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Soils in forest ecosystems bear a high potential as carbon (C) sinks in the mitigation of climate change. The amount and characteristics of soil organic matter (SOM) are driven by inputs, transformation, degradation and stabilization of organic substances. While tree species fuel the C cycle by producing aboveground and belowground litter, soil microorganisms are crucial for litter degradation as well as the formation and stabilization of SOM. Nonetheless, our knowledge about the tree species effect on the SOM status is limited, inconsistent and blurred. The investigation of tree species effects on SOM is challenging because in long-established forest ecosystems the spatial distribution of tree species is a result of the interplay of environmental factors including climate, geomorphology and soil chemistry. Moreover, tree distribution can further vary with forest successional stage and silvicultural management. Since these factors also directly affect the soil C-status, it is difficult to identify a pure “tree species effect” on the SOM status at regular forested sites. It therefore remains unclear in how far tree species-specific litter with different quality influences the microbial driven turnover and formation of SOM.
Tree species effects on SOM and related soil microbial properties were investigated by examining soil profiles (comprising organic forest floor horizons and mineral soil layers) in different forest stands at the recultivated spoil heap ‘Sophienhöhe’ located at the lignite open-cast mine Hambach near Jülich, Germany. The afforested sites comprised monocultural stands of Douglas fir (Pseudotsuga menziesii), black pine (Pinus nigra), European beech (Fagus sylvatica) and red oak (Quercus rubra) as well as a mixed deciduous stand site planted mainly with hornbeam (Carpinus betulus), lime (Tilia cordata) and common oak (Quercus robur) that were grown for 35 years under identical soil and geomorphological conditions. Because the parent material used for site recultivation was free from organic matter or coal material, the SOM accumulation is entirely the result of in situ soil development due to the impact of tree species.
The first study revealed that tree species had a significant effect on soil organic carbon (SOC) stocks, stoichiometric patterns of C, nitrogen (N), sulfur (S), hydrogen (H) and oxygen (O) as well as the microbial biomass carbon (MBC) content in the forest floor and the top mineral soil layers (0-5 cm, 5-10 cm, 10-30 cm). In general, forest floor SOC stocks were significantly higher at coniferous forest stands compared to deciduous tree species, whereas in mineral soil layers the differences were smaller. Thus, the impact of tree species decreased with increasing soil depth. By investigating the linkage of the natural abundance of 13C and 15N in the soil depth gradients with C:N and O:C stoichiometry, the second study showed that differences in SOC stocks and SOM quality resulted from a tree species-dependent turnover of SOM. Significantly higher turnover of organic matter in soils under deciduous tree species depended to 46 % on the quality of litterfall and root inputs (N content, C:N, O:C ratio), and on the initial isotopic signatures of litterfall. Hence, SOM composition and turnover also depends on additional – presumably microbially driven – factors. The subsequent results of the third study revealed that differences in SOM composition and related soil microbial properties were linked to different microbial communities. Phospholipid fatty acid (PLFA) patterns in the soil profiles indicated that the supply and availability of C and nutrient-rich substrates drive the distribution of fungi, Gram-positive (G+) bacteria and Gram-negative (G−) bacteria between tree species and along the soil depth gradients. The fourth study investigated the molecular composition of extractable soil microbial biomass-derived (SMB) and SOM-derived compounds by electrospray ionization Fourier transformation ion cyclotron resonance mass spectrometry (ESI-FT-ICR-MS). This was complemented by the analysis of nine monosaccharides representing microbial or plant origin. Microbially derived compounds substantially contributed to SOM and the contribution increased with soil depth. The supply of tree species-specific substrates resulted in different chemical composition of SMB with largest differences between deciduous and coniferous stands. At the same time, microorganisms contributed to SOM resulting in a strong similarity in the composition of SOM and SMB.
Overall, the complex interplay of tree species-specific litter inputs and the ability, activity and efficiency of the associated soil fauna and microbial community in metabolizing the organic substrates leads to significant differences in the amount, distribution, quality and consequently, the stability of SOM. These findings are useful for a targeted cultivation of tree species to optimize soil C sequestration and other forest ecosystems services.
Climate fluctuations and the pyroclastic depositions from volcanic activity both influence ecosystem functioning and biogeochemical cycling in terrestrial and marine environments globally. These controlling factors are crucial for the evolution and fate of the pristine but fragile fjord ecosystem in the Magellanic moorlands (~53°S) of southernmost Patagonia, which is considered a critical hotspot for organic carbon burial and marine bioproductivity. At this active continental margin in the core zone of the southern westerly wind belt (SWW), frequent Plinian eruptions and the extremely variable, hyper-humid climate should have efficiently shaped ecosystem functioning and land-to-fjord mass transfer throughout the Late Holocene. However, a better understanding of the complex process network defining the biogeochemical cycling at this land-to-fjord continuum principally requires a detailed knowledge of substrate weathering and pedogenesis in the context of the extreme climate. Yet, research on soils, the ubiquitous presence of tephra and the associated chemical weathering, secondary mineral (trans)formation and organic matter (OM) turnover processes is rare in this remote region. This complicates an accurate reconstruction of the ecosystem´s potentially sensitive response to past environmental impacts, including the dynamics of Late Holocene land-to-fjord fluxes as a function of volcanic activity and strong hydroclimate variability.
Against this background, this PhD thesis aims to disentangle the controlling factors that modulate the terrigenous element mobilization and export mechanisms in the hyper-humid Patagonian Andes and assesses their significance for fjord primary productivity over the past 4.5 kyrs BP. For the first time, distinct biogeochemical characteristics of the regional weathering system serve as major criterion in paleoenvironmental reconstruction in the area. This approach includes broad-scale mineralogical and geochemical analyses of basement lithologies, four soil profiles, volcanic ash deposits, the non-karst stalagmite MA1 and two lacustrine sediment cores. In order to pay special attention to the possibly important temporal variations of pedosphere-atmosphere interaction and ecological consequences initiated by volcanic eruptions, the novel data were evaluated together with previously published reconstructions of paleoclimate and paleoenvironmental conditions.
The devastative high-tephra loading of a single eruption from Mt. Burney volcano (MB2 at 4.216 kyrs BP) sustainably transformed this vulnerable fjord ecosystem, while acidic peaty Andosols developed from ~2.5 kyrs BP onwards after the recovery from millennium-scale acidification. The special setting is dominated by most variable redox-pH conditions, profound volcanic ash weathering and intense OM turnover processes, which are closely linked and ultimately regulated by SWW-induced water-level fluctuations. Constant nutrient supply though sea spray deposition represents a further important control on peat accumulation and OM turnover dynamics. These extreme environmental conditions constrain the biogeochemical framework for an extended land-to-fjord export of leachates comprising various organic and inorganic colloids (i.e., Al-humus complexes and Fe-(hydr)oxides). Such tephra- and/or Andosol-sourced flux contains high proportions of terrigenous organic carbon (OCterr) and mobilized essential (micro)nutrients, e.g., bio-available Fe, that are beneficial for fjord bioproductivity. It can be assumed that this supply of bio-available Fe produced by specific Fe-(hydr)oxide (trans)formation processes from tephra components may outlast more than 6 kyrs and surpasses the contribution from basement rock weathering and glacial meltwaters. However, the land-to-fjord exports of OCterr and bio-available Fe occur mostly asynchronous and are determined by the frequency and duration of redox cycles in soils or are initiated by SWW-induced extreme weather events.
The verification of (crypto)tephra layers embedded stalagmite MA1 enabled the accurate dating of three smaller Late Holocene eruptions from Mt. Burney (MB3 at 2.291 kyrs BP and MB4 at 0.853 kyrs BP) and Aguilera (A1 at 2.978 kyrs BP) volcanoes. Irrespective of the improvement of the regional tephrochronology, the obtained precise 230Th/U-ages allowed constraints on the ecological consequences caused by these Plinian eruptions. The deposition of these thin tephra layers should have entailed a very beneficial short-term stimulation of fjord bioproductivity with bio-available Fe and other (micro)nutrients, which affected the entire area between 52°S and 53°S 30´, respectively. For such beneficial effects, the thickness of tephra deposited to this highly vulnerable peatland ecosystem should be below a threshold of 1 cm.
The Late Holocene element mobilization and land-to-fjord transport was mainly controlled by (i) volcanic activity and tephra thickness, (ii) SWW-induced and southern hemispheric climate variability and (iii) the current state of the ecosystem. The influence of cascading climate and environmental impacts on OCterr and Fe-(hydr)oxide fluxes to can be categorized by four individual, in part overlapping scenarios. These different scenarios take into account the previously specified fundamental biogeochemical mechanisms and define frequently recurring patterns of ecosystem feedbacks governing the land-to-fjord mass transfer in the hyper-humid Patagonian Andes on the centennial-scale. This PhD thesis provides first evidence for a primarily tephra-sourced, continuous and long-lasting (micro)nutrient fertilization for phytoplankton growth in South Patagonian fjords, which is ultimately modulated by variations in SWW-intensity. It highlights the climate sensitivity of such critical land-to-fjord element transport and particularly emphasizes the important but so far underappreciated significance of volcanic ash inputs for biogeochemical cycles at active continental margins.