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• Importance of SOM and CaCO 3 for aggregate formation depends on aggregate size class. • CaCO 3 controls aggregation of microaggregates in highly calcareous soils. • In calcareous soils SOM dynamics are predominantly associated with macroaggregate formation. • CaCO 3 does not improve the stabilisation of SOM. • CaCO 3 affects water (in)stability of aggregates. In soil, carbonates are important inorganic binding agents, controlling aggregate formation and soil structural stability. Until today, a clear understanding of the aggregate forming mechanisms in calcareous soil is missing. The objective of this study was to elucidate the initial aggregate formation in calcareous loess from the point when soil organic matter content was low and soil properties were mostly controlled by the parent material. We used a space-for-time chronosequence approach on agriculturally reclaimed loess soils in an open-cast lignite mining area in Garzweiler, Germany. We selected six time points from 0 to 24 years after reclamation in order to investigate the early processes of soil aggregate formation. Samples from two sampling depths (1–5 cm and 16–20 cm) were wet sieved into four size classes: silt- and clay-sized fraction (<63 µm), large microaggregates (63–200 µm), small macroaggregates (200–630 µm), and large macroaggregates (>630 µm). Each aggregate size class was characterized for soil organic carbon, total nitrogen and CaCO 3 content. Organic matter amendment induced the formation of large macroaggregates (>630 µm), but we did not detect an accumulation of soil organic carbon neither in microaggregates nor in the silt- and clay-sized fraction. The silt- and clay-sized fraction contained large amounts of CaCO 3 throughout the whole space-for-time chronosequence. This finding shows that in undeveloped loess material, microaggregation is controlled by cementation by carbonates inherited from the loess. These carbonates were likely easily dissolved during contact with water, therefore, aggregates in the early stage of soil development were not water-stable. Soil aggregate formation on young calcareous soils consists of two main mechanisms, 1. fresh soil organic matter predominantly contributes to macroaggregate formation, and 2. intrinsic cementing of loess through carbonates affects microaggregate stability. Therefore, the early stages of soil development in loess material do not show an aggregate hierarchy as we would expect in soils with high silt and clay content, such as Cambisols and Luvisols. [ABSTRACT FROM AUTHOR]

Wildfires change the chemical composition of soil organic matter (SOM). Since the effects of fires on organic matter (OM) in soil solution are largely unknown, we sought to compare the quality of dissolved organic matter (DOM) and total organic matter (TOM = DOM + particulate OM) between burned and control sites. The sites were subjected to a low-intensity surface fire in a coniferous forest in Germany dominated by spodic Cambisols derived from Triassic sandstone. Soil solutions from three different soil horizons (O, Ah, Bw), and throughfall (TF) were analyzed using solid-state 13 C NMR spectroscopy, allowing us to track the initial fire impact on OM vertically through the soil profile and 70 days after the fire. In addition, organic layer samples were analyzed by 13 C NMR spectroscopy to compare the OM quality. Under control conditions, properties of SOM influence the chemical composition of DOM and TOM in soil solutions. However, with fire, there is an initial increase in aromatic C in SOM, but not in DOM and TOM. Seventy days after the fire treatment, the aromatic C fraction in soil solutions of O and Ah layers increased, possibly due to accelerated oxidation processes, which would make the aromatic C more water-soluble. Our findings highlight the importance of short-term low-intensity fire-induced changes on forest soils that are useful to those seeking to better understand and model the temporal variability in the response of soil chemistry to fire to improve our knowledge of TOM and DOM dynamics in soils. [ABSTRACT FROM AUTHOR]

Temporal dynamics create unique and often ephemeral conditions that can influence soil microbial biogeography at different spatial scales. This study investigated the relation between decimeter to meter spatial variability of soil microbial community structure, plant diversity, and soil properties at six dates from April through November. We also explored the robustness of these interactions over time. An historically unfertilized, unplowed grassland in southwest Germany was selected to characterize how seasonal variability in the composition of plant communities and substrate quality changed the biogeography of soil microorganisms at the plot scale (10 m × 10 m). Microbial community spatial structure was positively correlated with the local environment, i.e. physical and chemical soil properties, in spring and autumn, while the density and diversity of plants had an additional effect in the summer period. Spatial relationships among plant and microbial communities were detected only in the early summer and autumn periods when aboveground biomass increase was most rapid and its influence on soil microbial communities was greatest due to increased demand by plants for nutrients. Individual properties exhibited varying degrees of spatial structure over the season. Differential responses of Gram positive and Gram negative bacterial communities to seasonal shifts in soil nutrients were detected. We concluded that spatial distribution patterns of soil microorganisms change over a season and that chemical soil properties are more important controlling factors than plant density and diversity. Finer spatial resolution, such as the mm to cm scale, as well as taxonomic resolution of microbial groups, could help determine the importance of plant species density, composition, and growth stage in shaping microbial community composition and spatial patterns. [ABSTRACT FROM AUTHOR]