Assessing mechanical aggregate resilience of forest soils using a laser diffractometer with and without pre-treatment 

Soil aggregates influence soil physical functioning, water retention, infiltration, and soil resistance to erosion and is thus important for a wide range of soil ecosystem services. However, the characteristics of soil particle aggregates and their stability in forest soils are not sufficiently understood. In this study, we investigate how soil physical and chemical properties influence mechanical aggregate resistance by combining laser diffraction grain size analyses with an extensive set of physical and chemical parameters.Particle size analyses (PSA) were conducted using a Malvern (Mastersizer 2000 and Mastersizer 3000E) laser diffractometer on 1862 soil samples from Austrian forest sites in Styria, Upper and Lower Austria, and Burgenland taken in four depth levels (0-10, 10-20, 20-50 and 50-80 cm). Each sample was analysed twice: once without ultrasonic pre-treatment, preserving naturally occurring aggregates, and once after 4 min ultrasonic dispersion (Bandelin Sonorex RK 255, 35 kHz) to achieve complete disaggregation. Differences between the two treatments provide a proxy for aggregate stability. In addition, the same samples were characterised by a comprehensive set of physical and chemical analyses, including density of mineral soil, pH, content of carbonate, organic carbon, and total nitrogen, C/N ratio, plant-available nutrients, exchangeable cations, base saturation, and selected trace elements. These parameters capture potential key physical and chemical controls on aggregation, such as carbonate cementation. In a next step, we will analyse differences between dispersed and non-dispersed PSAs statistically and relate them to chemical and physical soil properties in order to identify controls on aggregate resilience. We assume that in particular carbonate cementation and cation exchange capacity affect the mechanical stability of soil aggregates and their response to ultrasonic dispersion. By linking grain size distribution changes to soil chemistry, this study aims to improve the understanding of physical and chemical controls on soil aggregation and to highlight the importance of considering pre-treatment effects when interpreting laser diffraction data.   This work was carried out within the FORSITE I, FORSITE II and FORSITE II+ projects, funded by the Federal Ministry of Agriculture and Forestry, Climate and Environmental Protection, Regions and Water Management, Republic of Austria.