Comparing wet and optimal moisture sieving for soil aggregate fractionation: impacts on C and N pools and microbial activity

The turnover and content of carbon (C) and nitrogen (N) in soil aggregates are influenced by land use and soil texture, with fractionation methods further contributing to differences between macro- and microaggregates by altering their composition and microbial activity. Our work investigated total and available C and N pools, alongside enzyme activities related to C and N cycling, in aggregates separated by wet and optimal moisture sieving from grassland and cropland Luvisol soils with sandy and loamy textures. The following incubation, with and without the ¹⁴C-glucose addition during 30 d, aimed to reveal the differences in the utilization strategies of soil organic matter by microorganisms inhabiting macro- and microaggregates. Optimal moisture sieving preserved higher dissolved N and peroxidase activity in macroaggregates, and 2.14 times higher microbial biomass N in microaggregates compared to wet sieving. Grasslands had higher C and N pools and associated enzyme activities than croplands, while loamy soils outperformed sandy soils in nutrient retention and microbial activity support. Wet sieving altered the relative content of -C=O and C-H bonds, especially in sandy soils. Sandy and loamy soils differed in the intensity of CO₃²⁻, SiO₂, and Al-OH bonds of clay minerals; still, they showed no differences in the functional groups of organic compounds. Optimal moisture sieving was more effective in preserving the available N pool inside. Loamy soils and grasslands demonstrated higher N and biological activity levels than sandy soils and croplands. This study underscores the critical role of fractionation methods, particularly optimal moisture sieving, in preserving soil nutrients and microbial activity, which is essential for understanding nutrient cycling in different ecosystems.