Soil organic matter (SOM) 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 molecular changes of POM during decomposition at a sub-millimetre scale by combining direct measurements of POM elemental and molecular composition with laboratory imaging visible–near-infrared (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 imaging VNIR spectroscopy 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.

中文翻译：

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VNIR高光谱成像解析不同土壤深度分解POM的空间分子异质性

土壤有机质 (SOM) 由具有不同功能和反应性的组分组成。其中，颗粒有机物 (POM) 是土壤中新输入有机物的主要产物，其化学归宿对应于 SOM 分解级联的第一阶段，最终导致有机相和矿物相结合。我们旨在通过将 POM 元素和分子组成的直接测量与实验室成像可见 - 近红外 (VNIR) 光谱相结合，在亚毫米尺度上研究 POM 在分解过程中的分子变化。为此，我们建立了一个孵化实验来比较秸秆和堆肥绿肥的分子组成，这些材料的 C/N 比差异很大，在 Luvisol 的重组表土或底土中分解过程中，并在孵化开始和结束时以高空间和光谱分辨率记录完整土壤核心的高光谱图像。高光谱成像与机器学习集成成功结合，生成了 POM 烷基/ON 烷基比和 C/N 的精确映射，揭示了秸秆和绿肥成分的空间异质性。我们发现两种类型的有机改良剂在孵化后的表层土壤中比在底土中的降解程度更高。我们还测量了秸秆分子变化的一致趋势，烷基/ON 烷基比从 0.06 略微增加到 0.07，C/N 下降了约 40 个单位。绿肥材料非常不均匀，孵化后没有检测到明显的分子变化。