Fine Earth Aggregates and Aggregating Particles at the Calhoun Critical Zone Observatory.

Soil aggregation influences nearly all soil and Earth surface properties and processes. Although physical and chemical interactions between individual primary particles serve as the building blocks of soil aggregation, quantifying and characterizing these micron scale process is limited by traditional analyses that use sieves, shakers, centrifuges, and balances. We have developed a straightforward and relatively rapid method with laser diffraction technology to characterize the physical properties of aggregates in the fine Earth fraction of soil (referred to as “fine Earth aggregates”) and the individual aggregating particles. We analyzed fine Earth aggregates (FEAs) and aggregating particles (APs) in 89 samples from a variety of soil and subsoil horizons in eleven soil pits at the Calhoun Critical Zone Observatory. We detect FEAs in 72% of the samples, ranging in size from 21 to 864 μm in diameter. Variation in both FEA and AP size varies significantly (p=0.05) by soil horizon. FEAs can occlude more than 80% of the total soil surface area in illuvial subsoil horizons (Bt horizons) where percent clay and the proportion of FEAs are positively correlated (R²= 0.82). Although percent clay and the proportion of FEAs are also positively correlated in surface horizons (R²= 0.48), per unit clay A and E horizons have a higher proportion of FEAs due to the influence of organic matter. At low organic matter concentrations a threshold of 4-5% of clay volume by percentage is needed to FEAs to form. Our methods for calculating fine Earth aggregation opens the door to large-scale spatial and temporal studies that will allow us to better understand the pedogenic processes behind soil aggregation, a property central to the understanding of the Earth’s critical zone.