Materials and methods
Soil sample and incubation experiment
The soil used for the incubation experiment was sampled from an agricultural trial field at ‘Campus Klein-Altendorf’ experimental research station (
50°37’16”N; 6°59’53”E), University of Bonn, Germany. The mean annual air temperature at
Bonn-Rohleber is 10.3 °C and the mean annual precipitation is 816 mm for the period 1991-2010. The soil is a Haplic Luvisol
\cite{iussworkinggroupwrbWorldReferenceBase2015} derived from quaternary Loess deposits, with a pH
water of 6.5 and a texture dominated by silt particles, with an enrichment in clay with depth (clay/silt/sand ⋍ 25-43/50-68/4-6 %). The organic C and total N contents are respectively 8.0 and 1.1 mg g
-1 in the topsoil and 3.0 and 0.5 mg g
-1 in the subsoil. Two soil depths (0-30 cm, and 30-60 cm), hereafter named topsoil and subsoil corresponding to the ploughed layer and the non-ploughed upper subsoil, were collected using a backhoe and directly sieved to 5 cm.
Soil cores were prepared in 90-cm high PVC pipes (⌀=7.5 cm) by inserting, from bottom to top, 20 cm of quartz sand, 30 cm of subsoil, 5 cm of mixed topsoil and subsoil, and 25 cm of topsoil. Soils were amended with two types of organic materials, wheat straw and green manure, added either to the topsoil or to the subsoil. Straw material corresponds to harvest residues whereas the composted green manure was a mixture of green waste (trees, bushes and shrubs) from public green areas and parks.
The organic material was incorporated by mixing it with the soil (1:4 volume mixing ratio for organic:soil) before packing the cores. Sixteen soil cores were prepared, corresponding to duplicates of the two types of organic material amendments (straw or green manure), admixed into one of the two soil depths (denoted top- or sub- ) for initial sampling (T0) and after 180 days (T1) of incubation at 20 °C and 60 % maximum water-holding capacity determined gravimetrically.
VNIR hyperspectral imaging
Before recording hyperspectral images, soil cores were cut lengthwise, from bottom to top, into two equal halves and then dried at room temperature. For imaging, samples were illuminated with two 150-W halogen lamps. Hyperspectral images were recorded using a Hyspex VNIR-1800 camera (Norsk elektro optikk, Norway) after automatic dark background correction. The sensor was equipped with a 30-cm lens, giving a final field of view of approximately 9 cm for the 1800 detectors (53 × 53 µm2 per pixel). For each pixel, light reflectance intensity was measured for 186 bands in the region 400-990 nm (spectral resolution of 3.17 nm per band).
To account for potential unevenness in illumination and spectral response at different horizontal locations in the core, the spectral intensity (I) of the raw images of the soil cores were normalised to the defined reflectance (R) of calibration target for each wavelength (λ) and pixel (x):