The role of colloids and nanoparticles in the soil microaggregate formation, stability, and architecture

Krause, Lars; Klumpp, Erwin (Thesis advisor); Schäffer, Andreas (Thesis advisor)

Aachen (2019, 2020)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2019

Abstract

The soil represents a dynamic and highly complex ecosystem at small spatial scales; ecological and physical soil functions such as the allocation of microbial habitats, storage of organic carbon (OC), and water are related to the composition and architecture of soil microaggregates <250 µm (SMA) including small (<20 µm) SMA as fundamental building units of larger soil aggregate structures. Especially fine colloids (<0.45 µm) as the smallest building units have the potential to stabilize aggregates across different hierarchy levels since they can be either highly reactive or mobile in dependency of changing environmental conditions. In this thesis, the size distribution and composition of small SMA was investigated with respect to their i) potential abiotic and biotic stabilizing agents such as Fe (hydr)oxides, OC, and microorganisms, ii) aggregation capability to larger structures, and iii) allocation of microbial habitats. Small SMA were isolated from macroaggregates (>250 µm) of five arable German Luvisols with a gradient of clay content. Additionally, the small SMA development from the colloidal model minerals montmorillonite and goethite was investigated in presence of microorganisms. A fractionation scheme was established to isolate free and occluded small SMA from macroaggregates at defined stability levels using ultrasonication at different energy levels (60, 250, or 440 J mL-1), wet sieving, and filtration. The composition and size distribution of small SMA were analyzed using asymmetric flow field-flow-fractionation (AF4), inductively coupled plasma mass spectrometry (ICP-MS), OC detector (OCD), laser diffraction, and microparticle detector (XPT) including image analysis. The multilateral impact between microorganisms, microbial survival, aggregates, and aggregation was investigated by viability stains, microscopy and image analysis. The mass distribution of SMA grouped the Luvisols into those with small (19, 22, and 24%) and large (32 and 34%) clay contents. The finer textured soils exhibited larger portions of occluded SMA with a size distribution peaking at 5-6 µm. Yet the occluded small SMA in the finer textured soils showed an enrichment of colloids <1 µm. The OC content was enriched in small SMA fractions, but more in the coarse textured sites, whereas the opposite was true for the large (250-20 µm) SMA. Both, the clay content and fine colloids increased the abundance and stability of occluded small SMA. The size distribution of small SMA changed with the applied ultrasonic energy and upon the chemical removal of cementing and gluing agents, such as Fe (hydr)oxides or OC. The application of 440 J mL-1 increased the abundance of colloids at the expense of larger subfractions (>1 µm). Removing Fe with dithionite-citrate-bicarbonate (DCB) shifted the size to a larger equivalent diameter, while destruction of OC with sodium hypochlorite (NaOCl) reduced it. The isolated and chemically treated small SMA were allowed to reaggregate during a repeated wetting and drying procedure in which small SMA reaggregated to particles with larger diameters up to 10 µm, including a decrease of the colloid fraction. Intriguingly, this gain in size was exceeded after Fe removal but it was not affected by OC removal. According to these changes, cementing functions could be attributed to Fe and aluminum (Al) (hydr)oxides since they impacted the stability of small SMA primarily via pore-filling, cementing the aggregates to a smaller size. In contrast, the gluing role of OC was restricted to the colloidal size range, holding them together to small SMA within defined size ranges when OC was present, and releasing colloids when OC was absent. The impact of biotic and abiotic factors on the small SMA formation was investigated on model minerals and microorganisms with different capabilities to produce extracellular polymeric substances (EPS) as components of biofilms. The model minerals were incubated with Pseudomonas protegens strain CHA0 and mutant strain CHA211 with an increased capability for EPS production, and Gordonia alkanivorans strain MoAcy 2. The incubation was conducted including wetting and drying cycles. Results showed the dynamic development of aggregates including a rapid occlusion of montmorillonite and goethite, followed by the colonization of small SMA increasing the aggregate size. Depending on the microbial taxa, the development of the aggregate size distribution varied which was modulated by desiccation. Depending on the strain, up to two-fold larger aggregates were formed once microorganisms were present. Larger SMA harbored living bacteria including a sheltering effect upon repeated desiccation. An enhanced production capability of EPS resulted in a five-fold larger survival rate after desiccation. These results highlighted the strongly linked interaction feedbacks between biotic and abiotic factors during aggregation which need to be considered to understand and predict the aggregate formation and development. Overall, the thesis contributed to a better understanding of the dynamic SMA formation process in which the colloidal sized building units are of particular relevance across different aggregate hierarchy levels.

Institutions

• Department of Biology [160000]
• Chair of Environmental Biology and Chemodynamics [162710]