Process sequence of soil aggregate formation disentangled through multi-isotope labelling

Microaggregates (250 µm) that resisted 60 J mL−1 ultrasonic dispersion. Afterwards, we assessed the C, N, Fe, and Si stable isotope composition in each size fraction. After four weeks we found a rapid build-up of stable macroaggregates comprising almost 50 % of soil mass in the treatment with plants and respective soil rooting, but only 5 % when plants were absent. The formation of these stable macroaggregates proceeded with time. Soil organic carbon (SOC) contents were elevated by 15 % in the large macroaggregates induced by plant growth. However, the recovery of EPS-derived 13C was below 20 % after 4 weeks, indicating rapid turnover in treatments both with and without plants. The remaining EPS-derived C was mainly found in macroaggregates when plants were present and in the occluded small microaggregates (<20 µm) when plants were absent. The excess of bacterial 15N closely followed the pattern of EPS-derived 13C (R2 = 0.72). In contrast to the organic gluing agents, the goethite-57Fe and montmorillonite-29Si were relatively equally distributed across all size fractions. Overall, microaggregates were formed within weeks. Roots enforced this process by stabilizing microaggregates within stable macroaggregates. As time proceeded the labelled organic components decomposed, while the labelled secondary oxides and clay minerals increasingly contributed to aggregate stabilization and turnover at the scale of months and beyond. Consequently, the well-known hierarchical organization of aggregation follows a clear chronological sequence of stabilization and turnover processes

Initial microaggregate formation: Association of microorganisms to montmorillonite-goethite aggregates under wetting and drying cycles

There is an intimate relationship between microorganisms and the formation and stability of soil microaggregates, realized by the immobilization and occlusion of microorganisms. Little is known about the initial aggregate formation phase and the role of microorganisms in this process under the impact of environmental changes such as wetting and drying. We investigated this initial aggregate formation process of montmorillonite and goethite in combination with two bacterial strains, Pseudomonas protegens strain CHA0 and Gordonia alkanivorans strain MoAcy 2, in the presence or absence of stress conditions in form of wetting and drying cycles for up to eight days. Montmorillonite and goethite formed microaggregates instantaneously, the size of these aggregates being enhanced in the presence of microorganisms, resulting in up to twofold larger aggregates. This increase in aggregate size was strain-dependent. However, the aggregates that developed during the first 48 h broke into smaller structures later on. A microscopic analysis of the microaggregates revealed that notably the larger microaggregates harbored bacteria and that microaggregates had a sheltering effect on living cells, especially when exposed to desiccation stress. Additionally, aggregate formation was analyzed in the presence of a Pseudomonas protegens mutant strain (CHA211) with increased production capability of extracellular polymeric substances (EPS). About fivefold higher survival rates of culturable cells were observed after desiccation for this EPS overproducing mutant strain in comparison to the wild-type. Our results hint at an aggregate formation process characterized by a rapid occlusion of mineral compounds, and, after the addition of microorganisms, the bacterial colonization of small microaggregates, leading to an increase in aggregate size. The further development of the aggregate size distribution varied depending on the presence of microbial taxa and was modulated by environmental conditions like desiccation events

Wet sieving versus dry crushing: Soil microaggregates reveal different physical structure, bacterial diversity and organic matter composition in a clay gradient

Gefördert im Rahmen des Projekts DEA