Clay fraction properties and grassland management imprint on soil organic matter composition and stability at molecular level

The dynamics of soil carbon in grassland are partly determined by soil organic matter (SOM) composition. However, it remains unclear which role grassland management plays in the interplay between SOM composition and carbon dynamics. Using pyrolysis-field ionization mass spectrometry (Py-FIMS), we studied the effect of meadow, mown pasture and pasture on the molecular SOM composition in German topsoils. In sandy soils of the Schorfheide-Chorin region, SOM composition and stability were strongly affected by clay contents and concentrations of crystalline Fe-oxides. Here, the grassland management type influenced lipid proportions, which accounted for a maximum of 11.1% of the total ion intensity (TII) under mown pasture. In the Hainich-Dun region, SOM composition was mainly related to the SOM decomposition stage (abundance of potentially recalcitrant compounds) but not to minerals. Compound classes of carbohydrates (4.3% TII), phenols and lignin monomers (8.5% TII), N-containing compounds (2.2% TII) and peptides (4.6% TII) were highest under meadow, while compound classes of lignin dimers (3.4% TII) and lipids (8.1% TII) were highest under pasture. In the Schwabische Alb region, the proportion of free fatty acids (1.6 to 2.3% TII) was positively related to the C/N ratio (r = 0.86); SOM stability was positively affected by poorly crystalline Fe-oxide content (r = 0.85). The results suggest that grassland management is affecting SOM composition and stability and thus influence SOM dynamics in grasslands. However, the proportion and composition (Fe-oxide content) of the soil clay fraction overrode grassland management effects if soil clay/OC ratios were <10.