Loess-derived soils under stress: Lessons learned from pedogenic response patterns

Pedogenesis is the complex interplay of biogeophyical and biogeochemical mechanisms operating simultaneously. Resolving the interactions and feedbacks requires experimental and analytical approaches that provide and integrate multiple, highly resolved signals into a coherent process-based framework (Totsche et al. 2024). Here, we investigate the response of loess-derived Regosol and Luvisol topsoils to controlled hydraulic and chemical stress using water-unsaturated column experiments conducted over seven months. Effluent, including pH, electrical conductivity (EC), major ions and elements, inorganic carbon, particle concentrations, and organic matter (OM) quality and quantity,  was monitored with high frequency.

To evaluate parameter interdependence and their joint response to forced perturbations, we applied a multivariate decomposition. Our analysis yielded distinct “effluent patterns” that represent recurring combinations of physicochemical parameters that evolve coherently over time. These patterns reflect the interplay among multiple processes, including solute release, particle mobilization, and OM transport, rather than isolated parameter responses.

The effluent course of both soils was dominated by four patterns, which together explained approximately 90% of the total variance. Pattern one reflects EC-driven transport and tracer-induced cation exchange, integrating conservative solute movement with the release of exchangeable mono- and polyvalent cations. Pattern two combines particle export with the mobilization of aluminum, iron, and phosphorus and fluorescence signatures of recalcitrant OM, indicating the destabilization and transport of organo-mineral associations (Lehmann et al., 2021). Pattern three is dominated by inorganic carbon dynamics and alkaline earth cations, revealing carbonate dissolution and diffusion-controlled release processes, following hydraulic stress. The fourth pattern is linked to the reversible exchange of surface-associated OM, coupled to the dynamics of monovalent cations, and the re-establishment of cation bridging after chemical perturbation (see Ritschel et al., 2023).

Clear differences in pattern expression were observed between soil types. Regosol responded to chemical stress primarily through carbonate dissolution and cation exchange, thereby buffering ionic strength gradients and limiting particle mobilization. In contrast, the Luvisol exhibited pronounced disaggregation and enhanced particle and hydrophobic, pedogenic OM export under electrolyte shifts, reflecting advanced pedogenic development and reduced stress resistance.

By capturing these contrasting responses to the forced stresses, we demonstrate how soil development governs the susceptibility of soils to environmental perturbations and, consequently, the (im-)mobilization pathways of particles, ions, and OM. The study addresses challenges associated with fluctuations in salinity, wetting and drying cycles, and the extensive use of liquid mineral fertilizers, as well as their effects on soil aggregation, organic matter dynamics, and nutrient availability. Together, these findings provide the basis for a conceptual framework for enhancing soil resilience in vulnerable agroecosystems under changing climate/environmental conditions.

 

Lehmann, K., Lehmann, R., Totsche, K. U. (2021) Event-driven dynamics of the total mobile inventory in undisturbed soil account for significant fluxes of particulate organic carbon. Sci. Total Environ. 756, 143774, doi: 10.1016/j.scitotenv.2020.143774

Totsche, K.U., Ray, N. and Kögel-Knabner, I. (2024), Structure–function co-evolution during pedogenesis—Microaggregate development and turnover in soils. J. Plant Nutr. Soil Sci., 187: 5-16. https://doi.org/10.1002/jpln.202400012

Ritschel, T., Aehnelt, M., Totsche, K.U., (2023). Organic matter governs weathering rates and microstructure evolution during early pedogenesis. Geoderma 429, 116269, https://doi.org/10.1016/j.geoderma.2022.116269