and control), age and soil depth were then used as inputs for linear mixed effect (LME) models fitted by restricted maximum likelihood (REML). Finally, the significance of microsite, age and soil depth was assessed using the likelihood ratio test. All computations were made in R software (R Development Core Team, 2013). General linear models were fitted with lm function. lme4 R package, particularly the lmer function, was used for model fitting ( Bates et al., 2014).