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.
<p><span>O</span><span>rganic matter added to agricultural soil </span><span>determines</span><span> the C balance and the nutrient cycling in these ecosystems. Organic fertilisation can result </span><span>in</span><span> the accumulation of </span><span>C in soil but can also stimulate the decomposition of the existing </span><span>soil </span><span>C pool, as the incorporation of an easily accessible energy-rich substrate often trigger the growth and activity of decomposer. We monitor</span><span>ed</span><span> the fate of two types of organic material (wheat straw and green manure) during </span><span>the first stages of </span><span>their decomposition in</span><span>to</span><span> the soil. For this, we incubated 1-m soil columns </span><span>amended</span> <span>with the two </span><span>organic fertilisers either into the topsoil or into the subsoil. We </span><span>measured changes</span><span> in C and N contents, and </span><span>used <sup>13</sup>C-NMR to resolve the </span><span>structural group composition of the added organic material. We also </span><span>scanned </span><span>the incubated samples </span><span>with a hyperspectral camera </span><span>and developed predictive models for C to N and for alkyl to O-alkyl ratios at a very fine spatial resolution (53 x 53 </span><span>&#181;m</span><sup><span>2</span></sup><span> per pixel</span><span>) for </span><span>organic particles in </span><span>the whole soil cores.</span></p><p><span>T</span><span>he approach based on hyperspectral imaging</span><span> was </span><span>successful</span><span> to follow the decomposition dynamics of POM </span><span>during</span><span> the incubation, and the associated d</span><span>ecreases</span><span> in C to N and in</span><span>creases in</span><span> alkyl to O-alkyl ratios </span><span>at a very fine spatial resolution, showing how different part</span><span>s</span><span> of the organic particles underwent distinct decomposition. </span><span>We also observed </span><span>contrasting</span><span> decomposition dynamics between the wheat straw and the green manure. </span><span>This method can bring new information about the first steps of fresh organic matter decomposition in soils and develop our general understanding of the soil organic matter decomposition continuum.</span></p>
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