Chemical characteristics of physically separated soil organic matter fractions in contrasting arable soils

Particle size-based macro organic matter (OM) (>0.05 mm) and density-based light OM (0.05 mm and density 0.05 mm and density >1.8 g × cm -3) from the macro OM fraction that was previously wet sieved from the bulk soil. Dense POM and coarse POM differed in their fraction mass, organic C content, C/N ratio, and contributions to the total soil OM and the magnitudes of these differences changed with soil types. Across four soil types, 13% to 22% macro organic C was present in the heavy macro OM fraction under a complete dispersion of 2 to 0.05 mm aggregates. Noncellulosic carbohydrates were identical and mainly plant-derived compounds in dense and coarse POM fractions but were dominated by microbially derived monomers in heavy macro OM fraction. Amino sugars in both POM fractions were dominated by fungal-derived compounds, whereas heavy macro OM fraction contained fungal-amino sugars only. Macro OM had contents of total noncellulosic sugars and total amino sugars similar to those of dense POM (i.e., light OM), but macro OM contained more microbially synthesized neutral sugars and fungal-amino sugar N than dense POM. In addition, macro OM and dense POM were not parallel soil OM fractions, in terms of recovered amounts of organic C and N, neutral carbohydrates, and amino sugars from contrasting soils. Our results suggested that macro OM and light OM fractions responded differently in different soil types