Molecular Composition of Soil Organic Matter Fractions Under Long‐Term Post‐Agricultural Restoration Across a Large Climate Gradient

ABSTRACT Long‐term post‐agricultural natural restoration generally rebuilds the pool of soil organic matter (SOM); however, it is not clear about the shifts in molecular composition within SOM physical fractions across a large climate gradient. We investigated this question in three zonal soils: Ferralic Cambisol (subtropic), Calcaric Cambisol (warm temperate), and Luvic Phaeozem (mid‐temperate) under 27‐year post‐agricultural restoration. Pyrolysis‐gas chromatography/mass spectrometry (Py‐GC/MS) is a useful method for analyzing SOM molecular composition. In this research, molecular features of coarse particulate organic matter (cPOM), fine particulate organic matter (fPOM), and mineral‐associated organic matter (MAOM) were measured by Py‐GC/MS. Among the three zonal soils, SOM molecular compositions varied greatly. The Luvic Phaeozem contained a greater relative abundance of N‐bearing compounds, while the Calcaric Cambisol had a greater relative abundance of aliphatics. Within the Luvic Phaeozem and Ferralic Cambisol with higher clay contents, physical size class, not post‐agricultural restoration, primarily shifted the molecular feature of SOM, with more lignin derivatives and aromatics in the POM, while enrichment of N‐bearing compounds in the MAOM. In contrast, both physical size class and post‐agricultural restoration had limited influences on SOM molecular composition in the Calcaric Cambisol with lower clay content. In conclusion, the molecular composition of bulk SOM was dependent on soil types across the climate gradient, but not on post‐agricultural restoration. Physical size class greatly shifted the molecular feature of SOM (especially for N‐bearing compounds) in soils with high clay contents, but not for the soil with low clay contents.