Resistance of loess-derived soils to environmental stress: lessivation revisited

The stability of soils is determined by the presence of (in-)organic aggregation agents, including inorganic cements and polyvalent cations, as well as organic binding and bridging agents. These components strengthen the resistance of soils and their (micro-)aggregates against mechanical and physicochemical stress caused by fluctuating water and changes in the composition of infiltrates. Consequently, the availability, distribution and stability of these aggregation agents directly influences particle (im-)mobilisation and translocation (lessivation), a key process in the formation of diagnostic soil horizons, particularly in the context of pedogenesis on loess substrates. Given that soils developed from loess are agriculturally valuable but sensitive to environmental stress, we explored the susceptibility of topsoils from Regosols and Luvisols to particle release under osmotic and hydraulic stress. We conducted a series of water-unsaturated column experiments at controlled boundary conditions over a duration of seven months and recorded the response of those soils to physicochemical and hydraulic stress at high temporal resolution.

Our experimental pedogenesis study revealed that both soil types tolerated hydraulic stresses well during drainage or ponding but responded sensitively to varying ionic strengths. Following an initially electrostatically induced particle immobilization during a pulse of high ionic strength, we observed a significant particle release driven by peptization. Particularly in Luvisol columns, a notable release of hydrophobic organic material, originally stabilized within aggregates, was observed. In contrast, the effluents of Regosol were primarily characterized by carbonate dissolution products. A large proportion of the released calcium ions was immobilized in the solid phase and contributed to the formation of cation bridges, while inorganic carbon became increasingly enriched in the effluents. In Luvisol, progressive depletion of calcium at cation exchange sites and limited availability of carbonates were also observed, which increased soil susceptibility to environmental stresses, resulting in an irreversible loss of (in-)organic aggregation agents. This induces the translocation of iron, aluminum, phosphorus, and predominantly hydrophobic organic matter via organo-mineral associations into deeper soil horizons.

Our experiments show that with ongoing pedogenesis, common stresses such as alternating water supply, ponding, or drainage have a significantly lower impact on the composition of mobile soil inventory compared to disruptive stresses, such as rapid increases in ionic strength, e.g., due to intense wetting-drying cycles, road salt application, or fertilization.  Therefore, stress induced by chemical gradients appears to play a more critical role in the extent of lessivation than mechanical stress resulting from flow. Our results also suggest a need for gentle and sustainable soil management practices to preserve the stabilizing function of aggregation agents in soil.