Structure Regeneration in 2mm Sieved Soils of Varying Clay Mineralogy.

Soil structure is essential to all soil ecosystem functions and services. Activities that impact on structure has significant effects on soil functions. This study examines the mechanisms involved in restoration of structure in 2-mm sieved soils of varying clay mineralogy. Totally seven soils, six similarly-textured (5 illites and 1 kaolinite sandy loam) soils representing a gradient in organic carbon and a smectite-rich soil, were subjected to organic amendment or no amendment, and incubated for a 22-month period in small, portable lysimeters. The soils were disturbed mechanically by simulated tillage periodically and exposed to field conditions (freeze-thaw and wet-dry cycles) during the regeneration period. Intact soil cores (100 cm³) were extracted from lysimeters and soil-water retention, gas diffusion coefficient, and air permeability were measured. Macro-aggregates of four size classes were retrieved from the bulk soil and aggregate tensile strength evaluated. After the incubation period, there were significant decreases in bulk density and increases in pores >100 µm for lysimeter samples compared with initial conditions. Soil structure complexity, quantified by measured soil gas diffusivity, air permeability, and derived pore network indices was greater for incubated samples than SR. Significant macro-aggregation was seen for all samples regardless of clay type, organic matter content, or amendment. Soil workability calculated from aggregate tensile strength was highest for the kaolinite-rich soil and lowest for the smectite-rich soil and for the illites, and found dependent on organic matter and clay contents. In summary, 22-month exposure of 2-mm sieved soil to field conditions yielded significant structure regeneration with an impact of clay mineralogy mainly on gas transport and aggregate characteristics, and only minimal short-term effects of organic amendment.