Divergent chemical compositions of soil organic matter size fractions under long-term amendments across a climate gradient

The chemical composition of soil organic matter (SOM) is the basis for its stabilization and functions. However, it is not clear how SOM chemical composition varies among size fractions under long-term fertilizations across a climate gradient. We explored this question in three soils: Ferralic Cambisol (subtropics), Calcaric Cambisol (warm temperate zone) and Luvic Phaeozem (mid-temperate zone) under five amendments: Control, N, NPK, NPK + Straw and NPK + Manure. The molecular composition of coarse particulate (cPOM, 250–2000 μm), fine particulate (fPOM, 53–250 μm) and mineral-associated OM (MAOM, <53 μm) were measured by pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). Among the three soils, the molecular composition of whole SOM diverged greatly, with much more relative abundances of aliphatic compounds and N-containing compounds in the Calcaric Cambisol and the Luvic Phaeozem, respectively. Within the Luvic Phaeozem (clay loam) and Ferralic Cambisol (clay), the molecular composition of SOM mainly varied among physical sizes, with selective preservations of aromatics and lignin in POM and enrichments of N-containing compounds in MAOM, but not amending regimes (even straw and manure incorporations). By contrast, the molecular composition of SOM changed little among physical sizes under various amendments in the Calcaric Cambisol (sandy loam). In conclusion, soil type, not quantity and quality of amendments, primarily regulated the molecular composition of SOM across the climate gradient. Moreover, the divergence of SOM molecular composition among physical size fractions was related to soil texture, with higher divergence in clay and clay loam but reverse in sandy loam soil.