Soil organic matter is composed of fractions with different functions
and reactivity. Among these, particulate organic matter (POM) is the
main educt of new inputs of organic matter in soils and its chemical
fate corresponds to the first stages of the SOM decomposition cascade
ultimately leading to the association of organic and mineral phases. We
aimed at investigating the POM molecular changes during decomposition at
a sub-millimetre scale by combining direct measurements of POM elemental
and molecular composition with laboratory imaging VNIR spectroscopy. For
this, we set up an incubation experiment to compare the molecular
composition of straw and composted green manure, materials greatly
differing in their C/N ratio, during their decomposition in
reconstituted topsoil or subsoil of a Luvisol, and recorded
hyperspectral images at high spatial and spectral resolutions of
complete soil cores at the start and end of the incubation.
Hyperspectral imaging was successfully combined with machine learning
ensembles to produce a precise mapping of POM alkyl/O-N alkyl ratio and
C/N, revealing the spatial heterogeneity in the composition of both
straw and green manure. We found that both types of organic amendment
were more degraded in the reconstituted topsoil than in subsoil after
the incubation. We also measured consistent trends in molecular changes
undergone by straw, with the alkyl/O-N alkyl ratio slightly increasing
from 0.06 to 0.07, and C/N dropping by about 40 units. The green manure
material was very heterogeneous, with no clear molecular changes
detected as a result of incubation.
The visualisation approach presented here enables high-resolution
mapping of the spatial distribution of the molecular characteristics of
organic particles in soil cores, and offers opportunities to disentangle
the roles of POM chemistry and morphology during the first steps of the
decomposition cascade of organic matter in soils.