Chemical Characteristics of Physically Separated Soil Organic Matter Fractions in Contrasting Arable Soils

Abstract

Particle size–based macro organic matter (OM) (>0.05 mm) and density-based light OM (<1.8 g ⋅ cm⁻³) fractions are often used as indicators of short-term changes in soil OM status as affected by agronomic practices, but their differences in chemical composition have not been clearly addressed. This study aimed to identify compositions of noncellulosic and amino sugars of macro OM and light OM in contrasting soils. Four loamy soil types were sampled from four respective areas characterized by different climates, parent materials, and cropping systems. Dense particulate OM (POM, density <1.8 g ⋅ cm⁻³) was directly extracted from the bulk soil, whereas coarse POM (particle size >0.05 mm and density <1.8 g ⋅ cm⁻³) was obtained after separating the heavy macro OM fraction (particle size >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.