Organic matter input determines structure development and aggregate formation in artificial soils

The formation of aggregates is considered to happen by clustering and cohesion of mineral particles and organic matter. Up to now, little is known about the role of different organic matter types within this process. We developed an experimental set-up to study the influence of organic carbon (OC) derived from particulate organic matter (POM-C) and from dissolved organic matter (DOM-C) for aggregate formation under controlled laboratory conditions. We designed artificial soil microcosms with loamy texture, mimicking an arable Cambisol, and performed an incubation for 30 days under constant water tension. The microcosms received either POM-C as milled hay litter, DOM-C as solution derived from hay or no additional OC input. We characterized the developed soil matrix by analysing aggregate size distribution and composition as well as the associated pore system. After incubation, we observed the formation of mostly large, water-stable macroaggregates (630–3000 μm) in all treatments with OC addition. POM-C input led to an increase in mass contribution of the large macroaggregates from 6% to 88 ± 6%, when compared to the original particle size distribution, while DOM-C input led to an increase to 59 ± 9%. The size fractions of the small microaggregates and primary particles (<63 μm) were present in a mass contribution of 11 ± 5% for the POM-C addition and 35 ± 7% for the DOM-C addition. The small microaggregates had a predominant size of 2–20 μm. The OC distribution was closely linked to the aggregate size distribution, indicating a functional link concerning the aggregate forming process. In POM-C microcosms, 95% of the OC present after incubation was stored in large macroaggregates. In DOM-C microcosms, the OC was bimodally distributed between large macroaggregates (59 ± 0.8%) and small microaggregates (33 ± 0.4%). CO₂ release was highest with DOM-C input, indicating that approximately 41% of the organic matter input was mineralized. High-resolution μCT scanning was used on subsample cores to investigate 3D pore structure differences. POM-C microcosms had a decreased macroporosity (>50 μm pore size) and highly connected pores (Euler number change from −1519 to −5767) after incubation. This indicates a reorganization of the pore space towards a fine, connected pore system with improved gas, water and nutrient exchange and thus implications for the microbial microenvironment. DOM-C microcosms also had an increased pore connectivity (Euler number change from −122 to −665) after incubation, but overall values were smaller than in the POM-C microcosms, indicating that changes in pore structure depend on OM type initiating aggregation. We demonstrate that aggregate formation in a loamy textured artificial soil is possible within 30 days and can take place in the absence of physical interference like stirring or repeated wet-dry cycles. Different organic matter types induce specific formation of aggregates and associated pore system as the effect of microbial decomposition of the OC input and the interaction of OC compounds with mineral surfaces.