For the cores amended with green manure, there was no shift in the mean predicted alkyl ratio and C/N after incubation (Table 2), a result of the very high coefficients of variance, which range from 55 to 82 % for the predicted alkyl ratio and from 41 to 52 % for C/N . Nevertheless, the distributions of the predicted green manure material statistically differed before and after incubation as well as between the topsoil and subsoil. The alkyl ratio of green manure decreased during the incubation at both depths, and was larger in the topsoil at both sampling times (Table 2). There were greater differences in the composition of green manure material admixed to the topsoil and subsoil before incubation than after incubation, with D reduced by around 50 % for the alkyl ratio and 75 % for the C/N ratio after incubation (Figure 2 and Table 3).

Fine-scale mapping of POM composition

The high resolution of the VNIR imaging allowed us to identify and characterise organic particles ranging from a few millimetres down to sub-millimetre scale (Figures 3 and 4). For both green manure and straw, their chemical composition, i.e. alkyl ratio and C/N, could be mapped at a very fine scale, with the maps revealing the spatial heterogeneity of both indicators for the two types of OM. This heterogeneity could be seen in both very small particles of a few hundreds of µm², as well as within large particles of several mm². Even in the case of straw, which was a lot more homogeneous than green manure, the chemical maps highlighted the spatial differences in the molecular composition.